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doi:10.2204/iodp.proc.309312.103.2006

Conclusions

Drilling in Hole 1256D during three scientific ocean drilling cruises, ODP Leg 206 (Wilson, Teagle, Acton, et al., 2003) and Expeditions 309 and 312, completed the first sampling of an intact section of ocean crust from lavas, through the sheeted dike complex, and into the uppermost gabbros of the plutonic section. Expedition 312 was the last cruise of IODP Phase 1 and the final scientific drilling voyage of the JOIDES Resolution before she undergoes major refit and renaming. The successful accomplishment of the longstanding scientific ocean drilling ambition of coring to gabbros is a fitting finale to the JOIDES Resolution’s achievements and IODP Phase 1.

Expedition 309/312 deepened Hole 1256D by 755.1 m to 1507.1 mbsf. This reference site now penetrates through 250 m of Miocene to Holocene sediments and 754 m of extrusive rocks comprising 284 m of lavas that solidified off-axis, including the >74 m thick lava pond, underlain by 534 m of axial sheet and massive lava flows. A 57 m thick lithologic transition zone divides the lavas from a thin, 346 m thick sheeted dike complex, the lower 58 m of which exhibits granoblastic textures resulting from contact metamorphism. Similar metamorphism has been described locally in ophiolites but never before from the seafloor. The uppermost gabbroic rocks encountered in Hole 1256D intrude the base of the sheeted dike complex at 1406.6 mbsf along a gently to moderately dipping boundary. This melt body has an apparent thickness of ~52 m downhole and on average a fractionated composition that precludes direct formation from a mantle-derived magma. A 24 m thick screen of granoblastically recrystallized dikes separates the upper gabbro body (Gabbro 1) from a second complex zone of plutonic rocks (Gabbro 2) that intrudes the dike screen at 1483 mbsf and continues to the current bottom of Hole 1256D. The lowermost rock sampled during Expedition 312 is a greenschist-facies dike without a granoblastic texture that must postdate the intrusion of the gabbros.

Drilling this first section to gabbros is a major engineering achievement and fulfills the ambitions of marine geologists and geophysicists since the inception of scientific ocean drilling in 1968. However, with only ~100 m penetration into a complicated dike/​gabbro boundary zone, many questions regarding the accretion of the ocean crust still remain. With the hole open and clear of debris, our understanding of mid-ocean-ridge processes could be greatly improved by further deepening of Hole 1256D. Here we highlight possible future advances that could be achieved with a timely return to Site 1256.

The three-dimensional geometry of the gabbroic bodies is difficult to ascertain from a single ~6 cm wide drill core, but if the bodies have significant lateral continuity (~1000 m), at 52 and 24 m thick the gabbro bodies intersected by Hole 1256D would have dimensions appropriate for an axial low-velocity zone imaged by multichannel seismic experiments at intermediate to fast spreading ridges. Cumulate gabbros have yet to be encountered. High-level mafic cumulate rocks that balance the fractionated compositions of the dikes and lavas are a predicted consequence of a “gabbro glacier” mode of accretion, and cumulates should occur within a few hundred meters of the dike/​gabbro boundary and the current bottom of the hole. The absence of such rocks would support a sheeted sill–type mode of accretion with mafic cumulates occurring toward the base of the crust (>5 km). Interestingly, both gabbro bodies (Gabbros 1 and 2) are intrusive and, therefore, relatively late (but probably still axial) in the accretion sequence, and the intrusive boundaries appear to have gentle to moderate dips (25°–45°). Without cumulate rocks or thick sequences of gabbroic rocks at Site 1256, we are not yet in a position to test models of crustal accretion at mid-ocean ridges, but the key samples may be nearby.

Understanding the seismic structure of the ocean crust requires calibration of remotely obtained regional geophysical data against measurements of physical properties and petrology of geological samples recovered from within deep ocean boreholes.  Hole 504B remains the only site whether the seismic Layer 2/3 boundary has been penetrated (e.g., Detrick et al., 1994). At that location, the change in seismic gradient clearly occurs within the dikes and the Layer 2/3 transition most probably reflects changes in bulk physical properties associated with an increase in hydrothermal alteration grade from where albite and chlorite are the dominant secondary minerals to where amphibole and plagioclase dominate. In Hole 1256D, gabbros have been recovered from crust clearly within seismic Layer 2 on the basis of shipboard and wireline physical property measurements, with the depth of the Layer 2/3 boundary estimated during site survey seismic experiments to be between 1450 and 1750 mbsf. Drilling deeper at Site 1256 would provide a second test of the geological meaning of the seismic layering of the ocean crust.

The upper gabbros are more strongly hydrothermally altered than the immediately overlying sheeted dikes that have been subjected to contact metamorphic recrystallization. The balance between conductive and hydrothermal cooling is key to understanding the thermal structure of the ocean crust as well as for estimating the magnitude of hydrothermal chemical exchanges between the crust and oceans. Were the upper gabbros, particularly cumulate gabbros, cooled by conduction or hydrothermal fluids? Were hydrothermal interactions pervasive or restricted to veins, and did alteration occur at black smoker or higher temperatures (350°–800°C) or did most of the hydrothermal interactions occur later at subgreenschist-facies conditions (e.g., prehnite and clays) some distance away from the ridge?

Site 1256 was deliberately located ~5 km from the 5Cr/5Bn magnetic reversal. To date, all rocks recovered from Hole 1256D appear to be similarly (reversely) magnetized. Depending on the accretion processes and the rate of cooling of the lower ocean crust, we may expect deeper rocks to be normally magnetized if the crust took longer than ~70 k.y. to cool below the temperature (~400°C) at which a reversal would be preserved. Whether this reversal is present and, if present, at what depth it occurs would provide unique constraints on lower crustal cooling rates.

IODP Expedition 309/312, which followed ODP Leg 206, has made significant progress in recovering an intact section of the upper oceanic crust. The inverse relationship between spreading rate and depth to axial melt lenses has been confirmed, supporting the strategy of drilling in crust formed at a superfast spreading rate to achieve the first penetration of a complete upper crustal section. A return expedition to Hole 1256D could deepen the hole sufficiently into the plutonic rocks to obtain definitive answers to longstanding questions about the mechanisms of crustal accretion.

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