IODP

doi:10.2204/iodp.pr.335.2011

Background and geological setting

Drilling a deep hole through intact ocean crust formed at a fast spreading rate has been one of the prime motivations for scientific ocean drilling since its inception (Teagle and Ildefonse, 2011). Fast-spreading ocean crust is targeted because geological and geophysical observations indicate that long distances of the ridge crests behave relatively uniformly. Consequently, we should be able to extrapolate the findings from a few deep penetrations to describe a significant portion of the Earth’s surface (~30%). Only through the recovery of a significant section of cumulate gabbro underlying the dikes and erupted lavas will we be able to test competing models of magmatic accretion at fast-spreading mid-ocean ridges and evaluate the impact of these processes on the wider Earth system.

Integrated Ocean Drilling Program (IODP) Expedition 335 (13 April–3 June 2011) is the fourth scientific drilling cruise of the Superfast Spreading Crust campaign (Ocean Drilling Program [ODP] Proposal 522Full-2; D.S. Wilson et al., unpubl. data) to ODP Hole 1256D (644.163′N, 9156.061′W) to deepen this ocean crust reference penetration a significant distance into cumulate gabbros. Hole 1256D is located on 15 Ma crust in the eastern equatorial Pacific Ocean in oceanic basement that formed during a sustained episode of superfast ocean ridge spreading (>200 mm/y) (Wilson, 1996) (Figs. F1, F2). Ocean crust formed at a superfast spreading rate was deliberately targeted because there is strong evidence from mid-ocean-ridge seismic experiments that gabbros occur at shallower depths in intact ocean crust with higher spreading rates. Consequently, the often difficult-to-drill upper ocean crust should be relatively thin. Expedition 335 follows ODP Leg 206 in 2002 and IODP Expedition 309/312 in 2005 (Wilson, Teagle, Acton, et al., 2003; Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, et al., 2006), which prepared the first scientific borehole for deep drilling by installing a large reentry cone secured with almost 270 m of 16 inch casing through the 250 m thick sedimentary overburden and cemented into the uppermost basement (Fig. F2). Hole 1256D was then deepened through an ~810 m thick sequence of lavas and a thin (~346 m) sheeted dike complex, the lower 60 m of which is strongly contact metamorphosed to granoblastic textures. The first gabbroic rocks were encountered at 1407 meters below seafloor (mbsf), where the hole entered a complex dike–gabbro transition zone that includes two 20 to 50 m thick gabbro lenses intruded into granoblastic dikes. As of the end of Expedition 312, Hole 1256D had a total depth of 1507.1 mbsf and was open to its full depth. Hole 1256D is poised at a depth where, with a few hundred meters of penetration, cumulate gabbros should be recovered for the first time from in situ lower ocean crust. These will reveal hitherto unavailable fundamental observations regarding the processes that form new crust at the mid-ocean ridges and the chemical exchanges between the crust, oceans, and mantle.

The scientific justifications for deep drilling of fast-spreading ocean crust will be further explored in the Expedition reports section of the Expedition 335 volume. The specific tests of ocean crust accretion models planned for Expedition 335 are described below.

Rationale for the Superfast Spreading Crust campaign and location of Site 1256

The key to proposing the Superfast Spreading Crust campaign (Leg 206 and Expeditions 309/312 and 335) 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 (ODP Proposal 522Full-2; D.S. Wilson et al., unpubl. data). Drilling lavas and dikes has proved problematic in past scientific ocean drilling, resulting in poor hole conditions, slow penetration rates, and low core recovery (Figs. F3, F4). The recognition that crust formed at a relatively fast spreading rate is a compelling target for deep drilling follows the observation that there is 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. F5). Since the Purdy et al. (1992) compilation, careful velocity analysis, summarized by Hooft et al. (1996), has refined the conversion from traveltime to depth, and data from additional sites have been collected (Carbotte et al., 1997). The fastest rate spreading centers surveyed with modern multichannel seismic reflection, ~140 mm/y full rate at 14–18S on the East Pacific Rise (EPR), show reflectors, interpreted as the axial melt lens, at depths from 940 to 1260 mbsf (Detrick et al., 1993; Kent et al., 1994; Hooft et al., 1994, 1996). At 9–16N on the EPR where spreading rates are 80–110 mm/y, depths to the melt reflector are mostly 1350–1650 mbsf, where well determined (Kent et al., 1994; Hooft et al., 1996; Carbotte et al., 1997). The implication from reflection seismic studies of axial low-velocity zones is that crust formed at superfast spreading rates (>200 mm/y) should have a thin dike section and cumulate gabbros should occur at relatively shallow depths.

