Integrated Ocean Drilling Program, 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 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 successfully deepened to a total depth of 1255 meters below seafloor (mbsf) (1005 m subbasement), having penetrated through >800 m of extrusive normal mid-ocean-ridge basalt, and entered a region dominated by intrusive rocks with numerous subvertical chilled dike margins. 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 spectacular mineralized breccias denoting a lithologic transition zone. The extrusive lavas are less hydrothermally altered than other basement sites (e.g., Sites 417 and 418 and Holes 504B and 896A), and there is no systematic change with depth from oxidizing to reducing seawater alteration. Instead, oxidizing alteration occurs irregularly with depth, most commonly associated with steeply dipping vein networks. Below 1061 mbsf, massive basalts, some with doleritic textures, dominate the sheeted intrusives. Numerous subvertical dikes, commonly with brecciated and mineralized chilled margins, crosscut the sheeted intrusives. These rocks are altered under greenschist facies hydrothermal conditions and have significantly higher thermal conductivity and P-wave velocity. During Expedition 309 Hole 1256D was exited cleanly, and the hole is in excellent condition and ready for deepening. At 1255 mbsf, Hole 1256D is tantalizingly close to the minimum estimated depth for the frozen axial magma chamber predicted to be at 1275-1525 mbsf. IODP Expedition 312 will return to this site in late 2005, and, despite the grueling 15 m/day pace of advance and assuming further benign drilling conditions, is set to deepen Hole 1256D by a further 500 m. The total depth would then be well beyond where geophysical interpretations predict gabbros to occur.