Integrated Ocean Drilling Program (IODP) Expeditions 309 and 312 will complete drilling of a continuous section through volcanic basement and the underlying sheeted dike complex and into the uppermost plutonic rocks at Ocean Drilling Program (ODP) Site 1256. The crust at this site formed at a superfast (>200 mm/y) spreading rate ~15 m.y. ago at the East Pacific Rise. In preparation for deep drilling, during ODP Leg 206 a reentry cone and 16 inch casing were installed and cemented into basement in Hole 1256D and coring was completed to 500 meters subbasement (msb) within the volcanic section.
The relationship between ocean ridge spreading rate and depth to axial low-velocity zones, interpreted to be subaxial melt lenses, predicts that the dike–gabbro transition should occur at 900–1300 msb at Site 1256. We estimate reaching a depth of 1450 msb during Expeditions 309 and 312 and so should penetrate gabbroic rocks, allowing determination of the lithology and structure of the upper oceanic crust and definition of the thickness of the volcanic and sheeted dike sections. Recovered cores will enable investigations of magmatic processes within the axial melt lens and determination of whether upper gabbros are cumulates complementary to dikes and lavas or coarse-grained equivalents of extrusive rocks. Mineral studies will enable determination of cooling rates within the melt lens, and structural studies will examine the roles of faulting and fracturing in crustal accretion and hydrothermal circulation.
We will correlate remotely measured geophysical parameters of the crust with basic geological observations. Core observations and downhole logging will allow calibration of seismic velocities with lithology and enable determination of the relationships between seismic transitions (e.g., the Layer 2/3 boundary) and lithology and metamorphism. Magnetic measurements will establish the contribution of different lithologies and alteration to marine magnetic anomalies. We will investigate interactions between magmatic processes, rock type, and hydrothermal water-rock interaction, including testing the alteration "stratigraphy" within extrusive volcanic rocks and sheeted dikes established in Hole 504B, as well as in the underlying gabbroic rocks. Core studies will determine the nature of the deep subsurface reaction and shallower mixing zones, establish geochemical budgets for the Site 1256 crust, and examine the depth extent of the subsurface biosphere in igneous basement.
Initial logging during Expedition 309 will sample borehole fluid and determine temperature and hole conditions. Full logging suites collected at or near the end of Expedition 309 and at the end of Expedition 312 will relate recovered material to the crust and core and borehole geophysical measurements to remote surveys of crustal properties.