The theoretical basis for expecting an inverse relation between spreading rate and melt lens depth is fairly straightforward if one considers a gabbro glacier style of crustal formation (e.g., Henstock et al., 1993; Phipps Morgan and Chen, 1993; Quick and Denlinger, 1993). The latent heat released in crystallizing the gabbroic crust must be conducted through the lid of the melt lens to the base of the axial hydrothermal system, which then advects the heat to the ocean. The temperature contrast across the lid is governed by the properties of magma (1100–1200C) and thermodynamic properties of seawater (350–450C where circulating in large volumes) and will vary only slightly with spreading rate and ridge depth (e.g., Jupp and Schultz, 2000). The heat flux through the lid per unit ridge length will therefore be proportional to the width of the lens and inversely proportional to the lid thickness. For reasons that are not understood, seismic observations show uniform width of the melt lens, independent of spreading rate. With width and temperature contrast not varying, the extra heat supplied by more magma at faster spreading rates must be conducted through a thinner lid (dike layer) to maintain steady state (see Phipps Morgan and Chen [1993] for a more complete discussion). This analysis leads to the prediction that to reach cumulate gabbros in normal oceanic crust with minimal drilling, it is therefore best to target crust formed at the fastest possible spreading rates (Wilson et al., 2006).

A setting similar to the modern well-surveyed area at 14–18S could be expected to reach gabbro at a depth of ~1400 m, based on 1100 m to the axial magma chamber reflector and subsequent burial by an additional 300 m of extrusives (Kent et al., 1994). At faster rates, depths could be hundreds of meters shallower. In contrast, seismic velocity inversions at the axes of the Juan de Fuca Ridge and Valu Fa Ridge, Lau Basin, are at depths of ~3 km (Purdy et al., 1992), at intermediate spreading rates comparable to Deep Sea Drilling Project (DSDP)/ODP Site 504.

Superfast spreading rate crust in the eastern equatorial Pacific Ocean

The identification of magnetic anomalies formed at the southern end of the Pacific/Cocos plate boundary led to the recognition of crust formed at full spreading rates of ~200–220 mm/y from ~20 to 11 Ma (Wilson, 1996) (Fig. F1), 30% to 40% faster than the fastest modern spreading rate. This episode of superfast spreading ended with a reorganization of plate motions at 10–11 Ma. The southern limit of crust formed at the superfast rates is the trace of the Cocos-Nazca-Pacific triple junction, as Nazca-Pacific and Cocos-Nazca spreading rates were not as fast. The older age limit of this spreading episode is hard to determine with the limited mapping and poor magnetic geometry of the Pacific plate, but it is at least 18 Ma. The northern limit of this province is entirely gradational, with rates dropping to ~150 mm/y somewhat north of the Clipperton Fracture Zone.

The estimated depth to an axial melt lens for ocean crust formed at such a superfast spreading rate is ~800–1000 m. The anticipated depth to the gabbros for Site 1256 is ~1100–1300 m, allowing for a reasonable thickness (~300 m) of near-axial lava flows (e.g., Hooft et al., 1996) (Table T1; see “Headline results from previous drilling at Site 1256: ODP Leg 206 and IODP Expedition 309/312”), making this region a compelling target for deep drilling to help us understand crustal accretion processes at mid-ocean ridges.

