Fluid-rock interactions in the lower oceanic crust: Thermodynamic models of hydrothermal alteration

Rocks from the lower oceanic crust show both petrologic and isotopic evidence for extensive alteration by circulating hydrothermal fluids derived from seawater. Deeply circulating fluids may have a substantial, but as yet largely undetermined, impact on the thermal regime of spreading centers, magmatic processes at mid‐ocean ridges, and elemental fluxes at ridge hydrothermal systems. In the present study, the consequences of water‐rock interactions during the penetration of seawater into hot lower oceanic crust were investigated using thermodynamic reaction path models. Alteration assemblages predicted for reaction of an evolving seawater with olivine gabbro, gabbronorite, and troctolite during heating from 300° to 900°C closely resemble alteration assemblages observed in rock samples from the lower oceanic crust. These assemblages include sodic plagioclase + actinolite ± diopsidic clinopyroxene ± epidote ± chlorite ± prehnite ± analcime ± quartz at lower temperatures (300°–500°C) and plagioclase + hornblende ± diopside ± chlorite ± trace magnetite at intermediate temperatures (500–700°C). The relative abundances of these alteration products show much greater variation with temperature than with water/rock (W/R) ratio, indicating that variations in the amounts of hydrated minerals such as amphiboles may not be an accurate indication of the W/R ratio experienced by rocks in the lower crust since the same amount of fluid may produce widely differing amounts of amphibole at slightly different temperatures. Depending on the temperature of interaction, some rocks in the lower oceanic crust may have experienced much higher W/R ratios than has previously been proposed based on the amount of amphibole present. At higher temperatures (750°–900°C),. the models predict that very little alteration occurs before the fluid equilibrates with the rock, producing only trace amounts of amphibole as an alteration phase, with the remainder of the rock retaining its igneous mineral composition essentially unchanged. This result suggests that extensive penetration of rocks by aqueous fluids may occur in the lower ocean crust at temperatures >∼700°C and leave very little conspicuous petrologic evidence for their presence. Such cryptic alteration is consistent with isotopic evidence for hydrothermal alteration of oxygen and Sr isotopes by seawater in lower ocean crust rock samples which otherwise retain their igneous compositions and textures. Taken together, the model results al