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 similar to170 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 delta(18)O = 8.7 - 25.7parts per thousand, deltaD = -69.4parts per thousand to -90.4parts per thousand, and delta(13)C = -2.7parts per thousand to 1.8parts per thousand. The intensity of alteration and the amount of sediment within the basement decrease with depth, leading to corresponding decreases in delta(18)O and deltaD. A SUPER composite, constructed to estimate the bulk composition of the upper crust, has delta(18)O = 12.0parts per thousand, deltaD = -87.0parts per thousand, and delta(13)C = 0.7parts per thousand. Compared to core descriptions and geophysical logs, the SUPER composite contains too much O-18-rich sediment (delta(18)O = 25.7parts per thousand), and a corrected delta(18)O value of 10.8parts per thousand is more reasonable for the upper crust at Site 801. These values are higher than those for other bulk upper oceanic basement sections (delta(18)O = 8.0 - 10.0parts per thousand) 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 O-18-rich secondary carbonate in veins and breccia cements contributed to the high bulk δO-18 of this old upper crustal section.