Geological setting of Site 1256

Site 1256 lies in 3635 m of water in the Guatemala Basin (644.2′N; 9156.1′W) on Cocos plate crust formed at ~15 Ma on the eastern flank of the EPR (Fig. F1). 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 and is centered in the zone that accreted at a superfast spreading rate (~200–220 mm/y full rate) (Wilson, 1996). The site lies ~1150 km east of the present crest of the EPR and ~530 km north of the Cocos-Nazca spreading center. The trace of the Cocos-Nazca-Pacific triple junction passes ~100 km to the southeast, with the elevated bathymetry of the Cocos Ridge recording the trail of the Galapagos plume, farther to the southeast (~500 km). 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. F6). This location was formed near the Equator within the equatorial high-productivity zone and initially endured high sedimentation rates (>30 m/m.y.) (e.g., Farrell et al., 1995; Wilson, Teagle, Acton, et al., 2003). Sediment thickness in the region is between 200 and 300 m and is 250 m at Site 1256 (Wilson, Teagle, Acton, et al., 2003).

The Guatemala Basin has relatively subdued bathymetry, and the immediate surroundings of Site 1256 (~300 km) are relatively unblemished by major seamount chains or large tectonic features high enough to penetrate the sediment cover (~200–300 m). Site 1256 was one of a number of sites approved by the ODP Site Survey panel (as Site GUATB-03C) (Fig. F7) for operations during Leg 206, and this specific location was selected from the regions surveyed to take advantage of faster upper crustal velocities, which correctly predicted the presence of massive basalt flows, and to avoid thrust faults that occur elsewhere in the region. The details of the 1999 Maurice Ewing site survey cruise operations are documented in Wilson, Teagle, Acton, et al. (2003) and Wilson et al. (2003).

Site 1256 has a seismic structure reminiscent of typical Pacific off-axis seafloor (Fig. F8). Upper Layer 2 velocities are 4.5–5 km/s, and the Layer 2–3 transition is between ~1200 and 1500 m subbasement (msb) (Fig. F9). The total crustal thickness at Site 1256 is estimated at ~5–5.5 km.

Site 1256 sits atop a region of smooth seafloor and basement topography (<10 m relief) (Fig. F7). Northeast of Site 1256 (15–20 km), a trail of ~500 m high circular seamounts rises a few hundred meters above the sediment blanket. Using the site survey multichannel seismic data (Wilson et al., 2003), we have constructed a geological sketch map of the uppermost basement in the GUATB-03 survey region (Fig. F10). Elsewhere in the region, 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 GUATB-03 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 (OBH) 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 velocity variations (Fig. F10A). Two principal features are apparent: a 5 to 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 EW9903-25.

The uppermost basement at Site 1256 is capped by a massive lava flow >74 m thick (Fig. F2). This flow is relatively unfractured, with shipboard physical property measurements on discrete samples indicating VP > 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 in excess of 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) make up most of the lava stratigraphy at Site 1256 (Fig. F2) (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, et al., 2006), 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). Subordinate pillow lavas are present in Hole 1256D (e.g., Tominaga et al., 2009), and because of the large number of fractures and pillow interstices, seismic velocities in these units 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.

At 15 Ma, Site 1256 is significantly older than Hole 504B (~6.9 Ma), and lower temperatures are predicted at mid-levels in the crust (~115C at 1500 msb), so high borehole temperatures should not become an issue until a significant thickness of gabbros is penetrated (Fig. F11). Heat flow (~113 mW/m2 in Hole 1256C) is close to that predicted from cooling plate models, indicating minimal active hydrothermal circulation, as was confirmed by diffusive pore water profiles in the Site 1256 sedimentary overburden (Wilson, Teagle, Acton, et al., 2003).

The only serious drawback to the GUATB-03/Site 1256 area as a crustal reference section for fast spreading rates is its low original latitude. This means that the determination of magnetic polarity from azimuthally unoriented core samples is nearly impossible. In addition, the nearly north–south ridge orientation makes the magnetic inclination insensitive to structural tilting. Magnetic logging with either the General Purpose Inclinometry Tool (GPIT) fluxgates that form part of the Formation MicroScanner/Dipole Sonic Imager (FMS-sonic) tool string (or a separate magnetic tool with gyroscopic orientation) should be adequate for polarity determination and reorientation of recovered cores to the geographic reference frame.

