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doi:10.2204/iodp.proc.309312.103.2006

Expedition 309

Predrilling experiments

The first scientific operation of Expedition 309 was the deployment of the wireline WSTP to measure ambient temperature and collect a sample of fluid at the bottom of Hole 1256D. These operations took place 929 days after the cessation of logging operations in Hole 1256D during Leg 206, which should have been sufficient time for the borehole to recover from drilling operations and return to ambient thermal and chemical conditions. Hole 1256D was reentered with a logging BHA, and pipe was tripped until the bit began taking weight at 725 mbsf, ~27 m above the maximum penetration during Leg 206. The bit was picked up ~2 m, and the WSTP was deployed to collect a water sample at this level. The characteristics of the WSTP have been previously described (see Davis, Mottl, Fisher, et al., 1992; Fisher, Urabe, Klaus, et al., 2005), and similar preparations were undertaken prior to deployment following intensive maintenance of the tool. After arming the sampler, the tool was lowered on the wireline to ~2 m below the throat of the logging bit. The first sample (WST/O 1; Table T11) was taken from 724.6 mbsf, very close to or possibly within the unconsolidated fill at the bottom of the hole. This resulted in the filters on the WSTP becoming clogged with silt and grit, and a partial sample of murky fluid with a high concentration of suspended particles was recovered. While the WSTP was being cleaned and prepared for a second sampling attempt, the APCT tool was lowered to measure the temperature of the bottom of the borehole (see below). A second water sample (WST/O 2) was then taken with the WSTP at 712.6 mbsf, and clear fluid was returned. The drill string was then withdrawn to 262 mbsf (7 m above the casing shoe), and two wireline logging strings (triple combo and FMS-sonic; see below) were deployed to determine the condition and gauge of Hole 1256D for comparison with measurements made at the end of Leg 206.

In addition, a temperature measurement and a sample of ocean bottom water were taken with the WSTP before the fifth reentry of Hole 1256D on 30 July 2005. The sample (WST/O 3) was taken from ~2 m above the seafloor (06°44.1727′N, 91°56.0506′W; 3633 meters below sea level [mbsl]) ~27 m northeast of Hole 1256D.

Temperature measurements in Hole 1256D

As the thermal state of Hole 1256D and the temperature of the water samples recovered are relevant to the discussion of the composition of borehole fluids, wireline temperature measurements using the APCT tool and the LDEO TAP tool will be described first. The results of the other predrilling wireline measurements are documented later in this section.

During the reconditioning of the WSTP for the retrieval of a second borehole fluid sample, the APCT tool was lowered to record the temperature at the bottom of the hole as insurance against failure of the TAP tool. The APCT tool is the same tool that was used for reading temperature in bottom water and upper sediments in Hole 1256B during Leg 206, providing the advantage of similar if not identical calibration. However, because the tool is housed in an APC cutting shoe, time for thermal equilibration is several minutes. The tool was lowered ~140 m/min, slow enough to record a constant temperature of 1.4°C between 2500 and 3600 mbsl, reproducing the bottom water measurement from Leg 206 (Wilson, Teagle, Acton, et al., 2003). The tool was first lowered to the base of the drill pipe at 713 mbsf, where the second borehole fluid sample was to be taken, and allowed to equilibrate for 7 min. A stable temperature of 64.5°C was reached. The tool was then lowered to 725 mbsf, the same depth where the WSTP obtained the murky water sample (WST/O 1), and allowed to equilibrate for 3 min, reaching a stable temperature of 65.8°C (Fig. F34). A 1.3°C temperature difference over 12 m is well above the temperature gradient expected from Hole 1256B measurements or the subsequent TAP tool results discussed below. The reading at the top of the hole fill is probably less prone to disturbance compared to the shallower (712.6 mbsf) measurement, which may have been influenced by entrained water that was brought down the hole with the drill string.

During subsequent triple combo logging runs, the temperature of Hole 1256D was recorded using the TAP tool during the downhole, uphole, repeat, and main runs (see below) with the base of the drill pipe withdrawn to within the casing. The TAP tool has a much lower thermal inertia than the APCT tool and is more responsive to temperature changes. The TAP tool temperature sensor is in an open compartment at the bottom of the triple combo tool string and is more protected from turbulence and entrainment of borehole fluid when the tool is being raised as opposed to lowered in the hole. As a result, temperature profiles recorded while logging uphole tend to be more regular, but because the tool string tends to entrain fluid and heat in its wake, borehole temperatures measured on the upward traverse are slightly higher than downward measurements and the second logging pass records higher temperatures than the first run.

The profiles show a steady increase in temperature from the base of the casing (269 mbsf) with a maximum temperature of 67.5°C recorded at the deepest logging depth (~725 mbsf) (Fig. F34). This temperature is slightly higher than that measured using the APCT tool (65.8°C), with the difference reflecting some combination of the difference in calibration between the tools and incomplete recovery from the disturbance caused by lowering the drill pipe to the top of the fill. Because the TAP tool is externally calibrated and the TAP tool ocean bottom temperature of 1.9°C matches regional measurements, we believe that temperatures measured in the borehole by this tool are closer to the true values than the APCT tool readings. Hence, TAP tool temperatures will be used in the following discussions of the borehole fluid chemistry.

Heat flow measured in the sedimentary section of Hole 1256B to a depth of 158 mbsf was 113 mW/m² (Wilson, Teagle, Acton, et al., 2003). In the basement section in Hole 1256D, thermal conductivity averaged 2.0 W/(m·K) in the ponded lava flow above 350 mbsf and 1.7 W/(m·K) below 350 mbsf. For a model of uniform heat flow of 113 mW/m² downhole, these conductivities predict a thermal gradient of 0.056 K/m above 350 mbsf and 0.067 K/m below 350 mbsf. These predictions are reasonably close to the observed gradients of 0.067 and 0.071 K/m, probably within uncertainties of thermal conductivity and probable slight disturbance of hole temperature. This is further evidence that there is little advection of heat by fluid in the Site 1256 basement or major vertical fluid movement in Hole 1256D.

Geochemistry of borehole fluids recovered from Hole 1256D

Each WSTP sample consists of two portions: a small volume (10–12 mL) of relatively pristine fluid captured within the Ti coil (denoted WST) and a larger volume (500–1000 mL) overflow sample (denoted WSO) collected in the tool housing. Upon recovery, splits of the samples were preserved for shore-based studies and analyzed aboard ship following established methods (Table T11; see the “Methods” chapter). Because of the small volume of fluid recovered from the Ti coil of the WSTP, not all shipboard analyses could be conducted on the more pristine samples. Analyses of the Ti coil and overflow samples were combined to establish a comprehensive characterization of borehole fluid. The first WSTP sample (WST/O 1) taken from the very bottom of the open hole (~724.6 mbsf) contained a very large component of suspended material and only a small overflow volume (~500 mL). The measured salinity of 26 suggests that the tool was not completely purged of deionized water before the filters became clogged and external fluid intake was curtailed. As such, we have concerns about the validity of the WST/O 1 samples, although most species are broadly in line with analyses of other borehole fluid samples. The second borehole fluid sample (WST/O 2) was taken ~12.5 m above the loose fill at the bottom of the hole, and clear fluid was recovered from both the Ti coil and the overflow chamber. Our preferred Hole 1256D borehole fluid composition (Table T11) is principally that analyzed from the Ti coil (WST 2), supplemented where necessary (pH, SiO₂, and NH₄⁺) with analyses from the overflow sample (WSO 2) where there was insufficient volume to complete the full suite of analyses. This preferred composition is used in the following diagrams and discussions. Similarly, our preferred composition of Site 1256 ocean-bottom seawater (WST/O 3) (Table T11) is derived from the Ti coil measurements, supplemented where necessary with analyses from the overflow volume.

Relative to Site 1256 bottom seawater, borehole fluid is hotter (~65°C) and slightly more neutral (pH = ~7.4) and has significantly lower alkalinity (0.85 mM). Salinity is unchanged (35), and sodium concentration and chlorinity are probably within experimental error. The largest changes are in concentrations of dissolved ions with major reductions in the concentrations of boron (–18%), sulfate (–19%), potassium (–41%), lithium (–47%), and magnesium (–55%). In contrast, the strontium concentration is slightly increased (18%) and calcium content is very strongly elevated (415%) (Fig. F35A).

Composition of uppermost basement fluid at Site 1256 has been estimated by extrapolating to basement pore water measurements made in the sedimentary overburden during Leg 206 (Table T11) (Wilson, Teagle, Acton, et al., 2003). Diffusive profiles were obtained for most elements, suggesting minimal advection of pore fluids through sediments at this site, giving confidence in the estimated uppermost basement fluid composition. An irregular pore water profile was measured for Sr as basement was approached, with significant uncertainty in the estimated Sr concentration (~120–220 µM). Compared to bottom seawater, uppermost basement fluid has lower concentrations of alkalinity (–66%), sulfate (–29%), silica (–10%), lithium (–38%), potassium (–15%), and magnesium (–41%) but strongly elevated Ca (132%) (Fig. F35B). Because of uncertainty in the Sr concentration of uppermost basement fluid, the behavior of this element is poorly constrained and it may exhibit strong enrichment (78%) or very slight depletion (–3%).

Compared to uppermost basement fluid estimated from Site 1256 pore waters, deep borehole fluid has similar pH, salinity, chlorinity, and alkalinity (Fig. F35C). Dissolved sulfate and silica concentrations in the borehole fluid are slightly (14%) to strongly (84%) increased, respectively. For the cations, Li (–15%), Mg (–25%), and K (–30%) concentrations are lower, whereas Ca is significantly higher (122%).

Comparisons with basement fluids from Site 504 and the eastern flank of the Juan de Fuca Ridge

The most detailed studies of the chemistry of basement fluids have been conducted in the region of the oceanic basement reference hole at Site 504 (ODP Sites 501, 504, 677, 678; see Mottl et al., 1983, 1985; Becker, Sakai, et al., 1988; Mottl and Gieskes, 1990; Becker, Foss, et al., 1992; Alt, Kinoshita, Stokking, et al., 1993) and during ODP Leg 168 and subsequent investigations of young buried basement on the eastern flank of the Juan de Fuca Ridge (Davis, Fisher, Firth, et al., 1997; Elderfield et al., 1999; Mottl et al., 2000; Wheat et al., 2000). These studies have involved both sediment pore water profiles and direct sampling of borehole fluids and have elucidated the main reactions occurring at low to moderate temperatures (<100°C) between buried ocean floor basalts and seawater-derived basement fluids. The processes are generally similar to those discerned from controlled laboratory fluid–rock exchange experiments (e.g., Mottl, 1983), and the resulting elemental changes to the fluids match the secondary mineralogy of altered basalts (e.g., Coggon et al., 2004). In general, during low-temperature seawater–basalt exchange, K and Mg (and Li) are lost from the fluid because of the formation of Mg saponite and celadonite in veins and filling vesicles. Groundmass minerals and phenocryst phases in basalts are commonly slightly altered unless alteration is particularly intense. The biggest change is in the replacement of mesostasis and partial replacement of volcanic glass by Mg saponite and celadonite. This leads to significant increases in fluid calcium (+ strontium) concentrations. However, strontium isotopic measurements indicate that in natural systems, an increase in basement fluid calcium concentrations includes a significant component of carbonate-derived calcium remobilized from the overlying sediments (Coggon et al., 2004).

A series of holes was drilled during Leg 168 into 0.6–3.5 m.y. old basement on Juan de Fuca Ridge (Sites 1023–1032; Davis, Fisher, Firth, et al., 1997). Because of the close proximity of Juan de Fuca Ridge to the North American continental margin, this young crust has been rapidly buried by 100–500 m of turbiditic and hemipelagic sediments. As a consequence of the burial of young ocean crust by a thick blanket of sediment, the temperature of the basement increases with distance from the ridge axis from 16° to 64°C and fluids display systematic chemical trends useful for comparison with Site 1256 basement and borehole fluids. Although it has been shown that there is no continuous pathway of eastward fluid flow from the unsedimented ridge axis to the buried flanks (Wheat et al., 2000), fluid compositions display a general evolution of reduced alkalinity, Mg, K, Na, SO₄, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr and increased Sr, Ca, and Cl with increasing basement age and temperature (Elderfield et al., 1999; Mottl et al., 2000) (Table T12).

For most species, both Site 1256 uppermost basement fluid and deep borehole fluid fall along irregular trends with temperature delineated by Juan de Fuca Ridge flank fluids (Fig. F36). With the exception of ODP Sites 1030 and 1031, which sit above a shallow basement high, there is a trend of decreasing Mg concentration with upper basement temperature (Fig. F36A). The ~35°C fluid from uppermost basement at Site 1256 lies directly along this trend, although the estimated composition of the uppermost basement fluid from Site 504 (Table T12) (Mottl et al., 1983) sits well above the Juan de Fuca profile. The ~67°C deep borehole fluid from Hole 1256D collected during Expedition 309 has a lower Mg concentration than the uppermost basement fluid, but the composition is still relatively high (~24 mM) compared to the hottest fluids collected from the Juan de Fuca Ridge (2–13 mM) (Table T12) and is indicative of fluid–rock Mg exchange at ~50° ± 5°C. Ca concentrations of both shallow and deep Site 1256 fluids are close to those predicted from Leg 168 samples, as are the concentrations of sulfate, alkalinity, K, and Li, although the behavior of Sr is poorly constrained. With the exception of Sr, which has a very high concentration (437 ppm) (Table T12), the uppermost basement fluid from Site 504 is also in close agreement with Leg 168 trends.

Systematic changes in chemistry occur in the Juan de Fuca fluids when compared to their Mg concentrations. With decreasing Mg concentration, calcium concentrations of Juan de Fuca basement fluids display a regular increase, and Site 1256 and 504 upper basement fluids fall along this trend. In contrast, deep borehole fluid from the bottom of Hole 1256D has a high Ca concentration (59 mM) (Fig. F37A) for Mg of ~24 mM.

When plotted against Mg concentration, other principal ions in the Hole 1256D borehole and uppermost basement fluids generally fall along rough trends defined from the Juan de Fuca Ridge flank transect. Alkalinity, sulfate, and K concentrations are generally lower with decreasing magnesium, although Li and Sr do not display any regular trends (Fig. F37). Site 1256 and 504 fluids display similar behavior, with the exception of Sr, which is poorly constrained at Site 1256 and very high at Site 504.

The disagreement between the concentration of Mg in hot deep borehole fluids with that predicted from uppermost basement fluids or laboratory experiments has previously been noted for wireline fluid samples recovered from Hole 504B (Becker, Foss, et al., 1992). Figure F38 shows the composition of the deepest borehole fluids collected in Hole 504B at the beginning of DSDP/ODP Legs 83, 111, and 137 (Mottl et al., 1985; Becker, Sakai, et al., 1988; Becker, Foss, et al., 1992) (Table T12) as well as the composition of the uppermost basement fluid estimated from pore water studies during DSDP Leg 69 (Mottl et al., 1983). Mg, Ca, K, and Sr concentrations of these deepest fluids are almost constant with depth despite the very large increase in borehole temperature (up to 158°C for the Leg 137 measurement) (Table T12). Sulfate displays a strong decrease with depth, whereas Li concentrations are higher. At Site 1256, similar behavior is exhibited by Mg with only a moderate difference (–25%) between the 35°C uppermost basement fluid and the 67°C deep borehole sample, and sulfate, K, Sr, and Li only show minor changes. There is a more pronounced change in Ca concentration with the fluid from the bottom of the hole having more than double the Ca content of the uppermost basement fluid, in contrast to minor variation in Ca for Hole 504B borehole fluids.

The near-uniform Mg and Ca concentrations of the most pristine borehole fluids, taken from the deepest available points in Hole 504B during the different drilling expeditions, suggests that a number of possible processes may be occurring in the Site 504 basement. It appears unlikely that surface seawater used to flush the hole before the commencement of logging operations remains a significant component of fluid in the borehole. If this surface water has reacted with the borehole wallrocks, one would predict strong compositional changes with increasing depth and temperature as drilling of the borehole progressed, yet this is not observed. Surface seawater appears to be displaced by basement formation fluids. Two possible mechanisms are suggested. Either the borehole is filled by basement fluid from the upper, more permeable parts of the crust and this fluid equilibrates thermally but not chemically with the wallrocks, or the formation fluid resident in the entire upper crust at Site 504 has a very uniform composition and there is presently very little fluid–rock exchange occurring at depth, perhaps due to very low fluid/​rock ratios. Elemental concentrations suggest that more reaction is occurring in Hole 1256D than in Hole 504B. Ca (and Li and K) concentrations are close to those expected for a fluid in chemical equilibrium with the basement at this level, and the Mg concentration is at least partially depleted toward the predicted composition (Figs. F36, F38).

In summary, at ~67°C, deep borehole fluid collected from the bottom of Hole 1256D (~725 mbsf) is close to the temperature predicted for a purely conductive thermal regime. There is little evidence for advective transport of either heat or chemicals. The deep fluid is significantly different from the estimated composition of the ~35°C uppermost basement fluid at Site 1256 (~250 mbsf), with moderately lower Li, K, and Mg concentrations but much higher dissolved silica and calcium concentrations in the deep fluid. Compared to the well-characterized basement fluids from the eastern flank of the Juan de Fuca Ridge, most ions are present in concentrations predicted for the in situ temperatures. The exception is the Mg concentration of deep borehole fluid, which at 24 mM is higher than would be expected for a fluid reacted with basement at >60°C.

Initial wireline logging of Hole 1256D

Following the second WSTP run, the drill string was raised to 262 mbsf (7 m above the casing shoe), and two wireline tool strings were deployed (Fig. F39). Heave conditions were good, typically <1.5 m throughout the logging operation. The first deployment was the triple combo tool string, which consisted of the HNGS, the APS, the HLDS, the DLL, and the TAP tool. The triple combo configuration was run to 725 mbsf, ~27 m above the maximum penetration during Leg 206 and the same level as the maximum depth reached by the WSTP and APCT tools. The first upward pass covered the interval from 725 to 633 mbsf, when data acquisition stopped because of a voltage drop. The caliper was closed, and the main pass was restarted from the bottom of the hole and proceeded successfully up to the casing shoe (269 mbsf).

The second tool string deployment consisted of the DSI, the SGT, the GPIT, and the FMS. A single pass successfully covered the interval from 725 mbsf to the casing shoe (269 mbsf). FMS arms were closed at 305 mbsf prior to the tool string entered the casing. Logging operations were completed at 0530 h on 18 July 2005.

During all logging runs, the WHC was turned on following the exit of the tools from pipe and was used continuously while the tool strings were in the open hole. Following acquisition, all logging data were transmitted to LDEO for depth and environmental correction processing (see “Downhole measurements” in the “Methods” chapter). All Expedition 309 logging data have been depth-shifted with respect to existing Leg 206 data, with the reference pass being the first Leg 206 FMS-sonic pass.

Data quality

The principal results from this initial phase of wireline logging are shown in Figures F39, F40, and F41. Borehole conditions are excellent, and no ledges or obstructions were encountered. Caliper readings from both the triple combo and FMS-sonic tool strings show good borehole conditions, with a diameter typically between 10 and 12 inches; 82% of the FMS caliper measurements are <12 inches (Fig. F42). The excellent hole conditions resulted in good measurements from the contact tools, such as density, porosity, and FMS. The FMS-sonic tool string followed a different pathway during the Expedition 309 pass than it followed during the Leg 206 passes (Fig. F43), and consequently in many intervals the FMS image coverage of the borehole wall has increased. Sonic velocities measured by the DSI appear to be of high quality.

