Interaction of groundwater with the serpentinized harzburgites of the upper mantle plays an important role in the geologic carbon cycle and serpentinization processes. In this study, an intact serpentinized harzburgite with more than 60% lizardite from the Semail ophiolite in Oman was reacted with water at near neutral pH and low PCO2 to observe its dissolution behavior and reaction path within the CaO-MgO-SiO2-CO2-H2O system at 20 °C and 30 bars. In the experiment, mass (i.e. H2O, ions) transfer is by diffusion and the selected temperature and pressure conditions mimic water-rock interactions at the shallow portions of most ultramafic rock hosted aquifers. Equilibrium activity-activity diagrams and thermodynamic data were used to compare the experimentally determined reaction path with the theoretical stability boundaries for minerals involved to investigate the evolution of bulk chemical composition, mineralogy, and water composition. The experiments demonstrated that (i) the water-rock interactions increased the pH of the aqueous phase from 5.9 to 7.5 over a period of 18 weeks; (ii) the dissolution of lizardite, orthopyroxene (enstatite), and clinopyroxene (diopside and augite) increased the concentration of Mg, Ca, and Si in the aqueous phase; (iii) the dissolution was incongruent with respect to Mg and Si, favoring Si release at pH > 6 due to (a) breaking of more reactive cation‑oxygen bonds that is consistent with the stoichiometry of magnesium for proton exchange reaction which favors a surface that is enriched in Mg at basic conditions, and (b) preferential dissolution of clinopyroxene Mg1.30Ca0.52Fe0.06Si2O6; (iv) the aqueous phase was undersaturated with respect to carbonate (e.g. calcite, magnesite, and hydromagnesite) and hydrous (e.g. lizardite, chrysotile, brucite, and talc) minerals; (v) the bulk chemical composition and mineralogy of the intact serpentinized harzburgite matrix did not change; (vi) the thermodynamic data can successfully predict water-intact serpentinized harzburgite behavior if the water chemical composition can be constrained; and (vii) no detectable macroscopic form of damage (e.g. microcracks forming as a result of differential stress fields due to changes in volume during chemical reaction) was observed.

中文翻译：

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20°C 和 30 bar 下水-蛇纹石化方辉橄榄岩相互作用期间的地球化学-流体力学耦合


地下水与上地幔蛇纹石化方辉橄榄岩的相互作用在地质碳循环和蛇纹石化过程中发挥着重要作用。在这项研究中，来自阿曼 Semail 蛇绿岩的完整蛇纹石化方辉橄榄岩（蜥蜴石含量超过 60%）与水在接近中性 pH 和低 PCO2 的条件下反应，观察其在 CaO-MgO-SiO2-CO2-H2O 中的溶解行为和反应路径。系统温度为 20 °C，压力为 30 bar。在实验中，质量（即H2O、离子）转移是通过扩散进行的，选定的温度和压力条件模拟了大多数超镁铁质岩石含水层浅部的水-岩石相互作用。使用平衡活性-活性图和热力学数据将实验确定的反应路径与所涉及矿物的理论稳定性边界进行比较，以研究本体化学成分、矿物学和水成分的演变。实验表明，(i) 水-岩石相互作用在 18 周内将水相的 pH 值从 5.9 提高到 7.5； (ii) 蜥蜴石、斜方辉石（顽火辉石）和单斜辉石（透辉石和辉石）的溶解增加了水相中 Mg、Ca 和 Si 的浓度； (iii) 溶解与 Mg 和 Si 不一致，有利于 Si 在 pH > 6 下释放，因为 (a) 更具反应活性的阳离子-氧键的断裂，这与质子交换反应中镁的化学计量一致，有利于碱性条件下富镁的表面，(b) 单斜辉石 Mg1.30Ca0.52Fe0.06Si2O6 优先溶解； (iv)水相相对于碳酸盐（例如方解石、菱镁矿和水菱镁矿）和含水相（例如 蜥蜴石、温石棉、水镁石和滑石）矿物； (v) 完整蛇纹石化方辉橄榄岩基质的整体化学成分和矿物学没有改变； (vi) 如果水的化学成分受到限制，热力学数据可以成功预测水完好的蛇纹石化方辉橄榄石的行为； (vii) 没有观察到可检测到的宏观形式的损坏（例如，由于化学反应过程中体积变化而导致差异应力场形成的微裂纹）。