Headline results from previous drilling at Site 1256: ODP Leg 206 and IODP Expedition 309/312

Hole 1256D in the eastern equatorial Pacific Ocean 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 preliminary coring to document the sedimentary overburden at Site 1256, operations during Leg 206 installed in Hole 1256D 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). Armoring the sediment/basement boundary reduces erosion of the borehole walls at this weak point and assists in clearing drill cuttings from the hole. The cone and casing facilitate multiple reentries and help maintain hole stability. The large-diameter casing was deployed to protect against the situation where one or two more casing strings (13⅜ and 10 ¾ inch) would be needed to stabilize the uppermost ~100 m of oceanic basement during the initiation of Hole 1256D. It also allows the possibility that future expeditions could insert further casing into the hole to isolate unstable portions of the very uppermost basement or improve the borehole hydrodynamics for clearing cuttings from the hole (see “Operations”). Because of the occurrence of a very thick massive lava flow at the top of basement, casing was not necessary for securing the uppermost portion of the hole.

The volcanic sequences at Site 1256 comprise a sequence of massive flows and thinner sheet flows with subordinate pillow basalt, hyaloclastite, and breccia (Fig. F12). The uppermost crust at Site 1256 comprises a ~100 m thick sequence of lava dominated by a single massive lava flow >75 m thick, requiring at least this much seafloor relief to pond the lava. On modern fast-spreading ridges, such topography does not normally develop until 5–10 km from the axis (Macdonald et al., 1996). Although this lava flow cooled off-axis it may have originated at the ridge axis before flowing on to the ridge flanks, as is observed for very large lava flows on the modern ocean floor (Macdonald et al., 1989). The lava sequence immediately below includes sheet and massive flows and minor pillow flows. Subvertical, elongate flow-top fractures filled with quenched glass and hyaloclastite in these lavas indicate flow lobe inflation requiring cooling on a subhorizontal surface off-axis (Umino et al., 2000). From this shipboard evidence, a total thickness of 284 m of lava that flowed and cooled off-axis was estimated (Table T1). Sheet flows and massive lava flows erupted at the ridge axis make up the remaining extrusive section to 1004 mbsf, before a lithologic transition is marked by subvertical intrusive contacts and mineralized breccia. This contrasts slightly with the volcanic stratigraphy for Hole 1256D developed from analysis of wireline geophysical imaging (Tominaga et al., 2009; Tominaga and Umino, 2010) that suggests <50% of the lava drilled crystallized within 1000 m of the axis but that the majority of the lava pile had formed within 3000 m of the ridge crest.

Rocks throughout pilot Hole 1256C and the uppermost parts of Hole 1256D exhibit dark gray background alteration where the rocks are slightly to moderately altered, olivine is replaced, and pore spaces are filled by saponite and minor pyrite as the result of low-temperature seawater interaction at relatively 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 and Teagle, 1999). The iron oxyhydroxide–rich brown mixed halos are later features that formed by circulation of oxidizing seawater. The 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 1 (lava pond) but is concentrated in zones of greater permeability and, consequently, increased fluid flow. 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 as the lava/dike boundary is approached or from interaction with more evolved fluid compositions (e.g., decreased K/Na and elevated silica). Black, brown, and mixed halos are absent in lowermost lavas (>900 mbsf), and dark gray background alteration related to abundant saponite and pyrite is ubiquitous. Vertical vein sets become more common below ~900 mbsf.

When compared with other basement sites (e.g., DSDP/ODP Sites 417 and 418 and Holes 504B and 896A), Hole 1256D contains a much smaller number of brown, mixed, and black alteration halos (Wilson, Teagle, Acton, et al., 2003; Alt, 2004). The abundance of carbonate veins in Hole 1256D is also lower than at many other sites (Coggon et al., 2010). Site 1256 is, however, quite similar to ODP Site 801, also in crust generated at a fast-spreading ridge, albeit 170 m.y. ago (Alt and Teagle, 2003). 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 (Wilson, Teagle, Acton, et al., 2003; Alt, 2004). In contrast, alteration appears to have been concentrated into 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.

