GEOCHEMICAL SIGNATURES OF WEATHERING AND SURFACE WATER CHEMISTRY IN THE LATE ARCHEAN
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Earth’s Archean surface environment was important for the origin and evolution of life. Here, it is proposed that the pH2,g controlled the redox state of the Archean atmosphere, consistent with the stabilities of detrital minerals, such as pyrite, siderite, and uraninite. In the marine environment, greenalite, siderite, and hematite, or amorphous precursors, were primary minerals. Magnetite was formed during diagenesis and metamorphism. Fluctuations of pH2,g or pH could oxidize elements such as Mo and Re. Organic acids were metastable in the surface waters at high pH2,g values, possibly stimulating biologic activity and the formation of organic hazes in the atmosphere. Reaction path models were used to simulate rainwater, weathering, and river water chemistry under present-day and late Archean conditions. The thermodynamic properties of ferrous iron and other end-member minerals important on the early Earth were estimated using an extended Linear Free Energy Relationship. Modeling of the present-day weathering of basalt + calcite produced hematite, kaolinite, Na-Mg-saponite, and chalcedony, and world average river water (WARW) after destruction of 10-4 moles kg-1 H2O. Late Archean weathering of olivine basalt + calcite produced kaolinite, chalcedony, and Fe(II)-rich clays, and WARW with low pH, and high HCO3- and Fe. The behavior of P, Mn, Cr, and Cu during late Archean weathering was investigated during weathering of olivine basalt. Apatite and Mn-olivine dissolved, producing high phosphate and Mn(II) in Archean WARW. Insoluble MnO2,cr and hematite formed during whiff of oxidants. Chromite hardly dissolved but Cr(OH)3,am dissolved completely, forming high Cr(III) waters. Weathering of chalcopyrite produced chalcocite and bornite, but whiffs of oxidants produced native copper, chalcocite, bornite, and hematite, and high dissolved Cu. Aqueous Cr2+ could be stable in hydrothermal solutions and a dominant species even with Cr(III)-Cl complexation whereas higher pressure favors Cr(III). For example, at 5 GPa and 1000 °C, Cr2+ is the dominant species at geologically reasonable pH and log fO2 values. However, at 5 GPa and 600 °C, Cr3+ and Cr2+ might coexist. It appears likely that Cr(II) could play a significant role in low pressure hydrothermal fluids and in subduction zone fluids.