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doi:10.2204/iodp.proc.309312.103.2006

Background and objectives

Hole 1256D in the eastern equatorial Pacific is the first basement borehole prepared with the infrastructure desirable for drilling a moderately deep hole into the oceanic crust (~1.5–2 km). Following the installation of 16 inch casing cemented into oceanic basement, 500 m of the uppermost ocean crust in Hole 1256D was drilled during Leg 206 (Wilson, Teagle, Acton, et al., 2003). Expedition 309 was the second scientific ocean drilling cruise in a multiphase mission to Hole 1256D to complete the recovery, for the first time, of a complete section of the upper oceanic crust from extrusive lavas down through the dikes and into the uppermost gabbros. Expedition 309 (July–August, 2005) was followed closely by Expedition 312 (November–December, 2005), which continued the pursuit of these goals.

Drilling a complete, in situ section of oceanic crust has been a major, unfulfilled ambition of Earth scientists since the inception of scientific ocean drilling. In the late 1950s, the audacious Project MoHole proposed to drill in the oceans completely through Earth’s crust and into the mantle (see Bascom, 1961; Greenberg, 1974). Although that project eventually stagnated, it stimulated a critical momentum for scientific ocean drilling. The principal emphasis of Project MoHole was to understand the nature of oceanic crust and the underlying uppermost mantle, but unfortunately, many of the key questions regarding the accretion and evolution of the oceanic crust remain unanswered despite a further 40 y of research. This is principally due to the paucity of deep drill holes into in situ oceanic crust (Fig. F1), which are essential to calibrate regional geophysical techniques such as seismics or magnetics and which provide cores for analytical constraints on magmatic, hydrothermal, and structural processes.

Expedition 309/312 built on the successes of Leg 206 to drill a complete section of the upper oceanic crust that formed at a superfast spreading rate. Following preliminary coring to document the sedimentary overburden at Site 1256, operations during Leg 206 in Hole 1256D included installation of a reentry cone supported by 20 inch casing and large-diameter (16 inch) casing all the way through the sediment cover and cemented 19 m into basement (Wilson, Teagle, Acton, et al., 2003). The cone and casing facilitated multiple reentries and helped maintain hole stability essential for deepening Hole 1256D through the dikes and into the gabbros. The large-diameter casing leaves open the possibility that one or two more casing strings could be installed in Hole 1256D should future expeditions need to isolate unstable portions of the hole. Armoring the sediment/basement boundary reduced erosion of the borehole walls at this weak point and assisted in the clearing of drill cuttings from the hole.

After these installations, the upper 502 m of the igneous crust was cored (during Leg 206) with moderate to high recovery (48%). The uppermost crust at this site comprises a sequence of massive flows and thinner sheet flows with subordinate pillow basalt, hyaloclastite, and breccias. The sequence has normal MORB (N-MORB) composition and is slightly to moderately altered. It was extruded over sufficient time to record stable geomagnetic field directions and to capture transitional directions in the upper units as the geomagnetic field reversed. Importantly, operations during Leg 206 in Hole 1256D concluded with the hole clean of debris, in excellent condition, and open to its full depth.

Expedition 309/312 continued operations in Hole 1256D (Fig. F2). The goal was to core through the remaining extrusive rocks and the underlying sheeted dike complex into plutonic rocks. This continuous section of in situ oceanic crust generated at a superfast spreading rate in the eastern Pacific was intended to

• Provide the first sampling of a complete section of oceanic crust from extrusive rocks through the dikes and into the gabbros,
• Confirm the nature of high-level axial magma chambers, and
• Define the relationship between magma chambers and their overlying lavas and the interactions between magmatic, hydrothermal, and tectonic processes.

Rationale for site selection and location criteria for deep drilling

The key to proposing the Superfast Spreading Rate Crust campaign (Leg 206; Expedition 309/312) was to identify a style of crustal accretion where the extrusive lavas and dikes overlying the gabbros are predicted to be relatively thin, thus increasing the likelihood of penetrating through the complete upper crustal section in the fewest drilling days. The recognition that crust formed at a superfast spreading rate is a compelling target for deep drilling follows the observation of an inverse relationship between the depth to axial low-velocity zones imaged by seismic experiments, interpreted to be melt lenses, and spreading rate (Purdy et al., 1992) (Fig. F3). Even allowing for additional thickness of lavas that flowed from the ridge axis to cover the immediate flanks, the uppermost gabbros should be at relatively shallow depths in superfast-spreading crust. The predicted depth to gabbros at Site 504 on the south flank of the intermediate spreading rate Costa Rica Rift is >2.5 km, whereas the depth to the axial low-velocity zones at typical fast spreading rate (~80–150 mm/y; full rate) crust on the EPR is ~1–2 km. The estimated depth to an axial melt lens for oceanic crust formed at a superfast spreading rate is ~700–1000 m, and the anticipated depth to gabbros for Site 1256 is ~1000–1300 m, allowing for a reasonable thickness (~300 m) of near-axial lava flows (e.g., Hooft et al., 1996; see "Predictions of depth to gabbros").