Below 1061 mbsf, subvertical intrusive contacts are numerous, indicating the start of a relatively thin, ~350 m thick sheeted dike complex dominated by massive basalt. Some basalts have doleritic textures, and many are crosscut by subvertical dikes with common strongly brecciated and mineralized chilled margins. There is no evidence from core or geophysical wireline logs for significant tilting of the dikes, consistent with subhorizontal seismic reflectors in the lower extrusive rocks that are continuous for several kilometers across the site (Hallenborg et al., 2003). Measurements of dike orientations by wireline imaging are in close agreement with direct measurements on recovered cores and indicate that the dikes at Site 1256 are slightly tilted away from the paleospreading axis (dip/dip direction = 79 ± 8/053 ± 23) (Tominaga et al., 2009).

The secondary mineralogy of the rocks indicates a stepwise increase in alteration grade downhole from in the lavas to the dikes, with low-temperature phases (<150C phyllosilicates and iron oxyhydroxides) in the lavas giving way to dikes partially altered to chlorite and other greenschist facies minerals (at temperatures greater than ~250C) (Fig. F12) (Alt et al., 2010). Within the dikes, alteration intensity and grade increase downward, with actinolite more abundant than chlorite below 1300 mbsf and hornblende present below 1350 mbsf, indicating temperatures approaching ~400C (Alt et al., 2010). The dikes have significantly lower porosity (mostly 0.5%–2%) and higher P-wave velocities and thermal conductivity than the lavas. Porosity decreases and P-wave velocity increases with depth in the dikes.

In the bottommost ~60 m of the sheeted dikes (1348–1407 mbsf), basalts are partially to completely recrystallized to distinctive granoblastic textures characterized by granular secondary clinopyroxene and lesser orthopyroxene resulting from contact metamorphism by underlying gabbroic intrusions (Figs. F12, F13). Textural changes in oxide minerals indicate that the zone of metamorphic recrystallization may extend for >90 m above the upper contact with the first gabbro interval. There is unequivocal evidence for hydrothermal alteration before and after the formation of the granoblastic textures and potentially small, irregular patches of partial melting (Koepke et al., 2008; France et al., 2009; Alt et al., 2010). Simple thermal calculations suggest that the two gabbro bodies so far intersected in Hole 1256D have insufficient thermal mass to be responsible for a 60 to 90 m thick high-temperature (600–900C) metamorphic halo if the intrusions are simple subhorizontal bodies (Koepke et al., 2008; Coggon et al., 2008). This feature requires either a much more substantial intrusion nearby or significant topography on the dike/gabbro boundary.

Aside from the granoblastic contact metamorphic assemblages in the basal dikes, hydrothermal mineralogy and inferred alteration temperatures of the lower dikes in Hole 1256D are generally similar to those in the lower dikes of Hole 504B (as high as ~400C). The fact that the dike section at Site 1256 is much thinner than the one at Site 504 (~350 versus ~1000 m), however, indicates a much steeper hydrothermal temperature gradient at Site 1256 (~0.5C/m versus 0.16C/m in Hole 504B).

Gabbro and trondjhemite intrusions into sheeted dikes at 1407 mbsf mark the top of the plutonic section (Figs. F12, F13). Two major bodies of gabbro were penetrated beneath this contact, with the 52 m thick upper gabbro (Gabbro 1) separated from the 24 m thick lower gabbro (Gabbro 2) by a 24 m screen of granoblastic basalts (Dike screen 1). The textures and rock types observed in Hole 1256D are reminiscent of varitextured gabbros thought to represent a frozen melt lens between the sheeted dike complex and above the cumulate rocks in many ophiolites (e.g., MacLeod and Yaouancq, 2000; France et al., 2009).