Results

The primary purpose of initial logging operations was to check Hole 1256D for borehole wall breakouts and variations in hole diameter through comparison with measurements made at the end of Leg 206 (Wilson, Teagle, Acton, et al., 2003). Figures F40 and F41 present a selection of the Leg 206 and Expedition 309 logging measurements. The triple combo provided good data, and all measurements are in very good agreement with Leg 206 data (Fig. F40), with good repeatability of data from successive passes. Furthermore, acoustic velocity measurements (P-wave, S-wave, and Stoneley wave) appear to be less noisy and generally of higher quality than Leg 206 measurements (Fig. F41).

The size and shape of Hole 1256D are recorded by two orthogonal calipers (C1 and C2) on the FMS-sonic tool string and by a single caliper on the HLDS of the triple combo tool string (Figs. F40, F42). The caliper readings from the FMS tool are generally more reliable than those from the HLDS, as the one-armed caliper is highly nonlinear and directional, generally tracking the major axis of elliptical holes. Initial comparison of FMS caliper measurements from Leg 206 and Expedition 309 show that the borehole shape remains very similar. Hole 1256D has been little degraded between Leg 206 and Expedition 309. The most in-gauge portions of the penetration correspond to the lava pond (from the casing shoe to 350 mbsf) and the lowest part of the hole (from 605 to 750 mbsf). In several intervals the diameter of the borehole locally exceeds the limits of both the FMS and HLDS calipers (>16 inches [40 cm]). These wider sections occur at 350–403, 423–435, 451–465, 539–570, and 675–688 mbsf. In some intervals, measurements from the two orthogonal FMS calipers are slightly different between Leg 206 and Expedition 309, for instance in the intervals from 300 to 350 and 656 to 673 mbsf. However, these differences in borehole shape occur in intervals where C1 and C2 indicate that the borehole is strongly elliptical (i.e., C1 ≠ C2). Consequently, these slightly different caliper measurements between Leg 206 and Expedition 309 are mostly likely related to differences in the rotation of the tool string (Fig. F42). In several narrow zones (517, 597, 602, and 685 mbsf), strong differences between C1 and C2 are identified in Leg 206 and Expedition 309 caliper measurements. These intervals may correspond to borehole breakouts, and the north–south orientation of the borehole enlargements suggests a west–east maximum stress direction. During Leg 206, one tight spot was recorded at 486 mbsf. This tight spot is still present in Expedition 309 caliper data, and a new narrow zone is indicated at 472 mbsf (9.27 inches).

Igneous petrology

Hole 1256D was reentered during Expedition 309, and basement was cored from 752 to 1255 mbsf. The cores were labeled continuously from the last core (Core 206-1256D-74R) of Leg 206. Cores 309-1256D-75R through 170R were drilled during Expedition 309. Recovery varied from 14%–45% in sheet flows to 20%–100% in massive basalts. Recovery over the total cored interval averaged ~36%.

Hole 1256D basement is divided into igneous units (Fig. F44) based on criteria presented in the “Methods” chapter. Units identified during Expedition 309 are labeled continuously from the last rocks recovered during Leg 206 (Wilson, Teagle, Acton, et al., 2003), starting with Unit 1256D-27. In total, 39 new units were identified during this cruise (Units 1256D-27 through 65) (Table T13). Some units have been divided into subunits in order to highlight dike contacts or minor lithologic changes such as grain variations occurring within a unit (see the “Methods” chapter).

Unit descriptions

Sheet and massive flows (752–1004 mbsf)

Sheet flows (Units 1256D-27 through 29, 32 through 33, 35, 37, and 38)

Thin basaltic sheet flows (with individual cooling units <3 m) make up 80% of the total sheet and massive flow section (penetrated during Leg 206 and Expedition 309). Sheet flows make up only 65% of the portion of this section drilled during Expedition 309, indicating an increased proportion of massive flows. Where contacts are recovered, individual flows or cooling units are separated by chilled margins commonly with fresh or altered glass or, more rarely, volcanic breccia (Tables T13, T14). In general, the recovered glassy margins are planar (Fig. F45).

Curved glassy margins have been recovered in only a few isolated pieces (e.g., intervals 309-1256D-81R-1, 16–22 cm, Unit 1256D-29b [Fig. F46], and 91R-1, 51–56 cm, Unit 1256D-33a). Because of the sparseness of radial pipe vesicles oriented perpendicular to the glassy margin, we have not classified these rocks as pillow lavas. Where contacts were not recovered, individual flows were distinguished by systematic changes in grain size. Using these criteria, minimum thicknesses of individual flows or cooling units range between 0.11 and 1.68 m with an average thickness of 0.55 ± 0.35 m (1 σ = standard deviation). No intercalated marine sediments were found between flows, and most sheet flows are nonvesicular.

Close inspection of glassy chilled margins shows that groundmass texture varies from the chilled margin to the inner part of the rock (e.g., Samples 309-1256D-113R-1, 20–24 cm [Thin Section 60], and 128R-1, 65–70 cm [Thin Section 91]). This is due to a decrease in the cooling rate. The best preserved glassy chilled margins exhibit four distinct zones (Fig. F47). From the margin inward, these zones are

• Zone 1: fresh glass, though commonly altered to phyllosilicates (saponite);
• Zone 2: glass with isolated spherulites;
• Zone 3: coalescent spherulites; and
• Zone 4: Fibrous clinopyroxene coalesced with plagioclase in variolitic textures.

The outermost zone (glassy; Zone 1) is preserved only in rare cases and is absent from most chilled margins (e.g., Sample 309-1256D-113R-1, 20–24 cm [Thin Section 60]). Rare phenocrysts or microphenocrysts are present in Zone 1, where they commonly form glomerocrysts. In Zone 2, spherulites consist of spherical aggregates with radial plumose/​fibrous crystallites. In the outer part, spherulites are isolated in the altered glassy groundmass. Toward the flow interior, spherulites increase in number and size and coalesce (Fig. F48A). Tiny titanomagnetite grains are commonly disseminated along the spherulite boundaries. Coalescent spherulites form a brown layer (Zone 3) parallel to the glassy margin. In this layer, many spherulites have tiny microlites of acicular plagioclase. The inner boundary of Zone 3 is commonly marked by unoriented plagioclase microlites concentrated in a thin layer preferentially oriented parallel to the chilled margin (Fig. F48B). Zone 3 gradually changes to Zone 4 where the very fine (<10 µm) variolitic texture is characterized by fibrous clinopyroxene branching from plagioclase laths (Fig. F49A). Thin layers of flattened varioles are parallel to the chilled margin (e.g., Sample 309-1256D-128R-1, 65–70 cm [Thin Section 91]).

The flows are predominately aphyric (<1% phenocrysts) (Fig. F50), and grain size ranges from glassy at the chilled margins to cryptocrystalline or microcrystalline. Rare sheet flow interiors are fine grained (e.g., interval 309-1256D-80R-2, 60–142 cm). Sheet flow Units 1256D-28, 35b, and 37 have plagioclase, clinopyroxene, and olivine phenocrysts in decreasing abundance (Fig. F51). Phenocrysts commonly form glomeroporphyritic textures. The groundmass of sheet flows generally consists of plagioclase and clinopyroxene microlites, with interstitial titanomagnetite and altered glass, similar to those described during Leg 206. Both clinopyroxene and plagioclase are radially arranged to form fan-shaped aggregates. These varioles tend to be more flattened as they grow larger (Fig. F49B).

In addition to these varioles, flow interiors exhibit isolated microlites with dendritic features. Plagioclase microlites commonly have swallow-tail extensions or exhibit hollow cores, features suggestive of a high degree of undercooling (Bryan, 1972).

Unit 1256D-35c contains three small (0.5–2.2 cm) holocrystalline gabbroic xenoliths (intervals 309-1256D-107R-1, 44–52 cm, 108R-1, 20–36 cm, and 108R-1, 132–138 cm) consisting of fine-grained olivine, plagioclase, and clinopyroxene (Fig. F52). The sheet flow hosting these xenoliths, however, shows no discernible difference in whole-rock composition compared to the other sheet flows of this sequence (see “Geochemistry”).

Massive flows (Units 1256D-30, 31, 34, 36, and 39)

The remainder of the sheet and massive flows consists of massive lava flows >3 m thick (35% of the sheet and massive flow interval drilled during Expedition 309, corresponding to 32 m of recovered core). Minimum unit thicknesses (cumulative thickness calculated using only pieces recovered) vary from 3.2 to 11.3 m (average = 6.3 m). The flows are aphyric to sparsely phyric (rarely moderately phyric) and predominantly cryptocrystalline to microcrystalline basalts. The thickest, Unit 1256D-31, is a single cooling unit of fine-grained basalt below a 12 cm cryptocrystalline to microcrystalline upper contact (Fig. F53). Cored thickness of this unit is 26 m (Sections 309-1256D-85R-3 through 88R-1), of which 11.3 m was recovered. In contrast to the sheet flows, fine-grained rocks are more common in the massive lavas (Fig. F44). Except for Unit 1256D-39a, all massive lavas are aphyric and nonvesicular. The basalt of Unit 1256D-39a is sparsely plagioclase-olivine-phyric and is moderately vesicular (8%) (Fig. F54; see "General description of petrography"). Vesicles (0.6–1.2 mm) are spherical and filled with saponite (see “Alteration”).

Microscopic observations show that the fine-grained and microcrystalline rocks collected from the massive flows have intergranular to intersertal groundmass textures (Fig. F55). Platy and skeletal laths of plagioclase build crystal networks. Interstices between plagioclase laths are filled by either granular to fibrous clinopyroxene or by altered glass accompanied by titanomagnetite and sulfides. In some cases, interstices also contain anhedral plagioclase crystals with quartz intergrowths, acicular apatite grains, and granular to skeletal titanomagnetite crystals. Rare segregation vesicles are present in the interiors of massive flows (e.g., interval 309-1256D-106R-1, 70–80 cm). These vesicles are spherical and partially to totally filled with saponite, titanomagnetite grains, and acicular plagioclase microlites. Such vesicles may suggest that vapor differentiation has occurred in the thicker massive flows with migration of residual liquid toward cavities through a permeable but rigid crystal network (Anderson et al., 1984).

Transition zone (1004–1061 mbsf)

Cataclastic massive unit (Unit 1256D-40)

Unit 40 (interval 309-1256D-117R-1, 85 cm, to 118R-1, 66 cm) consists of heterogeneous rocks of different grain sizes. The upper part of this unit (interval 309-1256D-117R-1 [Pieces 9–14, 97–142 cm]) has a complex structure with clasts of cryptocrystalline basalt (Fig. F56) in a fine- to medium-grained basaltic breccia. The breccia contains highly altered glass clasts and is disrupted by an intensive network of thin chlorite-smectite veins. Thin section examination (Sample 309-1256D-117R-1, 122–125 cm [Thin Section 67]) shows fractured crystals, deformed and cemented by a banded matrix that shows flow structures (see “Structural geology”). With increasing distance from the top of the unit, the igneous texture becomes better preserved and more homogeneous, mesostasis becomes less abundant, and crystals are less fractured (interval 309-1256D-117R-2, 9–72 cm, through 118R-1, 0–66 cm). The lower part of the cataclastic massive unit comprises fine-grained dolerite with a partially developed subophitic texture (Sample 309-1256D-117R-2, 23–26 cm [Thin Section 68]). Constituent minerals are plagioclase (mostly as subhedral laths but with anhedral laths in interstitial zones), clinopyroxene, titanomagnetite, pigeonite, and apatite.

A few pieces, similar to the disrupted rocks of this cataclastic unit, occur in intensely veined sheet flows in Unit 1256D-37 (Section 309-1256D-113R-1 [Pieces 5, 6, and 8]) and Unit 1256D-41 (Sections 119R-1 [Pieces 1 and 3] and 120R-1 [Piece 7]) and may therefore support the interpretation of this unit as an altered, fractured dike margin (or a large-scale fault zone).

Sheet flows (Units 1256D-41 and 43)

Sheet flows are a major component of the transition zone, and their absence defines the upper boundary of the dike section. Seventeen flow units were identified from decreases in grain size or from the presence of brecciated flow margins. Seven are in Unit 1256D-41, and ten are in Unit 1256D-43. Both units consist of cryptocrystalline to microcrystalline aphyric basalt with rare (<0.5 vol%) plagioclase and clinopyroxene phenocrysts. The last recovery of a fresh glassy margin (not associated with a dike contact or clastic brecciation) was in the lower half of Unit 1256D-43 (interval 309-1256D-128R-1 [Piece 10, 64–72 cm]; 1056.8 mbsf) (Table T14).

Mineralized volcanic breccia (Unit 1256D-42)

One characteristic feature of the transition zone is the increased presence of volcanic breccias compared to the overlying sheet and massive flows. Volcanic breccias are useful for subdividing sheet flows or massive basalts into individual units (e.g., 25 cm of breccia at the top of Unit 1256D-45; interval 309-1256D-135R-1 [Pieces 10–14, 112–136 cm]). In interval 309-1256D-122R-1, 25 cm, to 123R-1, 109 cm, however, 2.8 m of spectacular volcanic breccia and breccia intercalated with basalt was recovered and defines Unit 1256D-42 (Fig. F57). This unit can be further subdivided based on abundance of basaltic rocks. The upper part, Unit 1256D-42a, consists solely of volcanic breccia (interval 309-1256D-122R-1, 25–149 cm, through 122R-2, 0–30 cm), but in Unit 1256D-42b the breccia is intercalated with aphyric cryptocrystalline to microcrystalline basaltic sheet flows. The volcanic breccias of Unit 1256D-42b are present in seven intervals between 309-1256D-122R-2, 61 cm, and 123R-1, 82 cm (Fig. F57). These breccias comprise angular to subangular aphyric cryptocrystalline basaltic clasts (0.5–4.5 cm) and subangular to elongate clasts of altered glass, with rare flame-shaped clasts (0.1–1.5 cm), cemented by chalcedony, saponite, calcium carbonate, albite, anhydrite, and sulfides (Fig. F57; see “Alteration” and “Structural geology”). The size of individual clasts increases toward the bottom of the brecciated interval. Some of the basaltic clasts have narrow altered spherulitic chilled margins. Immobile incompatible trace element ratios indicate no significant compositional difference between these clasts and the intercalated cryptocrystalline sheet flows (see “Geochemistry”). In thin section, these cryptocrystalline clasts have thin alteration rims, but igneous textures are preserved in the clast cores (e.g., Sample 309-1256D-122R-2, 74–77 cm [Thin Section 81]). Glassy fragments are more abundant than cryptocrystalline clasts. Phenocrysts and microphenocrysts of olivine and plagioclase are present in basaltic clasts. Microlites of plagioclase and fibrous clinopyroxene form isolated varioles or flow structures defined by alternating microlite-rich layers and flattened spherulites. Smaller clasts are more fractured, and the original igneous texture has been completely replaced by secondary phyllosilicates (Sample 309-1256D-122R-1, 90–93 cm [Thin Section 77]).

Sheeted dike complex (1061–1255 mbsf)

Massive basalt (Units 1256D-44 through 65)

Below 1061 mbsf, thin sheet flows are not present and most rocks are similar to the massive basalt units of the sheet and massive flows. These massive basalts are commonly aphyric and nonvesicular. Rare units are cryptocrystalline to microcrystalline (Units 1256D-56a, 57a, and 58), and microcrystalline (Units 1256D-44 through 46, 49 through 50, 52a, 54 through 55, and 59 through 65) to fine-grained rocks (Units 1256D-47, 48, 51, and 53a) predominate. In contrast to shallower depths in Hole 1256D, subvertical intrusive contacts (dikes) are common (Fig. F44). These intrusive contacts are used to define units and subunits (see the “Methods” chapter).

The massive basalts of the sheeted dike complex are holocrystalline and commonly doleritic. Chlorite is the major secondary mineral, replacing plagioclase and clinopyroxene (see “Alteration”). Constituent minerals are plagioclase, clinopyroxene, titanomagnetite, apatite, and rare pigeonite and olivine. Rocks exhibit seriate textures and are generally aphyric. Other textural features include intergranular to intersertal textures where the interstices between plagioclase laths are occupied by clinopyroxene, anhedral plagioclase, titanomagnetite, and glass (Fig. F58). Apatite is observed exclusively within altered plagioclase. This altered plagioclase commonly exhibits myrmerkite textures (Fig. F59). Ophitic to subophitic textures are rarely developed in coarse-grained intervals. Simple zoning patterns are common in larger plagioclase grains. Rare olivine is always completely replaced by phyllosilicates (chlorite). Alteration patches are commonly seen in coarser grained rocks (see “Alteration”).

Intrusive contacts (within massive Units 1256D-45 through 48, 52 through 57, 60, 61, and 64)

The sparseness of vesicles and predominance of vertical features suggests that the massive basalts deeper than 1061 mbsf represent the beginning of the sheeted dike complex. This conclusion is confirmed by later drilling during Expedition 312.

Units with intrusive contacts are marked on Figure F44 and presented in Table T15. There are two types of contacts: sharp or irregular direct contacts and brecciated contacts (see “Structural geology”). Most contacts belong to the latter category with one- to several-centimeter-wide brecciated zones. All contacts have developed chilled margins (Fig. F60A, F60B). The chilled margins of the dikes are generally composed of glassy to cryptocrystalline aphyric rock that is quenched against the cryptocrystalline to fine-grained massive basaltic host. Contact breccias are generally composed of fragments of altered glass initially quenched at the chilled margin and angular–subangular basaltic clasts of the host rock (less common); they can be cemented by anhydrite, chlorite, and sulfide (see “Alteration”). A spectacular example of this is the >50 cm long vertical contact in interval 309-1256D-140R-1, 26–80 cm (Unit 1256D-47), showing a sulfide-impregnated dike margin breccia with complex intrusive relationships and intricate multiple margin-parallel sulfide veins and crosscutting anhydrite veins (Fig. F61). Recovery of dike contacts becomes more common below Unit 1256D-52 at 1151 mbsf. Many of these dike contacts are vertical to subvertical in oriented pieces (in Sections 309-1256D-136R-1, 140R-1, 149R-1, 150R-1, 151R-1, 153R-2, 157R-1, 161R-1, 161R-2, 163R-1, 165R-3, and 166R-1). Where the actual contact is not recovered, dike proximity is suggested by chilled margins in single isolated pieces (e.g., interval 309-1256D-154R-1, 0–13 cm). In interval 309-1256D-155R-1 (Piece 20, 84–90 cm), the chilled margin forms an embayment into the host rock, indicating that it was not rigid during the intrusion (Fig. F62A). Further evidence for multiple intrusions is seen in Sections 309-1256D-161R-1 through 161R-2 (Unit 1256D-56b), where at least two intrusions are present: an inner sparsely clinopyroxene-olivine-plagioclase phyric dike intruding a sparsely clinopyroxene-olivine-plagioclase phyric spherulitic cryptocrystalline rock, itself chilled against an aphyric microcrystalline host rock (Fig. F62B). Another example of dike intrusion in a ductile host rock is observed in interval 309-1256D-163R-1, 113–122 cm. This contact is lobate and highly complex with fractured pieces of the chilled margin dispersed in the host rock (Fig. F62C).

Chemical analyses of one dike and its host rock (Samples 309-1256D-149R-1 [Piece 11A, 54–57 cm; Piece 3, 7–10 cm]) do not reveal any significant geochemical differences (see “Geochemistry”).

