Minerals (Jul 2020)
In Situ Formation of Monohydrocalcite in Alkaline Saline Lakes of the Valley of Gobi Lakes: Prediction for Mg, Ca, and Total Dissolved Carbonate Concentrations in Enceladus’ Ocean and Alkaline-Carbonate Ocean Worlds
Abstract
The nature of mineral precipitations in terrestrial alkaline soda lakes provides insights into the water chemistry of subsurface oceans on icy bodies in the outer solar system. Saturation analyses of terrestrial alkaline lakes have shown that the solution chemistries of lake waters are generally controlled by the presence of monohydrocalcite (MHC) and amorphous Mg-carbonate (AMC). However, direct observations of the formation of these metastable carbonates in natural alkaline lakes have been limited. This study provides evidence of in situ MHC formation in alkaline lakes, based on the water chemistry and mineralogy of suspended matter in Olgoy, Boon Tsagaan, and Orog Lakes (Valley of Gobi Lakes, Mongolia). The solution chemistries were close to saturation with respect to MHC and AMC, consistent with other alkaline lakes worldwide. Suspended matter was separated by the ultracentrifugation of lake water following freeze-drying. Our results show that MHC is the common mineral phase in the suspended matter. These observations confirm that MHC is the direct authigenic product of evaporation in alkaline lakes. The carbonate fraction in suspended matter from Olgoy Lake has a Mg/Ca ratio of 0.4, suggesting the formation of AMC in association with MHC. Based on the dissolution equilibria of AMC and MHC, we predict the Mg2+, Ca2+, and total dissolved carbonate concentrations in Enceladus’ ocean to be ~1 mmol/kg, ~10 μmol/kg, and 0.06–0.2 mol/kg, respectively, in the presence of AMC and MHC. We propose that the measurements of Mg contents in plumes will be key to constraining the total dissolved carbonate concentrations and chemical affinities of subsurface oceans on Enceladus and other alkaline-carbonate ocean worlds.
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