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doi:10.2204/iodp.proc.309312.101.2006

Scientific results

Expedition 309, Superfast Spreading Rate Crust 2, successfully deepened Hole 1256D by 503 m to a total depth of 1255.1 mbsf, or 1005.1 msb. At the end of Expedition 309, Hole 1256D penetrated a total of >750 m of extrusive lavas and proceeded a further 250 m into sheeted dikes. At 1255 mbsf, Hole 1256D was tantalizingly close to the predicted minimum estimated depth for the frozen axial magma chambers (1275 mbsf). Following the completion of a comprehensive wireline logging program, Hole 1256D was successfully exited and left clear of equipment with only minor unconsolidated fill at the bottom of the hole.

Expedition 309 (July–August 2005) was followed closely by Expedition 312, Superfast Spreading Rate Crust 3 (November–December 2005). During Expedition 312, Hole 1256D was deepened by 252.0 m to 1507.1 mbsf (1257.1 msb), successfully achieving the main goal of the Superfast Spreading Crust mission, penetration through lavas and dikes into gabbros. The hole now extends through the 345.7 m thick sheeted dike complex and 100.5 m into gabbroic rocks. The latter were first encountered at 1406.6 mbsf, near the middle of the depth range predicted from geophysical observations (Fig. F12). A complete suite of wireline logging, including a vertical seismic profile, was carried out, and the hole remains clear and open for future drilling deeper into the plutonic foundation of the crust.

Predrilling experiments during Expedition 309

The first scientific operation of Expedition 309 was to deploy the wireline Water Sampling Temperature Probe (WSTP) to collect a sample of the fluid at the bottom of Hole 1256D and measure the ambient thermal conditions in the hole using the Advanced Piston Corer Temperature (APCT) and Lamont-Doherty Earth Observatory (LDEO) Temperature/​Acceleration/​Pressure (TAP) tools. The WSTP was followed by wireline logging runs using the triple combo and FMS-sonic tool strings to assess the condition and caliper of the hole and check for borehole wall breakouts and unstable regions. There was ~27 m of loose fill in the bottom of Hole 1256D that precluded running the wireline tools to the full depth of the hole (752 mbsf).

The temperature profile made with the TAP tool on the triple combo tool string shows a steady increase in temperature from the base of the casing (269 mbsf) to a maximum temperature of 67.5°C recorded at the deepest logging depth (~725 mbsf). 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 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 the thermal conductivity measurements and the inevitable slight disturbance of the ambient hole temperature due to the passage of the drill string and wireline tools. This suggests that there is little advection of heat by fluid in the Site 1256 basement or major vertical fluid movements in Hole 1256D.

The 67.5°C deep borehole fluid collected from the bottom of Hole 1256D (~725 mbsf) is chemically very distinct from seawater. Relative to Site 1256 bottom seawater, the borehole fluid is hotter, slightly more neutral (pH = ~7.4), and has significantly lower alkalinity (0.85 mM). Salinity is unchanged (35‰). The largest changes are in the 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 the calcium content is very strongly elevated (415%). The deep borehole fluid is also significantly different from the composition of the ~35°C uppermost (~250 mbsf) basement fluid estimated from pore water chemical gradients measured at Site 1256 during Leg 206 (Wilson, Teagle, Acton, et al., 2003). The deep borehole fluid has lower lithium (–15%), magnesium (–25%), and potassium (–30%) concentrations but higher dissolved silica (14%), sulfate (84%), and calcium (122%) concentrations relative to the uppermost basement fluid.

When the deep borehole fluid is compared to the well-characterized basement fluids from the eastern flank of the Juan de Fuca Ridge (Elderfield et al., 1999), most ions are present in concentrations similar to those predicted for a 67.5°C fluid. The exception is the Mg concentration of the deep borehole fluid, which at ~24 mM is higher than would be expected for a fluid reacted with basement at >60°C. The disagreement between the concentration of Mg in hot deep borehole fluids and that predicted from uppermost basement fluids or laboratory experiments has previously been noted for wireline fluid samples recovered from Hole 504B (e.g., Becker, Foss, et al., 1992). However, the chemistry of the deep borehole fluid from Hole 1256D is closer to equilibrium than the most pristine borehole fluids, which were taken from the deepest available points in Hole 504B during the different drilling expeditions there. Those fluids have Mg concentrations similar to those of the uppermost basement fluid at Site 504. In contrast, the fluid from the bottom of Hole 1256D has Ca (and Li and K) concentrations close to those expected for a fluid in chemical equilibrium with 67.5°C basement and, unlike Hole 504B, the Mg concentration is at least partially decreased toward the predicted composition.

The primary purpose of the initial logging operations was to check Hole 1256D for borehole wall breakouts and variations in hole diameter though comparison with measurements made at the end of Leg 206. The initial phase of wireline logging indicated that, before the commencement of drilling, borehole conditions in Hole 1256D were 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. The FMS-sonic tool string followed a different pathway during the Expedition 309 pass compared with the Leg 206 passes, and consequently, in many intervals the FMS image coverage of the borehole wall has increased. Sonic velocities measured by the Dipole Sonic Imager appear to be of high quality. Several narrow zones (at 517, 597, 602, and 685 mbsf) with strong differences in the orthogonally arranged calipers on the FMS-sonic tool string were identified during both Leg 206 and Expedition 309. These intervals may correspond to borehole breakouts, and the north–south orientation of the borehole enlargements suggests a west–east maximum stress direction. A tight spot recognized during Leg 206 at 486 mbsf was also recorded during the Expedition 309 predrilling passes, and a new zone was identified at 472 mbsf (9.3 inches).

Ocean crust formed at superfast spreading rate: deep drilling the ocean basement in Hole 1256D

During Expedition 309, Hole 1256D was reentered with a rotary core barrel (RCB) assembly and C-9 coring bit at 2325 h on 18 July 2005, and ~27 m of loose fill was cleared from the bottom of the hole so that it was open to the full depth achieved during Leg 206 (752 mbsf; 502 msb). Rotary coring of the basement continued until 20 August (~33 days), when the hole was conditioned for wireline logging operations. A total of nine C-9 RCB hard-formation coring bits were used, and Hole 1256D was deepened by ~503 m to 1255.1 mbsf (~1005.1 msb; Cores 309-1256D-75R through 170R).

Hole 1256D was reentered at 2030 h on 15 November 2005 during Expedition 312. Following ~5½ days of remedial washing and reaming, the hole was opened and cleared of debris to the depth reached during Expedition 309. Rotary coring of basement proceeded for ~23 days, interrupted by ~5 days of fishing and milling when the cones were lost from a C-7 RCB coring bit. A total of seven RCB coring bits (six C-9 and one C-7) were used, and the hole was deepened by 252.0 m to 1507.1 mbsf (1257.1 msb), reached at 0250 h on 19 December. The hole was then prepared for a full suite of wireline logging. Logging tools did not penetrate past 1432 mbsf (1182 msb), indicating an obstruction ~75 m above the bottom of the hole. The hole was exited cleanly and remains open for further drilling.

Core recovery averaged 36% in Hole 1256D, with the highest rates in the lava pond and the lowest rates in the lowermost dikes (Table T3; Figure F13). Rate of penetration (ROP) varied similarly, with the highest rates in the lavas and the lowest rates in the lower dikes.

At 1257.1 msb, Hole 1256D is the fourth deepest hole drilled into oceanic basement since the launch of scientific ocean drilling in 1968 and the second deepest penetration into in situ ocean crust (Fig. F14). Hole 504B, deepened during seven DSDP and ODP legs into 6.9 Ma crust on the southern flank of the intermediate-spreading Costa Rica Rift, remains the deepest penetration of in situ ocean crust. Prior to Expedition 309/312, Hole 504B was the only hole to sample a complete sequence of extrusive rocks as well as the transition from extrusive rocks to sheeted dikes (Alt, Kinoshita, Stokking, et al., 1993).

Preliminary subdivision of the upper oceanic crust at Site 1256

To facilitate description and discussion of the crustal stratigraphy at Site 1256 and assist in the interpretation of cores recovered during Expedition 309/312, we present a subdivision of the upper crust sampled so far in Hole 1256D (Table T4; Fig. F15). Detailed descriptions are presented in the following sections.

The upper crust at Site 1256 can be portioned into six basement subdivisions, which, in descending order down the hole, are the lava pond, inflated flows, sheet and massive flows, transition zone, sheeted dikes, and plutonic section (Table T4).

Lava pond

The lava pond caps the uppermost crust at Site 1256. This domain includes Units 1256C-1 through 18 and 1256D-1 (~250–350.3 mbsf). The uppermost lavas were not recovered in Hole 1256D because 16 inch casing was set 19.5 m into basement and the interval was not cored. In Hole 1256C, the rocks immediately below the sediments comprise thin basaltic sheet flows a few tens of centimeters to ~3 m thick separated by chilled margins and rare intervals of recrystallized sediment (Units 1256C-1 through 17). The massive ponded flow, sensu stricto (Units 1256C-18 and 1256D-1), is defined at its top by a ~75 cm rind of glassy to cryptocrystalline aphyric basalt that overlies ~30 and ~74 m of fine-grained basalt in Holes 1256C and 1256D, respectively. The massive ponded flow becomes abruptly cryptocrystalline ~1.5 m above its base. Although the massive flow is much thicker in Hole 1256D than in 1256C, it is interpreted as a single lava body whose interior was liquid at the same time in both locations. The dramatic increase in thickness over 30 m of lateral distance and a total thickness in excess of 74 m indicates that there was at least this much paleotopography in order to pool the lava. On fast-spreading ridges, such topography does not normally develop until ~5–10 km from the axis (e.g., Macdonald et al., 1989), and we suspect that these lavas flowed a significant distance off axis before ponding in a faulted depression.