General description of petrography

The mineralogy, grain size, and modal analyses of thin sections are recorded in thin section description sheets and shown in Table T16. A complete list of digital photomicrographs available from the IODP data librarian is given in the photomicrograph log (see PHOTOLOG.XLS in “Supplementary material”).

Phenocrysts

Although basalts are mostly aphyric (81% of the thin sections), rare units are composed of sparsely (13%) to moderately (3%) phyric lavas. Plagioclase is the dominant phenocryst phase; with abundances up to 6.8 vol%, it represents 50% of the phenocrysts recognized during Expedition 309 with lesser amounts of clinopyroxene (up to 7 vol%; ~35%). These two phases commonly form glomerocrysts (Fig. F63). Olivine phenocrysts are less common (up to 7.8 vol%; ~14%). Phenocryst content ranges up to 10 vol% in four moderately phyric intervals at 759.8, 832.6, 951.1, and 990.5 mbsf (Units 1256D-28, 31, 35b, and 39a) (Figs. F44, F64). In Unit 1256D-28 (Sections 309-1256D-77R-1 through 78R-1; 759.8–763.6 mbsf), total phenocryst content is 9.2 vol% with plagioclase as the dominant phenocryst phase (74%). At 832.6 mbsf (Unit 1256D-31; Section 309-1256D-87R-2) and 951.1 mbsf (Unit 35b; Section 106R-1), these moderately phyric intervals (8.9 and 5 vol%, respectively) are dominated by clinopyroxene (76% and 52%, respectively). The next moderately phyric interval (990.5 mbsf; Unit 39a; Section 309-1256D-114R-2) has 9.9 vol% phenocrysts with olivine (60%) and plagioclase (40%) as the principal phases. Generally, the abundance of all phenocryst phases decreases with depth (Figs. F44, F64, F65).

Plagioclase

The maximum length of plagioclase phenocrysts is commonly between 0.1 and 1.2 mm, but rare crystals can be as long as 3.0 mm. Plagioclase phenocrysts occur most commonly as euhedral to subhedral laths, arranged as plagioclase ± clinopyroxene glomerocrysts; discrete euhedral phenocrysts are also present. Most plagioclase phenocrysts are unzoned or only slightly zoned. Some phenocrysts partially include smaller plagioclase laths. Glass inclusions are present in the inner cores or near the rims of plagioclase (Fig. F66).

In the sheet and massive flows, alteration features include resorption, development of clay minerals in cracks (Fig. F67), and partial replacement by albite. In the transition zone and the sheeted dikes, plagioclase is commonly completely replaced by chlorite/saponite minerals. A fresh rim commonly mantles altered phenocrysts (See “Alteration”). Microphenocrysts of plagioclase make up to 3.9 vol% of basalts. Generally, they exhibit the same features as plagioclase phenocrysts.

Clinopyroxene (augite)

Clinopyroxene phenocrysts commonly range from 0.2 to 1.5 mm, but rare phenocrysts can be as large as 3.6 mm. Most crystals are anhedral to subhedral (Fig. F68), but some phenocrysts have more euhedral and equant habits. Commonly, clinopyroxene partially encloses plagioclase phenocrysts forming glomerocrysts with subophitic textures.

Clinopyroxene phenocrysts are unzoned and better preserved than plagioclase and olivine. These rarely show resorption features. Melt inclusions are rare, mostly occurring near fractures.

Olivine

Fresh olivine is not present, and recognition is based on the euhedral shapes of the pseudomorphs. Olivine is altered to secondary saponite and chlorite (Figs. F68, F69). Interstitial glass is also altered to saponite or chlorite and can form in angular areas between plagioclase microlites, making the recognition of groundmass-size olivine difficult. Olivine phenocrysts are generally larger than other phenocryst phases. Maximum grain size varies from 0.5 to 1.2 mm with rare phenocrysts up to 6.0 mm.

Groundmass

Groundmass phases include plagioclase, augite, titanomagnetite, sulfides, and glass with minor pigeonite, apatite, and granophyric intergrowths of quartz and sodic plagioclase. Plagioclase is the dominant groundmass mineral, occurring in a variety of habits, most commonly as euhedral to subhedral, acicular, bladed, or skeletal laths. Augite occurs in a variety of habits. In most cryptocrystalline samples, augite is fibrous and nucleated from plagioclase microlites. When groundmass grain size is coarser, euhedral to subhedral grains are intergranular between plagioclase laths. Pigeonite is much less abundant than augite (e.g., Samples 309-1256D-111R-1, 85–87 cm, and 153R-2, 65–68 cm). It has only been detected in the coarser grained rocks of the transition zone and the sheeted dikes (Units 1256D-40 and 53a). Pigeonite is pale yellow in cross-polarized light and exhibits a rounded subhedral shape (Fig. F70) due to late crystallization between earlier formed crystals (Philpotts, 2003). Titanomagnetite is ubiquitous as euhedral to anhedral and skeletal grains in cryptocrystalline to microcrystalline rocks. Larger equant habits are more common in coarser grained rocks.

Comparison with Leg 206 basalts

The petrography of the sheet and massive flows of Hole 1256D recovered during Expedition 309 (725–1004 mbsf) is similar to that of the Leg 206 crustal section. In contrast, the transition zone and sheeted dikes show significant lithological differences, such as increased abundance of contact breccia, decreased abundance of fresh glass, and common dike contacts in the sheeted dikes. Furthermore, the cataclastic massive unit, the mineralized volcanic breccia, and the many vertical intrusive contacts indicate major differences in style of emplacement.

Mineralogical characteristics of basalts drilled during Expedition 309 are similar to Leg 206 basalts except for the following:

• More than 60% of Leg 206 basalts are sparsely phyric with olivine, plagioclase, and clinopyroxene phenocrysts (Wilson, Teagle, Acton, et al., 2003); aphyric basalts make up <40% of recovery. In contrast, Expedition 309 recovered >80% aphyric basalts (Fig. F65A). This difference is illustrated by the downhole decrease in total phenocryst content (Fig. F64).
• More than half of Leg 206 basalts have three major phenocryst phases (clinopyroxene-plagioclase-olivine phyric). In contrast, most Expedition 309 basalts have only one or two phenocryst phases except for eight samples (Fig. F65B).
• Phenocryst-bearing Leg 206 basalts are dominantly olivine-phyric (>80%). In Expedition 309 basalts, plagioclase is the most common phenocryst phase and olivine is rare (Fig. F65C).

Geochemistry

A total of 76 whole-rock samples from Hole 1256D were analyzed for major and trace element concentrations by ICP-AES (see the “Methods” chapter). Generally, the freshest rocks from each igneous unit were selected to obtain a downhole record of primary magmatic compositions. Nevertheless, even the freshest samples analyzed have some background alteration, with secondary minerals visible in thin section. Alteration is generally more intense in coarser grained rocks, and cryptocrystalline rocks were selected where possible. Some highly altered rocks were analyzed to investigate element mobility during hydrothermal alteration.

All ICP-AES values have been normalized to 100% with total Fe recalculated as FeO. Elemental analyses and loss on ignition (LOI) data are presented in Table T17. During the 20 shipboard ICP-AES runs, within-run reproducibility was monitored through multiple analyses of shipboard standards BAS-140 and BAS-206 (two MORB standards from ODP Legs 140 and 206, respectively) and Sample 309-1256D-84R-1, 54–58 cm (a sample from Expedition 309) (Table T18). ICP-AES precision for most major elements is ±3% with the exception of TiO₂ (6%), K₂O (14%), and P₂O₅ (20%). The reproducibility for trace elements is ±5% for V, Zr, Y, and Sr, 5%–10% for Zn, Co, Sc, and Ba, 12% for Cu, 34% for Ni, and 38% for Cr (Table T18). Nb was excluded from the analysis after Run 3 because of low Nb concentrations close to the levels of the working blank (Table T18; see Table T13 in the “Methods” chapter).

Downhole geochemistry

Downhole variations in geochemistry are presented with igneous units and rock type in Figure F71. Although there is considerable scatter in the data for any given element downhole, subtle but important geochemical differences can be recognized for different basement subdivisions.

The lava pond (~250–350.3 mbsf) that forms the uppermost basement at Site 1256 is characterized by high TiO₂ (1.9 ± 0.3 wt%), P₂O₅ (0.16 ± 0.03 wt%), Zr (121 ± 15 ppm), Y (43 ± 5 ppm), and V (435 ± 46 ppm) concentrations and low Mg# (46 ± 3) (Shipboard Scientific Party, 2003). This unit shows a pronounced downhole decrease in TiO₂ (2.5–1.8 wt%), K₂O (0.17–0.06 wt%), Zr (127–101 ppm), Y (45–36 ppm), and Sr (134–99 ppm), whereas Mg# increases from 43 to 50 (Figs. F71, F72). This may indicate that these lavas are more evolved compared to underlying basalts at Site 1256.

The inflated flows (350.3–533.9 mbsf) are a sequence of massive flows, pillow lavas, and sheet flows below the lava pond. Major and trace element concentrations of this sequence follow a general trend of fractionation typical of MORB. Within this sequence, rare “high-Zr Lavas” (Shipboard Scientific Party, 2003) are characterized by higher concentrations of Zr, Sr, and Cr, resulting in elevated Zr/TiO₂ and Zr/Y ratios (Figs. F71, F72) compared to other basalts (Shipboard Scientific Party, 2003). Based on distinct immobile element Zr/TiO₂ and Zr/Y, the Shipboard Scientific Party (2003) concluded that this is a primary magmatic feature.

The sheet and massive flows (533.9–1004.2 mbsf) are a sequence of lava sheet flows with subordinate massive lava flows. The major and trace element concentrations of this unit are similar to the inflated flows above; however, there is an increase in the scatter of Zr/Y. Two major cyclic variations of Mg# occur within this unit at depth intervals ~600 to ~710 mbsf and ~750–908 mbsf. In each cycle, Mg# increases with depth, suggesting successive episodes of fractionation and magma injection (Figs. F72, F73). At ~650 mbsf, three analyses (Samples 206-1256D-55R-2, 64–66 cm, 57R-2, 30–42 cm, and 57R-3, 1–3 cm) do not follow the general trend and have high MgO concentrations (>9 wt%). Sample 206-1256D-57R-3, 1–3 cm, is a highly altered cryptocrystalline basalt (Shipboard Scientific Party, 2003), and the increase in Mg is probably the result of intense seawater alteration.

At ~600 mbsf, there is a distinct steplike change in FeO (9–13 wt%), Zr (80–89 ppm), Cr (367–45 ppm), V (317–380 ppm), Ni (127–51 ppm), and Mg# (62–52) (Figs. F71, F72). This change may correspond to an increase in the abundance of clinopyroxene and olivine phenocrysts at ~600–700 mbsf (Shipboard Scientific Party, 2003).

A second offset in the downhole plots at ~750 mbsf corresponds to the start of analyses conducted during Expedition 309. Based on reproducibility of the internal standard BAS-206 used during both Leg 206 and Expedition 309, this offset cannot be solely caused by analytical error and must be attributed to natural downhole variation. There is, however, a systematic difference between Sc measured during Leg 206 and Sc measured during Expedition 309 (6–10 ppm lower in Expedition 309 results) that may result from analytical problems.

Between ~750 and 908 mbsf, MgO (7.2–8.4 wt%) and CaO (11–12 wt%) tend to increase with depth, whereas TiO₂ (1.5–1.2 wt%), FeO (12.3–10 wt%), Na₂O (3.1–2.3 wt%), V (380–284 ppm), and Y (39–28 ppm) decrease. Below 908 mbsf, Al₂O₃, FeO, MgO, CaO, Na₂O, K₂O, Zn, Co, V, Zr, and Sr do not change with depth, whereas SiO₂ (50.8–52.4 wt%) and Ba (7.3–21 ppm) increase with depth (Fig. F71). Above and below 908 mbsf, Zr/TiO₂ and Zr/Y increase with depth and have steplike offsets at 908 mbsf from 62.1 to 55.4 for Zr/TiO₂ and from 2.7 to 2.2 for Zr/Y (Fig. F72). Steplike offsets also occur in the downhole trends in FeO (10.0–11.6 wt%), MgO (8.4–7.9 wt%), CaO (12.6–11.8 wt%), Al₂O₃ (15.0–14.2 wt%), Sr (113–92 ppm), and Sr/CaO (9.0–7.7). Additionally, the slopes of Mg# and CaO/Al₂O₃ change at 908 mbsf from increasing downhole to steady values, suggesting two episodes of magma injection and evolution (Fig. F73). This change in the downhole trend of element concentrations at ~908 mbsf coincides with Unit 1256D-33b, the bottom of which marks a transition in the eruption style from sheet flows to massive flows (see “Igneous petrology”). The presence of gabbroic xenoliths in Section 309-1256D-107R-1 at ~955 mbsf (Unit 1256D-35c; see “Igneous petrology”) could be indicative of assimilation and may be one possible explanation for the different geochemical trends.

The transition zone (1004.1–1060.9 mbsf) comprises sheet flows with breccias, rare dikes, and a change from saponite to chlorite as the main secondary mineral, signifying a change to subgreenschist-facies alteration. Within this zone, elevated Na₂O (3.5 ± 1.4 wt%) and Sr/CaO (11 ± 4) (ppm/wt%), as well as lower CaO (9 ± 3 wt%), reflect replacement of primary plagioclase by albite (see “Alteration”) and a shift to higher (>200°C) hydrothermal alteration temperatures.

The upper dikes (1060.9–1348.3 mbsf) are massive basalts with common subvertical intrusive contacts and have relatively high P-wave velocity and thermal conductivity (see “Physical properties”). Downhole geochemical compositions are similar to the transition zone; however, from ~1100 to 1150 mbsf decreases in TiO₂ (1.6–1.1 wt%), Zr (107–62 ppm), and Y (43–27 ppm) concentrations coincide with increases in magnetic susceptibility and gamma ray attenuation (GRA) density (see “Physical properties”), an increase in the abundance of olivine and clinopyroxene phenocrysts (see “Igneous petrology”), and an increase in the abundance of vertical dikes. There is an offset at ~1125 mbsf in Al₂O₃ (15.5–14.5 wt%), FeO (9.7–11.1 wt%), Mg# (60.2–55.4), Zr/TiO₂ (68.7–56.2), and Zr/Y (3.0–2.3) (Figs. F71, F72). Below ~1200 mbsf, an increase in the scatter of the Mg#, Zr, Y, and Zr/TiO₂ is associated with an increased occurrence of dikes that may indicate multiple magma sources.

Analyses of cryptocrystalline basalts that are unambiguously dikes are chemically indistinguishable from massive basalts into which these dikes were intruded. There do not appear to be any systematic geochemical differences between sheet flows, massive flows, and dikes.

Bulk geochemistry

Compositional ranges for representative elements (normalized to 100%) of fresh samples are 48–55 wt% SiO₂, 9.4–14.0 wt% FeO, 6.2–8.9 wt% MgO, 7.1–12.8 wt% CaO, 1.8–5.0 wt% Na₂O, 21–367 ppm Cr, 71–129 ppm Sr, 56–133 ppm Zr, and 1–37 ppm Ba. Mg# ranges 45–62, with an average value of 53. These values broadly overlap results from Leg 206 (Fig. F74) and correspond to typical values for MORB (Su and Langmuir, 2003).

Plots of all elements analyzed versus MgO are presented in Figure F74. Despite considerable scatter in these plots, linear trends are present between TiO₂, FeO, CaO, Na₂O, and Zr versus MgO, likely resulting from magmatic fractionation (Fig. F74).

As for Leg 206 basalts, TiO₂ and Y show good positive linear correlations with Zr, due to their similar geochemical behavior (Fig. F75). During Leg 206, basalts above 750 mbsf were classified into three groups—high Zr-TiO₂, low Zr-TiO₂, and high Zr—based on a distinct gap in TiO₂-Zr concentrations. With additional data analyzed during Expedition 309, this data gap disappears and the threefold subdivision is probably not valid (Fig. F75). Rare samples from the lava pond in Hole 1256D fall off the dominant Y versus Zr and TiO₂ versus Zr trends, suggesting a possible minor variation in source composition.

Basalts from different igneous subdivisions in Holes 1256C and 1256D have MgO in the range of 6–9 wt%, and when trace element compositions of Site 1256 basalts are compared to compilations of EPR MORB, they are within one standard deviation of the average, albeit on the relatively trace element–depleted side (Fig. F76). Note that Site 1256 basalts have higher Zr/Y and Zr/TiO₂ than the highly trace element–depleted MORB from Hole 504B near the Costa Rica Rift (Fig. F77).

The lava pond basalts from Site 1256 are the only sequence that is relatively enriched in V and depleted in Cr compared to EPR MORB. The lava pond includes rocks with the highest incompatible element (Zr, TiO₂, Y, and V) concentrations and the most depletion in compatible elements (Cr and Ni), suggesting that it is more evolved than the other basalts from Site 1256 (Fig. F76).

Compared with first-order mid-ocean-ridge segments along the EPR, basalts from Site 1256 have very low Zr/TiO₂ and Zr/Y (Fig. F78). Although there is overlap among segments and a large scatter in data for each segment, Zr/TiO₂ and Zr/Y appear to decrease with increasing spreading rate. The origin of this relationship is unclear, but spreading rate may affect extents of magma fractionation or partial melting of the mantle or it may instead reflect regional scale mantle heterogeneity.

Geochemistry of altered samples

To make a preliminary assessment of element mobility during low- to high-temperature hydrothermal alteration, analyses of a few of the most altered rocks were made. An altered glass Sample 309-1256D-80R-1, 55–56 cm, from Unit 1256D-29b has been normalized against a fresh sample of aphyric microcrystalline basalt from the same unit, Sample 84R-1, 54–58 cm. The altered sample is strongly enriched (enrichment factor in parenthesis) in K₂O (2.43), Cr (2.82), Ni (2.03), Sr (2.01), and Ba (9.73) and strongly depleted in P₂O₅ (0.39), MnO (0.59), and Na₂O (0.83) (Fig. F79A; Table T19). The geochemical changes relate to celadonite filling interstices and the alteration of glass by celadonite, saponite, and Fe oxyhydroxides as observed in thin sections.

The most altered rock analyzed during Expedition 309, Sample 309-1256D-122R-1, 125–130 cm, an aphyric glassy basalt, has been normalized against the closest fresh aphyric microcrystalline basalt (Sample 124R-1, 60–63 cm; Unit 1256D-43). The altered sample is enriched in TiO₂ (1.36), MnO (1.43), Na₂O (1.91), K₂O (2.09), P₂O₅ (2.02), Cr (1.27), V (1.39), Zr (1.45), and Y (1.46) and depleted in CaO (0.79), Zn (0.69), Cu (0.67), and Sr (0.75) (Fig. F79A; Table T19). The geochemical changes relate to saponite and chlorite filling amygdules and to the alteration of glass and plagioclase by saponite, chlorite, and albite as observed in thin sections and X-ray diffraction (XRD) analyses.

A highly altered microcrystalline basalt, Sample 309-1256D-128R-1, 59–65 cm, has been normalized against the freshest aphyric microcrystalline basalt, also from Unit 1256D-43 (Sample 124R-1, 60–63 cm). The altered sample is enriched in MgO (1.16), Na₂O (2.41), K₂O (1.50), P₂O₅ (1.2), Zn (1.33), Cr (2.44), Ni (1.38), and Ba (1.5) and depleted in CaO (0.62) and Co (0.85) (Fig. F79A; Table T19). The geochemical changes are likely caused by the filling of interstices, alteration of olivine and plagioclase phenocrysts and glass by chlorite/saponite and minor titanite as observed in thin sections, and replacement of primary plagioclase by albite.