Site selection

A recent reconsideration of magnetic anomalies formed at the southern end of the Pacific/Cocos plate boundary has identified crust formed at a full spreading rate of ~220 mm/y from 20 to 11 Ma (Wilson, 1996) (Fig. F2). This is significantly faster than the present fastest spreading rate (~145 mm/y) for crust forming at ~30°S on the EPR. From this region created by superfast spreading, a single drill site in the Guatemala Basin, initially designated GUATB-03C and now known as Site 1256, was selected on young, ~15 m.y. old oceanic crust. The details of site survey operations and the reasons for the selection of this particular site are outlined in Wilson, Teagle, Acton, et al. (2003). Deep drilling at Site 1256 characterizes one end-member style of mid-ocean-ridge accretion.

In addition to the shallow depth to gabbros predicted from formation at a superfast spreading rate, Site 1256 has a number of specific attributes that make it an excellent opportunity to sample a complete section of upper oceanic crust. Site 1256 formed at an equatorial latitude (Fig. F4), and high equatorial productivity resulted in high sedimentation rates (>30 m/m.y.) and rapid burial of young basement. A thick sediment blanket was needed to enable the installation of a reentry cone with 20 inch casing that formed the foundation for deployment of the second, 16 inch diameter casing string that was cemented 19 m into the uppermost basement. At 15 Ma, Site 1256 is significantly older than the crust at Hole 504B (6.9 Ma), and lower temperatures are anticipated at midlevels of the crust. As such, high basement temperatures that can preclude drilling operations should not be reached until gabbroic rocks have been penetrated. Logistically, Site 1256 has a number of advantages. It is ~3.5 days steaming from the Panama Canal, and the short transit time allows for maximum time on site during drilling expeditions. As transfer between the Pacific and Atlantic oceans is common because of the scheduling demands of scientific drilling, close proximity to the canal has allowed the timely rescheduling of return visits to the site.

Geological setting of Hole 1256D

Site 1256 (6°44.2′N, 91°56.1′W) lies in 3635 m of water in the Guatemala Basin on Cocos plate crust formed ~15 m.y. ago on the eastern flank of the EPR (Figs. F2, F4). Three regions were surveyed during the 1999 Maurice Ewing site survey cruise (Wilson et al., 2003) with Site C in the GAUTB-03 region selected as the preferred location for deep drilling (Fig. F5) (Wilson, Teagle, Acton, et al., 2003). The depth of the site is close to that predicted from bathymetry models of plate cooling (e.g., Parsons and Sclater, 1977). The site sits astride the magnetic anomaly 5Bn–5Br transition in magnetic polarity (Fig. F5A). This crust accreted at a superfast spreading rate (~220 mm/y; full rate) (Wilson, 1996) and lies ~1150 km east of the present crest of the EPR and ~530 km north of the Cocos Ridge. The site formed on a ridge segment at least 400 km in length, ~100 km north of the ridge-ridge-ridge triple junction between the Cocos, Pacific, and Nazca plates (Fig. F4). This location was initially at an equatorial latitude within the equatorial high-productivity zone and endured high sedimentation rates (>30 m/m.y.) (e.g., Farrell et al., 1995; Wilson, Teagle, Acton, et al., 2003). The sediment thickness in the region is between 200 and 300 m and is 250 m at Site 1256 (Wilson, Teagle, Acton, et al., 2003).

Site 1256 has a seismic structure reminiscent of typical Pacific off-axis seafloor (Fig. F6). Upper Layer 2 velocities are 4.5–5 km/s and the Layer 2–3 transition is between ~1200 and 1500 msb (Fig. F7). The total crustal thickness at Site 1256 is estimated at ~5–5.5 km. Further to the northeast of Site 1256 (15–20 km), a trail of ~500 m high circular seamounts rise a few hundred meters above the sediment blanket (Fig. F5B). Using the site survey multichannel seismic data (Wilson et al., 2003), we constructed a geological sketch map of the uppermost basement in the GUATB-03 survey region (Fig. F8). The bathymetry in the GUATB-03 survey areas is generally subdued, and Site 1256 sits atop a region of smooth basement topography (<10 m relief). Elsewhere in the region, however, the top of basement shows a number of offsets along northwest-striking normal faults, and an abyssal hill relief of as much as 100 m is apparent in the southwest part of the area. Relief to the northeast is lower and less organized. In the northeastern sector of the GUAT-3B region, there is evidence for a basement thrust fault with a strike approximately orthogonal to the regional fabric (Wilson et al., 2003; Hallenborg et al., 2003). This feature dips gently to the northwest (~15°) and is clearly discernible to a depth of ~1.3 km on seismic Line EW9903-28 (Wilson et al., 2003), but the feature is less pronounced on seismic Line EW9903-27, indicating that the offset on the thrust decreases to the southwest.