Inflated flows

Immediately underlying the lava pond is a sequence of massive flows, pillow lavas, and sheet flows (Units 1256D-2 through 15; 350.3–533.9 mbsf) grouped together as the inflated flows. Although rocks exhibiting a number of eruptive styles are included here, the criterion for subdivision is the occurrence of subvertical elongate fractures filled with quenched glass and hyaloclastite (e.g., Sections 206-1256D-21R-1 and 40R-1) at the top of the lava flows. These features indicate flow-lobe inflation that requires eruption onto a subhorizontal surface at less than a few degrees (Umino et al., 2000, 2002), suggesting off-axial flows.

Sheet and massive flows

The bulk of the extrusive lavas at Site 1256 are included in the sheet and massive flows (Units 1256D-16 through 39b; 533.9–1004.2 mbsf). A total of 218 m of this subdivision was drilled during Leg 206, with a further 252 m of penetration during Expedition 309. This sequence consists of sheet flows tens of centimeters to ~3 m thick with subordinate massive flows >3 to 16 m thick and uncommon breccias. The flows are aphyric to sparsely phyric, cryptocrystalline to microcrystalline basalts. Units are distinguished by the presence of chilled margins or by grain-size variations. Throughout this interval, glassy chilled margins are common.

Transition zone

It is the very essence of a transitional sequence that boundaries are loosely defined and subjective. In Hole 1256D, the transition zone from Units 1256D-40 through 44a (1004.2–1060.9 mbsf) is identified by the occurrence of a number of criteria and different rock types as opposed to the appearance of one specific feature. Shore-based analysis of wireline logs and further petrographic and geochemical investigations will help refine the boundaries of this zone. Most of the rocks within the transition zone are aphyric, cryptocrystalline sheet flows. The top of the transition zone is marked by a cataclastic massive unit (Section 309-1256D-117R-1, 85 cm). This comprises subvertically oriented cryptocrystalline basalt clasts hosted within a very highly altered fine-grained basalt that has been incipiently brecciated and deformed along numerous fine veins and cataclastic stringers. Core 309-1256D-120R (~1018 mbsf) includes the first sign of a subvertical intrusive contact other than the single occurrence further upcore in Section 206-1256D-32R-2 at ~475 mbsf. Dike chilled margins become more common downhole, although extrusive textures and vesicles are still encountered. It should be noted that subvertical fracture sets possibly indicative of diking into the host rocks near Hole 1256D are common from ~900 mbsf. Breccias of various styles are common in the transition zone, including a spectacular mineralized volcanic breccia that comprises Unit 1256D-42a (interval 309-1256D-122R-1, 20 cm, to 122R-2, 30 cm; ~1028 mbsf). In the transition zone, secondary mineral assemblages (chlorite-smectite, albite, chlorite, actinolite, anhydrite ± minor prehnite, epidote, and laumonite) indicative of hydrothermal alteration at subgreenschist to greenschist facies temperatures start to become more common.

Sheeted dikes

Hole 1256D penetrates a 345.7 m thick sheeted dike complex from 1060.9 to 1406.6 mbsf (Units 1256D-44a through 80b). The upper boundary is defined by a change from sheet flows to massive basalts (in Unit 1256D-44a, Core 309-1256D-129R). Below that level, subvertical intrusive contacts are common, and these can be sharp or irregular and lobate, the latter style indicating the intrusion of magma into hypersolidus rocks (e.g., Section 309-1256D-149R-1, 30–97 cm; 1156 mbsf). Extrusive rocks may be present below 1060.9 mbsf, but there are no unambiguous indicators of eruption. Groundmass grain sizes vary from glassy to microcrystalline to fine grained with holocrystalline doleritic textures. No fresh glass was found in the sheeted dikes, but altered glass is present along some dike chilled margins and associated breccias in the upper half of the dike section. There is a step change in physical properties downward into the sheeted dikes, with significant increases in average thermal conductivity (from 1.8 ± 0.2 to 2.1 ± 0.1 W/[m·K]) and seismic velocity (5.4 ± 0.3 to 5.9 ± 0.1 km/s). The average porosity of massive units decreases from 4% ± 1% to 2% ± 1% across the 1060.9 mbsf boundary. Subvertical contacts that grade continuously from glassy chilled margins to microcrystalline to fine-grained massive basalt were not recovered from the upper half of the sheeted dikes cored during Expedition 309 but are common in the lower portion from Expedition 312. Greenschist and subgreenschist minerals occur in the upper dikes, and alteration intensity and grade generally increase downhole in the dikes below ~1300 mbsf. The amount of recrystallization and the abundance of actinolite increase below this depth, and secondary plagioclase and hornblende first appear in small amounts below ~1350 mbsf.

Granoblastic dikes

In the lower portion of the sheeted dikes, from 1348.3 to 1406.6 mbsf (Units 1256D-78 through 80b), the rocks are highly to completely altered and are locally recrystallized to granoblastic textures, leading to their designation as the granoblastic dikes. These rocks contain irregularly distributed local granoblastic patches, where the rock is completely recrystallized to secondary plagioclase and equant secondary clinopyroxene, magnetite, ilmenite, and rare orthopyroxene. The mineralogy and textures indicate recrystallization at high temperatures.

Plutonic section

The first gabbroic rocks were encountered at 1406.6 mbsf (Unit 1256D-81), where plutonic rocks intruded the overlying sheeted dikes. The plutonic section extends from 1406.6 mbsf to the bottom of the hole at 1507.1 mbsf (Units 1256D-81 through 95). This section consists of a 52.3 m thick Gabbro 1 unit (1406.6–1458.9 mbsf; Units 1256D-81 through 89b) and a 24 m thick Gabbro 2 unit (1483.1–1507.1 mbsf; Units 1256D-91 through 95) that are intrusive into metamorphosed basaltic dikes. The gabbroic rocks are fine to coarse grained (mostly medium grained) and range from gabbro to disseminated oxide gabbro, oxide gabbro, orthopyroxene-bearing gabbro, gabbronorite, trondjhemite, and quartz-rich oxide diorite (or FeTi diorite). The rocks are highly altered to amphibole, chlorite, plagioclase, titanite, and minor laumontite and epidote, with chlorite and epidote more abundant in the Gabbro 2 unit. Stoped basalt fragments are common in Gabbro 2.

The two gabbro units are separated by a 24.2 m thick dike screen (Unit 1256D-90a), consisting of fine-grained, highly to completely altered cryptocrystalline basalt dikes. These rocks commonly display granoblastic textures like those in the basal sheeted dikes. At the base of the drilled section is a 12.1 m thick gabbronorite unit of uncertain origin (Unit 1256D-94 through 95). This may be a metabasalt derived from younger sheeted dikes or a fine-grained intrusive gabbronorite. This lowermost section of the hole also contains a late basalt dike (Unit 1256D-95; 1502.6 mbsf) that does not exhibit granoblastic texture or other evidence of high-temperature metamorphism associated with gabbro intrusion.

Igneous petrology

Basement rocks recovered during Expedition 309/312 in Hole 1256D from 752 to 1507.1 mbsf were divided into 69 igneous units (1256D-27 through 95), labeled continuously from the last rocks recovered during Leg 206 (Table T4; Fig. F16) (Wilson, Teagle, Acton, et al., 2003).

Sheet and massive flows

The sheet and massive flows (Units 1256D-16 through 39; 533.9–1004.2 mbsf) are mainly composed of sheet flows and massive flows. Sheet flows (with individual cooling units ranging from tens of centimeters to <3 m thick) make up 80% of the total sheet and massive flows cored during Leg 206 and Expedition 309. However, in the portion of this subdivision drilled during Expedition 309, sheet flows account for only 65% of the rock recovered, indicating a prevalence of massive flows deeper in the section. Individual flows are commonly separated by chilled margins containing altered or fresh glass. Where contacts were not recovered, individual flows can be 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. The flows are predominantly aphyric (<1% phenocrysts), and grain size ranges from glassy at the chilled margins to cryptocrystalline or microcrystalline (Fig. F17). Rare sheet flow interiors are fine grained. 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. Where phenocrysts occur (for example, in Units 1256D-28, 35b, and 37), these rocks have plagioclase, clinopyroxene, and olivine phenocrysts, in order of decreasing abundance, commonly clustered in a glomeroporphyritic texture (Fig. F18). 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. F19).

In contrast to the thinner sheet flows, minimum thicknesses of the massive flow units vary from 3.2 to 11.3 m with an average of 6.3 m (cumulative thickness calculated using only the pieces recovered). The thickest, Unit 1256D-31, consists of a single cooling unit of fine-grained basalt below a 12 cm cryptocrystalline to microcrystalline upper contact (Fig. F20). A total of 26 m of this unit was cored, of which 11.3 m was recovered. In contrast to the sheet flows, fine-grained rocks are more common in the massive lavas (Fig. F16). The massive flows are aphyric and nonvesicular, with the exception of Unit 1256D-39a. This basalt is sparsely clinopyroxene-olivine-plagioclase-phyric and is moderately vesicular (8%) (Fig. F21). Thin section observations show that the most finely grained rocks collected from the massive flows have intergranular to intersertal groundmass textures (Fig. F22).

