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.

中文翻译：

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戈壁湖盐湖碱性盐湖中单氢方解石的原位形成：对土卫二海洋和碱性碳酸盐海洋世界中镁，钙和总溶解碳酸盐浓度的预测

陆地碱式苏打湖中矿物沉淀的性质为深入了解太阳系外部冰层上的地下海洋的水化学提供了见识。陆地碱性湖泊的饱和度分析表明，湖泊水的溶液化学通常受单氢方解石（MHC）和无定形碳酸镁（AMC）的存在控制。但是，对在天然碱性湖泊中形成这些亚稳碳酸盐的直接观察是有限的。这项研究提供了基于Olgoy，Bon Tsagaan和Orog Lakes（蒙古Gobi Lakes谷）的悬浮物的水化学和矿物学的证据，从而证明了碱性湖中MHC的形成。MHC和AMC的溶液化学接近饱和，这与全球其他碱性湖泊一致。冷冻干燥后，通过湖水的超速离心分离来分离悬浮物。我们的结果表明，MHC是悬浮物中常见的矿物相。这些观察结果证实，MHC是碱性湖泊中蒸发的直接自生产物。来自Olgoy Lake的悬浮物中的碳酸盐分数的Mg / Ca比为0.4，表明与MHC结合形成了AMC。根据AMC和MHC的溶解平衡，我们可以预测Mg 提示AMC与MHC相关联。根据AMC和MHC的溶解平衡，我们可以预测Mg 提示AMC与MHC相关联。根据AMC和MHC的溶解平衡，我们可以预测Mg在存在AMC和MHC的情况下，土卫二海洋中的2 ⁺，Ca ²⁺和总溶解碳酸盐浓度分别为〜1 mmol / kg，〜10μmol/ kg和0.06-0.2 mol / kg。我们建议测量烟羽中的Mg含量将是限制Enceladus和其他碱性碳酸盐海洋世界中地下海洋的总溶解碳酸盐浓度和化学亲和力的关键。