Additional processing (A.J. Harding, unpubl. data) of ocean-bottom hydrophone recordings indicates discernible variation in the average seismic velocity (~4.54–4.88 km/s) of the uppermost (~100 m) basement and regional coherence in the velocity variations (Fig. F8A). Two principal features are apparent: a 5–10 km wide zone of relatively high upper basement velocities (>4.82 km/s) that can be traced ~20 km to the edge of data coverage southeast of Site 1256 and a relatively low velocity (4.66–4.54 km/s) bull's-eye centered around the crossing point of seismic Lines EW9903-21 and 25.

The uppermost basement at Site 1256 is capped by a massive lava flow >74 m thick. This flow is relatively unfractured with shipboard physical property measurements on discrete samples indicating V_P > 5.5 km/s (Wilson, Teagle, Acton, et al., 2003). As such, it is likely that the area of relatively high uppermost basement seismic velocities represents the extent of the massive flow penetrated during Leg 206 in Holes 1256C and 1256D. Assuming an average thickness of 40 m, this would conservatively suggest an eruption volume >3 km³, plausibly >10 km³. This is extremely large when compared to the size of mid-ocean-ridge axial low-velocity zones that are thought to be high-level melt lenses, which typically have volumes ~0.05–0.15 km³ per kilometer of ridge axis and generally appear to be only partially molten (Singh et al., 1998).

Sheet flows (<3 m thick) and massive flows (>3 m thick) make up most of the lava stratigraphy cored so far at Site 1256, and such lava morphologies have been shown to dominate crust formed at fast spreading rates, away from segment tips (e.g., White et al., 2000, 2002). Only rare pillow lavas are present in Hole 1256D, and because of the large number of fractures and pillow interstices, seismic velocities of pillow lavas are generally lower than more massive lava flows. We speculate that the bull's-eye of relatively low seismic velocities is a thick pile of dominantly pillowed lava flows.

Scientific objectives of Expeditions 309 and 312

Expedition 309/312 continued the drilling of a continuous section through volcanic basement and the underlying sheeted dike complex into the uppermost plutonic rocks in Hole 1256D. Cores recovered from and wireline measurements made in Hole 1256D provide unique information to address the following specific scientific objectives.

Test the prediction, from the correlation of spreading rate with decreasing depth to the axial melt lens (e.g., Purdy et al., 1992), that gabbros representing the crystallized melt lens will be encountered at 1000–1300 msb at Site 1256.

The transition from sheeted dikes to gabbros has never been drilled, and this remains an important objective in achieving a complete or even composite crustal section. The dike–gabbro transition and the uppermost plutonic rocks are assumed to be frozen axial melt lenses and the fossil thermal boundary layer between magma chambers and vigorous hydrothermal circulation. Detailed knowledge of the dike–gabbro transition zone is critical to discerning the mechanisms of crustal accretion. The textures and chemistries of the uppermost gabbros are presently unknown but are central to understanding crustal construction; to date, samples are lacking that link gabbroic rocks to the overlying lavas, leading to the following questions:

• What is the geological nature of the low-velocity zones imaged by multichannel seismic reflection studies at the axes of mid-ocean ridges?
• Are the upper gabbros cumulate rocks from which magmas were expelled to form the dikes and lavas that then subsided to form the lower crust? Or are the uppermost gabbros coarse-grained chemical equivalents of the dikes and extrusive rocks frozen at the base of the sheeted dikes?
• Does most of the crustal accretion occur at deeper levels through the intrusion of multiple thin sills?
• What are the cooling rates of magma chambers?

These questions can be answered through petrographic and geochemical (major and trace elements) studies of gabbros (e.g., Natland and Dick, 1996; Kelemen et al., 1997; Manning et al., 2000; MacLeod and Yaoancq, 2000; Coogan et al., 2002a, 2002b) and the overlying lavas and their mineral constituents.

Determine the lithology and structure of the upper oceanic crust from a superfast spreading rate end-member.

Some basic observations regarding the architecture of ocean crust, including the lithology, geochemistry, and thicknesses of the volcanic and sheeted dike sections and how these vary with spreading rate or tectonic setting, are not well known. Karson (2002) provides estimates of the thicknesses of lavas and sheeted dikes from crust generated at fast and intermediate spreading rates (600–900 m lavas and 300–1000 m dikes at Hess Deep; 500–1300 m lavas and 500 to >1000 m dikes in Hole 504B and the Blanco Fracture Zone). With the exception of the incomplete section in Hole 504B, these estimates are based on observations of highly disrupted exposures, where structural complexities and the uniqueness of the geological environments indicate that such estimates should be treated with caution. Results of Expedition 309/312 will determine the thicknesses of these upper crustal units at Site 1256 as well as document the styles of deformation and magmatic accretion. Although studies of tectonic exposures of oceanic crust suggest that intense brittle deformation, faulting and localized zones of fracturing, and large amounts of dike rotation are common within sheeted dike complexes in crust formed at fast and intermediate spreading rates (Karson, 2002; Karson et al., 2002; Stewart et al., 2005), it is difficult to separate primary mid-ocean-ridge geometries from deformation related to the exposure in these tectonic windows. In contrast, large blocks of the sheeted dike complexes in the Semail ophiolite in Oman exhibit little of such faulting and distributed fracturing (Umino et al., 2003). Seismic profiles of the Site 1256 region show well-developed subhorizontal reflectors to ~1000 msb (Fig. F6), providing little evidence for rotation of the upper crust in this region.