Transition zone

The transition zone (Units 1256D-40 through 43; 1004.2–1060.9 mbsf) is characterized by increasing abundance of volcanic breccias interbedded within sheet flows. The top of this zone is defined by the cataclastic massive unit (1256D-40; interval 309-1256D-117R-1, 85 cm, through 118R-1, 66 cm). The upper part of this unit (interval 309-1256D-117R-1 [Pieces 9–14, 97–142 cm]) has a complex structure with fine- to medium-grained basalt in contact with brecciated clasts of cryptocrystalline basalt (Fig. F23). The fine- to medium-grained basalt contains highly altered glass clasts and is disrupted by an intensive network of thin chlorite-smectite veins imparting an incipient cataclastic texture. Thin section examination of these rocks (Sample 309-1256D-117R-1, 122–125 cm) shows that fractured crystals have a seriate texture, deformed and cemented by a banded matrix that shows flow structures. With increasing distance below the top of the unit, the igneous texture is better preserved and more homogeneous, mesostasis is less abundant, and crystals are less fractured (interval 309-1256D-117R-2, 9–72 cm, through 118R-1, 0–66 cm). In this lower part of Unit 1256D-40, the cataclastic massive unit consists of fine-grained dolerite with a partially developed subophitic texture (interval 309-1256D-117R-2, 23–26 cm). A few pieces, similar to the disrupted rocks of the upper part of this cataclastic unit, occur in intensively veined sheet flows in Units 1256D-37 and 41. This may support the interpretation that these disrupted rocks indicate the nearby presence of a dike or fault zone. Interval 309-1256D-120R-1, 9–26 cm, of the transition zone, however, captures the first unambiguous subvertical intrusive contact.

A second type of breccia is present in interval 309-1256D-122R-1, 25 cm, through 123R-1, 109 cm, where 2.8 m of mineralized volcanic breccia and breccia intercalated with basalt was recovered and defines Unit 1256D-42 (Fig. F24). 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 breccias are intercalated with aphyric, cryptocrystalline to microcrystalline basaltic sheet flows. 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-shape clasts (0.1–1.5 cm), cemented by chalcedony, saponite, calcium carbonate, albite, anhydrite, and sulfides (Fig. F24).

The transition zone also hosts the last occurrence of a glassy margin not associated with either a dike contact or clastic brecciation. This margin was recovered in the lower half of sheet flow Unit 1256D-43 (~1060 mbsf).

Sheeted dikes

The upper boundary of the sheeted dikes (Units 1256D-44 through 65; 1060.9–1406.6 mbsf) is defined by a distinct change from sheet flows to massive basalts at 1060.9 mbsf. Extrusive rocks could be present in the upper portion of this section, but there is no conclusive evidence of extrusive origin for any of the units in the upper portion of this section. The massive basalts are most commonly aphyric and nonvesicular. Most rocks at depths <1255 mbsf are microcrystalline (Units 1256D-44 through 46, 49 through 50, 52a, 54 through 55, and 59 through 65). Fine-grained rocks are less common (Units 1256D-47, 48, 51, and 53a), and rare units are cryptocrystalline to microcrystalline (Units 1256D-56a, 57a, and 58) basalt. In thin section, the massive basalts are holocrystalline with predominantly intergranular textures (Fig. F25), ranging to intersertal and, occasionally, subophitic. No basaltic units in the sheeted dikes above 1255 mbsf show gradations from a chilled margin through crypto- and microcrystalline rock to fine grained material, whereas such gradations are common at deeper levels of the dikes.

Between ~1255 and 1406 mbsf, we recovered aphyric fine-grained to microcrystalline basalts with some cryptocrystalline intervals. This interval is divided into 15 lithologic units (1256D-66 through 80; 1255–1406 mbsf), based primarily on abrupt changes in texture and/or grain size. Three subvertical intrusive contacts were recovered from unit boundaries in this zone (Units 1256D-68/69, 75/76, and 80/81) (Fig. F26), in addition to cryptocrystalline material at the Unit 1256D-77/78 boundary and in Unit 1256D-71, which most likely represents the contact between Units 1256D-70 and 72. Three other units (1256D-70, 71, and 73) have internal contacts. In addition, several near-complete cooling units grade from cryptocrystalline margins to fine-grained centers and back to cryptocrystalline margins. Together, these lithologic data indicate that there are at least 9 individual dikes among the 15 sheeted dike units below 1255 mbsf (Fig. F27).

Subvertical intrusive dike contacts are common in the sheeted dike complex. In general, two types of contacts can be distinguished: sharp or irregular direct contacts and brecciated contacts. Most contacts belong to the latter category, with brecciated zones one to several centimeters wide along the contact. All contacts have developed chilled margins. The chilled margins of the dikes are composed of glassy to cryptocrystalline aphyric basalts that have quenched against cryptocrystalline to fine-grained, massive basaltic hosts. Glassy material is common at chilled margins above 1255 mbsf but is less abundant at greater depths. Breccias at the contacts in the upper dike section (<1255 mbsf) comprise fragments of altered glass initially quenched at the chilled margin with subordinate, angular to subangular clasts of the host rock and cemented by anhydrite, chlorite, and sulfide (Fig. F28). One spectacular example is a >50 cm long vertical contact in interval 309-1256D-140R-1, 26–80 cm (Unit 1256D-47), with a sulfide-impregnated dike margin breccia with complex intrusive relationships and intricate multiple margin-parallel sulfide veins crosscut by anhydrite veins. Below 1255 mbsf, brecciated dike margins contain chlorite, amphibole, oxides, quartz, and sulfides filling fractures, and in some cases contain highly altered material that may be altered xenoliths.

In interval 309-1256D-155R-1 (Piece 20, 84–90 cm), the chilled margin forms a convex lobe, indicating that the host rock was not rigid during the intrusion (Fig. F29A). Further evidence for multiple intrusions is seen in Sections 309-1256D-161R-1 to 161R-2 (Unit 1256D-56b), where at least two intrusions are present. An inner sparsely clinopyroxene-olivine-plagioclase phyric dike has intruded a sparsely clinopyroxene-olivine-plagioclase-phyric spherulitic cryptocrystalline rock, itself chilled against an aphyric microcrystalline host rock (Fig. F29B). Another example of dike intrusion into 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. F29C).

The overall mineralogical characteristics of basaltic basement drilled during Expedition 309/312 are similar to overlying Leg 206 basalts, although there are some important differences. More than 60% of the basalts drilled during Leg 206 are sparsely phyric with olivine, plagioclase, and clinopyroxene phenocrysts (Wilson, Teagle, Acton, et al., 2003), and aphyric rocks comprise <40% of the Leg 206 basalts. In contrast, the vast majority of basalts recovered during Expedition 309/312 are aphyric (>80%) (Figs. F16, F30). This difference is demonstrated by the downhole variation of phenocryst contents and by a decrease with depth of the total phenocryst content (Fig. F16). More than half of Leg 206 basalts have three major phenocryst phases (clinopyroxene, olivine, and plagioclase), whereas only rare samples (eight) of the Expedition 309/312 basalts have more than two phenocryst phases (Figs. F16, F30). Phenocryst-bearing basalts collected during Leg 206 are dominantly olivine-phyric (>80%), whereas in those from Expedition 309/312, plagioclase is the most common phenocryst phase and olivine is the least abundant among the three phenocryst phases (Figs. F16, F30). The different observation teams during Leg 206 and Expedition 309 may account for some of this difference, but such bias should be relatively minor. The general change from sparsely phyric to aphyric and from dominantly olivine to plagioclase phenocrysts appears to be a genuine downhole trend. A second trend is that sheeted dike basalts recovered below 1255 mbsf during Expedition 312 are predominantly fine grained, whereas those from Expedition 309 above 1255 mbsf are predominantly microcrystalline.

All basalts of the sheeted dike complex have been hydrothermally altered. The uppermost units are partially altered to subgreenschist to greenschist minerals, but the intensity and grade of metamorphism increase downhole. Beginning at 1348.3 mbsf, the basalts are highly to completely altered to amphibole-bearing assemblages with localized domains characterized by secondary clinopyroxene and lesser orthopyroxene. These domains typically exhibit granoblastic textures (Fig. F31), and their onset defines the “granoblastic dike” interval (1348.3–1406.6 mbsf) at the base of the sheeted dike complex.

Plutonic section

At 1406.6 mbsf in Hole 1256D, we encountered a gabbro dike, marking the end of the sheeted dike complex and the beginning of the 100.5 m thick plutonic section, which extends to 1507.1 mbsf. Intrusive relationships show that the plutonic section is the product of a series of discrete magmatic events beginning with the emplacement of an unknown number of sheeted dikes. At least two gabbro bodies intruded this sheeted dike sequence, followed some time later by a small crosscutting basalt intrusion at the base of the drilled section.

The plutonic section of Hole 1256D includes two medium-grained gabbroic intervals: Gabbro 1 (1406.6–1458.9 mbsf; Units 1256D-81 through 89) and Gabbro 2 (1483.1–1507.1 mbsf; Units 1256D-91 through 95) (Fig. F16). These gabbroic units are intrusive into the basal dikes of the sheeted dike complex and are responsible for the contact metamorphic effects in the granoblastic dikes. Gabbro 1 and Gabbro 2 are separated by the dike screen (1458.9–1483.1 mbsf; Unit 1256D-90), a granulite-grade metabasalt interval with a well-developed granoblastic metamorphic texture and containing secondary clinopyroxene and orthopyroxene.

With the exception of a narrow (<1 m) quartz-rich oxide diorite dike that is compositionally equivalent to an oceanic FeTi basalt (Unit 1256D-82), Gabbro 1 is mineralogically uniform but texturally heterogeneous (Fig. F32). It is divided into eight units (1256D-81 and 83 through 89), primarily on the basis of variations in dominant texture and/or grain size. Gabbro 1 gabbros are commonly oxide bearing, and oxide abundance decreases irregularly downhole. Olivine is present in significant amounts only in the lower two units (1256D-88 and 89).

