University of Michigan, United States

The Superfast Spreading Rate Crust mission is a multicruise program to drill, for the first time, a complete section of the upper oceanic crust from the extrusive lavas, through the dikes, and into the underlying gabbros. Hole 1256D was initiated during Ocean Drilling Program Leg 206 in the eastern equatorial Pacific and is drilled into 15 Ma crust that formed at the East Pacific Rise during a period of superfast spreading (>200 mm/y). This site is chosen to exploit the inverse relationship between spreading rate and the depth to axial low-velocity zones, thought to be magma chambers now frozen as gabbros, observed from seismic experiments. During Integrated Ocean Drilling Program (IODP) Expedition 309, Hole 1256D was deepened to a total depth of 1255 meters below seafloor (mbsf) (1005 m subbasement), having penetrated >800 m of extrusive normal mid-ocean-ridge basalt, and entered the sheeted dike complex. IODP Expedition 312 returned to Hole 1256D and deepened it to 1507.1 mbsf. The hole now extends through the 346 m sheeted dike complex and 100.5 m into the upper portions of the plutonic section. The uppermost crust at Site 1256 comprises a >74 m thick ponded lava overlying massive, sheet, and minor pillow flows, some of which exhibit inflation structures requiring eruption onto a subhorizontal surface. This suggests a total thickness of off-axis lavas of 284 m. Sheet and massive lava flows make up the remaining extrusive section (534-1004 mbsf) above subvertical cataclastic zones, intrusive contacts, and mineralized breccias denoting a lithologic transition zone. Below 1061 mbsf, massive basalts, some with doleritic textures, dominate the sheeted dikes, which exhibit increased thermal conductivity and P-wave velocity. Numerous subvertical dikes, commonly with brecciated and mineralized chilled margins, crosscut the sheeted dikes. The upper dikes (<1255 mbsf) contain greenschist-facies minerals, actinolite becomes abundant below approximately 1300 mbsf, and hornblende and secondary plagioclase are present below approximately 1350 mbsf, reflecting a steep thermal gradient in the dikes. Superimposed on this is recrystallization of the lowermost approximately 60 m of dikes to granoblastic textures as the result of intrusion of underlying gabbros. An upper gabbroic body intrudes the sheeted dikes at 1406.6 mbsf and is separated from a lower gabbroic body at 1483-1507.1 mbsf by an intervening metamorphosed dike screen. Gabbroic rocks are highly altered and fine to coarse grained (mostly medium grained), range from gabbro to oxide gabbro and gabbronorite, and include differentiated rocks (trondjhemite and quartz-rich oxide diorite). The base of the section contains a gabbronorite of uncertain origin (intrusive gabbronorite or metamorphosed dike) and is cut by a late basalt dike. Physical properties change downward across the dike/gabbro contact (increased porosity and decreased velocity and density).