The compositions of the most altered samples reflect the physical and chemical conditions of hydrothermal alteration. In general, altered rocks above 900 mbsf are enriched in K₂O and depleted in Na₂O, MnO, and P₂O₅, whereas altered rocks below 900 mbsf have higher Na₂O, K₂O, and P₂O₅ but lower CaO concentrations (Fig. F71; Table T19).

Hydrothermal and tectonic breccias tend to be the sites of the most intense alteration in the ocean crust. The mineralized volcanic breccia (Sample 309-1256D-122R-1, 90–100 cm) has been normalized against the nearest fresh aphyric microcrystalline basalt from Unit 1256D-43 (Sample 124R-1, 60–63 cm). The mineralized sample is enriched in FeO (1.8), Na₂O (1.83), K₂O (2.04), P₂O₅ (1.45), Zn (4.47), Sc (1.95), Zr (1.22), V (1.16), Ba (4.21), and Y (1.21) and depleted in CaO (0.26), Ni (0.81), Cu (0.69), and Sr (0.59) (Fig. F79B; Table T19). The geochemical changes relate to the filling of interstices and veins with saponite and albite as observed in XRD and in thin section.

Sample 309-1256D-140R-1, 74–79 cm, is a mineralized dike margin breccia and has been normalized to the nearest fresh, sparsely plagioclase-clinopyroxene phyric microcrystalline basalt (Unit 1256D-46a; Sample 138R-1, 9–14 cm). The mineralized sample is strongly enriched in FeO (1.71), Cu (367), Co (2.69), Zn (2.46), and Sc (1.55) and depleted in CaO (0.36), Na₂O (0.25), K₂O (0.39), Ni (0.81), Cr (0.68), Zr (0.78), Y (0.70), Sr (0.41), and Ba (0.25) (Fig. F79B; Table T19). The geochemical changes may relate to mineralization of veins containing pyrite, chlorite, anhydrite, and calcite as observed in XRD measurements and the replacement of primary plagioclase and clinopyroxene by chlorite and minor albite as observed in thin section.

Alteration

Hydrothermal exchange between the oceans and the oceanic crust significantly affects the composition of seawater and crust and influences the physical and magnetic properties of the basement. Seawater chemical and isotopic signatures in altered oceanic crust are recycled with the crust at subduction zones, influencing arc volcanism and leading to mantle heterogeneities. One of the principal objectives of the multicruise mission (Leg 206 and Expedition 309/312) was to investigate the types and distribution of alteration effects and processes that occur in a section of upper crust that formed at a superfast spreading rate (200–220 mm/y) and to test whether these differ from those documented in crust formed at slow and intermediate spreading rates. Also of particular interest during Expedition 309 was the transition between low-temperature alteration and high-temperature hydrothermal alteration in a continuous section of in situ oceanic crust, which to date has only been described in Hole 504B.

The upper 502 m of lavas from Hole 1256D, drilled during Leg 206, was slightly to moderately altered to a dark gray saponite + pyrite background alteration as a result of low-temperature interaction with seawater (Wilson, Teagle, Acton, et al., 2003). Background alteration occurs by the pseudomorphic replacement of groundmass and phenocryst olivine, the replacement of interstitial mesostasis, and the filling of vesicles and interstitial voids. Veins in these basalts are predominantly filled with saponite ± pyrite and more rarely silica, celadonite, calcium carbonate, and/or Fe oxyhydroxide (Table T20). Vein-related alteration is manifest as colored alteration halos flanking some veins. Halos are predominantly black and more rarely brown, with celadonite, saponite, and Fe oxyhydroxide within the host rock groundmass. In some samples black and brown halos are superimposed, resulting in “mixed” halos.

During Expedition 309, we recorded the effects of fluid–rock exchange, based on color in hand specimen and on the presence and abundance of secondary minerals in hand specimen and in thin sections, checked by XRD. Various types of alteration are described: background alteration, alteration halos related to veins and patches of alteration, and veins and breccias (Table T20; see 309VEIN.XLS in “Supplementary material”).

During Expedition 309, 5160 veins were recorded from Hole 1256D in Sections 309-1256D-75R-1 through 170R-3, giving an average of 28 veins/m of recovered core (Fig. F80). Veins make up 1.2 vol% of total core recovered (Fig. F81). The total number, average vein frequency, and volume percent of veins in the combined Leg 206 and Expedition 309 Hole 1256D cores are 10,196, 25 veins/m, and 1.1%, respectively. Table T20 provides a statistical summary of vein and breccia abundances and types for Hole 1256D.

Two very different types of alteration were encountered in the section of Hole 1256D drilled during Expedition 309. At the top of this section (752 mbsf), rocks that had reacted with seawater at low temperatures, similar to the range of conditions encountered by basalts drilled during Leg 206, were recovered in Cores 309-1256D-75R through 112R (752–981 mbsf). Below this depth, secondary minerals indicative of higher temperature hydrothermal alteration were observed (Table T21). The transition between these two types of alteration is defined according to the following criteria (Table T22):

• The top of the alteration transition zone is identified at ~981 mbsf (in Core 309-1256D-112R) by the tentative identification of chlorite-smectite in thin section.
• The base of the alteration transition zone occurs at ~1027 mbsf (in Core 309-1256D-122R) with the occurrence of the mineralized volcanic breccia (See “Igneous petrology” and “Structural geology”).
• The first appearance of actinolite in veins occurs in Sample 309-1256D-122R-1, 35–38 cm, at 1027.65 mbsf within the mineralized volcanic breccia, coincident with a change in dominant phyllosilicate mineralogy from chlorite-smectite to chlorite.

Phyllosilicates (saponite and/or chlorite-smectite and/or chlorite and all the mixed layer intermediaries) are the main secondary minerals throughout Hole 1256D. Their exact nature reflects the temperature of crystallization; it is therefore important to distinguish them, but this can only be done confidently by careful XRD glycolation and heating experiments. The different chlorite-smectite mixed layers cannot be distinguished in hand specimen. However, in thin section, saponite (pale olive-brown to olive-green, non- to slightly pleochroic, first-order interference colors) is clearly distinguished from chlorite (green, slightly pleochroic, anomalous blue to tan interference colors). All intermediary saponite/​chlorite mixtures have intermediate optical features. As glycolation and heating experiments were not carried out on the ship, thin section observations of smectite-chlorite or chlorite were used to calibrate the vein and alteration logs. Consequently, the downhole distribution of phyllosilicates in Hole 1256D (Figs. F80, F81) is preliminary and will be refined by postcruise studies.

Summary of Hole 1256D alteration stratigraphy

A general alteration stratigraphy in basalts recovered during Leg 206 and Expedition 309 is envisaged as follows. From 250 to 850 mbsf, saponite and celadonite are the most abundant secondary minerals in background alteration, halos, and veins. They occur together with iron oxyhydroxides and lesser amounts of calcite, pyrite, chalcedony, and quartz (Fig. F82). In this interval, secondary minerals are either evenly distributed in the groundmass or replace plagioclase and olivine phenocrysts and fill vesicles and amygdules. These secondary minerals formed at low temperatures (<100°C) during basalt alteration by seawater (Honnorez, 1981; Alt et al., 1986, 1996b; Laverne et al., 1996).

Rocks from Sections 309-1256D-75R-1 through 91R-1 (752–850 mbsf) are slightly to moderately altered (2%–50%) and show pervasive uniform background alteration from 85% to 100% dark gray (see 309ALT.XLS in “Supplementary material”). Dark gray basalts represent the least intensive but most pervasive alteration, and saponite is the dominant secondary mineral. Black, brown, and mixed halos and dark patches are common in rocks down to 850 mbsf and are associated with veins filled by saponite, celadonite, and iron oxyhydroxides. These halos result from the replacement of the host rock groundmass, as well as olivine and plagioclase phenocrysts. The formation of black halos derives from an early low-temperature seawater-basalt interaction under anoxic conditions, which initiated during cooling of the lava within 1–2 m.y. of basalt emplacement (Böhlke et al., 1980; Honnorez, 1981; Laverne, 1993; summary in Alt, 2004). Later interaction of basalts with cold oxidizing seawater produced brown halos characterized by replacement of primary phases by saponite and iron oxyhydroxides.

From 850 to 981 mbsf, black, brown, and mixed halos are absent (Fig. F83) and dark gray background alteration related to abundant saponite and pyrite is ubiquitous. These rocks, as well as saponite- and pyrite-bearing intervals cored during Leg 206 (e.g., interval 554–562 mbsf), result from the interaction of basalt with low-temperature basement fluids that have chemically evolved from seawater through water-rock reaction. Hydroschorlomite (a Ti-, Ca-, and Fe-rich hydrogarnet) occurs in alteration halos and host rocks in Sections 309-1256D-75R-1 through 99R-2 (752–919.1 mbsf). This mineral is also present in Leg 206 basalts in Sections 206-1256D-59R-2 through 74R-2 (661.7–749.3 mbsf) (Laverne, 2006; Laverne et al., 2006) and is thought to represent alteration of titanomagnetite under conditions transitional from low-temperature to hydrothermal alteration and formation of titanite. The interval from 981 to 1027 mbsf is the alteration transition zone and is characterized by the presence of mixed-layer chlorite-smectite instead of pure saponite and an increase in the occurrence of anhydrite. This alteration mineral assemblage suggests slightly elevated temperatures (100°–200°C) compared to those found shallower in the crust.

Below ~1028 mbsf, green to dark green background alteration is more widespread and locally composes up to 100% of the rock. Alteration halos contain a greater proportion of secondary minerals, and we observe the first occurrences of actinolite, prehnite, titanite, and epidote at 1027, 1032, 1051, and 1095 mbsf, respectively, together with anhydrite in both veins and host rocks (see 309DIST.XLS in “Supplementary material”). These minerals are indicative of hydrothermal alteration under subgreenschist- to greenschist-facies conditions. The principal features of these alteration zones are detailed in the following section.

Low-temperature alteration and the alteration transition zone (Sections 206-1256D-2R-1 through 309-1256D-122R-1; 250–1027 mbsf)

Low-temperature background alteration

All rocks from Sections 309-1256D-75R-1 through 122R-1 (752–1027.3 mbsf) are slightly to moderately altered (2%–50%) and show pervasive background alteration (see 309ALT.XLS in “Supplementary material”). In Sections 309-1256D-75R-1 through 116R-2, background alteration is uniform and basalts are 85%–100% dark gray in hand specimen because of the presence of saponite, which fills vesicles and amygdules and replaces olivine, plagioclase, and clinopyroxene phenocrysts and the interstitial mesostasis, with minor chalcedony and calcite filling vesicles, amygdules, and miarolitic voids (Fig. F84). There is minor replacement of plagioclase by albite (Fig. F85), and disseminated secondary pyrite and rare chalcopyrite occur throughout the cores. The proportion of secondary minerals in a given rock is a function of rock texture and primary mineralogy, with greater amounts of saponite in rocks containing more abundant olivine, mesostasis, and vesicles.

In interval 309-1256D-78R-3, 11–80 cm, through 85R-6, 0–88 cm (766.4–818.3 mbsf), common, centimeter-scale, and irregular dark patches are present. These are volumes of rock that contain more abundant primary intercrystalline pore space that is now filled with secondary saponite and minor pyrite, thus imparting the dark color to the rock.

Low-temperature vein minerals

The predominant vein mineral phases related to low-temperature alteration in Hole 1256D include saponite, celadonite, iron oxyhydroxides, chalcedony, and minor pyrite. This assemblage is similar to the alteration observed during Leg 206 (Table T20). Celadonite is commonly intergrown with iron oxyhydroxides and overgrown by later saponite (Fig. F86). Celadonite is present in 290 veins (5.6% of the total) in Expedition 309 cores (Table T20; see 309VEIN.XLS in “Supplementary material”). Iron oxyhydroxides were recorded in 505 veins (9.8% of the total) within Expedition 309 cores. Iron oxyhydroxides are not present below 926 mbsf (Core 309-1256D-102R). Saponite-bearing veins are the most abundant with 2799 in total or 54% of all veins (and 87% of veins Cores 309-1256D-75R through 112R) and 0.85% by volume of recovered core. Saponite veins range from 0.1 to 15 mm thick and commonly occur in association with pyrite, quartz, and chalcedony (Fig. F87) and more rarely include calcite, anhydrite, and laumontite (confirmed by thin section and XRD) (Table T23). In rare cases, saponite is only an accessory phase to these minerals.

Low-temperature vein-related halos

Alteration related to veins is common and developed locally as differently colored halos adjacent to vein margins. Specific vein-related alteration types identified in Hole 1256D include black, brown, and mixed halos; simple light green, light gray, and dark green halos; and discontinuous pyrite halos (Table T21). Mixed halos are the dominant halo type down to 850 mbsf (~1% by volume of core) and are strongly developed at 770–800 and 811–820 mbsf. Brown and black halos make up 0.1% and 0.45%, respectively, by volume of the cores, and their occurrence is restricted to 754–820 mbsf in Expedition 309 cores (Fig. F83; see 309ALT.XLS in “Supplementary material”).

Black halos

The term "black halos" refers to celadonite-bearing dark green, dark gray, and black halos. They are characterized by the presence of celadonite, which replaces olivine and interstitial material and fills pore spaces (Fig. F88) together with saponite. Celadonite is identified by its green color in thin section or its blue-green color and brittle texture in hand specimen. Saponite is more abundant than celadonite and fills vesicles and interstitial voids and replaces olivine, plagioclase phenocrysts, and groundmass (Fig. F89). Iron oxyhydroxides may also be present in minor amounts, intergrown with or staining celadonite and saponite. Black halos range in width from 0.1 to 20 mm but are most commonly 2–10 mm wide. The deepest celadonite vein recorded in the vein log (see 309VEIN.XLS in “Supplementary material”) is in Sample 309-1256D-91R-1, 31–34 cm (864.2 mbsf). The deepest black halo associated with a celadonite-bearing vein observed in thin section occurs in Sample 309-1256D-96R-1, 110–112 cm (894.1 mbsf).

Brown halos

Brown alteration halos occur adjacent to veins containing iron oxyhydroxides, celadonite, saponite, and silica minerals and commonly range in width from 0.1 to 5 mm (Figs. F90, F91). The orange-brown color of these halos results from staining of primary minerals, filling of vesicles and interstitial voids, and replacement of olivine by iron oxyhydroxides (Fig. F92). The deepest occurrence of brown halos observed in hand specimen is in Sample 309-1256D-101R-2, 24–24 cm (918 mbsf), but brown halos are observed in thin section down to Sample 85R-2, 104–107 cm (821 mbsf), in which a 1 mm thick brown halo flanks an iron oxyhydroxide– and chalcedony-bearing vein. Rare iron oxyhydroxides occur within the groundmass of other samples, with the deepest iron oxyhydroxides observed in thin section in Sample 309-1256D-101R-1, 76–80 cm (917.76 mbsf) (Fig. F82).

Mixed halos

Mixed alteration halos result from superposition of brown halos on black halos, ranging in width from 0.3 to 50 mm but most commonly 0.8–10 mm. In hand specimen, mixed halos are mainly associated with iron oxyhydroxides, celadonite, and saponite (± chalcedony ± calcite) veins. Mixed halos commonly exhibit a color zonation that reflects the abundance and distribution of secondary minerals. The color zonation varies because of the relations between the brown, black, and dark gray or dark green halos. A reddish brown or yellowish brown zone is commonly developed immediately adjacent to the vein but is surrounded by a grayish black to dark green zone adjacent to the dark gray host rock (e.g., Samples 309-1256D-85R-2, 100–112 cm, 85R-2, 65–95 cm, and 85R-2, 95–125 cm) (Fig. F93). In several samples, black halos up to 20 mm wide form the inner part of the mixed halo, adjacent to an outer 10–15 mm wide brown halo (e.g., Samples 309-1256D-82R-1, 80–83 cm, 83R-2, 129–129 cm, 84R-1, 3–7 cm, and 84R-1, 122–133 cm) (Fig. F94). The dark outer portions of the halos are mineralogically indistinct from the normal black halos described above with saponite and celadonite replacing olivine and filling vesicles and interstitial spaces. The inner brown zone is similar in thin section to the brown halos described above. The occurrence of an inner brown halo is consistent with the formation of an early black, saponite/​celadonite-bearing alteration halo, which is subsequently partially overprinted by an iron oxyhydroxide–rich brown halo. There can be variations in the compositions of mixed halos. For example, in thin sections of Samples 309-1256D-80R-2, 109–111 cm, and 80R-3, 15–19 cm, the mixed halo (light gray, brown, and dark gray) flanks a 0.2 mm thick vein with iron oxyhydoxides along the margins and celadonite at the center (iron oxyhydroxides and celadonite). Saponite is present only in the surrounding background alteration. In Samples 309-1256D-85R-1, 28–37 cm, and 101R-1, 133–134 cm, 20 mm thick mixed halos are separated from dark gray basalt by a pyrite front. The deepest occurrence of a mixed halo in hand specimen was noted in Sample 309-1256D-101R-1, 133–134 cm (917 mbsf).

Pyrite-rich halos

Pyrite-rich halos are present in Sections 309-1256D-96R-1 through 128R-1 (893.0–1056.9 mbsf). Rare occurrences of pyrite-rich halos are also recorded in Cores 309-1256D-134R, 157R, and 168R. They are characterized by the presence of sulfides (pyrite and, locally, chalcopyrite), either disseminated or as a 0.3–6 mm pyrite front, at the boundary between halo and the host rock (Fig. F95). Pyrite-rich halos are dark gray or dark green and contain chlorite-smectite as a major secondary mineral. These halos are commonly discontinuous and their maximum thickness is 0.7 mm. They are the least abundant halo recovered during Expedition 309 and comprise only 0.06% by volume of the recovered cored material (Fig. F83; see 309VEIN.XLS in “Supplementary material”).

Low-temperature simple dark to light green and dark to light gray halos

Below the top of Section 309-1256D-84R-2 (803.3 mbsf), alteration halos are described as simple dark gray and light green (Table T21; see 309VEIN.XLS in “Supplementary material”) and are saponitic, without celadonite and iron oxyhydroxides. In Sections 309-1256D-91R-1 through 128R-1, simple dark gray and light green halos are very common and range in width from 0.1 to 12 mm but are most commonly 0.4–2 mm thick. The rocks in these halos are slightly altered, with 5% of saponite and possible saponite/​chlorite occurring in vesicles and amygdules and partly replacing plagioclase, clinopyroxene, and olivine phenocrysts (Figs. F96, F97, F98).

Alteration of glass

Glass occurs in minor amounts in intervals 309-1256D-80R-1, 12–19 cm, 80R-1, 36–38 cm, 81R-1, 1–5 cm, 90R-1, 6–10 cm, and 113R-1, 4–35 cm. Except in Sample 309-1256D-90R-1, 6–10 cm, which is described in the breccia section, glass is black or dark green in hand specimen and commonly contains numerous light green saponitic veins <0.5 mm thick, thus conferring an almost hyaloclastite appearance to these intervals (Fig. F99). In Sample 309-1256D-113R-1, 20–24 cm, glass is completely altered to isotropic yellowish brown palagonite and minor pyrite. Minor cracks are filled by chalcedony.