Gabbro 2 consists of three separate igneous intervals: an upper medium-grained orthopyroxene-bearing gabbro that grades to gabbronorite near its margins (Units 1256D-91 through 93), a fine-grained gabbronorite of which the origin is unclear (Unit 1256D-94), and a later basaltic crosscutting intrusion (Unit 1256D-95). The upper orthopyroxene-bearing gabbro interval of Gabbro 2 is characterized by an absence of fresh olivine, high but variable orthopyroxene contents (5%–25%) and considerable local heterogeneity (Fig. F33). Oxides are present throughout, mostly up to 5% and rarely higher. Oxide abundance generally diminishes downhole. At the upper margin of this zone, gabbronorite intrudes and invades the metabasalt of the dike screen, isolating and detaching centimeter-sized blocks of metabasalt (Fig. F34). Similar detached blocks of pinkish orthopyroxene-rich cryptocrystalline to fine-grained metabasalt are also present close to a lower contact, at the base of Unit 1256D-93.

Below the medium-grained orthopyroxene-bearing gabbro of Gabbro 2 is a fine-grained “gabbronorite” of uncertain origin (Unit 1256D-94). It is extensively recrystallized with well-developed granoblastic textures, although relict igneous, intergranular textures are preserved in some places, especially away from the upper boundary. There is some ambiguity as to whether this unit is a metabasalt or a fine-grained intrusive gabbronorite. The textures of rocks from Unit 1256D-94 are similar to those from Unit 1256D-90 of the dike screen, and both of these units appear to show an increase in modal orthopyroxene at their contacts with the orthopyroxene-bearing gabbro.

The last unit recovered during Expedition 312, Unit 1256D-95, begins at 1502.6 mbsf. Unit 1256D-95 is a relatively light gray cryptocrystalline basalt that is distinguished by the presence of abundant pink clinopyroxene (titanaugite?) and by its relatively low primary oxide content. It is also distinct from the other basaltic rocks of the plutonic section in its relatively low metamorphic grade well-preserved igneous texture and relatively low degree of alteration, which are roughly equivalent to those of much shallower units around Unit 1256D-70. We interpret this unit as a crosscutting, relatively enriched basalt dike or sill that intruded after significant cooling had occurred.

Geochemistry

The freshest rocks from each igneous unit were selected for elemental analysis by shipboard inductively coupled plasma–atomic emission spectroscopy (ICP-AES) (Fig. F35). All samples from Expedition 309 have compositions within the range of MORB with SiO₂ = 48–55 wt%, FeO = 9.4–14.0 wt%, MgO = 6.2–8.9 wt%, CaO = 7.1–12.8 wt%, Na₂O = 1.8–5.0 wt%, Cr = 21–367 ppm, Sr = 71–129 ppm, Zr = 56–133 ppm, and Ba = 1–37 ppm. The range of the Mg# is 45–62 (average = 53). These values broadly overlap the results from Leg 206 (Fig. F36) and correspond to typical values for MORB (Su and Langmuir, 2003). Analyses of cryptocrystalline basalts from Expedition 309 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 (Fig. F35).

Drilling in Hole 1256D during Expedition 312 recovered basaltic dikes, gabbros, quartz-oxide diorite, and a trondjhemite dike. Several rocks recovered from the junk baskets during mill runs were analyzed and are labeled as “ghost cores.” The rocks fell into the bottom of the hole, so it is unknown at what depth the rocks originated. Basaltic dikes recovered from the sheeted dike complex during Expedition 312 have variable contents of major and trace elements and span nearly the entire range of rocks cored from higher in the hole. However, new minima for Zn and V concentrations and maxima for Ni and Cr abundances are found in two analyses of the sheeted dikes. Most dikes fall along a fractional crystallization trend defined by rocks from Hole 1256D. Although there is considerable scatter in the data, linear magmatic fractionation trends are present for TiO₂, FeO, CaO, Na₂O, and Zr versus MgO (Fig. F36). This trend appears to be largely controlled by the fractional crystallization of clinopyroxene and plagioclase (Fig. F37), suggesting these rocks are fairly evolved.

There are subtle variations in the basalt chemistry downhole, with a number of step changes or reversals of fractionation trends possibly indicating cycles of fractionation, replenishment, and, perhaps, assimilation (e.g., at ~600, 750, 908, and 1125 mbsf). However, dike compositions from Expedition 312 are variable and do not define obvious trends downhole.

Gabbros in the plutonic section were recovered in two sequences, separated by a dike screen (Table T4). Gabbros from Gabbro 1 have variable chemical compositions ranging from more fractionated at the top of the sequence toward less evolved deeper down. The lowermost gabbro from the Gabbro 1 sequence has high MgO (11.4 wt%) and in thin section appears to have accumulated olivine. The Gabbro 2 sequence contains generally more fractionated rocks than Gabbro 1; however, there is again a general trend toward less evolved compositions downcore. For example, TiO₂ is 2.5 wt% at the top of Gabbro 2 and 1.2 wt% at the base and FeO is 16.4 wt% at the top and ~10 wt% at the bottom (Fig. F35). Therefore, systematic fractional crystallization occurred in these gabbros, although their lithofacies are very heterogeneous.

Only two of the leucocratic rocks recovered during Expedition 312 were analyzed. The first leucocratic rock encountered was a thin dike within a basaltic dike unit. This rock has high silica (72 wt%) and Zr (~840 ppm), low TiO₂ and FeO, and is unlike any other rock from Hole 1256D. Chemically and petrographically, it is a trondjhemite and likely a very late-stage liquid remaining after extensive fractionation took place in the melt lens. The second leucocratic rock recovered was a quartz-rich oxide diorite. This unit has unusual geochemical characteristics with very high FeO and TiO₂ contents and is similar to an evolved FeTi basalt.

Similar to analyses of basalts from Leg 206, Ti and Y in basaltic rocks from Expedition 309/312 show good positive linear correlations with Zr, due to their similar geochemical behavior (Fig. F38). Leg 206 basalts above 750 mbsf were classified into three groups based on a distinct gap in the Ti-Zr concentration: high Zr-Ti, low Zr-Ti, and high Zr. With additional data analyzed during Expedition 309/312, this data gap disappears and the three-fold subdivision is probably not valid. Rare samples from the inflated flows in Hole 1256D fall off the Y versus Zr and Ti versus Zr trends, suggesting some minor variation in source composition. Several samples from Expedition 312 also show similar deviations.

Basalts from different igneous subdivisions in Holes 1256D and 1256C all have MgO in the range 5–9 wt%, and when trace element compositions of Site 1256 basalts are compared to compilations of EPR MORB, they are on the relatively trace element–depleted side of average EPR MORB (Figs. F38, F39). Note that Site 1256 basalts have higher Zr/Y and lower Ti/Zr ratios than the strongly trace element–depleted MORB from Hole 504B (Figs. F39, F40), suggesting a more depleted mantle source or larger degrees of partial melting in the latter.

Compared with other first-order mid-ocean-ridge segments along the EPR, basalts from Site 1256 have very low Zr/TiO₂ and Zr/Y ratios (Figs. F40, F41). Although there is overlap among segments and large scatter in the data for each segment, Zr/TiO₂ and Zr/Y ratios appear to decrease with increasing spreading rate. It should be emphasized that Site 1256 basalts have considerably lower Zr/Y and higher Ti/Zr ratios at given MgO wt% than the present-day MORB from the EPR between 5°N and 10°N, the closest data to the projected position of Site 1256 15 m.y. ago at ~1°N on the EPR (Fig. F39). If mantle sources were similar during the past 15 m.y. at the EPR, the above lines of evidence suggest a higher degree of partial melting at ~15 Ma at Site 1256 associated with the higher rate of spreading. Otherwise, the data must reflect regional-scale mantle heterogeneity.

Of all the basalts from Site 1256, the lava pond is the only unit that is relatively enriched in V and depleted in Cr compared to EPR MORB. The lava pond includes the rocks with the highest incompatible element concentrations (Zr, TiO₂, Y, and V) and the most depleted compatible element concentrations (Cr and Ni), suggesting that it is more evolved than the rest of the basalts from Hole 1256D (Fig. F39).

Alteration

One of the principal objectives of the combined missions (Leg 206 and Expedition 309/312) is to investigate the alteration processes that occur in a section of upper crust that formed at superfast spreading rates (200–220 mm/y) to test whether these differ from those documented in crust formed at slow and intermediate spreading rates. Of particular interest during Expedition 309/312 is the opportunity to observe the transition between low-temperature alteration and high-temperature hydrothermal alteration in a continuous section of oceanic crust. To date, this transition has only been described in Hole 504B. All rocks in the ocean crust intersected by Hole 1256D are partially altered to secondary minerals, and the products of fluid-rock exchange are manifest by background alteration, alteration halos related to veins, isolated alteration patches, and veins and breccias (Figs. F42, F43, F44).

Two main alteration types were encountered in the section of Hole 1256D drilled during Expedition 309. From 752 to 965 mbsf, rocks that had reacted with seawater at low temperatures, similar to the range of conditions encountered in Leg 206 cores, are present. The background alteration is uniform, from 85% to 100% dark gray, because of the presence of saponite filling vesicles and replacing olivine and plagioclase, clinopyroxene phenocrysts and chalcedony and calcium carbonate filling vesicles, and miarolitic voids (Fig. F42). The predominant vein mineral phases related to low-temperature alteration in Hole 1256D include saponite, celadonite, iron oxyhydroxides, chalcedony, and minor pyrite (Figs. F43, F44). Celadonite is commonly intergrown with iron oxyhydroxides with later overgrowths of saponite. Specific vein-related alteration types identified in Hole 1256D include black halos, brown halos, mixed halos, simple light green, dark green, and light gray halos, and discontinuous pyrite halos (Fig. F42). Black, brown, and mixed halos and dark patches are common throughout the rocks from 752 to 918 mbsf and are related to veins filled by saponite, celadonite, and iron oxyhydroxides. These halos result from the more pervasive 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., 1981; Honnorez, 1981; Laverne, 1993; summary in Alt, 2004). Subsequent interaction of the basalts with cold oxidizing seawater produces brown halos characterized by replacement of primary phases by saponite and iron oxyhydroxides. From 918 to 964 mbsf, black, brown, and mixed halos are absent (Fig. F42) and dark gray background alteration with abundant saponite and pyrite is ubiquitous. These rocks, as well as saponite- and pyrite-bearing intervals cored during Leg 206 (e.g., 554–562 mbsf), result from the interaction of basalt with low-temperature basement fluids that have chemically evolved from seawater through water–rock reactions.