Drilling the sheeted dike complex at Site 1256 will enable evaluation of whether faulting and fracturing in tectonic exposures are representative of oceanic crust or whether they may be related to their tectonic setting. Most dikes in sheeted dike complexes in tectonic exposures of crust generated at intermediate and fast spreading rates and in Hole 504B in intermediate-spreading-rate crust generally dip away from the spreading axis, suggesting tectonic rotation of crustal blocks (Karson, 2002). Do such rotations also occur in crust generated at superfast spreading rates, and are they similar, or is the crust less tectonically disrupted? A single drill hole may not conclusively answer this question but should provide important constraints.

Correlate and calibrate seismic and magnetic imaging of the crustal structure with basic geological observations.

Ground-truthing regional geophysical techniques such as seismic and magnetic imaging is a key goal of the IODP Initial Science Plan (International Working Group, 2001) and related documents (e.g., COMPLEX; Pisias and Delaney, 1999). A fundamental question we will address in this experiment is how velocity changes within seismic Layer 2 and the Layer 2–Layer 3 transition relate to physical, lithological, structural, and alteration variations in the volcanic rocks, dikes, and gabbros. At Site 504, in crust generated at an intermediate-rate spreading ridge, the Layer 2–Layer 3 transition lies within the 1 km thick sheeted dike complex and coincides with a metamorphic change (Detrick et al., 1994; Alt et al., 1996a), but it is unknown whether the results from Hole 504B are representative of ocean crust in general or of crust generated at different spreading rates. Is the depth to gabbros shallower in crust generated at a superfast spreading rate as predicted, and what are the relative thicknesses of volcanic and dike sections compared with crust constructed at slow or intermediate spreading rates?

Marine magnetic anomalies are one of the key observations that led to the development of plate tectonic theory, through recognition that the ocean crust records the changing polarity of Earth’s magnetic field through time (Vine and Matthews, 1963). It is generally assumed that micrometer-sized grains of titanomagnetite within the erupted basalts are the principal recorders of marine magnetic anomalies. However, recent studies of tectonically exhumed lower crustal rocks and serpentinized upper mantle indicate that these deeper rocks may also be a significant source of the magnetic stripes (Hosford et al., 2003). Coring a complete section through the sheeted dike complex will allow evaluation of the contribution of these rocks to marine magnetic anomalies. Whether these deeper rocks have a significant influence on the magnetic field in undisrupted crust is unknown, as is the extent of secondary magnetite growth in gabbros and mantle assemblages away from transform faults. Sampling the plutonic layers of the crust will test the Vine-Matthews hypothesis by characterizing the magnetic properties of gabbros through drilling normal ocean crust on a well-defined magnetic stripe, away from transform faults.

Investigate the interactions between magmatic and alteration processes, including the relationships between extrusive volcanic rocks, sheeted dikes, and underlying gabbroic rocks.

Little information presently exists on the heterogeneity of hydrothermal alteration in the upper crust or the variability of associated thermal, fluid, and chemical fluxes. How these phenomena vary at similar and different spreading rates is unknown. Metamorphic assemblages and analyses of secondary minerals in material recovered by deep drilling can provide limits on the amount of heat removed by hydrothermal systems and place important constraints on the geometry of magmatic accretion and the thermal history of both the upper and lower crust (e.g., Manning et al., 2000; MacLeod and Yaoancq, 2000; Coogan et al., 2002a, 2002b). Fluid flow paths, the extent of alteration, and the nature of deep subsurface reaction and shallower mixing zones are all critical components of our understanding of hydrothermal processes that can only be tackled by drilling. These problems can be addressed by examining the stratigraphy and relative chronology of alteration within the extrusive lavas and dikes, by determining whether disseminated sulfide mineralization resulting from fluid mixing and a large step in thermal conditions is present at the volcanic–dike transition (as in Hole 504B and many ophiolites), and by evaluating the grade and intensity of alteration in the lower dikes and upper gabbros. The lowermost dikes and upper gabbros have been identified as the conductive boundary layer between the magma chambers and the axial high-temperature hydrothermal systems and as the subsurface reaction zone where downwelling fluids acquire black-smoker chemistry (Alt, 1995; Alt et al., 1996a; Vanko and Laverne, 1998; Gillis et al., 2005). However, extensive regions of this style of alteration or zones of focused discharge are poorly known, and information from ophiolites may not be applicable to in situ ocean crust (Richardson et al., 1987; Schiffman and Smith, 1988; Bickle and Teagle, 1992; Gillis and Roberts, 1999). Drilling beyond the boundary between the lower dikes and upper gabbros will help trace recharge fluid compositions, estimate hydrothermal fluid fluxes (e.g., Teagle et al., 1998, 2003; Gillis et al., 2005), and integrate the thermal requirements of hydrothermal alteration in sheeted dikes and underlying gabbros with the magmatic processes in the melt lens. Detailed logging of cores combined with geochemical analyses will enable determination of geochemical budgets for hydrothermal alteration (e.g., Alt et al., 1996a; Alt and Teagle, 2000; Bach et al., 2003). Is there a balance between the effects of low-temperature alteration of lavas versus high-temperature hydrothermal alteration of dikes and gabbros? This is a critical check on global budgets for many elements (Mg, K, ⁸⁷Sr, U, and ¹⁸O) presently estimated from vent fluid chemistries, riverine inputs, and thermal models (e.g., review of Elderfield and Schultz, 1996).

