Stable isotopic composition of upper oceanic crust formed at a fast spreading ridge, ODP Site 801

[1] ODP Legs 129 and 185 sampled the upper 474 m of 170 Ma ocean crust in the western Pacific in order to investigate alteration processes in fast spread crust and to determine inputs to subduction. Fourteen composite bulk samples of altered upper oceanic crust from Site 801 have dO = 8.7–25.7%, dD = 69.4% to 90.4%, and dC = 2.7% to 1.8%. The intensity of alteration and the amount of sediment within the basement decrease with depth, leading to corresponding decreases in dO and dD. A SUPER composite, constructed to estimate the bulk composition of the upper crust, has dO = 12.0%, dD = 87.0%, and dC = 0.7%. Compared to core descriptions and geophysical logs, the SUPER composite contains too much O-rich sediment (dO = 25.7%), and a corrected dO value of 10.8% is more reasonable for the upper crust at Site 801. These values are higher than those for other bulk upper oceanic basement sections (dO = 8.0–10.0%) and result in part from: (1) intense low-temperature (<100 C) hydrothermal alteration of the upper 100 m of tholeiitic basement at Site 801 that may not be representative of material subducting in the western Pacific and (2) an aging effect, whereby progressive addition of Orich secondary carbonate in veins and breccia cements contributed to the high bulk dO of this old upper crustal section.