The interval from 964 to 1028 mbsf is characterized by the presence of pyrite-rich alteration halos, mixed-layer chlorite-smectite, and anhydrite (Fig. F45). This alteration mineral assemblage suggests more elevated temperatures (100°–200°C) than shallower in the crust. Below ~1028 mbsf, green and dark green background alteration, particularly in the coarser grained rocks, occurs as a consequence of the moderate to complete replacement (up to 100%) of basaltic clasts and glass in the mineralized volcanic breccia to saponite/​chlorite and minor talc. The first occurrences of actinolite, prehnite, titanite, and epidote are recorded at 1027, 1032, 1051, and 1095 mbsf, respectively (Fig. F45). These minerals are indicative of hydrothermal alteration under subgreenschist to greenschist facies conditions. In this part of the crust, 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. These halos comprise 10%–100% secondary minerals with chlorite, actinolite, titanite, albite, and pyrite replacing plagioclase and clinopyroxene and filling interstitial spaces, along with minor quartz, chalcopyrite, epidote, and prehnite.

The upper sheeted dikes in Hole 1256D are slightly to completely altered by hydrothermal fluids. Most rocks display dark gray alteration in which clinopyroxene is partially altered to dusty, corroded clinopyroxene and actinolite and plagioclase is partially replaced by albite and chlorite. Titanite is a common accessory. Interstitial glass is replaced by chlorite, but below ~1300 mbsf actinolite is more abundant. Veins are common features of all cores and are present at a frequency of ~30 veins/m. Chlorite is the most common vein filling, although quartz, pyrite, chalcopyrite, actinolite, prehnite, laumontite, and calcite are also common vein fillings. Anhydrite veins are common to ~1200 mbsf. Many veins have 1–10 mm wide halos in which the wallrock is highly altered to chlorite, albite, actinolite, titanite, quartz, pyrite, calcite, and prehnite that replace plagioclase and clinopyroxene and fill interstitial pore space. Halos commonly comprise a narrow (1–10 mm) dark green chlorite-rich margin adjacent to veins surrounded by a wider (5–20 mm) light gray halo. Crosscutting relationships indicate that groundmass replacement and vein filling by chlorite, titanite, albite, actinolite, and pyrite are relatively early, but this alteration can be overprinted by hydrothermal veins composed of quartz, chlorite, epidote, pyrite, chalcopyrite, and rare sphalerite. Late crosscutting assemblages that probably formed at lower temperatures (100°–250°C?) include anhydrite, prehnite, laumontite, and calcite.

Hydrothermal alteration is most spectacularly manifest by two phenomena: (1) centimeter-scale hydrothermal alteration patches and (2) mineralized dike margins. Alteration patches comprise centimeter-scale zones of 100% hydrothermal minerals, most commonly quartz, prehnite, laumontite, chlorite, anhydrite, and calcite either replacing altered rock or filling pore space, surrounded by dark chloritic halos in which there is near complete replacement of groundmass phases. Secondary magnetite is a common accessory phase. Alteration patches have no clear relationship to veins and are common features throughout the dikes.

Deeper than ~1320 mbsf, actinolite becomes dominant over chlorite (Fig. F45). Clinopyroxene is partially to completely replaced by actinolite with abundant magnetite inclusions, and actinolite is the most common vein filling. Quartz-chlorite alteration patches in the upper dikes (above ~1300 mbsf) give way to rare actinolite-rich patches, but the latter are small (~5 mm). Brown pleochroic hornblende in veins and replacing clinopyroxene is common below ~1340 mbsf.

Many of the subvertical dike margins encountered are disrupted by complex vein networks that brecciate the chilled contacts with intense hydrothermal recrystallization of the surrounding groundmass to chlorite, actinolite, quartz, pyrite, and chalcopyrite. Deeper in the sheeted dikes, below ~1300 mbsf, sulfide mineralization at the dike margins is less abundant, but many intrusive contacts are lined with secondary magnetite.

There is a profound change in texture and secondary mineralogy in basalts deeper than 1348 mbsf, with the dikes exhibiting a distinctive granoblastic texture in which significant proportions of these rocks are thoroughly recrystallized to microcrystalline granular aggregates of secondary clinopyroxene, orthopyroxene, actinolitic hornblende, plagioclase, and subrounded blebs of magnetite and ilmenite. The granoblastic assemblage is heterogeneous, and only rarely are large areas completely recrystallized. More commonly, the granoblastic dikes have only a minor proportion of completely recrystallized 0.5–1 mm patches included within zones in which there is only minor replacement of the original igneous texture by clinopyroxene and orthopyroxene. Subrounded, equant secondary magnetite is commonly the most visible indicator of partial recrystallization. The granoblastic assemblage can also occur as bands and veins. The zone of most intense development of the granoblastic texture is from 1370 to 1397 mbsf, and the basalts directly overlying the gabbros are only partially to strongly recrystallized. If the relative ferocity of recrystallization is principally the result of the reheating of these rocks, as appears likely, the occurrence of the most extensively granoblastic texture ~10–35 m above the dike/​gabbro contact possibly indicates this boundary is not horizontal. The relative timing of hydrothermal alteration and recrystallization of the granoblastic dikes is difficult to discern. Secondary clinopyroxene can be host to magnetite inclusions, perhaps indicating growth from magnetite-riddled actinolite, which is a common alteration product of igneous clinopyroxene at higher levels in the dikes. Crosscutting hornblende, actinolite, and chlorite veins indicate that significant hydrothermal alteration postdates the granoblastic recrystallization.

Gabbros of the plutonic section are dark gray to dark gray-green, with a greenish hue signifying more extensive replacement of clinopyroxene by actinolitic hornblende (Fig. F46). Plagioclase, when altered, generally appears whiter than the igneous feldspar and commonly has rims replaced by secondary feldspar and actinolite + chlorite. The intensity of gabbro alteration is strongly dependent on the grain size of the rock. Gabbro with large (~10–15 mm) ophitic clinopyroxenes intergrown with plagioclase is much less altered than the irregular coarser grained zones between. The intrusive margins at the top of the plutonic section and the boundaries with the dike screen were the loci for extensive hydrothermal alteration. Leucocratic rocks (e.g., Units 1256D-82, 90b, 90c, and 90f) are more altered than the host gabbros or dikes, although this may reflect the relative abundance of such rock types near intrusive margins and the narrow (~5–15 mm) width of many leucocratic intrusions. These leucocratic rocks are commonly altered to amphibole, secondary plagioclase, chlorite, epidote, prehnite, and titanite. The host rocks around leucocratic dikes are commonly intensively altered, with 2–15 mm dark green amphibole and chlorite-rich halos with replacement of clinopyroxene by actinolitic hornblende and plagioclase to secondary feldspar, actinolitic hornblende, prehnite, and epidote.

The basalts in the dike screen within the plutonic section are dark gray fine-grained to cryptocrystalline basaltic dikes that are partially recrystallized to granular assemblages with smooth annealed grain boundaries. The igneous texture of the dikes is maintained by plagioclase, but clinopyroxene is partially to completely recrystallized to subrounded grains. Titanomagnetite is partially recrystallized to subrounded grains, but many grains have more angular shapes than those within the granoblastic dikes overlying the upper gabbro. The lowermost unit recovered in Hole 1256D is a late dike in which clinopyroxene is altered to dusty clinopyroxene-actinolite, actinolite, chlorite, and secondary plagioclase while maintaining the primary intergranular igneous texture. The absence of granoblastic texture indicates that this basalt was intruded into the lower dike screen and possibly the overlying gabbros.

The volcanic rocks from Hole 1256D are generally less altered compared to most other basement sites (e.g., Sites 417 and 418, Holes 504B and 896A). The lavas of Hole 1256D contain a much smaller proportion of oxidized alteration halos compared to Holes 504B and 896A, and these effects do not systematically diminish with depth (Fig. F47). Instead, the effects of alteration by oxidizing seawater occur irregularly with depth associated with steeply dipping vein networks. The amount of calcite within Hole 1256D is also very low compared to other basement penetrations.

Although pyrite is abundant in the lower lavas and upper sheeted dikes at Site 1256, intense quartz-epidote-Fe, Cu, Zn, and Pb sulfide stockwork-like mineralization like that in Hole 504B is not present in the transition zone. However, anhydrite, which is sparse in Hole 504B (Teagle et al., 1998), is abundant at Site 1256. At both Sites 504 and 1256, there are stepwise increases downward in alteration temperatures at the transition zone (from ~100°C to ~250°C), and alteration temperatures within the sheeted dikes at both sites increase downward, from ~250°C at the top to ~400°C at the base of upper dikes. This change in alteration temperature occurs over ~1 km in Hole 504B, however, whereas in Hole 1256D this occurs over only 300 m. This is independent of the superimposed heating and recrystallization to granoblastic textures in the basal dikes of Hole 1256D, related to the gabbro intrusions that resulted in recrystallization at temperatures possibly >800°C.