During the last decade, several studies have shown that the upper volcanic part of the modern oceanic crust is a habitat for microorganisms. In this environment, microbes colonize fractures in glassy basaltic rocks, extracting energy and/or nutrients from the glass by dissolving it and leaving behind biomarkers that reveal their former presence (Furnes and Staudigel, 1999; Banerjee and Muehlenbachs, 2003). Microbial alteration of volcanic glass has been shown to decrease with basement depth at other sites (Furnes and Staudigel, 1999). Temperature and depth limits to subbasement microbiological activity can be investigated by deep sampling and study of microbial alteration textures, chemical and isotopic indicators, and molecular microbiology (e.g., Blake et al., 2001; Alt et al., 2003; Banerjee and Muehlenbachs, 2003).

Principal results of Leg 206

The major objectives of Leg 206 were to establish a cased reentry hole that would be open for future drilling and to achieve a target penetration >500 msb. Before basement drilling was initiated, a series of holes was drilled to thoroughly characterize the sediments and magnetostratigraphy and biostratigraphy of the sedimentary overburden and to determine the casing depth into basement for the main hole. Four holes were drilled during Leg 206 with Holes 1256A, 1256B, and 1256C recovering a near-complete record of the 250 m thick sedimentary overburden. Pilot Hole 1256C penetrated 88.5 m into basement, and Hole 1256D was the cased reentry hole, with a large reentry cone supported by 95 m of 20 inch casing and 296.5 m of 16 inch casing cemented into the uppermost basement. TD of penetration of Hole 1256D during Leg 206 was 752 mbsf, including 502 m drilled into basement. Recovery of igneous rocks was good, excellent in places, with average recovery rates of 61.3% and 47.8% in Holes 1256C and 1256D, respectively.

The sedimentary overburden is divided into two units: Unit I (0–40.6 mbsf) is clay rich, with a few carbonate-rich layers; Unit II (40.6–250.7 mbsf) is predominantly biogenic carbonate. The interval 111–115 mbsf is rich in biogenic silica, which forms a distinct diatom mat, deposited at ~10.8 Ma. Chert nodules are a common feature below 111 mbsf, and red-brown iron oxide–rich silicified sediments that may be recrystallized metalliferous sediments directly overlie the basement (within ~1 m). The primary control on the interstitial water chemistry at Site 1256 is diffusion between seawater and basement fluids, with a continuous chert bed at 158 mbsf providing a low-diffusivity barrier. The calcareous microfossil biostratigraphy is in good agreement with the magnetostratigraphy. Calculated sedimentation rates vary from ~6 to 36 m/m.y. and decrease with time as the site moved away from the high-productivity zone near the paleoequator (Wilson, Teagle, Acton, et al., 2003).

Approximately 60% of the igneous basement in Holes 1256C and 1256D consists of thin (tens of centimeters to <3 m) basaltic sheet flows separated by chilled margins (Fig. F9). Massive flows (>3 m thick) are the second most common rock type, including the thick ponded flow near the top of the holes. Minor intervals of pillow lavas (~20 m) and hyaloclastite (a few meters) and a single dike were recovered in Hole 1256D. The low proportion of pillow lavas (<10%) indicates rapid lava emplacement on low topographic relief, consistent with thermal-model predictions of <1 km vertical thickness of the dike zone. The transition from axial eruptions to lavas that flowed out onto the ridge flanks was not determined during Leg 206; however, the thickness of the massive ponded flow requires significant basement relief in order to pool the lava, and this would only be developed significantly off axis (>5 km).