Gabbro 1 is mineralogically and texturally heterogeneous, comprising gabbros, oxide gabbros, quartz-rich oxide diorites, and small trondjhemite dikelets. Oxide abundance decreases irregularly downhole, and olivine is present in significant amounts only in the lower part of Gabbro 1. These rocks are moderately to highly altered by hydrothermal fluids to actinolite, hornblende, secondary plagioclase, epidote, chlorite, prehnite, and laumontite.

The intervening Dike screen 1 is interpreted as an interval of sheeted dikes captured between the two intrusions of gabbros. Dike screen 1 consists of fine-grained metabasalt similar to the granoblastic dikes overlying Gabbro 1. Dike screen 1 is cut by a number of small quartz gabbro and tonalite dikelets of variable thickness (1–10 cm), grain size, and composition.

Gabbro 2 comprises gabbro, oxide gabbro, and subordinate orthopyroxene-bearing gabbro and trondjhemite that are similarly altered to Gabbro 1 and has clear intrusive contacts with the overlying granoblastic Dike screen 1. Partially resorbed stoped dike clasts are entrained within both the upper and lower margins of Gabbro 2 (Fig. F13G). Gabbro 2 is characterized by an absence of fresh olivine, high but variable orthopyroxene contents (5%–25%), and considerable local heterogeneity. Oxide abundance generally diminishes downhole. The predominant rock type is orthopyroxene-bearing gabbro with gabbronorite in the marginal units. The lowermost rock recovered from Hole 1256D during Expedition 312 was a highly altered actinolite-bearing basaltic dike that lacks granoblastic textures; hence, it is interpreted to be a late dike that postdates the intrusion of the lower gabbro.

Relative to other well-studied upper ocean crust sections (e.g., Karson, 2002), Site 1256 shows a thick lava sequence and a thin dike sequence. Steady-state thermal models require that the conductive lid separating magma from rapidly circulating seawater thins as spreading rate increases, indicating that the thin dike sequence is a direct consequence of the high spreading rate. A thick flow sequence with many massive individual flows and few pillow lavas is a reasonable consequence of short vertical transport distance from the magma chamber and is similar to observations from the mid-segments of the fastest spreading ridges in the modern ocean (White et al., 2002). This is in direct contrast to spreading models developed from observations of tectonically disrupted fast-spread crust exposed in Hess Deep (Karson et al., 2002) that suggest regions of high magma supply should have thin lavas and thick dikes. Similarly, there is little evidence for tilting (at most a few degrees) in Hole 1256D and no evidence for significant faulting. In contrast, the upper crust exposed at Hess Deep shows significant faulting and rotation within the dike complex (Karson et al., 2002), indicating that observations from that tectonic window may be site specific and not widely applicable to intact ocean crust. The massive lava flow at the top of the Site 1256 basement indicates that faults with ~50–100 m throws must exist relatively near to the ridge axis even in superfast spreading rate crust to provide the necessary relief to pond of the lava. At fast-spreading ridges, such relief is typically developed ~5 to 10 km from the axis.