Structure

Basalts recovered during Expedition 309/312 exhibit brittle structures and minor brittle-ductile structures. The main structural features are represented by veins, vein networks, fractures, cataclastic zones, shear veins, microfaults, and breccia (Fig. F48). Primary igneous structures include syn- to late-magmatic structures, partially linked to flow and solidification of lava. Three main types of breccia were recovered during Expedition 309 (<1255 mbsf): incipient breccia, hyaloclastite, and hydrothermal breccia. Breccias recovered during Expedition 312 (>1255 mbsf) are mainly associated with chilled dike margins. Observed petrofabrics include magmatic flow indicators, local cataclastic domains related to alteration and/or intrusive processes, and recrystallization textures. Unique to gabbroic rocks are magmatic patches, boundaries between contrasting melts and xenoliths, and textural and compositional bands.

In the sheet and massive flows (752–1004.2 mbsf), structures and fracturing are heterogeneously partitioned and are most intensely developed at the top of the massive flows. Vertical sets of veins, cataclastic zones, and shear veins are present in massive units, whereas breccias (incipient breccia) are more common in sheet flows. On the whole, the vertical vein sets become more common from ~900 mbsf. Most structures are related to the cooling of lava and are represented by curved, radial, Y-shaped, and irregular veins filled with secondary minerals.

The transition zone (1004.2–1060.9 mbsf) is characterized by steeply dipping chilled dike margins and the presence of cataclastic zones, breccias (mostly hyaloclastite), and vertical veins. The cataclastic massive unit in Section 309-1256D-117R-1 consists of rounded to angular clasts of dolerite and glassy spherulitic to variolitic basalt. Three to four centimeters of cataclasite separates doleritic basalt fragments from chilled fragments (Fig. F49). The cataclastic 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 have cataclasite and protocataclasite textures and are cut by ultracataclasite and gouge. The crosscutting relationships between the different types of fault rocks are visible in a thin section (Fig. F49). 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 the thin section, fragments of plagioclase exhibit 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 intensity of fracturing downhole (Fig. F50) is mostly slight with the exception of the mineralized volcanic breccia (Unit 1256D-42; Sections 309-1256D-122R-1 and 122R-2), a hyaloclastite with abundant sulfide minerals. 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. F51). Basalt clasts exhibit the textural features of sheet flows, such as spherulitic to variolitic textures (see Fig. F51) and lava flow–related folding. Clasts are embedded in a scarce fine-grained clay matrix cemented by sulfides, carbonate, and silica. In interval 309-1256D-122R-1, 52–125 cm, the mineralized volcanic breccia grades from an almost pure hyaloclastite with rare sulfides to mineralized hyaloclastite. There is a concomitant increase in basalt clasts and matrix volume with respect to glassy clasts (Fig. F52).

Numerous chilled margins were recovered in cores from the transition zone and the sheeted dikes, and these contacts are increasingly common with depth. Below ~1004 mbsf, where such contacts are subvertical, they are interpreted as dike contacts, and these become very common in the sheeted dikes. Chilled margins range from lobate and interfingered to sharp. Chilled margins also contain flow banding, stretched spherulites, and injections of basalt. Many of these chilled margins are associated with, or highly disrupted by, diffuse veining and brecciation (Fig. F53). Veins and breccia domains both cut and are cut by chilled margins, and alteration of dikes is enhanced at chilled margins. Multiple dikes and banded dikes also occur. The sheeted dikes are also characterized by the first notable occurrence of systematic conjugate veins. From 1090.7 mbsf (Unit 1256D-45) to 1255 mbsf, all the structural features, except shear veins, are common and more abundant. Shear veins are present only in the uppermost portion of the massive basalt (Unit 1256D-44). Veins and vein networks contain indications for oblique vein opening. Only in sparse and localized shear veins is there evidence for significant noncoaxial strain. Veins are the most common brittle structure in the cores, but fractures are widespread as well. Most of the gently dipping fractures are drilling induced and may have nucleated on preexisting cooling joints and other planes of weakness, thereby hindering recovery.

Gabbroic rocks contain fabrics and structures related to melt transport. Unique to gabbroic rocks are magmatic patches, boundaries between contrasting (now gabbroic) melts and xenoliths, and textural and compositional bands. Leucocratic intrusions form bands and patches with local evidence of high-temperature shear.

Some brittle structures in the gabbros formed at high temperatures, including domains of intense microfracture of plagioclase that are largely annealed by secondary plagioclase. Alteration of the gabbros and dikes is pervasive but is enhanced in areas of veining and at intrusive contacts.

The true dip of the chilled margins measured during Expedition 309 ranges from 50° to 90° with a mode at ~70°–75° (Fig. F54). Preliminary interpretation of FMS and UBI images during Expedition 309 indicates that these features dip steeply to the northeast. The population of structures measured during Expedition 312 has systematic changes in orientation downhole. With the exception of fractures, all structures become more steeply dipping with depth. At 1406 mbsf, gabbro intrudes the dikes with a 45° dipping contact, below which structures separate into two different populations. Veins below 1406 mbsf are steeply dipping, similar to their orientation in the dikes and lavas. Leucocratic intrusions into the gabbro also have moderate to steep dips. In contrast, igneous contacts, magmatic flow fabrics, and magmatic banding in the gabbros are moderately dipping, similar to the orientation of the contact between the gabbro and the dike complex.

There is no evidence in the recovered cores for significant tilt or strain during accretion and seafloor spreading of the Site 1256 crust.

Paleomagnetism

The primary goal of paleomagnetic studies is to assess the roles of different rock types that make up the upper oceanic crust in generating marine magnetic anomalies. Magnetic remanence data were collected before and after progressive alternating-field or thermal demagnetization. Most 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.

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 the paleomagnetic results from Expedition 309. Rocks from the lower parts of Expedition 309 recovery have higher coercivities and a pronounced increase in the apparent quality of data that occurs over the interval 970–1030 mbsf. Because of the equatorial paleolatitude of the site, polarity remains ambiguous until absolute declinations can be obtained based on orienting pieces relative to the downhole logging images of the borehole wall. The component of the drilling overprint that may remain would affect inclination more than declination, so for samples for which data analysis suggests that much of the drilling overprint has been removed (e.g., Fig. F55), generally from >1000 mbsf, declination values will be reliable enough to determine polarity in oriented pieces. If the number of oriented pieces is small but the polarity pattern is clear from those pieces or from measurements of the downhole magnetic field, declinations from the more stable unoriented samples should be adequate for orienting pieces for structural purposes once the polarity has been determined separately.

The generally positive inclinations for Expedition 309 samples are not what is expected for the low paleolatitude. The most obvious possibility is that a significant portion of the drilling overprint remains on nearly all of the samples. A potential alternative is that there is a pervasive present-field overprint. Another alternative, tectonic tilting, cannot be entirely discounted. 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.

Plots of the magnetic intensity against depth show a recurrent concave pattern (Fig. F56), which shows reasonable agreement with the cryptocrystalline boundaries of igneous units and subunits. Higher 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 the size and distribution of primary minerals (e.g., Petersen et al., 1979), in particular titanomagnetite. About 70% of the igneous units and subunits drilled during Expedition 309 show repeated concave patterns (Fig. F56), suggesting the presence of multiple cooling units (with the observed magnetic intensity pattern) within each lithologic unit. Our calculations suggest that the average thickness of these cooling units is ~1.0 ± 0.5 m (Fig. F56).

Paleomagnetic measurements on Expedition 312 samples were performed on discrete samples of the working half and intact pieces of the archive half. Demagnetization was largely performed by alternating field, and eight samples were thermally demagnetized. Most 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.

Natural remanent magnetization (NRM) directions of discrete cubes are consistently steep (inclination at least 45° and usually >60°) and generally have southerly declinations, as expected if the NRM is dominated by drilling overprint. Most samples tend steadily toward shallower inclinations during progressive alternating-field demagnetization (Figs. F57, F58), although few reach a stable direction reproduced on several successive demagnetization steps. Directional scatter at high demagnetization fields is fairly minor. Thermal demagnetization results show a gradual drop in intensity with temperature, with the final 5%–10% removed at 570° or 580°C temperature steps, indicating nearly pure magnetite as the primary carrier.

Archive-half pieces generally trend toward shallow directions, but the maximum alternating field was 40 mT, so less of the overprint is removed. However, because the radially inward component of the drilling overprint adds differently to the predrilling magnetization for the two halves of the core, archive-half measurements are useful for evaluating how much of the overprint is removed and in conjunction with working half samples in order to get cleaner predrilling directions. For about half of the working-half samples, we were able to estimate a preoverprint direction from the intersection of well-constrained great circle trend fit separately to working half and archive half demagnetization trends. This estimate always has shallower inclination than the estimate from the working half alone and in a few cases reaches negative values (Fig. F59).

On Figure F60, we plot the downcore variations of the best estimate of declination and inclination for Expedition 309/312 samples. Squares represent directions for working-half samples. Shallow inclination is expected at the equatorial paleolatitude, but nearly half of our estimates remain steeper than 20° and none show negative inclination, indicating a remaining bias toward drilling overprinted directions. On Figure F61, NRM, ratio of magnetization after 20mT versus NRM, and susceptibility are represented.

Measured NRM intensities are probably several times higher than the predrilling values. Demagnetized inclinations approach the expected nearly horizontal values, but trends continuing to shallow at the last demagnetization suggest that overprint remains. Because of the steep orientation of the overprint, declinations are less sensitive to incomplete removal of overprint and in most cases should be adequate for approximate strike of structural features. For pieces that can be oriented by core-log integration, the declinations should also be reliable enough to determine magnetic polarity.

