Hydrothermal alteration of upper oceanic crust formed at a fast-spreading ridge: mineral, chemical, and isotopic evidence from ODP Site 801

ODP Hole 801C penetrates >400 m into 170-Ma oceanic basement formed at a fast-spreading ridge. Most basalts are slightly (10-20%) recrystallized to saponite, calcite, minor celadonite and iron oxyhydroxides, and trace pyrite. Temperatures estimated from oxygen isotope data for secondary minerals are 5-100 degreesC, increasing downward. At the earliest stage, dark celadonitic alteration halos formed along fractures and celadonite, and quartz and chalcedony formed in veins from low-temperature (