The uppermost lavas, sampled only in Hole 1256C because of setting casing in Hole 1256D, are composed of thin basaltic sheet flows a few tens of centimeters to ~3 m thick, separated by chilled margins and containing rare intervals of recrystallized sediment. Units 1256C-18 and 1256D-1 each consist of a single cooling unit of cryptocrystalline to fine-grained basalt, interpreted to be a ponded lava flow, which serves as a clear marker unit for correlation of the igneous stratigraphy between holes. A total of 32 m of this unit was cored in Hole 1256C, of which 29 m was recovered. This ponded flow is much thicker 30 m away in Hole 1256D, where it has a minimum thickness of 74.2 m, indicating steep paleotopography. The groundmass of the interior of the flow is fine grained but deformed and thermally metamorphosed ~1.5 m from its base.

The remainder of this section in Hole 1256D (with the exception of Units 1256D-3, 4a, 4c, 8c, 16d, and 21) consists of sheet flows tens of centimeters to ~3 m thick with uncommon massive flows 3.5–16 m thick. These sheet and massive flows are aphyric to sparsely phyric cryptocrystalline to microcrystalline basalt and are distinguished by common chilled margins with fresh or altered glass.

One ~20 m thick interval of aphyric to sparsely phyric cryptocrystalline pillow basalt with glassy chilled margins was recovered from near the top of the section (Unit 1256D-3) as well as two ≥0.3 m thick interval of volcanic breccia composed of angular fragments of cryptocrystalline basalt embedded in a matrix of altered glass (Unit 1256D-4a).

Basalts show a large variation in grain size and textures from holohyaline in the outermost chilled margins of lava flows to coarse subophitic textures in the lava pond. Basaltic lavas are dominantly aphyric to sparsely phyric, but where phenocrysts are present, olivine is the dominant phase, with subordinate plagioclase, minor clinopyroxene, and rare spinel. Measurements of petrographically fresh samples by shipboard inductively coupled plasma–atomic emission spectroscopy (ICP-AES) revealed general downhole variations with Mg# (= 100 × Mg/[Mg + Fe]), Cr, Ni, and Ca/Al ratios broadly increasing with depth, whereas TiO₂, Fe₂O₃, Zr, Y, Nb, V, and Sr broadly decrease with depth, although smaller scale variations are superimposed on these broad trends (Fig. F9). All Leg 206 lavas from Site 1256 plot in the N-MORB field on a Zr-Y-Nb ternary diagram.

In the lavas directly below the large massive flow, a sharp increase in Mg# accompanies an increase in incompatible element concentrations (Fig. F9). The combination of high Mg# and high incompatible element concentrations argues against differentiation as the cause of the enrichments and suggests variation in the primitive magma composition.

Rocks throughout Holes 1256C and 1256D exhibit dark gray background alteration, where the rocks are slightly to moderately altered and olivine is replaced and pore spaces are filled by saponite and minor pyrite as the result of low-temperature seawater interaction at low cumulative seawater/rock ratios. Vein-related alteration is manifest as different-colored alteration halos along veins. Black halos contain celadonite and have been interpreted to result from the presence of upwelling distal low-temperature hydrothermal fluids enriched in iron, silica, and alkalis (Edmond et al., 1979; see summary in Alt, 1999). Iron oxyhydroxide–rich brown mixed halos are later features that formed by circulation of oxidizing seawater. Brown halos have a similar origin and formed along fractures that were not bordered by previously formed black halos. This vein-related alteration occurs irregularly throughout Hole 1256D below the massive Unit 1256D-1 but is concentrated in two zones of greater permeability and, consequently, increased fluid flow at 350–450 and 635–750 mbsf. The appearance of albite and saponite partially replacing plagioclase below 625 mbsf indicates a change in alteration conditions. This change may result in part because of slightly higher temperatures at depth near the lava/dike boundary or from interaction with more evolved fluid compositions (e.g., decreased K/Na and elevated silica).

When compared with other basement sites, Hole 1256D (Fig. F10) contains a much smaller proportion of brown, mixed, and black alteration halos. The abundance of carbonate veins in Hole 1256D is also lower than at many other sites. Site 1256 is, however, quite similar to ODP Site 801 in the western Pacific Ocean, also in crust generated at a fast-spreading ridge, albeit 170 m.y. ago. One important feature is the lack of any oxidation gradient with depth in Hole 1256D, in contrast to the stepwise disappearance of iron oxyhydroxide and celadonite in Hole 504B and the general downhole decrease in seawater effects at Sites 417 and 418. In contrast, alteration appears to have been concentrated in different zones that may be related to the architecture of the basement such as lava morphology, distribution of breccia, and fracturing and the influence of these on porosity and permeability. Clearly, there is greater variation in the processes of alteration occurring in the oceanic crust than is recorded at Sites 504, 417, and 418, which have served as reference sections to date. This illustrates the point that models for the formation and alteration of oceanic basement based on crust formed at slow and intermediate spreading rates cannot automatically be applied to crust generated at fast spreading rates. Deeper penetration of fast-spreading crust is required in order to fully understand alteration processes and geochemical budgets, as well as igneous and structural processes involved in the construction of the crust.