Marine seismologists have long been dividing the ocean crust into seismic layers: Layer 1 comprises low-velocity sediments (VP < 3 km/s); Layer 2 has low velocity and a high velocity gradient, with VP typically ranging from ~3.5 to ~6.7 km/s; whereas Layer 3 has high velocity and a low velocity gradient (VP ranges from 6.7 to 7.1 km/s). There is a widespread perception that Layer 3 is equivalent to gabbro, even though Hole 504B has penetrated Layer 3 but not gabbro (Alt et al., 1996; Detrick et al., 1994). From regional seismic refraction data, the transition from seismic Layer 2 to Layer 3 at Site 1256 occurs at ~1450–1750 mbsf (1200–1500 msb) (Wilson et al., 2003) (Figs. F9, F12). Shipboard determinations of seismic velocities of discrete samples are in close agreement with in situ measurements by wireline tools to ~1320 mbsf, above the granoblastic dikes interval; below that depth, velocities are significantly lower than the sonic log, and the gabbro velocities range between ~5.3 and 6.4 km/s (Swift et al., 2008). Contrary to expectation, porosity increases and P-wave velocities decrease stepwise downward from the lowermost dikes into the uppermost gabbro in Hole 1256D, as the result of the contact metamorphism of the granoblastic dikes and the strong hydrothermal alteration of the uppermost gabbros (Fig. F12). Porosity and velocity then increase downhole in the gabbro but are still <6.5 km/s. Similar trends for porosity and velocity were observed in postcruise laboratory measurements at ambient pressure, albeit on a small suite of samples (Violay et al., 2010). However, the compressional velocities on these samples were generally ~800 m/s higher in the gabbroic section, with VP averaging ~6.4 km/s from the middle part of gabbro to the bottom of Gabbro 2. This difference may arise from the incomplete saturation of the samples during shipboard measurements. Wireline velocity measurements end at the top of gabbro, but we interpret the gabbro intervals as within Layer 2 because a smoothed extrapolation of the downhole velocities will either have velocities <6.5 km/s, still characteristic of Layer 2, or will have an exceptionally high gradient to higher velocities, also characteristic of Layer 2. Encountering gabbro at a depth clearly within Layer 2 reinforces previous inferences that factors including porosity and alteration (Detrick et al., 1994; Alt et al., 1996; Carlson, 2010) are more important than rock type or grain size in controlling the location of the Layer 2–3 transition. The position of the dike/gabbro boundary therefore appears to have little control over the seismic velocity structure of the crust (Alt et al., 1996; Detrick et al., 1994). Unfortunately, because the transition from Layer 2 to Layer 3 most likely lies beneath Hole 1256D, we cannot yet determine what controls this transition at Site 1256.

Summary of whole-rock igneous geochemistry from Site 1256

Flows and dikes from Hole 1256D show a wide range of magmatic fractionation, from fairly primitive to evolved (Figs. F12, F14). Shallower than 600 mbsf, magma compositions are bimodal, with relatively evolved thick flows and more primitive thin flows. The lava pond includes rocks with the highest incompatible element compositions (Zr, TiO2, Y, and V) and lowest compatible element concentration (Cr and Ni), suggesting that it is more evolved than other basalt from Site 1256 (Fig. F15). The initial division (based on Leg 206 lavas only) into high Zr-TiO2, low Zr-TiO2, and high Zr disappears with more data from Expedition 309/312. Rare samples from the lava pond fall off the dominant Y versus Zr and TiO2 versus Zr trends, suggesting possible minor variation in source compositions.

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). Downhole geochemical compositions within the dike section are variable and do not define trends. Primary and evolved compositions are closely juxtaposed, as would be expected for vertically intruded magmas. The range of major element compositions in the dikes is similar to that of the overlying rocks, and the average composition of the dikes is indistinguishable from the average composition of the lavas (Figs. F12, F14).

The gabbros have highly variable bulk compositions. The uppermost gabbros have geochemical characteristics similar to the overlying dikes with mid-ocean-ridge basalt (MORB) chemistries (MgO ~7–8 wt%; Zr ~47–65 ppm). Conversely, deeper in Gabbro 2 the rocks are significantly less fractionated and there are general downhole trends of increasing MgO, CaO, and Ni and decreasing FeO, Zr, and Y. The uppermost rocks of Gabbro 2 are fractionated with MgO contents of 6.1 wt%, but lower in the sequence MgO reaches 9.3 wt%. Decreasing concentrations of FeO and TiO2 downhole suggest that Gabbro 2 is fractionated similarly to Gabbro 1. The intrusive nature of both gabbro bodies and the chemical variations within them suggest that they intruded into the base of the sheeted dikes and underwent minor internal fractionation in situ resulting in the observed general geochemical stratification. Partially resorbed xenoliths of granoblastic dikes within Gabbro 2 indicate that stoping of the intruded dikes may have contaminated the gabbro compositions.