Downhole variations in magnetic patterns for Expedition 312 samples are minor, with demagnetization behavior of dikes indistinguishable from that of gabbros (Figs. F57, F58). NRM intensity is widely scattered in both dikes and gabbros and the distributions overlap, but intensities in the dikes are higher. When viewing the results for all of Hole 1256D (see Wilson, Teagle, Acton, et al., 2003), the striking variation is between the zones of low-temperature alteration shallower than 1000 mbsf and high-temperature alteration deeper. The shallow section is highly variable, with a majority of samples severely to completely overprinted but a small fraction showing minor overprint that appears completely removed by demagnetization. In contrast, the lower section is consistently moderately overprinted but appears to have a small fraction of the overprint remaining after maximum demagnetization. The difference probably arises from variations in grain size and oxidation of primary titanomagnetites in the shallow section, contrasted with nearly pure magnetite that is either secondary or altered to exsolve a Ti-bearing phase such as ilmenite in the hydrothermally altered section.

Integration of sample measurements with measurements of the magnetic field in the borehole should allow progress in characterizing the crustal magnetization. The amplitude of the marine magnetic anomalies in the area of the site has been satisfactorily modeled by Wilson (1996) with a layer 500 m thick magnetized at 10 A/m. A layer 1250 m thick with a magnetization of 4 A/m would, of course, produce an equivalent anomaly. An average predrilling magnetization of 2–5 A/m is within the plausible range for the dikes and gabbros recovered at Site 1256, so they remain candidates for a significant fraction of the source of marine magnetic anomalies.

Physical properties

Downhole changes in physical properties reflect changes in lithology and alteration throughout the Expedition 309/312 sections. P-wave velocities of Expedition 309 basalts (752–1255 mbsf) range from 4.8 to 6.1 km/s (average = 5.5 ± 0.3 km/s) (Fig. F62). This average value is similar to those estimated at a regional scale based on seismic refraction data and is consistent with shipboard values from Leg 206. From 752 to 1106 mbsf, average V_P increases ~0.05 km/s for each 50 m down Hole 1256D to nearly 6.0 km/s at 1130.6 mbsf. V_P is slightly higher below 1060 mbsf (5.8 ± 0.1 km/s) than above (5.4 ± 0.3 km/s), but everywhere it may be reduced locally by alteration and fracturing.

Very little net change occurs in compressional velocity over the ~250 m cored during Expedition 312 (1255–1507 mbsf) (Fig. F62). This observation masks considerable variation occurring in three asymmetric cycles. Each cycle is composed of an increase at gradients of ~1 km/s in 50 m and terminates with an abrupt decrease of up to 0.5 km/s in <7 m. The three sharp boundaries occur at the top of intrusive units. The shallowest boundary coincides with the top of Unit 1256D-76 near 1325 mbsf, where the velocity drops by ~0.4 km/s. The deeper two boundaries coincide with contacts between dikes and gabbros. One occurs near 1407 mbsf at the base of the granoblastic dikes (contact between Units 1256D-80 and 81), where the velocity drops ~0.5 km/s. The other boundary is a drop of 0.3 km/s at the top of Gabbro 2 (Unit 1256D-91). Velocities in the upper sections of the two gabbro units vary considerably, reflecting changes in porosity and density (Fig. F62). Velocity is highest in highly metamorphosed granoblastic basalt and lowest and most variable in intrusive gabbros. Thus, intrusive cycles appear to control vertical changes in velocity.

The average grain density of Expedition 309 basalts is 2.94 ± 0.04 g/cm³, and the average bulk density is 2.86 ± 0.07 g/cm³, similar to basalts recovered during Leg 206 (2.92 ± 0.07 and 2.82 ± 0.10 g/cm³, respectively) (Fig. F62). Densities of discrete samples do not show a strong downhole increase with depth, 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). Porosity values range from 2% to 14% (average = 4%). There is a decrease in porosity from the massive units above 1060 mbsf to those below this level: 4% ± 1% to 2% ± 1%, respectively (Fig. F62).

Following depth gradients that begin near 1060 mbsf, bulk density increases with depth and porosity decreases in the sheeted dikes (Fig. F63). Across the lithologic contact at 1407 mbsf, density drops from 3.04 g/cm³ in the lower granoblastic dikes to 2.88–2.90 g/cm³ in the uppermost gabbros. Porosity increases from 0.1–1% to 1–8% across the boundary. Both bulk and grain densities vary more in the gabbro than in the lower dikes, consistent with the observed variability in petrology and alteration. For all Expedition 312 samples, average bulk density is 2.97 ± 0.09 g/cm³, average grain density is 2.99 ± 0.08 g/cm³, and average porosity is 1.2% ± 1.4%.

Thermal conductivity measurements yielded values of 1.7–3.1 W/(m·K) (average = 2.0 ± 0.3 W/[m·K]) over the depth range of ~752–1255 mbsf (Fig. F62). Average thermal conductivity from the top of the sheet and massive flows (533 mbsf) to 1060 mbsf is 1.8 ± 0.2 W/(m·K). There is a significant increase in thermal conductivity starting in the transition zone and a distinct stepwise increase to 2.1 ± 0.1 W/(m·K) at 1060 mbsf, at the top of the sheeted intrusives (Fig. F62). Of the major rock types recovered during Expedition 309 and Leg 206, massive basalts and dikes of the sheeted dikes have significantly higher average thermal conductivities than massive flows, sheet flows, pillows, and hyaloclastites (Fig. F62). The mineralized volcanic breccia has the highest thermal conductivity of all rocks measured at Site 1256: 2–3.1 W/(m·K). Thermal conductivity increases by ~15% through the sheeted dikes from ~2.0 W/(m·K) at 1060 mbsf to 2.3 W/(m·K) (Fig. F62). In the upper gabbros, the median value of thermal conductivity drops to ~2.2 W/(m·K) and variability increases. Thermal conductivity again increases in the gabbro and dike screen, reaching values of 2.5–2.7 W/(m·K) at the base of the hole.

Whole-round cores were run through the multisensor track (MST) prior to splitting. Rather than considering all MST data, during Expedition 309 only measurements from the middle of pieces >8 cm were used for analysis. Magnetic susceptibility in Expedition 309 rocks ranged from ~0 to 10,000 × 10^–5 SI, with the highest values corresponding to massive lava flows, massive basalts, and dikes. In the transition zone and into the sheeted dikes (1004.2–1255.1 mbsf), variability in magnetic susceptibility does not correspond to rock type and appears to be more influenced by intensity and style of alteration.

Magnetic susceptibility varies downhole with a wavelength of 100–140 m. Because magnetite appears in thin sections as a secondary product in Expedition 312 rocks, it is possible that these cycles represent variations in the nature or degree of alteration. In Expedition 312 dikes, spikes in magnetic susceptibility reach 14,000 SI units over depth intervals of 2–4 m. Magnetic susceptibility is generally low (~2,000 SI units) in the gabbro units with the exception of very high spikes, reaching 20,000 SI units in the uppermost 1–5 m. Large magnetite crystals are likely the source of the high magnetic susceptibility.

After corrections, gamma ray attenuation (GRA) bulk density steadily increases with depth independent of changes in lithology and alteration from 2.7 g/cm³ in the Upper Dikes to 3.0–3.2 g/cm³ at the base of the hole. In most cores examined during Expedition 312, natural gamma ray counts are 0–4 cps, but higher values (5–11 cps) were measured in eight samples in the upper dikes and gabbro units.

Downhole measurements

Following the completion of drilling during Expedition 309, a wiper trip was run over the complete basement interval and the hole was prepared for wireline logging operations. In all, five tool strings were used in the following order: the triple combo, the FMS-sonic, the UBI, the WST, and a second run of the FMS-sonic. All deployments were successful except the WST, which suffered from wireline difficulties caused by running this light tool into the open hole. The vertical deviation measured at 1200 mbsf reaches 4.3°, and the hole azimuth varies between 250° and 290°. Caliper readings from both the triple combo and FMS-sonic tool strings show generally good borehole conditions (Fig. F64). The average hole diameter measurements from the FMS-sonic calipers 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. Comparison of the caliper data from the pre- and postdrilling operations of the upper 500 m shows that the borehole is being progressively enlarged with continued drilling.

Overall, combined results of standard geophysical measurements and FMS and UBI images suggest that the section drilled during Expedition 309 may be separated into subsections, continuous with the three logging intervals distinguished during Leg 206 (see Wilson, Teagle, Acton, et al., 2003):

• Logging Interval I (base of casing to 346 mbsf) is characterized by high resistivity (up to 100 Ωm) and monotonous FMS and UBI images and corresponds to the massive ponded lava.
• Logging Interval II (346–532 mbsf) is marked by a sharp increase in natural gamma ray log values, variable geophysical properties, and distinctive electrical and acoustic images that indicate the presence of pillow lavas intercalated with sheet and massive flows and hyaloclastites.
• Logging Interval III (532–920 mbsf) was identified during Leg 206 to 752 mbsf by a decrease in the range of variation of physical properties and electrical images that agree with the petrological interpretation of this zone as a sequence of massive and sheet flows. In the section drilled during Expedition 309, this interval has moderate resistivity values (commonly between 10 and 100 Ωm) with very high, short-wavelength frequency variability. Natural radioactivity is highly variable in this interval but is usually >2 gAPI. Intervals of high natural radioactivity (>8 gAPI) are present at 770–774, 784–796, and 842–878 mbsf. An extremely high value (37 gAPI) of natural radioactivity is recorded at 886 mbsf. At 785–843 and 853–920 mbsf, electrical resistivity increases with depth from 8.6 to 770 Ωm and 9.1 to 106 Ωm, respectively. Similar trends are recorded with the FMS-sonic tool where compressional velocities increase from 4 to up to 6 km/s. Logging Interval III is characterized by alternating layers of thin flows, breccias, and massive units.
• Logging Interval IV extends from 920 to 1061 mbsf. This interval is characterized by long-wavelength, large-amplitude resistivity variations. Within this interval, low-resistivity zones are generally associated with high natural radioactivity values. The lowest resistivity values are recorded at 919–927, 1028–1032, and 1047–1054 mbsf. The interval between 919 and 927 mbsf shows perturbations in the temperature profile that may indicate a highly fractured or faulted zone where fluids are circulating. The interval 1028–1032 mbsf is associated with an increase in the natural radioactivity and can confidently be correlated with the mineralized volcanic breccia (Cores 309-1256D-122R through 123R; Unit 1256D-42). Logging Interval IV coincides with volcanic breccias interbedded within sheet flows as described in cores.
• Logging Interval V extends from 1061 mbsf to the bottom of the hole and corresponds to the sheeted intrusives. This interval is characterized by high electrical resistivities (generally >100 Ωm) as high as 2500 Ωm at 1161 mbsf. Furthermore, extremely low (<2 gAPI) and constant natural radioactivity is recorded in this interval. Below 1028 mbsf, P-wave values increase and velocities higher than 6 km/s become common. In this interval, density is generally in the range 2.8–2.9 g/cm³. FMS and UBI images show the common presence of subvertical, highly conductive features that dip steeply (~80°–85°) to the northeast and are interpreted to be dike margins. These regions have abundant horizontal fractures and veins.

