Abstract

Experimental interaction of clay-rich marine sediments with sea water at 200 and 300 °C, 500 bars, and a water/rock mass ratio of 5 resulted in the generation of relatively acid, oxidizing conditions. The development of acidity is attributed to the formation of smectite, whereas the oxidation state of the fluids was dependent on the relative proportions of ferromanganese oxide phase, ferrous-rich silicate phases, and organic carbon. An experiment conducted with an oxidized sediment at 300 °C thus produced higher aqueous concentrations of Mn and associated base metals at a relatively high fO2 (∼10−15), but an organic carbon-rich sediment yielded fluids characterized by a lower fO2 (∼10−20), lower concentrations of Mn, higher Fe, and exhibited substantial thermal alteration and dissolution of organic components. Silica concentrations in the experimental fluids ranged from cristobalite to amorphous silica saturation levels in response to the dissolution of amorphous silica.

Comparison of fluid data and mineralogical assemblages from the Gulf Coast and Salton Sea regions with experimental results suggests that alteration processes in sedimentary basins can, in large part, be attributed to water-rock interaction. Observed differences are apparently due to the higher degree of reaction in the geologic environment and to the increased stability of the illite-chlorite assemblage at the lower silica activity characteristic of geologic fluids.

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