Breccia

Low-temperature breccias comprise fragments of basalt or glass in a saponite and celadonite matrix. Incipient breccia in which highly altered basalts are crosscut by networks of fine veins (see “Structural geology”) was recovered in Sections 309-1256D-78R-1 through 119R-1 within the low-temperature zone. This makes up 48% of the total volume of breccias recovered during Expedition 309. Much of this type of rock is transitional from vein nets. The best example of this type of breccia recovered is Sample 309-1256D-108R-2, 11–28 cm (960 mbsf) (Fig. F100). The rock is composed of 80% moderately to highly altered lithic basaltic clasts and minor glass clasts that range in size from 1 to 80 mm. The basaltic clasts are angular and rimmed by green and dark gray alteration halos. Interstitial areas between the clasts are cemented by saponite, quartz, chalcedony, pyrite, and laumontite, and the glass clasts are commonly replaced by saponite (see 309DIST.XLS in “Supplementary material”). Olivine inside basaltic clasts is replaced by saponite.

Hyaloclastite is present in Sections 309-1256D-90R-1, 113R-1, and 114R-1. Sample 309-1256D-90R-1, 5–10 cm, is composed of 2–50 mm glassy clasts with a matrix that contains 60% saponite, 10% pyrite, 15% quartz, 5% anhydrite, and 10% late-stage calcite (Fig. F101). Glass clasts are 80%–90% altered with most small (<2 mm) clasts completely replaced by saponite. In the angular glass clasts, alteration is most intense in the corners of the clasts, but elsewhere the replacement of the glass is locally incomplete. This has led to relict clast boundaries and alteration rims with round kernels of less altered glass at their centers. Clasts have numerous microfractures that are filled with chalcedony and pyrite. Rare plagioclase phenocrysts within the glass are replaced by albite (Fig. F102).

Hydrothermal alteration (Sections 309-1256D-122R-2 through 170R-3; 1027–1255 mbsf)

Background alteration

Green and dark green background alteration occurs in Section 309-1256D-122R-1 (1027 mbsf) as a consequence of intense (locally up to 100%) replacement by saponite/​chlorite and minor talc of basaltic clasts and glass in the mineralized volcanic breccia (Figs. F103, F104). Cores 309-1256D-136R through 170R (1255.1 mbsf) show green to dark green background alteration (e.g., Figs. F105, F106), particularly in the coarser grained rocks (e.g., Fig. F107). These rocks are generally slightly to moderately altered, but two intervals are moderately to highly altered with 11%–60% secondary minerals in Sections 309-1256D-132R-1 through 142R-1 (1076.7–1124.5 mbsf) and 149R-1 through 153R-2 (1156.9–1177 mbsf). Plagioclase is partially replaced by albite and chlorite or chlorite-smectite (Fig. F108). Less commonly, plagioclase is partially replaced by epidote (Fig. F109), prehnite (Fig. F110), anhydrite, and quartz, even in regions where there is no well-defined alteration halo associated with veins. Clinopyroxene is partly replaced by actinolite and chlorite. All of these secondary minerals, along with titanite, replace mesostasis.

Hydrothermal veins

Chlorite was identified in veins in Section 309-1256D-117R-1 (1003.2 mbsf) (Figs. F80, F81). From Section 309-1256D-120R-1 (1017.7 mbsf), chlorite is more common and becomes the dominant vein mineral instead of chlorite-smectite, occurring in 930 veins (18% of the veins in Expedition 309 cores) (Figs. F80, F81). The deepest occurrence of chlorite-smectite in veins is at 1095.4 mbsf. In veins, chlorite occurs with quartz, pyrite, actinolite, anhydrite, prehnite, laumontite, and calcite (Figs. F111, F112). Pyrite was identified in 1154 veins in this alteration zone. Pyrite and chalcopyrite veins make up 36% of the total and account for <0.1% by volume of the recovered core. Vein pyrite commonly occurs with chlorite or chlorite-smectite; however, in 7.5% of sulfide-bearing veins, pyrite constitutes >90% of the material within the vein. A total of 254 silica mineral–bearing (quartz and chalcedony) veins, up to 5 mm wide, constitute 4.9% of the total vein count and <0.1% by volume in Expedition 309 cores (Table T20). Calcite was identified in 20 veins, accounting for only 0.8% of veins in the hydrothermal alteration zone (<0.1 vol%) (Table T20). Calcite is present as a minor component in veins throughout Hole 1256D associated with saponite, silica, and rare sulfide veins. Laumontite is first identified by XRD in Sample 309-1256D-91R-1, 12–14 cm (864.20 mbsf) (Table T23) and is present irregularly to Section 130R-1 (1030 mbsf). Laumontite was identified in 42 veins or 1.2% of total veins (<0.1 vol%) (Tables T20, T23). Anhydrite, quartz, and pyrite are common in chlorite-bearing veins along with minor titanite and calcite. Anhydrite is first observed in Section 309-1256D-81R-1 (788 mbsf) and is common at depths below Section 128R-1 (1056.1 mbsf) (Tables T20, T23; Figs. F111, F113). A total of 365 anhydrite-bearing veins were recorded, constituting 7.1% of total veins (<0.1 vol%). A total of 28 veins with >90% anhydrite were recorded. Rare epidote in veins occurs in Sections 309-1256D-135R-1, 145R-1, and 150R-1, either filling the entire vein or with chlorite, pyrite, and anhydrite (Fig. F114).

Timing

Spatial relationships of minerals in veins and host rocks (Figs. F115, F116, F117, F118, F119, F120) suggest the following successive stages of alteration:

1. Chlorite, titanite, albite (not in vein), actinolite, and pyrite;
2. Quartz, epidote, pyrite, chalcopyrite, and sphalerite; and
3. Anhydrite, prehnite, laumontite, and calcite.

Although Stage 3 minerals are grouped together, they did not form contemporaneously, and intensive shore-based observations of crystallization patterns will be required to clearly establish the timing. Stages 1 and 2 reflect alteration under greenschist-facies conditions, whereas Stage 3 reflects later mineral precipitation from fluids of varying chemistry at lower temperatures (100°–250°C) than Stages 1 and 2.

Simple and composite gray to green alteration halos generated under greenschist-facies conditions

Alteration halos in rocks from Section 309-1256D-128R-1 (1056.1 mbsf) occur as both simple dark gray, dark green, light gray, and light green halos and composite halos in which every combination of these colors is possible (Fig. F121). These halos range in width from 1 to 11 mm (average = 5 mm), depending on alteration intensity and groundmass size, and represent 0.4% by volume of the cores. The most intense development of these styles of halos occurs from 860 to 1115 mbsf. These halos comprise 10%–100% (average = 50%) secondary minerals, with chlorite (up to 43 vol%) and titanite (up to 5 vol%) together with actinolite, which becomes more common downhole (up to 43 vol%) (e.g., Figs. F122, F123, F124, F125, F126, F127), albite (up to 10 vol%), pyrite (up to 10 vol%), and minor quartz, chalcopyrite, and prehnite replacing plagioclase and clinopyroxene and filling interstitial spaces (e.g., Samples 309-1256D-149R-1, 93–98 cm, and 153R-2, 3–6 cm) (Figs. F126, F127).

Alteration patches

Alteration patches are highly recrystallized centimeter-scale areas, where the degree of recrystallization decreases from 100% in the center (recorded as amygdules in the alteration log; see 309ALT.XLS in “Supplementary material”) to 40%–60% in the external light gray rim. The center of these patches is composed of one or several of the following minerals: anhydrite, prehnite, quartz, zeolite, chlorite, and pyrite (Fig. F128). One spectacular highly recrystallized zone in Sample 309-1256D-156R-1, 68–74 cm, comprises a 4 mm × 10 mm central amygdule (100% secondary minerals) with, from center to rim, quartz, pyrite, chlorite, titanite, prehnite, anhydrite, and calcite surrounded by a dark alteration patch in which the percentage of secondary minerals is lower (95%) and rare primary minerals are present (Fig. F129).

Light green halos may flank amygdules, as in Sections 309-1256D-141R-1 and 154R-2, in which the light green color results from a high abundance of chlorite, quartz, and anhydrite (Fig. F130).

Alteration of glass

Below ~1028 mbsf, glass is mainly related to volcanic breccias, as in Cores 309-1256D-122R (1027.3 mbsf) through 128R (1056.1 mbsf), or it occurs in brecciated chilled margins related to the contact between dikes and the basaltic host rock (e.g., Core 153R; 1174.9 mbsf). Altered glass is light green to dark gray in hand specimen and yellow-brown when replaced by chlorite/saponite or green when replaced by chlorite and titanite in thin section (Fig. F131). In Sample 309-1256D-161R-2, 51–55 cm, glassy clasts of breccia exhibit spectacular successive rinds or more or less green (chlorite) and yellowish brown material developing from microfractures (0.1 mm thick) filled by prehnite at the center and yellowish brown material at the edge.

Breccia

Hyaloclastites altered at high temperature are present in Sections 309-1256D-123R-1 and 136R-1. These breccias comprise glass clasts within a matrix of chlorite-smectite, quartz, anhydrite, and disseminated pyrite. Although higher temperature phases are present, the style of alteration is very similar to that observed in low-temperature hyaloclastites with glass fragments surrounded by green chloritic alteration rims with numerous microfractures filled with pyrite and quartz.

Mineralized volcanic breccia is present in Sections 309-1256D-122R-1 and 122R-2, and similar rocks are present in Section 135R-1. This style of breccia accounts for ~30% of total recovered breccias. The mineralized volcanic breccia in Core 309-1256D-122R is composed of glass fragments and basaltic clasts cemented by saponite, anhydrite, pyrite, and chalcedony. Basaltic clasts are rimed by intensely altered composite dark-light gray-green halos where clinopyroxene is partially replaced by chlorite-smectite and plagioclase is partially replaced by albite. Some basalt clasts exhibit total replacement. Some clasts (Sample 309-1256D-122R-1, 90–93 cm, for example) (Fig. F132) have poorly defined, diffuse margins that grade into the cement and are characterized by light gray, light green, and dark green highly altered halos. The cement is pale green and composed of chlorite-smectite, anhydrite, pyrite, chalcopyrite, chalcedony, and late-stage calcite.

Within Section 309-1256D-122R-1, the mineralized volcanic breccia exhibits a gradation between a dark-green/bluish color in the upper part (Fig. F133) and light green in the lower part (Fig. F132). Such color changes are observed both in the cement and the clast, and the origin of the color is related to a decreasing abundance of glass fragments and an increase in basaltic clasts.

Hydrothermal breccia is observed in interval 309-1256D-122R-2, 62–85 cm (Fig. F134), and constitutes <10% of recovered breccia. It is composed of subangular basalt clasts and is supported by a cement of chlorite-smectite, quartz, and pyrite. The basalt lithic clasts are rimmed by composite dark green to light green chlorite-smectite halos. Background alteration within the clasts consists of quartz, chlorite-smectite, and anhedral pyrite replacing plagioclase, interstitial areas, and vesicles. Veins of chlorite-smectite and quartz with unclear relationships crosscut the clasts. Pyrite formed as late infill along with radial crystals of quartz in a cement composed of ~60% quartz, 30% pyrite, and 10% chlorite-smectite.

Summary

Two alteration types were encountered in the section of Hole 1256D drilled during Expedition 309 (Table T22). At the top of the Expedition 309 section, rocks that had reacted with seawater at low temperatures, similar to the range of conditions encountered by Leg 206 cores, are present down to ~981 mbsf. These rocks contain saponite, celadonite, iron oxyhydroxides, chalcedony, and pyrite. Vein-related alteration types identified in Hole 1256D include black, brown, and mixed halos; simple light green, light gray, dark green, and light gray halos; and discontinuous pyrite halos.

Below ~981 mbsf, secondary minerals indicative of higher temperature hydrothermal alteration were observed. The transition between these two types of alteration is defined according to the following criteria:

• The first appearance of chlorite-smectite in thin section at ~981 mbsf.
• The occurrence of the mineralized volcanic breccia at ~1028 mbsf.
• The first occurrences of actinolite, prehnite, titanite, and epidote recorded at 1027, 1032, 1051, and 1095 mbsf, respectively. These minerals are indicative of hydrothermal alteration under subgreenschist- to greenschist-facies conditions. Alteration halos occur both as simple dark gray, dark green, light gray, and light green halos and composite halos in which every combination of these colors is possible.

Structural geology

The primary igneous and postmagmatic structures described in cores recovered from Hole 1256D between 752 and 1255.1 mbsf during Expedition 309 are directly comparable to features observed during Leg 206. Primary igneous structures include syn- to late magmatic structures partially linked to flow and solidification of lava. Postmagmatic structures include veins, cataclastic zones and microfaults, joints, and breccias. The distribution of structural features in Hole 1256D documented during Expedition 309 is summarized in Figure F135. Our techniques and methods utilized are discussed in “Structural geology” in the “Methods” chapter with structural observations recorded on the structural description forms and the structural logs (see Table T5 in the “Methods” chapter and STRUCTUR.XLS and BRECLOG.XLS in “Supplementary material”). Textural features and composition of breccia are recorded in the breccia log (see Table T5 in the “Methods” chapter).

Primary igneous structures

Primary igneous features recognized in Hole 1256D include textural banding or modal layering, lava flow–related textures, alignment of vesicles, brittle microfracturing of minerals, igneous contacts, and late magmatic veining with associated fracturing. The downhole occurrence of these features is reported in Figure F135.

Igneous fabric

The most notable features are textural domains and/or modal domains (Smith, 2002; Phillpotts and Dickson, 2002) commonly preserved in sheet flows and localized toward the top of flows. Textural domains are defined by the occurrence of alternating bands with different crystal abundance or crystal alignment. Modal layering is defined by crystal abundance and type of minerals present.

Weak to pronounced shape-preferred orientation of plagioclase is observed near chilled margins of sheet flows in Sections 309-1256D-75R-1, 80R-3, and 98R-1. Aligned plagioclase laths (<0.1 mm) are either scattered within a glassy or cryptocrystalline groundmass or clustered in thin bands (Fig. F136). As minerals do not show crystal-plastic deformation or dynamic recrystallization, such alignment is induced by the flow of lava. Flow banding in glassy margins is commonly defined by aligned coalesced spherulites (intervals 309-1256D-80R-1 [Piece 10, 52–59 cm] and 96R-1 [Piece 21, 109–112 cm]).

Flow-related structures such as aligned and imbricate plagioclase laths have also been observed in dike chilled margins (e.g., Sections 309-1256D-161R-2 and 163R-2) (Fig. F137). Alternating plagioclase-rich layers and layers with flattened and stretched spherulites and amygdules (Section 309-1256D-80R-1) (Fig. F136), as well as changes in groundmass grain size (see “Igneous petrology”), define weak modal layering in some sheet flows.

Another type of modal layering is developed by pyroxene-plagioclase clusters with intervening submillimetric to millimetric patches of once glassy mesostasis, now replaced by clay minerals. These patches are locally lens shaped and are imperfectly aligned to form discontinuous layers (Sections 309-1256D-84R-1 and 128R-1) (Fig. F138). The dips of these alignment surfaces are generally horizontal, although in interval 309-1256D-84R-1 (Piece 5, 37–40 cm), in the massive flow Unit 1256D-30, altered glassy patches and intervening plagioclase laths are steeply oriented.

The dip of these primary igneous features varies from horizontal in the top and bottom parts of sheet flows (intervals 309-1256D-75R-1 [Piece 8, 45–52 cm], 76R-2 [Piece 1, 0–16 cm], and 98R-1 [Piece 3, 9–20 cm]) to nearly vertical at dike contacts (Sections 149R-1 and 161R-1) and, locally, in the interior part of flows (intervals 84R-1 [Piece 5, 29–44 cm] and 92R-1 [Piece 1, 0–6 cm]). Trails of subspherical vesicles are rare and do not have preferred orientations. This is as expected when sampling different portions of lobate lava flows with variable primary dips with a vertical drillhole.

Folding near chilled margins

At the top of sheet flows and in glassy breccia clasts near chilled glassy margins, the millimeter-scale layers of flattened and coalesced spherulites and layers of aligned plagioclase laths are folded within the glassy matrix (e.g., intervals 309-1256D-80R-3 [Piece 3, 15–19 cm] and 89R-1 [Piece 13, 64–67 cm]). In sheet flows, folds mainly have subhorizontal or gently dipping axial planes parallel to the chilled contact and show similar or isoclinal geometry. Intervals 309-1256D-80R-3 (Piece 3, 15–19 cm) and 128R-1 (Piece 2, 6–12 cm) show multiple refolding of the plagioclase layers (Fig. F139). Folding with vertical axial planes occurs as well and overprints the earlier subhorizontal folding (intervals 309-1256D-80R-3 [Piece 3, 15–19 cm] and 75R-1 [Piece 8, 45–52 cm]) (Figs. F139, F140). Folds are also preserved in dikes in Sections 309-1256D-149R-1 (Fig. F139C) and 163R-2. These folds have vertical axial planes subparallel to the contact with the host rock.

Crystal fracturing

In fine-grained to microcrystalline basalts, plagioclase laths and phenocrysts commonly exhibit microfractures. Fracturing is observed throughout all cores and is heterogeneously distributed. In plagioclase, microcracks are mostly planar and arranged radially from the twin planes. Healed arrays of solid and/or fluid inclusions are reactivated or cut by microcracks, mainly controlled by crystallographic cleavages. These features most commonly result from differential cooling and shrinkage. The intragranular microcracks are filled by alteration phases, principally saponite, and are the main sites for the initiation of primary mineral replacement (e.g., intervals 309-1256D-85R-1 [Piece 1, 119–122 cm], 85R-2 [Piece 2, 104–107 cm], 118R-1 [Piece 3, 11–13 cm], and 132R-1, 143–147 cm) (Fig. F141). From ~1000 mbsf downhole, plagioclase crystals exhibit microstructures characteristic of low-temperature (<400°C) deformation such as deformation twinning, flame perthite textures, and bending.

Intrusive contacts

Numerous chilled margins were recovered in cores from the transition zone and the sheeted dike complex. From ~1004 mbsf, where such contacts are subvertical, they are tentatively interpreted as dike contacts (e.g., Sections 309-1256D-117R-1, 136R-1, 137R-1, 140R-1, and 143R-1).

Chilled margins range from sharp chilled contacts (e.g., intervals 309-1256D-136R-1 [Piece 4, 12–18 cm], 151R-1, 23–39 and 120–125 cm, and 165R-3 [Piece 6, 53–59 cm]) to irregular contacts (e.g., interval 151R-1, 115–117 cm) (Fig. F142). Many of these dike chilled margins are associated with, or highly disrupted by, diffuse veining and brecciation (e.g., intervals 309-1256D-117R-1 [Piece 12, 121–131 cm] and 140R-1 [Piece 5, 27–63 cm] and Sections 137R-1, 143R-1, and 161R-1) (Fig. F142C). Multiple dikes and banded dikes (Fig. F143) occur in Sections 309-1256D-161R-1, 163R-2, and 166R-1; their true dip ranges from 50° to 90° with a mode at ~70°–75° (Fig. F144).