During Expedition 309 postdrilling logging, the temperature of Hole 1256D was recorded using the TAP tool (Fig. F65). The temperature record from the TAP tool clearly does not record the equilibrium thermal state of the crust because of ~33 days of fluid circulation during drilling, but it does provide important information on the cooling of the Site 1256 lithosphere. The maximum temperature in the hole is 60°C—much cooler than the equilibrium temperature of 105°C predicted from heat flow and temperatures measured during predrilling operation (see “Predrilling logging operations”). A temperature of 60°C is also significantly cooler than the equilibrium temperature measured in Hole 1256D at 724 mbsf before the commencement of coring during Expedition 309. There are clear perturbations in the temperature profile, with three intervals at ~691, 796, and 928 mbsf displaying negative temperature excursions that indicate a slower return toward the predicted equilibrium temperature (Fig. F65). In the same figure, the resistivity log and the FMS images (917–934 mbsf) show that these intervals have very low resistivity. The 928 mbsf perturbation corresponds to a change in rock type in the recovered cores from massive aphyric basalt to a cryptocrystalline to fine-grained sheet flow (Units 1256D-34b through 35a). The transient temperature anomaly probably indicates that this interval is a zone of high permeability that was preferentially invaded by the cold drilling fluids and is consequently recovering more slowly.

Expedition 312 downhole measurements in Hole 1256D were conducted after drilling and coring at Site 1256 terminated. The hole was conditioned and a wiper trip run in preparation for logging. After the bottom-hole assembly (BHA) was set at 280 mbsf the following six tool strings were deployed: the triple combo, Vertical Seismic Imager (VSI), FMS-sonic, UBI-sonic, FMS, and TAP-Dual Laterolog (DLL) tool strings.

At a depth of 1440 mbsf, the cable head tension of the first tool string decreased, indicating that the tools reached total depth, ~67 m above the total cored depth. All logging runs were conducted from this depth and provided high-quality data with an excellent overlap of logging results from Expedition 309. Borehole conditions were good during the six logging runs, and no major washouts or obstructions were encountered. The choice of tool strings and tool combinations were adjustments to the original logging plan after the failure of the DLL and the FMS tools in the first two uphole logging operations. Caliper readings from the triple combo and FMS-sonic tool strings indicate excellent borehole conditions in the newly drilled section below 1255 mbsf, with a hole diameter typically between 10 and 11 inches. However, compared to Expedition 309 Phase 1 and 2 caliper data, it is apparent that continued drilling led to progressive enlargement in the upper borehole sections (Fig. F64). The vertical deviation measured at 1427 mbsf reached 5.4°, and the hole azimuth ranged between 254° and 281° but was on average 271.5° for the newly drilled section in Hole 1256D.

Overall results obtained during Expedition 312 support the division of the lithology based on core description from recovered sample material. In general, the natural occurring radioactivity was measured continuously with each logging run and used to depth-match each logging run relative to the triple combo tool string logs.

Overall total gamma radiation remains relatively constant and well below 4 gAPI in the section logged during Expedition 312 (Fig. F66). The net measured formation resistivity increased with increasing depth possibly related to changes in metamorphism (see “Alteration”). This trend is interrupted at several depth intervals. Large decreases in resistivity locally result from enlarged borehole (e.g., ~1300 mbsf), but decreases in resistivity are also caused by brecciated lithologies or intrusive contacts unrelated to borehole size (e.g., at 1220, 1240, 1265, 1270, 1290, and 1320 mbsf). Below 1200 mbsf, shallow and deep resistivity measurements are 500–10,000 Ωm and 1000 –140,000 Ωm, respectively. These high readings differ strongly from the values reported during Expedition 309 and indicate a change in lithology. Strong decoupling between the shallow and deep resistivity measurements continues to total depth. The resistivity data observed in the sheeted dike complex suggests that the lithology may be divided into four sections based on variability and magnitude of electrical resistivity. These sections can be distinguished at 1060–1155, 1155–1275, 1275–1350, and 1350–1407 mbsf (Fig. F66). Overall density and neutron porosity range 1.5–3.09 g/cm³ and 2%–75%, respectively, but most variation remains small in the newly cored section of Hole 1256D. Distinct anomalies correlate with enlarged borehole diameters related to washouts. The average densities of the sheeted dike complex and the granoblastic dikes are 2.89 and 2.99 g/cm³, respectively. Density drops to an average of 2.95 g/cm³ in Gabbro 1 with a minimum of 2.93 g/cm³. This change in density is accompanied with a decrease in compressional velocity from 6.2 to 4.6 km/s.

A vertical seismic profile (VSP) was shot in Hole 1256D during Expedition 312 to determine interval velocities and to record seismograms for further analysis of the seismic properties of upper ocean crust. It was planned to collect data at stations spaced 22 m apart in the open borehole. The most serious problem was poor anchoring to the borehole wall in the upper borehole sections, indicated by harmonic ringing in the seismograms. In many cases, this could be reduced when the VSI tool was moved 1–5 m uphole to obtain secure clamping, but stations at 1054, 934, 811, 587, and 444 mbsf had to be abandoned, leaving gaps of up to 45 m between stations (Table T5). Inflexibility in the stacking process and lack of control on the picking routine in the automatic picking algorithm contributed to traveltime errors. Interval velocities determined at sea during the VSPs run during Leg 206 and Expedition 312 are preliminary and likely to change when the seismograms are examined in more detail on shore. In general, the VSP interval velocities parallel trends in the sonic log and the shipboard velocity measurements on recovered rock samples (Fig. F67). Although velocity magnitude differs among the various methodologies because of different frequencies of sound and the different confining pressures, trends with depth are similar. This similarity demonstrates the fundamental dependence of velocity fluctuations in uppermost crust on the primary eruptive process and the increase in velocity with depth on the increasing density of the rocks due to progressively higher temperature alteration and metamorphism. However, there are two unusually high velocity intervals, 7.6 km/s at 1339–1361 mbsf and 6.5 km/s at 880–903 mbsf, that are not matched by low velocities at neighboring stations. This likely results from traveltime errors at adjacent stations but could also be explained by a nearby high-velocity intrusive body not sampled by drilling.

Preliminary analysis of the resistivity and sonic image data from Expedition 312 indicates high-quality data were obtained over the logged borehole section. It is obvious that directly above the dike/​gabbro boundary the formations are characterized by discontinuous fractures, whereas the fractures in the gabbroic section are continuous. However, other data suggest that the vertically oriented high-amplitude zones in the UBI image may be tool-related artifacts rather than characteristics of the formation. These features are not observed in FMS images. FMS and UBI imagery complement each other and are crucial in reorienting core pieces. Oriented images provide essential information on reconstructing the in situ orientation of fractures and veins. Features presented in the UBI image at 1410 and 1418 mbsf have a northeast-oriented plunge and an approximate dip between 35° and 40° and may represent fractures. The same features are also evident on the resistivity image, where they represent zones of high conductivity.

The TAP tool was deployed at the beginning and the end of the logging operation during Expedition 312 to gain information on the thermal rebound in the borehole after coring ceased (Fig. F65). The bottom hole temperature was recorded three times, and increases from 64.24° to 67.90°C and 86.5°C were observed in time frames of ~5 h and 68.5 h, respectively. Perturbations are visible at 900–950 and 1350–1400 mbsf with negative deviation from the temperature profile (Fig. F65). These negative temperature anomalies indicate a slower return to equilibrium temperatures and may be due to a higher influx of seawater invasion during the drilling process.

Digital imaging

Rotary coring generally returns azimuthally unoriented samples, but 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 Deutsche Montan Technologie Digital Color CoreScan system during Expedition 309/312. Because of limited time between the collection of downhole logging data and the end of Expedition 309, only a few preliminary attempts at matching core images to logging images have been made. Some of these attempts show potentially good matches between the unrolled core images and the FMS and UBI data from Hole 1256D. Figure F68 shows an example using the largest piece recovered during Expedition 309 from 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.

During Expedition 312, a total of 165 whole-core images were scanned. The total scanned length (22.8 m) represents almost 49% of the interval cored. Although it was challenging to gain whole-core images from several cores because of low recovery, images of significant lithological features (e.g., the dike/​gabbro contact at Section 312-1256D-213R-1, 52 cm [1406.62 mbsf]) were satisfactorily obtained.

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