A Brine Evolution Model and Mineralogy of Chemical Sediments in a Volcanic Crater,Lake Kitagata,Uganda |
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Authors: | Lichun Ma Tim K Lowenstein James M Russell |
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Institution: | (1) Institute of Mineral Resources, Chinese Academy of Geological Sciences, 100037 Beijing, China;(2) Department of Geological Sciences, State University of New York, Binghamton, NY 13902, USA;(3) College of Resources and Environment, China Agricultural University, Beijing, 100193, China;(4) Department of Geological Sciences, Brown University, Box 1846, Providence, RI 02912, USA; |
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Abstract: | Lake Kitagata, Uganda, is a hypersaline crater lake with Na–SO4–Cl–HCO3–CO3 chemistry, high pH and relatively small amounts of SiO2. EQL/EVP, a brine evaporation equilibrium model (Risacher and Clement 2001), was used to model the major ion chemistry of the evolving brine and the order and masses of chemically precipitated sediments.
Chemical sediments in a 1.6-m-long sediment core from Lake Kitagata occur as primary chemical mud (calcite, magadiite NaSi7O13(OH)3·3H2O], burkeite Na6(CO3)(SO4)2]) and as diagenetic intrasediment growths (mirabilite (Na2SO4·10H2O)). Predicted mineral assemblages formed by evaporative concentration were compared with those observed in salt crusts along
the shoreline and in the core from the lake center. Most simulations match closely with observed natural assemblages. The
dominant inflow water, groundwater, plays a significant role in driving the chemical evolution of Lake Kitagata water and
mineral precipitation sequences. Simulated evaporation of Lake Kitagata waters cannot, however, explain the large masses of
magadiite found in cores and the formation of burkeite earlier in the evaporation sequence than predicted. The masses and
timing of formation of magadiite and burkeite may be explained by past groundwater inflow with higher alkalinity and SiO2 concentrations than exist today. |
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