There are linear trends between MgO versus TiO2, FeO, CaO, Na2O, and Zr, most likely resulting from fractional crystallization of a gabbro, with the fractionating assemblage consisting of clinopyroxene and plagioclase, as expected for relatively evolved basaltic magmas. The predicted composition of the cumulate gabbros generated during this fractional crystallization is very different from that of the gabbros drilled so far. Simple mass balance calculations indicate that the average basalt has lost >30% of its original liquid mass as solid gabbro, implying the presence of at least 300 m of cumulate gabbro in the crust below the present base of the hole (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, et al., 2006).

The gabbro compositions span a range similar to the flows and dikes but are on average more primitive, with higher MgO and lower FeO, albeit still within the range of EPR basalts (Figs. F14, F15). Even though it is less fractionated, the average gabbro composition is evolved relative to candidates for primary magma in equilibrium with mantle olivine. Possible primary mantle melt compositions should have Mg# of 70–78 and MgO of 9–14 wt%. All flows and dikes and most gabbros are too evolved to be candidates for primary magma. Therefore, the residue removed from primary magma to produce the observed gabbro and basalt compositions must occur below the present base of Hole 1256D.

The compositional ranges of fresh lava and dike samples correspond to typical values for MORB for most major elements and many trace elements (Su and Langmuir, 2003) and are similar to those observed for the northern EPR (Fig. F15). A few incompatible elements, including Na and Zr, have lower concentrations than observed for the modern EPR lavas, but the generally substantial overlap of compositions indicates similar process operated at the superfast spreading ridge that formed Site 1256 and the modern EPR. When trace elements are compared to EPR MORB, they are within one standard deviation of average, albeit on the relatively trace element–depleted side of MORB. For example, compared with first-order mid-ocean-ridge segments along the EPR, basalts from Site 1256 have low Zr/TiO2 and Zr/Y. Although there is overlap among the segments and a large scatter in the data for each segment, Zr/TiO2 and Zr/Y appear to decrease with increasing spreading rate (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, et al., 2006). The origin of this relationship remains unclear. Spreading rate may affect the extents of magma fractionation or partial melting of the mantle, or it may instead reflect regional scale mantle heterogeneity. To decrease trace element ratios to this extent would require ~30% more melting at the superfast ridge, but this appears unlikely because of normal crustal thickness (~5.5 km) at Site 1256 (Park et al., 2008).

The simplest model for mid-ocean-ridge magma plumbing is that the melt lens imaged by multichannel seismic (MCS) experiments is the magma chamber where crystal-rich residues are separated from the evolved lavas that reach the seafloor (Fig. F16). Hole 1256D gabbros are texturally and compositionally similar to varitextured gabbros at the base of the sheeted dike complex in Oman interpreted to be axial melt lenses (MacLeod and Yaouancq, 2000; France et al., 2009). If the gabbro bodies so far encountered in Hole 1256D were intruded on-axis, and if they extended roughly horizontally for at least hundreds of meters, at 52 and 24 m thick they would have dimensions appropriate for axial low-velocity zones imaged by MCS experiments at intermediate to fast-spreading ridges (Singh et al., 1998). However, these bodies could not have been the site of primary magmatic fractionation. Chilled margins against the underlying dike screens precludes segregating a crystal residue that subsides to form the lower crust as in the gabbro glacier model, and its overall fractionated composition requires that crystals have been segregated elsewhere. This implies that sills or other bodies containing cumulate materials must exist deeper in the crust and/or below the crust/mantle boundary, consistent with recent models based on lower crustal sections of ophiolites (e.g., Boudier et al., 1996; Kelemen et al., 1997; MacLeod and Yaouancq, 2000) and some marine geophysical experiments (Crawford and Webb, 2002; Dunn et al., 2001; Garmany, 1989; Nedimovic et al., 2005; Canales et al., 2009). However, the gabbro glacier mode of accretion or a combination of the gabbro glacier and sheeted sills models cannot yet be rejected, as fractionated gabbros in the dike–gabbro transition zone are not unexpected, and the predicted region of cumulate rocks could still exist just below the present maximum depth of Hole 1256D. Deepening Hole 1256D by as little as a few hundred meters would provide the critical samples that enable answering this outstanding basic question.