SILICATE DIAGENESIS IN ALKALINE LAKE BASINS

Beth Gierlowski-Kordesch had a keen appreciation for the variety of styles of interactions between water, minerals, and organisms in lake basins. Many silicate minerals can form authigenically or alter in alkaline lacustrine systems, particularly at high salinity. In volcaniclastic fluvio-lacustrine basins such as those of the East African Rift, tuffaceous deposits commonly provide weatherable materials that can be an important source of both detritus and dissolved solids. Many reactions can alter mineral assemblages in near-surface environments, including those associated with pedogenesis, transport, deposition, and early diagenesis. Significant weathering related reactions include oxidation, hydrolysis, and cation exchange. Reactions in depositional settings include reduction, precipitation directly from solution, dissolution-precipitation, exchange, or sorption reactions. Elevated P_CO2 due to hydrothermal or biogenic input can also be an important factor due to effects on pH. In general, increased salinity and alkalinity, such as occurs with hydrological closure, is associated with a transition from calcic to potassic and sodic authigenic silicates. Four related silicate families are found in such settings: zeolites, clay minerals, sodium silicates, and silica. Zeolites commonly show zonation or progression representing increasing salinity in an order such as chabazite > phillipsite > analcime. At high salinity with moderate to elevated aqueous silica, Mg-enrichment in clay minerals may occur, which can induce layer charge increase, K-fixation, and illitization. Sodium silicates such as magadiite and kenyaite represent hyperalkaline and high silica conditions. Cherty silica can form in a number of ways, including accumulations of diatomaceous ooze, recrystallization of amorphous silica, capillary or other evaporative precipitation, and leaching of sodium silicates. Although the potential mixtures of authigenic silicate minerals found in alkaline lacustrine deposits can be complex, the precipitation of such minerals is fundamentally controlled by their thermodynamic stabilities. Therefore, they can provide useful perspectives on paleolimnology when the confounding effects of kinetics and diagenesis can be constrained.