Hydrothermal alteration of oceanic crust in the West Philippine Sea Basin (Ocean Drilling Program Leg 195, Site 1201): inferences from a mineral chemistry investigation

Secondary minerals of a 91 meters-thick sequence of pillow basalts cored during ODP Leg 195 (Site 1201, West Philippine Basin) were investigated to reconstruct the hydrothermal alteration history and regime. The basement was first buried by red clays, and then by a thick turbidite sequence, thereby isolating it from seawater. The basalts are primitive to moderately fractionated, texturally variable from hypocrystalline and spherulitic to intersertal, sub-ophitic and intergranular. Relic primary minerals are plagioclase, clinopyroxene and opaques. Hydrothermal alteration pervasively affected the basalts, generating secondary clay minerals (mostly glauconite, minor Al-saponite and Fe-beidellite), ‘‘iddingsite’’, Ca–Na-zeolites, minor alkali-feldspar and calcite. The secondary mineral paragenesis and mutual relationships suggest that the hydrothermal alteration occurred under zeolite-facies conditions, at temperatures <100–150 °C. The main phase of alteration occurred under oxidizing conditions, with a high seawater/rock ratio, in an open-circulation regime, at temperatures of 30–60 °C, with precipitation of abundant glauconite and ‘‘iddingsite’’. A later stage of alteration occurred at ca. 70 °C, with precipitation of abundant Ca–Na-zeolites and minor calcite, in a more restricted circulation regime as a consequence of basement burial under the sedimentary cover, which supplied an altered, Ca-rich and Mg–K-sulfate-poor water causing precipitation of almost pure calcite.