Veins and joints

Veins are the most abundant structures observed. Veins are extensional open fractures filled by a variety of secondary minerals. In this group of structures we also include shear veins, which are veins with minor shear displacement, coated by slickenfibers or overlapping fibers (see “Structural geology” in the “Methods” chapter). Joints are open fractures with no differential displacement. We measured very few joints in oriented core pieces, and those measured commonly show planar morphology. Drilling-induced fractures (mostly saddle-shaped and disc fractures) (Fig. F145) are present, but these were not systematically recorded in the structural logs. Veins and joints resulting from fracturing during cooling were also not measured, as these cannot provide reliable data for establishing the regional stress field. Cooling vein geometries include Y-shaped intersections (triple junctions) and curved or sinuous, mostly steeply dipping veins intersected at high angle by thinner veins (Fig. F146). Cooling-related veins are more common in sheet flows than in massive units (e.g., Sections 309-1256D-77R-1, 95R-1, and 112R-1).

A total of 1076 veins were logged and measured in Hole 1256D during Expedition 309 (see STRUCTUR.XLS in “Supplementary material”). These veins, together with microfaults and breccia, give insight into the extent of fracturing in recovered rocks. However, the number of logged veins and joints underestimates the total number of veins present in the cores because we measured only those structures present in oriented pieces (see the “Methods” chapter).

Veins commonly have planar and curved morphologies. Their width ranges from <1 to 2 mm. Veins displaying an alteration halo in the adjacent wallrock commonly have a total width of ~5 mm (see “Alteration”). Many veins have irregular margins but a generally planar trend. An individual vein may branch into a number of diverging “splays” (see “Structural geology” in the “Methods” chapter) at their ends. In some cases, veins are oriented in en echelon and/or Riedel shear arrays or have dilated into tension gashes filled with secondary minerals (e.g., infilling of quartz in interval 309-1256D-82R-1 [Piece 2, 3–16 cm]). Such arrays can be used to define the sense of shear (Fig. F147). Multiple veins can exhibit anastomosing geometries, and where veining is intense, vein networks have developed (e.g., Sections 309-1256D-101R-1, 104R-1, and 117R-1) (Fig. F148). Vein networks also form sets of parallel veins (e.g., interval 309-1256D-100R-1 [Piece 13, 93–104 cm]). These sets of vertical veins are common throughout the hole, but their density increases from 1090.7 mbsf downhole in the sheeted dike complex (see Fig. F135). Where vein networks become complex, they can grade into an incipient brecciation texture (e.g., Section 206-1256D-59R-2) (see “Breccia,” below).

Veins (mostly planar) form conjugate sets with a dihedral angle of ~50°–60°, and these features are mostly developed in the massive flows (e.g., Sections 309-1256D-130R-1, 131R-1, 133R-1, 135R-1; massive Unit 1256D-44) (Fig. F149).

Shear veins are mostly planar, 0.5–2 mm wide, and commonly filled with dark green saponite and sulfides. The geometry and orientation of fibers or slickenlines in shear veins show both normal and reversed senses of shear. A normal sense of shear is most common throughout Hole 1256D, but a reversed sense of shear is mostly concentrated in shear veins from Cores 309-1256D-108R through 118R (Table T24; Fig. F150). Minor strike-slip shear veins occur in Cores 309-1256D-86R through 87R (from ~820 to ~830 mbsf) and in Cores 129R through 131R (from 1060 to ~1070 mbsf).

Minerals filling veins consist of saponite, chlorite-smectite, celadonite, iron oxides, sulfides, silica (quartz and chalcedony), anhydrite, minor calcite, prehnite, laumontite, and epidote. Veins are commonly composed of a combination of these minerals, and the end-member types were defined based on the most abundant infilling mineral (see “Alteration”). Crystal morphology of the infilling minerals may be fibrous, blocky, or composite. Saponite is the principal mineral in fibrous veins. Clay fibers are commonly straight and elongated at a high angle to the vein wall, but in curved or irregular veins, these fibers can be oblique or radiating from the vein walls. Nonfibrous vein minerals include iron oxides and oxyhydroxide, celadonite, and chalcedony. Many veins appear to have formed in several superposed cracking events. Such composite veins are commonly characterized by the coexistence of both fibrous and nonfibrous minerals (e.g., interval 309-1256D-77R-1 [Piece 22, 106–128 cm]). Figure F151 illustrates an example of a composite vein filled with fibrous saponite and fine-grained chalcedony along the vein wall and coarse-grained chalcedony in the vein center.

Cataclastic zones and microfaults

Cataclastic or fault zones recovered in Expedition 309 basalts are characterized by volumes of deformed wallrock accompanied by sets of subparallel anastomosing veins and shear veins that grade into vein networks and brecciated damage zones. Two main types of cataclastic zones have been observed: those related to shear veins and slickensides in fine-grained basalt and cataclastic zones that occur at the contact between fine-grained basalt and chilled margins.

The term “microfault” is retained when a slickenside with a small volume of fault rocks (millimeter scale) or no fault rocks is present (e.g., Sections 309-1256D-85R-4, 85R-5, and 87R-1). The infillings of the microfault are commonly affected by intense hydrothermal alteration where the basaltic groundmass is almost totally replaced by saponite. The cataclastic zones and microfaults are heterogeneously distributed downhole through the lava pile (Fig. F135). Most cataclastic zones have steep dips.

Examples of cataclastic/​fault zones associated with sets of veins and shear veins in fine-grained basalt are in Sections 309-1256D-85R-1 and 85R-2 (Fig. F152). The apparent width of these zones and their offset (where visible) is millimetric to centimetric. Cataclastic zones can be characterized by quite irregular and gradational margins, such as in Sections 309-1256D-85R-1 and 87R-1 (Fig. F153), or by straight and sharp margins, as in interval 117R-1, 116–118 cm. Cataclastic zones in interval 309-1256D-85R-1 (Piece 1E, 72–76 cm) are mostly composed of fragments of altered glass, broken crystals of clinopyroxene and plagioclase, and millimetric angular clasts of the host rock and of an altered very fine grained (<0.1–0.2 mm) to glassy matrix. Cataclastic zones with straight margins cut cataclastic zones with irregular morphologies (Fig. F154C, F154D) and are marked by altered glass (Fig. F154B, F154D). Flow structures are visible in thin section (Fig. F154B).

The best examples of cataclastic zones at the contact with chilled basalt occur in Sections 309-1256D-117R-1 and 140R-1. These are characterized by fine-grained to aphanitic cryptocrystalline protocataclasite to cataclasite and fault gouge (using the fault-rock terminology of Sibson, 1977), with local development of slightly to moderately foliated fabrics. The cataclastic massive unit in Section 309-1256D-117R-1 consists of rounded to angular clasts of dolerite and glassy spherulitic to variolitic basalt. A total of 3–4 cm of cataclasite separate doleritic basalt fragments from chilled fragments (Fig. F155). The damaged zone is characterized by a complex network of tiny veins, mostly dark green, dark brown, and light green, on the cut surface of the core. The light green veins have an aphanitic vitreous luster and disturb and cut across dark brown cataclastic saponite-bearing bands. These bands are composed of cataclasite and protocataclasite cut by veins of ultracataclasite and gouge. The crosscutting relationships between the different types of fault rocks are visible in thin section (Fig. F155B). Vein networks and cataclastic banding have caused incipient brecciation of the host rock, and larger fragments show only minor relative rotation. Flow-related microstructures and laminations are observed in very narrow (0.2–0.5 mm wide) veins. In thin section, fragments of plagioclase show intergranular and intragranular deformation. Clasts are surrounded by a banded matrix that displays flow textures and is made up of subangular and rounded fragments of minerals and altered glass of variable grain size.

The cataclastic zone in Section 309-1256D-140R-1 (Fig. F156) is characterized by flow structures and multiple veining. Veins have dilational features and different mineral fillings (see “Alteration”); clear crosscutting relationships are visible. Euhedral sulfides are disseminated in veins that cut through the cataclastic zone. Veins of chlorite and sulfide show shear bands and are cut by dilational quartz veins with stretched fibers and crack-and-seal growth steps. Cataclasite and protocataclasite consist of wallrock fragments and fragments of plagioclase and pyroxene in a quartz-bearing matrix.

Narrow cataclastic zones resembling veins cut through cryptocrystalline basalt in Section 309-1256D-143R-1 (Fig. F157A). Within the cataclastic zone, common sand-sized quartz and feldspar grains (interval 309-1256D-143R-1 [Piece 7, 34–38 cm]) (Fig. F157B) in a very fine grained prehnite-bearing matrix suggest the occurrence of fluid-assisted disaggregation of the host material.

A 3 mm wide cataclastic zone occurs in Section 309-1256D-139R-1 (Fig. F158) where primary minerals of the host basalt are partially replaced by chlorite, albite, and titanite. The cataclastic zone is composed of fine-grained to cryptocrystalline material derived from fracturing of the host rock and is cut by a syntectonic vein containing carbonate, sulfide, and very fine grained quartz. The cataclastic zone cuts a 1–2 cm wide brittle-ductile shear zone composed of 0.1 mm wide chlorite- and titanite-bearing shear bands, dilational quartz-bearing gashes, and one submillimetric cataclastic layer.

Breccia

Three main types of breccia were recovered in Hole 1256D during Expedition 309 (Table T25): incipient breccia, hyaloclastite, and hydrothermal breccia.

Incipient breccia includes microbreccia starting from veins or vein networks (Fig. F159) where contact with the basalt host rock is commonly preserved. Submillimetric to millimetric basalt clasts are isolated by veins and “float” within a vein matrix of clay minerals, silica, altered glass, and sulfides (see “Alteration”). In more intensely brecciated cores, breccia has a “jigsaw puzzle” texture in which clasts have angular shapes with straight boundaries and can be fitted back together, although some clasts are subrounded. Clasts are millimeter to centimeter sized and represent fragments of the basaltic host rock. Good examples of incipient breccias occur in intervals 309-1256D-83R-2 (Piece 5, 16–21 cm) and 108R-2 (Pieces 2–6, 4–30 cm), but such features occur irregularly in Cores 78R through 128R, mostly in small unoriented pieces. These characteristics suggest in situ formation, with fragmentation occurring along veins grading into a vein network that isolates clasts of intact basalt (Harper and Tartarotti, 1996). Disaggregation of the host basalt would be aided by contractive cooling joints. Even though the displacement of clasts away from the intact rock is small (millimeter scale), the nature of the matrix and the angular shape of clasts suggests that brecciation was assisted by relatively high fluid pressures.

Hyaloclastites are breccias composed of vitroclastic material produced by the interaction of water and hot magma or lava (Fisher and Schmincke, 1984). During Expedition 309, hyaloclastite was observed in Sections 309-1256D-90R-1 (inside sheet flow Unit 1256D-33a), 113R-1 (bottom part of sheet flow Unit 1256D-37), and 114R-1 (upper part of massive Unit 1256D-39a). Hyaloclastite comprises clasts of volcanic glass and altered glassy shards, with both rounded and cuspate shapes, surrounded by a matrix of fine-grained clay (see BRECLOG.XLS in “Supplementary material”). Glassy shards are millimeter sized and are replaced by colliform light brown and whitish minerals that grew from the rim to the core of the shard (see “Alteration”). In interval 309-1256D-90R-1 (Piece 2, 5–10 cm), platy clasts have a preferred orientation, defining weak layering (see Fig. F101).

Mineralized volcanic breccia

The mineralized volcanic breccia is a hyaloclastite characterized by the presence of sulfide minerals (see “Igneous petrology”). It occurs in Sections 309-1256D-122R-1 and 122R-2, where it forms 15–20 cm long pieces; it is also present in intervals 135R-1 (Pieces 10–13, 101–130 cm) and 137R-1 (Pieces 13–15, 84–105 cm). This volcanic breccia consists of aphyric basalt clasts with subangular to subrounded shapes ranging in size from 2 mm to 7 cm, volcanic glass clasts, glassy shards, and subrounded to rounded altered glassy shards (Fig. F160). Basalt clasts exhibit textural features of the sheet flow, such as spherulitic to variolitic textures (see Fig. F160) and lava flow–related folding. Clasts are embedded in a scarce, fine-grained clay matrix cemented by sulfides, carbonate, and silica (see “Alteration”). The mineralized volcanic breccia may be regarded as a subtype of hyaloclastite because of the heterogeneous nature of clasts, in spite of their common basaltic composition (Fisher and Schmincke, 1984). In Core 309-1256D-122R, the mineralized volcanic breccia grades from almost pure hyaloclastite with rare sulfides in the upper part of the core (interval 122R-1, 25–51 cm) to mineralized hyaloclastite in the lower part of Section 122R-1 (interval 122R-1, 52–125 cm) and the upper part of Section 122R-2 (interval 122R-2, 1–15 cm). From the upper to the lower parts, the mineralized volcanic breccia exhibits a relative increase in basalt clasts and matrix volume with respect to glassy clasts (Fig. F161A, F161B). Breccias also show variation in color, probably reflecting changes in alteration (see “Alteration”), as is also suggested by the irregular shapes of aphyric basalt clasts of the light green pieces, which merge with diffuse margins into the matrix (Fig. F162).

Hyaloclastite is also associated with multiple intrusive chilled contacts in the sheeted dike complex (e.g., Sections 309-1256D-161R-1 and 161R-2) (Fig. F163) and is composed of subrounded altered glassy clasts and basalt clasts. In interval 309-1256D-161R-1 (Piece 2, 9–15 cm), clasts consist of cuspate glassy shards with uncommon irregular rims. Clasts are embedded by altered glass.

Hydrothermal breccia is present in interval 309-1256D-122R-2 (Pieces 9–11, 62–85 cm). It consists of millimeter- to centimeter-sized irregular fragments of altered glass and angular aphyric cryptocrystalline basalt supported by cement made of quartz, carbonate, and sulfides (see Fig. F134; “Alteration”). Rims of basalt clasts are straight and regular and marked by a composite dark green to light green halo.

Structure orientation

Only true dip data obtained by measurement of structure orientations are considered because cores are not oriented with respect to geographic north. The absence of structural marker planes such as sedimentary bedding or regular modal layering of known orientation precludes the reorientation of measured veins and the calculation of dip azimuths. Integration of paleomagnetic data, wireline logs, and whole-round core scans may enable reorientation of veins on some larger pieces during shore-based analysis, which would enable calculation of true dip azimuths.

True dip values of all oriented structures (i.e., veins, shear veins, joints, and igneous contacts) for Expedition 309 are shown in Figure F164. Most structures are steeply dipping, with dip angles generally between 50° and 80° with a frequency mode of ~80°. Steeply dipping and vertical veins are common in the lower part of Hole 1256D below 1160 mbsf (Fig. F135).

The distribution of true dips between 989.9 and 1109.0 mbsf (Units 1256D-39 through 46) shows that the true dip values of planar structures from massive Units 1256D-39, 1256D-44, 1256D-45, and 1256D-46 have a bimodal distribution with the modes diverging at an angle of ~40°–50° (Fig. F165). This depth interval corresponds also to the first appearance of conjugate vein systems (Fig. F135).

True dip values of veins, shear veins, and microfaults from Hole 1256D are plotted in the rose diagrams in Figure F166. Veins are characterized by high dip values clustered around 70° and 90°, but secondary clusters with dips of 40° and 60° are also present. In contrast, shear veins are steeply dipping, having a mode around 80°.

Summary of structures

A summary of the distribution of the main structural features downhole is reported in Figure F135 and is compared with the intensity of fracturing and lithostratigraphy of Hole 1256D. In the sheet and massive flows (from 750 to 1004 mbsf), structures and fracturing are heterogeneously distributed. Vertical sets of veins, cataclastic zones, and shear veins are present in massive units; vertical veins become more common below ~900 mbsf. Breccias are more common in sheet flow units. Fracturing is most intensely developed at the top of massive flows.

In the transition zone (1004.2–1060.9 mbsf), breccias are the most common structural feature. The top of the zone is also characterized by the appearance of dike chilled margins and the concurrent presence of cataclastic zones, breccia, and vertical veins. The intensity of fracturing downhole is mostly slight with the exception of Unit 1256D-42 (i.e., the mineralized volcanic breccia).

In the sheeted dike complex, subvertical dike chilled margins are common. This zone is also characterized by the first notable occurrence of systematic conjugate veins. From 1090.7 mbsf (Unit 1256D-45) downhole, all the structural features except shear veins are common and more abundant. Shear veins are present in only the uppermost massive basalt (Unit 1256D-44).

Paleomagnetism

To characterize the paleomagnetic signal and resolve the magnetization components recorded in igneous rocks in Hole 1256D, we measured and analyzed the magnetic remanence of selected archive-half sections, discrete rock pieces (<15 cm long) from the archive half, and discrete cubes (~8 cm³ and 1 cm³) from the working half of the core (Tables T26, T27, T28). Remanence data were collected before and after progressive alternating-field or thermal demagnetization. Our primary goal is to assess the roles of different rock types that construct the upper oceanic crust in generating marine magnetic anomalies. To reach this goal, a variety of shore-based rock magnetic data will be required along with more remanence data to build upon our shipboard remanence observations. The data presented here provide some important information about the stability of the magnetization of the upper oceanic crust observed onboard.

All core pieces and discrete samples have a pronounced drilling overprint, which is characterized by a steep downward direction and a radial-horizontal component that points toward the center of the core. This was documented during Leg 206 (among others) through measurements of 25 separate pieces of a 1 cm whole round from interval 206-1256D-26R-4, 74–75 cm, located at 443.19 mbsf in Hole 1256D (Wilson, Teagle, Acton, et al., 2003). In the IODP core orientation system, this overprint results in a strong bias in the declinations toward 0° for archive-half samples and 180° for working-half samples (see “Core orientation” in the “Methods” chapter). Initial NRM measurements are thus characterized by very steep inclinations of ~70°–80° and scattered declinations commonly close to 0° or 180°. For a number of intervals, we were unable to remove this overprint with alternating-field or thermal demagnetization, with the magnetization not reaching a stable characteristic remanent magnetization (ChRM). However, for other intervals, especially below ~1000 mbsf, the overprint is not so severe and is at least partly removed by the demagnetization methods applied.

Measurements and results

Our data set was based on analyses of ~9 cm³ and 1 cm³ cubes cut from the working half of the core and measurements of whole pieces or subpieces from the archive half of the core. Archive pieces had to be large enough to be oriented accurately (generally at least 8 cm long) but small enough not to cause flux jumps in the magnetometer (generally <15 cm). Because the drilling overprint is stronger and more resistant to demagnetization near the outside of the core, archive-half pieces are less suitable than working-half discrete cubes, but because alternating-field demagnetization is nondestructive for purposes other than magnetic analysis, we were able to measure many more archive-half samples. All archive-half and most working-half samples were subject to alternating-field demagnetization (Table T26); 12 discrete working-half samples from the depth interval 753–928 mbsf were treated with thermal demagnetization (Table T27).

Stable remanent magnetic directions were calculated using principal component analysis (PCA) as proposed by Kirschvink (1980) to ensure that the directions showed a well-defined component trending to the origin of the demagnetization diagrams. Directions passing away from the origin of the diagram were rejected. In all cases, ChRM is defined from at least three and as many as eight successive directions. In order to assess the quality of the calculated directions, we used as a quality parameter the maximum angular deviation (MAD), as defined by Kirschvink (1980) and Butler (1992). Samples with MAD angles >15° are categorized as poor quality and therefore rejected. Samples that show no trends in directions from nearly vertical toward more horizontal inclinations were also excluded from the data sets.

Working-half alternating-field demagnetization

To isolate the ChRM component of magnetization of most discrete samples, alternating fields from 2 to 80 mT (the maximum field of the 2G rock magnetometer demagnetizer) were consistently applied at 2–4 mT increments below 20 mT and at 5 mT increments from 20 to 80 mT. Fields above 50 mT generally produced scattered directions and low intensities that were not used in analysis.