Structures in basement at Site 1256 include both igneous features and postmagmatic deformation. Igneous structures such as flow layering, preferred phenocryst orientation, or fine layering delineated by coalesced spherulites or vesicles are observed near the chilled margins of sheet flows but are best developed near the upper and basal margin of the massive ponded flow (Units 1256C-18 and 1256D-1). The massive ponded flow exhibits other features not observed in the rest of the hole, such as the folding of flow layering and shear-related structures, highlighting the complex internal dynamics occurring during emplacement and cooling of this large igneous body. Folds at the top of the ponded lava have gently dipping axial planes, whereas such features become steeper toward the bottom. Shear indicators such as pull-aparts and tension gashes, now filled with late-stage magma, are more common toward the base of the ponded lava flows.

Brittle deformation is common throughout the upper crust sampled by Holes 1256C and 1256D and includes veins with various morphologies, shear veins, joints, and breccias. There is no systematic variation in the structural attitude (true dip) with depth, and this probably reflects the influence of other factors such as grain size or lava morphology rather than regional tectonics or the local stress field. Aphyric basalt in sheet flows exhibits a more irregular fracture pattern than coarser grained lavas. Shear veins indicate both normal and reversed senses of shear, suggesting the occurrence of some, although probably local, compressional components. Shear veins are most common in the massive ponded lava (Units 1256C-18 and 1256D-1) and in sheet flows from Cores 206-1256D-27R through 43R, where the geometries of the infilling fibers indicate reversed sense of shear. Brecciated rocks of different styles occur throughout the cores but are most common in the sheet flows. Textural features indicate that most breccias formed either by reworking of lava top portions (both chilled and cryptocrystalline basalt) or by the fracturing of rock assisted by relatively high fluid pressures.

Basalt samples from Site 1256 show a strong tendency to have been partially or fully remagnetized during drilling, much more so than for most other DSDP and ODP sites. In most cases, however, a preoverprint component can be discerned, if not always measured accurately, with the shipboard equipment. For Hole 1256D, most samples from igneous Units 3 through 8a and 14 through 26 demagnetize to a shallow inclination, as expected for the equatorial paleolatitude. For Hole 1256C, all samples have steep inclinations and most are dominated by overprint, but a few samples from Units 1256C-3, 7, 18c, 18h, and 22 show evidence for a stable, steep component distinct from the overprint. The steep inclination may reflect eruption during the magnetic polarity transition between Chrons 5Br and 5Bn, which would imply transport of these uppermost lavas at least ~5 km from the ridge axis.

Basement rocks cored during Leg 206 at Site 1256 show little variation in physical properties with depth. The rocks in and above the massive ponded flow (Units 1256C-1 through 18 and 1256D-1; ~276–350 mbsf) have an average bulk density of 2.89 ± 0.03 g/cm³, which is not significantly different from the basalts below when considered together (2.8 ± 0.1 g/cm³). However, there is a significant decrease in average density (2.7 ± 0.1 g/cm³) for the lava flows immediately below Unit 1256D-1 from 350 to 451 mbsf (Units 1256D-2 through 8a). Porosity is low in basement rocks drilled during Leg 206, with an average of 5% ± 3% within a total range from 2% to 19%. The average thermal conductivity of basalts from Hole 1256D is 1.8 ± 0.1 W/(m·K). P-wave velocities (V_P) of discrete basalt samples from Leg 206 vary from 4.2 to 6.2 km/s with an average velocity of 5.4 ± 0.1 km/s. A notable exception to the uniform velocity structure of the upper 500 m of basement is a distinct decrease (and increase in variability) in V_P to 4.8 ± 0.3 km/s immediately below the massive ponded lava (Units 1256D-2 through 4c; 350–400 mbsf). Ponded lavas have a slightly higher discrete sample V_P (5.5 ± 0.1 km/s) than most of the rocks below, which have an average V_P of 5.4 ± 0.1 km/s. Magnetic susceptibility varies from ~0 to 10,000 × 10^–5 SI in the upper 500 m of Hole 1256D basement. Ponded lavas have an average magnetic susceptibility of 5100 × 10^–5 ± 900 × 10^–5 SI, and below 350.3–752 mbsf, magnetic susceptibility values increase systematically with depth, from ~1000 × 10^–5 to 5000 × 10^–5 SI, with an average magnetic susceptibility of 3000 × 10^–5 ± 1800 × 10^–5 SI. Natural gamma ray (NGR) measurements were rarely above background in basement rocks of Leg 206 with the exception of a potassium-rich zone (Unit 1256C-18; 294–308 mbsf) in the massive ponded lava.

