The hydrous alteration of ultramafic rocks, known as serpentinization, generates fluids that can fuel microbial communities and enable the synthesis of simple organic compounds. Serpentinization reactions can proceed even at the ambient, low-temperature conditions present in continental aquifers raising questions about the limits of life deep in the Earth's subsurface. The hydrous alteration of ultramafic rocks, known as serpentinization, generates fluids that can fuel microbial communities and enable the synthesis of simple organic compounds. Serpentinization reactions can proceed even at the ambient, low-temperature conditions present in continental aquifers raising questions about the limits of life deep in the Earth's subsurface. Through thermodynamic calculations, we investigate various reactions that facilitate the transformation of oxic, slightly acidic rainwater into reduced, hyperalkaline fluids during low-temperature serpentinization. We explore a suite of factors (variabilities in temperature, host-rock compositions, fluid salinity, and the buffering capacity of various serpentinization-relevant minerals) that offer broad insights into the chemical environments formed through low-temperature serpentinization. Results of calculations show that alteration of olivine-rich lithologies will lead to fluids constrained by the chrysotile-brucite-diopside equilibrium assemblage, close in pH to those measured from the most alkaline fluids hosted in ultramafic rocks. Variabilities in the compositions of fluids hosted by continental serpentinizing systems can be attributed to a shift from being in equilibrium with diopside to calcite, among other reactions. Results of calculations also show that it would be difficult to distinguish fluids reacting with either fresh or altered ultramafic rocks based solely on their pH, and total dissolved Ca, Mg and Si content. Our models also account for Fe incorporation into solid solutions of serpentine and brucite and show that the global H2 flux from continental serpentinization could be considerably lower than estimates based on iron oxidation to magnetite only. Lastly, we present the energetic landscape available to subsurface microorganisms by focusing on two microbial process using H2: methanogenesis and hydrogen oxidation. Limited but available energy (0.2–1.7 calories/kg fluid) can be exploited by methanogens, permitting the possibility of deep communities in serpentinizing aquifers. More energy is available for methanogenesis (0.2–6 calories/kg fluid) and hydrogen oxidation (0–17 calories/kg fluid) when upwelling, deep-seated, serpentinization-generated fluids mix with shallow groundwater. Ultimately, predictions set forth in this study provide a framework for testing ideas that can explain the compositions of fluids and microbial communities sampled at ultramafic environments here on Earth and perhaps in the near future, on ocean worlds in our solar system.

中文翻译：

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低温、大陆、蛇纹石化生成流体地球化学的热力学约束

超基性岩的含水蚀变，称为蛇纹石化，产生的流体可以为微生物群落提供燃料，并能够合成简单的有机化合物。即使在大陆含水层中存在的环境低温条件下，蛇纹石化反应也可以进行，这引发了关于地球地下深处生命极限的问题。超基性岩的含水蚀变，称为蛇纹石化，产生的流体可以为微生物群落提供燃料，并能够合成简单的有机化合物。即使在大陆含水层中存在的环境低温条件下，蛇纹石化反应也可以进行，这引发了关于地球地下深处生命极限的问题。通过热力学计算，我们研究了在低温蛇纹石化过程中促进含氧微酸性雨水转化为还原的高碱性液体的各种反应。我们探索了一系列因素（温度变化、主岩成分、流体盐度和各种蛇纹石化相关矿物的缓冲能力），这些因素为通过低温蛇纹石化形成的化学环境提供了广泛的见解。计算结果表明，富含橄榄石的岩性改变将导致流体受到温石棉-水镁石-透辉石平衡组合的约束，其 pH 值接近于从超基性岩中的碱性最强的流体测得的值。大陆蛇纹石化系统承载的流体成分的变化可归因于从与透辉石平衡到方解石的转变，以及其他反应。计算结果还表明，仅根据其 pH 值和总溶解的 Ca、Mg 和 Si 含量，很难区分与新鲜或蚀变的超基性岩反应的流体。我们的模型还考虑了 Fe 并入蛇纹石和水镁石的固溶体中，并表明大陆蛇纹石化的全球 H2 通量可能比仅基于铁氧化成磁铁矿的估计值低得多。最后，我们通过关注两种使用 H2 的微生物过程来展示地下微生物可用的能量景观：产甲烷和氢氧化。有限但可用的能量（0.2-1. 7 卡路里/千克液体）可以被产甲烷菌利用，从而使深部群落有可能在蛇纹石化含水层中形成。当上升流、深层蛇纹石化产生的流体与浅层地下水混合时，更多的能量可用于甲烷生成（0.2-6 卡路里/千克流体）和氢氧化（0-17 卡路里/千克流体）。最终，这项研究中提出的预测为测试想法提供了一个框架，可以解释在地球上的超镁铁质环境中采样的流体和微生物群落的组成，也许在不久的将来，在我们太阳系的海洋世界中。2-6 卡路里/公斤流体）和氢氧化（0-17 卡路里/公斤流体），当上升流、深层、蛇纹石化产生的流体与浅层地下水混合时。最终，这项研究中提出的预测提供了一个测试想法的框架，可以解释在地球上的超镁铁质环境中采样的流体和微生物群落的组成，也许在不久的将来，在我们太阳系的海洋世界中。2-6 卡路里/公斤流体）和氢氧化（0-17 卡路里/公斤流体），当上升流、深层、蛇纹石化产生的流体与浅层地下水混合时。最终，这项研究中提出的预测提供了一个测试想法的框架，可以解释在地球上的超镁铁质环境中采样的流体和微生物群落的组成，也许在不久的将来，在我们太阳系的海洋世界中。