Demagnetization behavior for several samples is shown in Figures F167, F168, F169, F170, and F171. Sample 309-1256D-88R-1, 78–80 cm (Fig. F167), shows marginally acceptable stability, as is typical of observations in depths from 752 to 1000 mbsf. Direction change is erratic, and MAD, at 13.8°, barely meets the acceptance criterion. Other samples, especially deeper than 1000 mbsf, show higher coercivities, smoother directional change, and possible approach close to a ChRM (Figs. F168, F169, F170, F171). Some of these more stable samples show NRM directions close to the expected working-half overprint (Figs. F168, F169) but show a steady trend toward shallower inclinations until directions become scattered at high demagnetization fields. These examples are typical for depths below 1000 mbsf. Because inclination changes significantly for these samples during demagnetization, it is difficult to judge how closely the ChRM direction is approached before the directions scatter. PCA analysis used here on usually 3–8 demagnetization steps gives an inclination commonly 5°–15° steeper than the direction at the highest step used for these samples. The most stable samples (Figs. F170, F171) show NRM directions relatively far from the overprint direction and lose intensity fairly slowly; however, even these samples show progressive change of inclination that makes it difficult to identify a precise ChRM inclination.

Working-half thermal demagnetization

Progressive thermal demagnetization experiments were conducted on 1 cm³ cubes to study their magnetic stability. Most samples have 50% of remanent magnetization removed between 200° and 300°C, with further magnetic intensity loss to 500°–600°C, temperatures at which >95% of remanent magnetization has been removed (Fig. F172). Above 500°C, intensities are low and directions are scattered, indicating that blocking temperature or Curie temperature has been exceeded or that magnetic minerals are being converted. Stable direction behavior and steady intensity loss to 500°C shown in Figure F172 indicate low-Ti titanomagnetite as the dominant magnetic carrier. Because of time constraints and lack of clearly superior magnetic cleaning by thermal demagnetization, only one batch of samples was thermally demagnetized.

Archive-half alternating-field demagnetization

We applied alternating fields from 5 to 40 mT at 5 mT increments to the studied archive-half pieces. NRMs were consistently close to the expected steep and northerly directions, those attributed to drilling-induced overprint. Inclinations generally became shallower during demagnetization but did not appear to reach a stable endpoint. Declinations rarely moved to southerly directions, indicating only partial removal of the drilling overprint.

Results overview

Direction results from PCA analysis are plotted as a function of depth for all samples in Figure F173. Intensities for NRM and selected alternating-field demagnetization levels are shown in Figure F174. Because these depth plots include all samples, with varying degrees of success at removing the drilling overprint, we also plot the ratio of intensity remaining after 20 mT alternating-field demagnetization to NRM intensity (Fig. F175). An index shown in the analysis of Leg 206 paleomagnetic results to be generally higher for samples with directional behavior, suggesting better removal of drilling overprint (Wilson, Teagle, Acton, et al., 2003). Generally higher coercivities in deeper depths of Expedition 309 recovery are shown by a pronounced increase in this ratio over the interval 970–1030 mbsf (Fig. F175).

At first glance, declinations in Figure F173 appear random. For archive-half samples, circular plots (Fig. F176) show that directions are clustered strongly about northerly directions. Working-half directions are much more azimuthally distributed. The difference probably results from (1) the greater strength of the drilling overprint on the outer margins of the archive-half pieces and (2) rejection of data in discrete samples from the working half.

One of the interesting points of the routine alternating-field demagnetization of the archive-half samples is the variability of the magnetic intensity after 30 mT. Downhole measurements of magnetic intensity show a recurrent concave pattern (Fig. F177), which shows a reasonable agreement correlation with boundaries of lithologic units and subunits (see “Igneous petrology”). High intensities are related to upper and lower boundaries of cooling units, whereas lower intensity peaks occur within units. Although further shore-based analyses are required, these trends probably result from changes in size and distribution of primary minerals (e.g., Petersen et al., 1979), titanomagnetite in particular. It is likely that magnetic intensity will be higher in portions where magnetic minerals are in the single or pseudosingle domain sizes (e.g., Petersen et al., 1979; Butler, 1992; Matzka et al., 2003), which correspond to those portions where mineral grain size is smaller (<0.05 µm; Bleil and Petersen, 1983). However, ~70% of the lithologic units and subunits show repeated concave patterns (Fig. F177A), suggesting the presence of multiple cooling units (with the observed magnetic intensity pattern) within a single lithologic unit. Downhole average thickness of these cooling units is ~1.0 ± 0.5 m (Fig. F177B).

Discussion

Because of the strong drilling overprint and uncertainty about how completely the overprint has been removed by demagnetization, we cannot yet make strong statements based on paleomagnetic results from Expedition 309. Because of the equatorial paleolatitude of the site, polarity remains ambiguous until absolute declinations can be obtained based on orienting pieces relative to downhole logging images of the borehole wall (see “Digital imaging” and “Downhole measurements”). The component of drilling overprint that may remain would affect inclination more than declination, so for samples such as those plotted in Figures F168, F169, F170, and F171, generally from cores deeper than 1000 mbsf, declination values will be reliable enough to determine polarity in azimuthally oriented pieces.

The generally positive inclinations are not what is expected for low paleolatitude. The most obvious possible explanation for the positive inclination is that a significant portion of drilling overprint may remain on nearly all of the samples. Potential alternatives that cannot be entirely discounted are (1) a pervasive present-field overprint and (2) tectonic tilting. However, any tilting must predate deposition of the ponded lava flows at the top of the section, and the nearly north–south original strike of the ridge axis does not provide a favorable orientation for changing inclination as a response to slip on ridge-parallel faults. Detailed postcruise demagnetization studies of a large number of discrete samples from the working half, integrated with rock-magnetic and other studies, should clarify many of the remaining unanswered questions from this work.

Physical properties

Shipboard physical property measurements were conducted on whole-round cores, split core halves, and discrete samples following the procedures described in “Physical properties” in the “Methods” chapter. Whole-core measurements of bulk magnetic susceptibility, GRA bulk density, and NGR were made on all recovered rocks using the shipboard multisensor track (MST). Discrete measurements of thermal conductivity were made on selected archive-half samples, and ~9 cm³ cubes were used for measurements of mass, volume, and compressional wave velocity (V_P). Measurement and sampling parameters (e.g., sampling intervals and sizes) are listed in Table T29. Figure F178 is a summary of discrete sample results, and Figures F179, F180, and F181 summarize MST results of Expedition 309. Throughout this section, average values are reported ±1 standard deviation.

Because of the fractured nature of basaltic rocks and incomplete recovery (average = ~36%) during the expedition, the diameter of recovered rocks varies and few long continuous sections of core were recovered (see “Digital imaging” for a discussion of the frequency of whole-round pieces >8 cm). Therefore, the quality of data acquired on the MST was degraded and the number of pieces available for half-round and discrete sample measurements was limited.

Density and porosity

Grain density, bulk density, and porosity were calculated from the wet mass, dry mass, and dry volume on 80 discrete samples (Table T30). Discrete sampling was limited by the dearth of homogeneous pieces that could be cut into ~9 cm³ minicubes without cracking. Where unbroken, coherent sections of the working-half cores were not available, some cubes were necessarily cut slightly <2 cm in one dimension. In a few cases, when no other sample of a particular rock type was available, small irregularly shaped pieces were used for mass and volume measurements. The Scientech 202 balances and Pentapycnometer have measurement precisions of 0.01 g and 0.02 cm³, respectively (Blum, 1997). During Expedition 309, repeat measurements of a calibration sphere and sample volumes yielded errors of <1%, corresponding to a density uncertainty of 0.03 g/cm³.

For all discrete samples measured during Expedition 309, average grain density and bulk density are 2.94 ± 0.04 and 2.86 ± 0.07 g/cm³, respectively. The densities of discrete samples do not show a strong downhole increase with depth (Fig. F178), even considering differences in rock type. Massive and sheet flow units have the same density within error (2.88 ± 0.04 and 2.86 ± 0.07 g/cm³). Average grain and bulk densities of basalts recovered during Leg 206 are similar at 2.92 ± 0.07 and 2.82 ± 0.10 g/cm³, respectively.

In general, any decrease in bulk density is accompanied by an increase in porosity. All porosity values from Expedition 309 except one are between 2% and 10%, with an average of 4%. There is a decrease in porosity from the massive units above 1060 mbsf to those below this level of 4% ± 1% to 2% ± 1% (Fig. F182). The mineralized volcanic breccia sample from Core 309-1256D-122R contains the highest porosity (14%). Some of the lack of variability in discrete sample density is likely due to sampling bias toward more robust pieces, which can be cut into cubes.

Discrete measurements of density are considered more precise, although GRA measurements were made every 2.5 cm of the whole-round cores to increase data coverage. Low recovery of large (>8 cm) pieces during Expedition 309, particularly in the sheet flow sections, resulted in a discontinuous GRA density record. In addition, any piece smaller than the whole-round diameter (~56–60 mm) produced a low, and in many cases negative, GRA density. Hence, all measurements <2 g/cm³ were discarded. Considering only GRA values >2 g/cm³, the average uncorrected GRA density for whole-round cores recovered during Expedition 309 is 2.5 ± 0.2 g/cm³ (Fig. F179). A preliminary volume correction can be made because the whole-round cores typically have a diameter of 58 mm rather than 66 mm (as is assumed by the Intelligent Manufacturing Software). Volume-corrected GRA density measurements of whole-core sections from Expedition 309 and Leg 206 are shown together in Figure F183. After correction, the average GRA density for Expedition 309 is 2.8 ± 0.2 g/cm³, well within the expected range for oceanic basalts (e.g., Telford et al., 1990) and within the error of the discrete bulk density results.

Although GRA density data are highly variable, density generally increases with depth. An exception to the depth trend is the lava pond that forms the uppermost basement (~250–350 mbsf) at Site 1256 and was cored during Leg 206. These rocks have a high corrected GRA density of 2.8 ± 0.1 g/cm³. From 350 to 1255 mbsf, GRA density increases from ~2.7 to 2.9 g/cm³. Below 1060 mbsf, average corrected GRA density is 2.9 ± 0.2 g/cm³. On average, Expedition 309 massive flows (2.80 ± 0.2 g/cm³) have the same corrected density as sheet flows (2.75 ± 0.2 g/cm³). The mineralized volcanic breccia in Section 309-1256D-122R-1 has a notably low average density, with a corrected GRA density average of 2.5 ± 0.1 g/cm³.

Compressional wave velocity

A total of 73 discrete cubes were measured for V_P (Table T30). Because only the most visually solid (crack free) pieces of core recovered can be used for V_P measurements, velocities reported here are likely higher than the bulk ocean crust at this site. The PWS3 instrument has a precision of 1 m/s and an accuracy of 2 m/s (Blum, 1997). Measurements made on the acrylic calibration standards were within 2 m/s of expected values. However, the imperfect cut surfaces of rock samples and/or heterogeneity within the samples increase the error in the V_P results. When a sample was not moved but measured 10 times, V_P measurements varied up to 20 m/s. When the same sample was placed at slightly different positions between the transducers (with or without adjusting the transducers) and remeasured, V_P repeatability was 40 m/s (~1% error).

The mean V_P is 5.5 ± 0.3 km/s and exhibits relatively small variations in depth through the cores recovered during Expedition 309 (Figs. F178, F184). From 752 to 1106 mbsf, average V_P increases ~0.05 km/s every 50 m down Hole 1256D to nearly 6.0 km/s at 1130.6 mbsf (Sample 309-1256D-143R-2, 2–4 cm). V_P is consistently higher and less variable below 1060 mbsf (5.8 ± 0.1 km/s) than above (5.4 ± 0.3 km/s).

Average horizontal and vertical V_P through Hole 1256D are 5.5 ± 0.3 km/s and are identical within error (Fig. F185A). However, below ~1060 mbsf, discrete samples have a slightly faster (~0.1 km/s) vertical V_P (Fig. F185B). V_P and porosity are inversely related with R² = 0.55. There is no clear relationship between velocity and rock type (Fig. F186), but this may reflect biased sampling of robust pieces of core. Porosity is frequently invoked as a major controlling factor on velocity (e.g., Carlson and Herrick, 1990), but much of the scatter in Site 1256 V_P data remains unexplained by porosity alone (Fig. F186). The intensity of alteration and the nature of the secondary minerals may also significantly influence V_P . In some locations, such as the volcanic mineralized breccia in Cores 309-1256D-122R and 123R (1028 mbsf), low velocity is coincident with high NGR counts (cf. Figs. F178, F181), likely dominated by ⁴⁰K (Jarrard et al., 2003), indicating a high abundance of clay minerals in these samples.

Thermal conductivity

Thermal conductivity was measured on 37 archive-half pieces (Table T31). Representative samples were chosen from pieces larger than ~7 cm. In some cores, no thermal conductivity measurements could be made because of widespread fracturing and poor recovery. Based on repeat measurements of samples and calibration standards, individual thermal conductivity values reported here have an instrument uncertainty of 0.1 W/(m·K) (~5%; Blum, 1997).

Thermal conductivity values are between 1.7 and 3.1 W/(m·K), with an overall average of 2.0 ± 0.3 W/(m·K) (Fig. F178). Of the major rock types recovered during Expedition 309 and Leg 206, massive basalts and dikes from the sheeted intrusives have significantly higher average thermal conductivity values than massive flows, sheet flows, pillows, and hyaloclastites from the overlying lavas (Fig. F187A). Sheet flows have the lowest thermal conductivity (1.7 ± 0.1 W/[m·K]). Mineralized volcanic breccia has the highest thermal conductivity (2–3.1 W/[m·K]) (Fig. F187A; Table T31) with a very high thermal conductivity (3.1 W/[m·K]) at Section 309-1256D-122R-1, 51 cm, which probably reflects the porous nature of the sample, the high degree of alteration, and the high sulfide connectivity (see “Alteration”).

The lava pond (~250–350.3 mbsf; Unit 1256D-1) has a relatively high thermal conductivity (2.0 ± 0.2 W/[m·K]) and overlies the inflated flows and sheet and massive flows, which have lower thermal conductivity (1.7 ± 0.1 and 1.8 ± 0.1 W/[m·K], respectively) (Fig. F187B). The transition zone has three thermal conductivity measurements from very different rock types (two in breccia and one in a dike), and therefore a wide range of thermal conductivity (2.4 ± 0.6 W/[m·K]) (Fig. F187B). There is a significant increase in thermal conductivity starting in the transition zone and a distinct steplike increase at the top of the sheeted dikes to 2.1 ± 0.1 W/(m·K) (1060.9 mbsf) (Fig. F187B).

Based on thermal conductivity measurements of samples from ~725 mbsf (~1.9 W/[m·K]) and downhole temperature measurements, heat flow in the basement at Site 1256 is calculated to be 113 mW/m² (for further description of heat flow, see “Predrilling experiments”). Subdividing the basement into two different thermal conductivity regions from 533 to 1004 mbsf, where thermal conductivity is 1.8 W/(m·K), and 1004 to 1255 mbsf, where thermal conductivity is 2.2 W/(m·K), and assuming constant heat flow through the crust, the predicted temperature is 85°C at 1004 mbsf and 98°C at 1255 mbsf. Pore water studies indicate the temperature gradient of Hole 1256D matches predictions of conductive heat flow, with little advective heat transport in the basement rocks.

Magnetic susceptibility

Magnetic susceptibility measurements were carried out using the MST at 2.5 cm intervals on whole-core sections prior to splitting. During Expedition 309, uncorrected magnetic susceptibility amplitudes vary from ~0 to ~13,000 × 10^–5 SI (Fig. F180A). Volume-corrected susceptibility has not been calculated for these cores, and all magnetic susceptibility data are presented with instrument units of SI. The uncertainty of magnetic susceptibility data is 5% (Blum, 1997).

Magnetic susceptibility is used as an indicator of variations in the type and concentration of magnetic minerals such as magnetite, hematite, goethite, and titanomagnetite in rocks. Variations in magnetic susceptibility can be attributed to (1) grain size of magnetic minerals, associated with type or thickness of flow, (2) style and degree of alteration, and (3) magnetic mineralogy, although it is not clear which factor dominates in these rocks. From the top of Hole 1256D through the sheet and massive flows (~250–1004.1 mbsf), massive flows generally have higher magnetic susceptibility than sheet flows. However, because recovered sheet flow pieces are commonly small, magnetic susceptibility data from sheet flows are more subject to edge effects. In the transition zone and into the sheeted dikes (1004.1–1255.1 mbsf), variability in magnetic susceptibility appears not to be dependent on rock type (Fig. F180) and may be more strongly influenced by the intensity and style of alteration (see “Alteration”).

Other factors also influence magnetic susceptibility in addition to rock type. For example, Units 1256D-27 through 29b (Sections 309-1256D-75R-1 through 80R-2; 752.0–803.9 mbsf) and Units 1256D-32 through 33 (Sections 88R-2 through 99R-1; 840.7–907.4 mbsf) are all sheet flows but have distinctly different magnetic susceptibility characteristics. A possible explanation for the difference is variation in the thickness of sheet flows, which affects both the abundance and size of magnetic mineral grains. The flows identified in Units 1256D-27 through 29b are thicker than ~1 m, but the flows in Units 1256D-32 through 33 can be as thin as a few tens of centimeters (see “Igneous petrology”). These changes in magnetic properties coincide with abundant sulfides in veins (pyrite and/or chalcopyrite) and pervasive destruction of primary titanomagnetite due to intense hydrothermal alteration. Zones of intense alteration coincide with regions of low magnetic susceptibility. In addition, both massive and sheet flow units show a qualitative increase in magnetic susceptibility near the top and bottom of the igneous units (see “Paleomagnetism”).

Natural gamma radiation

NGR measurements were carried out using the MST at 5 cm intervals on whole-core sections prior to splitting. NGR counts vary from 0 to 11 for a 20 s integration (Fig. F180). Of the 8642 NGR measurements made during this expedition, only 609 (7%) were above the detection limit of the instrument (0.01 counts). The average of the NGR values above the detection limit is 1.4. More than half of these values are <1 count.

A primary intended use of NGR data during this expedition was for comparison with downhole logging tool measurements (see “Downhole measurements”). In addition, NGR counts can also be used to qualitatively estimate the presence of significant potassium, which is a possible indicator of celadonite or other potassium-bearing minerals formed during low-temperature hydrothermal alteration. Although NGR rarely exceeds background noise and is strongly affected by low recovery (Fig. F180), some zones of high NGR, such as intervals 309-1256D-154R-1, 60–75 cm, and 161R-1, 1–9 cm, coincide with relatively high K₂O from ICP-AES analyses (0.08 and 0.13 wt%, respectively; see “Geochemistry”).

Multisensor track data reduction

The MST is designed to collect several types of data at regular intervals and is best suited for making physical property measurements on continuous sediment cores. Because cores recovered from Hole 1256D are discontinuous and individual pieces are separated by plastic spacers, the raw MST data are far from ideal and do not provide a continuous downhole evaluation of physical properties. Consequently, there is very large scatter in the MST measurements and we have filtered the data to include only pieces longer than 2 cm. After selecting pieces longer than 2 cm, edge effects are still apparent (Figs. F179B, F180B, F181B). Considering only the middle measurement from pieces longer than 8 cm eliminates much of the scatter but still likely underestimates the true values of GRA density, NGR, and magnetic susceptibility (Tables T32, T33, T34; Figs. F179C, F180C, F181C). Consequently, only pieces >8 cm were used to calculate average GRA density, magnetic susceptibility, and NGR for each unit (Figs. F179D, F180D, F181D).