A full suite of logging tools was run in Hole 1256D following the completion of coring operations. The tools utilized, in order of deployment, were the triple combo tool string, the FMS-sonic tool string, the BGR gyromagnetometer, the Ultrasonic Borehole Imager (UBI), and the Well Seismic Tool (WST). Logging showed that Hole 1256D was in excellent condition with no constrictions or ledges. Caliper readings from both the triple combo and the FMS-sonic tool strings showed the borehole diameter to be mostly between 11 and 14 inches, with only four short intervals >16 inches. Downhole measurements and images showed large variation, reflecting the massive units, lava flows, pillow lavas, and hyaloclastites recovered in Hole 1256D (Fig. F9).

Predictions of depth to gabbros

Confirmation of an inverse relationship between spreading rate and the depth to axial low-velocity zones imaged by multichannel seismic experiments across the axes of mid-ocean ridges and thought to be axial melt lenses is fundamental to the pursuit of the Superfast Spreading Rate Crust mission. Extrapolation of the depth to the low-velocity zones versus spreading rate relationship (Fig. F3) (Purdy et al., 1992) to a superfast spreading rate akin to that occurring during the accretion of crust at Site 1256 ~15 m.y. ago on the EPR (200–220 mm/y) (Wilson, 1996) indicates that the melt lens would have been located at depths between ~700 and 1000 m beneath the axis. As the new plate cools and moves away from the ridge axis, it will become buried by lavas that flow short distances down the ridge slope (~1–2 km), as well as by larger lava bodies that flow significant distances (~5–10 km) off axis, such as the >74 m thick massive lava pond (Units 1256C-18 and 1256D-1) that formed the uppermost crust at Site 1256 or similar features recognized on the modern EPR (e.g., Macdonald et al., 1989).

In planning Expedition 309/312, the depth to gabbros was estimated by assuming that the total thickness of near-axis and off-axis lavas was between 100 and 300 m, giving a total estimated depth to gabbros of between 825 and 1300 msb (Figs. F3, F11; Table T1). Our selection of 100–300 m of lava flows comes from a number of lines of evidence. Multichannel seismic experiments on the EPR estimate Layer 2A thicknesses of ~300 m in the near-axis region (Hooft et al., 1996; Carbotte et al., 1997a), although the geological nature of this seismic layer remains poorly understood.

Stronger evidence comes from petrologic descriptions of rocks from the uppermost basement at Site 1256, drilled during Leg 206. The very uppermost basement, designated the lava pond (Units 1256C-1 through 18 and 1256D-1; 250–350.3 mbsf) comprises thin basaltic sheet flows a few tens of centimeters to ~3 m thick (Units 1256C-1 through 17) overlying a massive ponded flow (Units 1256C-18 and 1256D-1) of ~30 to ~74 m of fine-grained basalt in Holes 1256C and 1256D, respectively (Wilson, Teagle, Acton, et al., 2003). Although the massive flow is much thicker in Hole 1256D than it is in Hole 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 maximum thickness >74 m requires at least this much paleotopography in order to pool the lava. On fast spreading rate ridges, such topography does not normally develop until ~5–10 km from the axis (e.g., Macdonald et al., 1989, 1996).

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 critical criteria for this subdivision is the occurrence of subvertical elongate fractures filled with quenched glass and hyaloclastite (e.g., Sections 206-1256D-21R-1 and 206-1256D-40R-1) at the top of the lava flows. These features indicate flow-lobe inflation, which requires eruption onto a subhorizontal surface with less than a few degrees slope (Umino et al., 2000, 2002), and it is therefore unlikely that such lavas formed directly at the ridge axis. Such inflation features are not observed in deeper cores in Hole 1256D. Taken together, the cumulative thickness of the lava pond and the inflated flows is ~280 m, close to our preferred estimate for the thickness of off-axis lava flows.

Seismic data from site survey Cruise EW9903 offer significant clues about the expected downhole lithologic variations. The velocity-depth function inferred from seismic refraction (Fig. F7) (A.J. Harding, unpubl. data) shows a uniform gradient from ~4.8 km/s at the sediment/basement interface to 5.3 km/s at ~600 msb, with the gradient then sharply increasing and velocity reaching 5.9 km/s at ~800 msb. The gradient abruptly returns to a moderate value, with velocity of 6.5 km/s at ~1250 msb. The gradient decreases gradually, with nearly uniform velocity near 6.8 km/s between 2000 and 3000 msb. Multichannel seismic reflection data (Fig. F6) (Hallenborg et al., 2003) show several nearly horizontal reflections with kilometers of horizontal extent at nearly 5.5 s traveltime, or roughly 800–900 msb. For the upper ~800 m, the relatively lower velocities and horizontal reflection character suggest that flows constitute a substantial fraction of the uppermost crust at Site 1256. At greater depth, the decrease in velocity gradient below ~1250 msb marks the seismic Layer 2/3 boundary. Unfortunately, the gradual nature of the change in gradient means that the depth of this transition cannot be assigned a depth more precise than 1250–1500 msb. Whether this boundary corresponds to the presence of gabbro remains to be tested. Figure F11 shows that predicted depths of penetration during Expedition 309/312 should allow testing of whether gabbro is present at these depths.

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