Microanalytical data on Plio-Pleistocene sediments from a Salton Sea Geothermal Field well define the evolution of textures and mineral assemblages from sediment to metamorphic rock in an open hydrothermal system.

At shallow depths detrital grains occur within a fine-grained matrix of I-S and illite. As depth and temperature increase, grains lose their detrital characteristics; they subsequently form a continuous intergrown array of irregular domains, with phyllosilicates occupying the progressively decreasing pore space.

Illite and chlorite are authigenic phyllosilicates down to 2500 m, where biotite occurs. Authigenic phyllosilicates occur as subhedral crystals (200–600 Å thick). Alkali feldspar has end-member compositions, implying exchange of alkalis with convected fluids.

Changes in texture imply dissolution of detrital phases and neocrystallization of authigenic phyllosilicates directly from pore fluids. Changes with depth are not time-dependent for phyllosilicates: they formed simultaneously at all depths as a result of a single hydrothermal event in an open system.

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