Examples of MST data for five different rock types— massive flow, sheet flow, mineralized volcanic breccia, the cataclastic massive unit, and a dike—are shown in Figure F188. The massive flow in interval 309-1256D-85R-1, 130–140 cm, has characteristically high magnetic susceptibility, high GRA density, and low to no NGR signal (Fig. F188A). NGR counts are occasionally above zero in massive flows because of pervasive background alteration and veins (see “Alteration”). The sheet flow in interval 309-1256D-95R-1, 84–94 cm, has characteristic and variable magnetic susceptibility and GRA density signals, mostly due to the fractured nature of these rocks and poor recovery (Fig. F188B). Sheet flows without veins or pervasive background alteration commonly have no detectable NGR. The spectacularly altered mineralized volcanic breccia (interval 309-1256D-122R-1, 90–100 cm) has low magnetic susceptibility, low, albeit variable, GRA density, and high NGR (Fig. F188C). In addition, this unit has distinctly low magnetic intensity (see “Paleomagnetism”). The cataclastic massive unit (interval 309-1256D-117R-1, 97–107 cm) (Fig. F188D) has variable magnetic susceptibility similar to that of the sheet flows but no detectable NGR signal and a low GRA density, likely due to the broken nature of the piece. The cataclastic massive unit is highly altered, mostly to saponite without celadonite. This replacement of groundmass by saponite without celadonite is typical of higher temperature alteration deeper (>918 mbsf) in Hole 1256D. The dike (interval 309-1256D-160R-1, 13–23 cm) has high magnetic susceptibility, high but variable GRA density, and nondetectable NGR (Fig. F188E).

Digital imaging

Rotary coring generally returns azimuthally unoriented samples. Cores can potentially be oriented by matching features observed in the core to features imaged by wireline logging of the borehole wall. For the purpose of obtaining orientation, all whole-round core pieces that were longer than ~80 mm and that could be rotated smoothly through 360° were imaged on the DMT Digital Color CoreScan system (DMT GeoTec, 1996; DMT GmbH, 2000a).

Prior to scanning, a red line was marked on each piece to indicate the position of the cut surface. Our convention was that the working half of the core was to the right of this line with the core upright. A letter “W” was marked on the working half to minimize the chances of sorting errors after cutting the core. Subsequent observations of the core such as vein strike or magnetic declination are oriented by the IODP convention relative to the cut surface (see Figs. F11 and F15 in the “Methods” chapter), so orienting the red line relative to a logging image allows orienting core observations to magnetic north.

Core image processing

For each piece scanned, the length of the piece was measured and the depth to the top of the piece was calculated. This information is required at the acquisition stage as it is entered in the DMT software Digicore (DMT GmbH, 2000a). Depths, lengths, and piece numbers of all scanned pieces are provided in a log (see SCANLOG.XLS in “Supplementary material”). In all, 531 pieces of total length 97.2 m were scanned, representing ~20% of the interval cored during Expedition 309.

The scanned images were then integrated for each core using the Core Recovery Quality Control program (DMT GmbH, 2000b). Images are plotted on a depth scale according to their IODP curated depths, leaving appropriate gaps where material was not scanned or not recovered. Depth profiles are output as EMF files, which can then be opened, edited, and saved in Adobe Illustrator format.

After whole-round scanning, core pieces were split and labeled according to IODP convention. The cut face of the archive half of every section was allowed to dry and was then scanned, prior to description by the petrologists, using the IODP Geotek digital imaging system. The slabbed images were added to the Adobe Illustrator files and aligned in depth alongside the whole-round images for comparison between external and internal features of the core. Figure F189 shows an example of a whole-round digital core image alongside the corresponding cut-face image for comparison. Planar features, such as veins and fractures, are linear in the slabbed core but appear as sinusoids on the whole-round image. These files are intended for use as a template for detailed postcruise structural and core-log integration studies, in particular for whole-round core and downhole log image correlation. They are available as “Supplementary material.” They will aid determination of core (and sample) depth with respect to downhole logs and of core reorientation with respect to geographic north as measured magnetically by the GPIT on the FMS-sonic and UBI tool strings.

Core-log integration

Because of limited time between the collection of downhole logging data and the end of the cruise, only a few preliminary attempts at matching core images to logging images have been made. Some of these attempts, using the methods described in Haggas et al. (2001), show potentially good matches between the unrolled core images and the FMS and UBI data from Hole 1256D. Figure F189 shows an example using the largest piece recovered during Expedition 309, Section 309-1256D-85R-1. Although it is hard to trace fractures as sinusoids through all four panels from the FMS pads, the spacing and dip of the fractures can be matched convincingly between the core images and the FMS images, with the cutting line on the north side of the core.

Downhole measurements

Operations

After reaching a depth of 1255.1 mbsf, Hole 1256D was conditioned and filled with a mixture of sepiolite drilling mud and seawater. Following a wiper trip, the pipe was tripped and a logging BHA with logging bit was run into the hole. The base of the BHA was set at 261 mbsf, and a total of five tool strings were deployed (Fig. F190). The first deployment consisted of the triple combo tool string, which contained the HNGS, the APS, the HLDS, the DLL, and the TAP tool. An obstruction was found at 1226 mbsf that prevented the tool string from proceeding to the bottom of the hole. The first pass covered the interval from 1226 to 555 mbsf. For data quality check, a short repeat pass was run between 945 and 840 mbsf.

The second tool string deployment was the FMS-sonic, which consisted of the DSI, the SGT, the GPIT, and the FMS. A successful main pass was made from 1224 to 555 mbsf, from slightly above the obstruction found during the previous run. Following completion of this first pass, the caliper arms failed to close. After ~2 h of unsuccessful attempts to close the calipers, it was clear that the tool was failing to respond. The tool string reentered the pipe with 700 lb overpull and was pulled back to the surface with the caliper arms extended. No external damage was noticed on the caliper, and no equipment was left in the hole.

The third run included the UBI, the GPIT, and the SGT. A sinker bar was added to speed up the lowering of the tool in the pipe. Furthermore, as the cable tension was low during the descent of the tool, we used the drilling pumps to wash the tool down the hole. The tool was not lowered to TD but only to 1215 mbsf to protect the ultrasonic subassembly located at the end of the tool string. The main pass covered the interval between 1215 and 707 mbsf. A repeat pass from 1215 to 1105 mbsf was also performed. This pass was devoted to test different acquisition settings in order to adjust the time window for detection of the first sonic wave arrival. The UBI was then lowered to 1215 mbsf again to record the main pass in high-resolution mode (120 m/h) to 707 mbsf. A repeat pass at standard resolution mode (280 m/h) was conducted from 1215 to 1005 mbrf. After these successful runs, 45 min was devoted to heave compensator tests with the Offshore Service Unit-F-model Modular Configuration MAXIS Electrical Capstan Capable (OSU-FMEC).

The fourth tool string consisted of the WST. The tool was lowered to the end of the pipe and halted in a safe position. After 1 h of marine mammal watch without a sighting, the generator-injector air gun was soft-started and pressure levels increased over half an hour. However, while running into the hole at ~20 m below the pipe the cable tension dropped 200 lb. Furthermore, the tool’s geophone background signal showed low noise when we tried to lower it, indicating that the tool was not moving down. The tool was then moved back inside the pipe and the WST was lowered twice, but we observed the same loss of cable tension in the open hole. While going back inside the pipe, 800 lb overpull was recorded. This overpull suggested that the cable had been damaged during the tool lowering attempts. The decision was made to retrieve the WST and inspect the tool and cable. Once at the surface, the tool was successfully tested. However, the cable showed six small kinks, starting from 3 m above the cable head to ~50 m. The decision was made to cut ~60 m of wireline cable and rehead the cable.

As we were now ahead of schedule, the FMS-sonic tool string, including the backup FMS probe, was deployed for the second time. During this fifth run, a single pass was achieved from 1216 (39 m above the bottom of the hole) to 305 mbsf, covering almost the entire Hole 1256D basement section. Furthermore, the DSI was run in cross-dipole mode. After this successful run, 45 min was devoted to further heave compensator tests with the OSU-FMEC.

During all logging runs, the WHC was turned on following the exit from pipe, and it was used continuously while the tool strings were in open hole. Sea conditions were favorable with maximum 1.5 m of heave. Logging operations and rig-down were completed by 0500 h on 24 August 2005.

Processing and data quality

Following acquisition, logging data were transmitted to LDEO for depth and environmental correction processing (see “Downhole measurements” in the “Methods” chapter) and the processed data were then returned to the ship. Depth matching was done by matching the postdrilling triple combo main pass to the predrilling logging results. All other postdrilling passes were then matched to this new triple combo pass.

Principal results are shown in Figure F191. Borehole conditions were excellent during the five runs, and no ledges or obstructions were encountered except for the one at the bottom of the hole. Caliper readings from both the triple combo and FMS-sonic tool strings show generally good borehole conditions (Fig. F192), with a diameter typically between 11 and 14 inches. The average hole diameter measurements are 11.25 inches for C1 and 10.90 inches for C2; this slight difference is the result of an elliptical borehole between 807 and 966 mbsf. Wide sections (>13 inches) are particularly common in this interval as well as between 1048 and 1060 mbsf. These enlarged hole sections affect measurements by tools that require eccentralization and good contact between the tool and the borehole wall (APS, HLDS, UBI, and FMS). Excellent hole conditions over the rest of the interval resulted in good measurements by these contact tools, particularly for the lowermost 300 m. Triple combo data are high quality, and there is an excellent overlap with the previous logging runs. Comparison of caliper data from pre- and postdrilling operations of the upper 500 m shows that progressive enlargement with continued drilling is evident (Fig. F192). For example, between 531 and 602 mbsf, sections wider than 13 inches were rare and localized during Leg 206 and predrilling operations. Such wide sections are now abundant, and calipers indicate that the borehole is elliptical (i.e., C1 ≠ C2). The borehole deviation is 4.3° at 1200 mbsf, and hole azimuth varies between 250° and 290°.

The FMS and UBI provided high-quality data. However, because the UBI has to be deployed very slowly (120 m/h), incomplete heave compensation and sticking of the tool influence the data quality. FMS and UBI data were processed to correct for acceleration and sticking that occurred during the uphole logging. The two passes of the FMS were depth-matched, with the predrilling pass as reference. Although the FMS images can be corrected with confidence, the UBI images still show artifacts of sticking. Further depth-matching of UBI data with FMS passes may alleviate some of those effects. Resistivity images were statically and dynamically normalized during conversion to color images. In most intervals, the coverage of the borehole wall by the two FMS passes is good and is complemented by UBI images. The DSI was operated with the modes of P and S monopole (medium frequency), lower dipole (standard frequency), upper dipole (standard frequency), and Stoneley (standard frequency) during the first FMS-sonic pass and cross-dipole during the second pass. Overall, logging data are of good to excellent quality for most of the measured parameters and images. Furthermore, all repeat measurements reproduced previously obtained data well.

Results and data overview

On each logging run, natural radioactivity was measured continuously with either the HNGS or SGT. The total gamma ray log exhibits a general but not monotonic decrease in radioactivity with depth (Fig. F193). Total natural radioactivity generally varies from 2 to 10 gAPI. Intervals of high natural radioactivity (>8 gAPI) are present between 770 and 774, 784 and 796, and 842 and 878 mbsf. In this latter interval, high natural radioactivity values are not associated with low-resistivity rock. This suggests that this interval may correspond to chemically different rocks. An extremely high natural radioactivity value (37 gAPI) is recorded at 886 mbsf, and it appears that uranium has the strongest influence on this high natural radioactivity value (Fig. F193). Below this depth, natural radioactivity becomes extremely low (2 gAPI on average) with only three narrow intervals where natural radioactivity reaches 6 gAPI (886–888, 944–947, and 1131–1132 mbsf).

Electrical resistivity measurements obtained with the DLL are highly variable with values ranging from 2.5 to 2500 Ωm. Generally, the lowest resistivity zones are well correlated with the lowest density and highest neutron porosity logs. The lowest resistivities are recorded between 919 and 927, 1028 and 1032, and 1047 and 1054 mbsf. The first interval is also associated with perturbations in the temperature profile (Fig. F194) that may indicate a highly fractured or faulted interval where fluids are circulating. The interval 1028–1032 mbsf is associated with an increase in natural radioactivity that may indicate an altered zone. This interval can confidently be correlated with the mineralized volcanic breccia (Cores 309-1256D-122R through 123R; Unit 1256D-42; see “Igneous petrology”). Between 785 and 843 mbsf and 853 and 920 mbsf, deep electrical resistivity increases with depth from 9 to 770 Ωm and 9 to 106 Ωm, respectively. The highest electrical resistivity values are recorded in the deepest part of the hole. Furthermore, a strong decoupling between the shallow and deep measurements is observed in this interval. Overall, electrical resistivity data suggest that the measured interval may be separated into three sections based on the variability and magnitude of the electrical resistivity (see “Discussion”).

Velocity data from the first FMS-sonic run are presented in Figure F195. P-wave velocities from the top of the logged section to 1028 mbsf are mostly in the range 4.5–6 km/s but are highly variable. Shear wave velocities follow similar trends to those of the P-wave velocity log. As recorded by electrical resistivity, two intervals from 785 to 843 and 853 to 920 mbsf show distinct trends, where the compressional velocities increase from 4 to 6 km/s for both trends. Below 1028 mbsf, P-wave values show a general increase with depth (compressional velocities >6 km/s become common). The exception to this trend is the intervals 1032–1036 and 1050–1054 mbsf, where extremely low compressional velocities (as well as low shear and Stoneley velocities) are measured (as low as 3.7 km/s). Two 1 m thick high-velocity intervals are identified at 1174 and 1177 mbsf. There is generally good agreement between logging P-wave and core P-wave values, with both data sets showing stepwise variations downhole. Postcruise processing and detailed analysis of cross-dipole measurements will determine any potential velocity anisotropy.

Density measurements are strongly influenced by hole conditions, especially in the upper logged interval to 967 mbsf. Below this depth, hole conditions are very good, and consequently reliable density and photoelectric effect (PEF) data are recorded. Bulk density values of the formation range from 1.13 g/cm³ at 925 mbsf to 3.3 g/cm³ at 1088 mbsf with a mean value of 2.75 g/cm³ for the entire logged section. Very low density values correspond to borehole washouts or fractured intervals and are generally correlated with high values in the porosity log. Density measured below 967 mbsf shows excellent correlation with density values from discrete laboratory measurements (see “Physical properties”). In this interval, density is generally in the 2.8–2.9 g/cm³ range. PEF shows variations between 2 and 5 b/e^–, with a mean value of 3.72 b/e^–.

Porosity measured in Hole 1256D is highly variable, as the porosity measurements are strongly influenced by hole conditions. Porosity ranges from 4.8% to 50%. High porosity values generally correlate with density and resistivity and correspond to intervals where the tool lost contact with the borehole wall. Absolute values in the neutron porosity log do not match discrete laboratory porosity measurements, which are usually lower. This may be caused by the presence of bound water in alteration minerals and/or sampling bias due to poor sampling by drilling and highly fractured material.

Initial analysis of imagery data (FMS and UBI) indicates that high-quality data were obtained for the vast majority of the hole. Examples of these data are presented in Figures F196 and F197 where FMS data are compared to UBI images. As the impedance contrast between different rock types drilled in Hole 1256D is high, very good results are obtained with the UBI in terms of lithologic changes, and excellent correlations can be made with the FMS (Figs. F196, F197). Furthermore, as these two tools provide oriented images, the continuous structural information gained from the images with respect to dip and azimuth of conductive fractures is a crucial contribution to the understanding of the tectonic evolution of the oceanic crust where Hole 1256D is drilled. On FMS and UBI images, mapping structural features consists of connecting a perfect plane (sinusoid) through the presumed geological object (mainly conductive features on FMS images or low-amplitude features on UBI images). Onboard preliminary analyses indicate a dominant northeast plunge direction for the subvertical features (Fig. F197) interpreted as dike margins (see “Structural geology”).

During the final logging run, the temperature of Hole 1256D was recorded using the TAP tool during the downhole and uphole runs as well as the main and repeat passes (Fig. F194). The borehole temperature measured uphole is slightly higher than the one measured downhole. The temperature record from the TAP tool must be interpreted with caution because the borehole had not reached thermal equilibrium following intense circulation of drilling fluid during the coring operation and wiper trip. Temperature data recorded immediately postdrilling give important information on cooling of the Site 1256 lithosphere and will be useful for planning future temperature-sensitive tool deployments. Maximum temperature in the hole is 60°C, significantly cooler than the 105°C predicted from heat flow and temperature measured during predrilling operations (see “Predrilling experiments”). Nevertheless, there are clear perturbations in the temperature profile. In Figure F194, three intervals centered approximately at 691, 796, and 928 mbsf display negative temperature anomalies that suggest a slower return to equilibrium. In the same figure, the resistivity log and the FMS images (interval 917–934 mbsf) show that these intervals have very low resistivity. The temperature anomaly at 928 mbsf indicates that this interval is probably a higher permeability section (faulted or highly fractured interval) that has been invaded by the cold drilling fluids, and hence cooled to lower temperatures, and is consequently recovering more slowly from the drilling process.

Discussion

Downhole geophysical measurements and images recorded in Hole 1256D show a high degree of variation, reflecting the massive units, thin flows, breccia, and dikes encountered in Hole 1256D (see “Igneous petrology”). Overall, combined results of FMS and UBI images and standard geophysical measurements suggest that the section drilled during Expedition 309 may be separated into three subsections. During Leg 206, three logging intervals were distinguished based on variations of the geophysical parameters (Wilson, Teagle, Acton, et al., 2003): Interval I (base of casing to 346 mbsf), Interval II (346–532 mbsf), and Interval III (532–752 mbsf). Preliminary interpretation of Expedition 309 data shows that Interval III continues to 920 mbsf.

Interval III (532–920 mbsf) has moderate resistivity values (commonly between 10 and 100 Ωm) with very high, short-wavelength variability frequency. Natural radioactivity is also highly variable in this interval but is usually >2 gAPI. This portion of the borehole is characterized by alternating layers of thin flows, breccias, and massive units.

Interval IV extends from 920 to 1060 mbsf and is characterized by long-wavelength, large-amplitude resistivity variations. Within this interval, low-resistivity zones are generally associated with high natural radioactivity values. This interval coincides with volcanic breccias interbedded within sheet flows described in cores.

Interval V corresponds to the sheeted intrusives from 1060 to 1226 mbsf. This interval is characterized by high electrical resistivity values and by a strong decoupling between shallow and deep electrical resistivities. Furthermore, extremely low (<2 gAPI) and constant natural radioactivity is recorded. On the FMS and UBI images, this interval is characterized by the presence of subvertical highly conductive features that may be interpreted as sheeted dike margins that dip steeply (~80°–85°) to the northeast. These regions have abundant of subhorizontal features possibly resembling fractures and veins.

Underway geophysics

We collected routine underway measurements of bathymetry and magnetic field on the outbound transit from Balboa, Panama, to Site 1256. Because the expedition plan was to deepen an existing hole at a well-mapped site, no additional surveys were conducted. Of the 1522 km of transit distance, 1025 km has bathymetric data and 1009 km has magnetic data. Because of minor maneuvering to maintain satellite communications, the track departs by several kilometers from the tracks of Leg 206, especially west of 88°W. This departure will allow mapping of local variations in magnetic anomaly strike.

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