Abstract A series of laboratory experiments was conducted to investigate serpentinization of olivine–pyroxene mixtures at 230 °C, with the objective of evaluating the effect of mixed compositions on Fe partitioning among product minerals, H2 generation, and reaction rates. An initial experiment reacted a mixture of 86 wt.% olivine and 14 wt.% orthopyroxene (Opx) with the same initial grain size for 387 days. The experiment resulted in extensive reaction (∼53% conversion), and solids recovered at termination of the experiment were dominated by Fe-bearing chrysotile and relict olivine along with minor brucite and magnetite. Only limited amounts of H2 were generated during the first ∼100 days of the experiment, but the rate of H2 generation then increased sharply coincident with an increase in pH from mildly alkaline to strongly alkaline conditions. Two shorter term experiments with the same reactants (26 and 113 days) produced a mixture of lizardite and talc that formed a thin coating on relict olivine and Opx grains, with virtually no generation of H2. Comparison of the results with reaction path models indicates that the Opx reacted about two times faster than olivine, which contrasts with some previous studies that suggested olivine should react more rapidly than Opx at the experimental conditions. The models also indicate that the long-term experiment transitioned from producing serpentine ± talc early in the early stages to precipitation of serpentine plus magnetite, with brucite beginning to precipitate only late in the experiment as Opx was depleted. The results indicate that overall reaction of olivine and Opx was initially relatively slow, but reaction rates accelerated substantially when the pH transitioned to strongly alkaline conditions. Serpentine and brucite precipitated from the olivine-Opx mixture had higher Fe contents than observed in olivine-only experiments at mildly alkaline pH, but had comparable Fe contents to reaction of olivine at strongly alkaline pH implying that higher pH may favor greater partitioning of Fe into serpentine and brucite and less into magnetite. Despite the presence of brucite, dissolved silica activities during the long-term olivine-Opx experiment maintained levels well above serpentine-brucite equilibrium. Instead, silica activities converged on levels close to metastable equilibrium between brucite and olivine. It is proposed that silica levels during the experiment may have been regulated by exchange of SiO2 between the fluid and a silica-depleted, brucite-like surface layer on dissolving olivine.

中文翻译：

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橄榄石-辉石混合物实验蛇纹石化过程中的氢生成和铁分配

摘要 进行了一系列实验室实验以研究橄榄石-辉石混合物在 230 °C 下的蛇纹石化，目的是评估混合成分对产品矿物中 Fe 分配、H2 生成和反应速率的影响。初始实验使具有相同初始粒度的 86 重量％橄榄石和 14 重量％斜方辉石 (Opx) 的混合物反应 387 天。该实验导致了广泛的反应（约 53% 的转化率），并且在实验结束时回收的固体主要是含铁的温石棉和残余橄榄石以及少量水镁石和磁铁矿。在实验的前 100 天只产生了有限数量的 H2，但是 H2 的产生率随后急剧增加，同时 pH 从弱碱性到强碱性条件增加。使用相同反应物（26 天和 113 天）进行的两个短期实验产生了蜥蜴石和滑石的混合物，在残存的橄榄石和 Opx 颗粒上形成了一层薄薄的涂层，几乎没有产生 H2。结果与反应路径模型的比较表明，Opx 的反应速度大约是橄榄石的两倍，这与之前的一些研究形成对比，这些研究表明橄榄石在实验条件下应该比 Opx 反应更快。这些模型还表明，长期实验从早期生产蛇纹石 ± 滑石转变为沉淀蛇纹石加磁铁矿，水镁石在实验后期才开始沉淀，因为 Opx 耗尽。结果表明，橄榄石和 Opx 的整体反应最初相对较慢，但当 pH 值转变为强碱性条件时，反应速率会显着加快。从橄榄石-Opx 混合物中沉淀的蛇纹石和水镁石的 Fe 含量高于在弱碱性 pH 下仅在橄榄石实验中观察到的 Fe 含量，但其 Fe 含量与橄榄石在强碱性 pH 下的反应相当，这意味着较高的 pH 可能有利于 Fe 更好地分配到蛇纹石和水镁石少成磁铁矿。尽管存在水镁石，但在长期橄榄石-Opx 实验中溶解的二氧化硅活性保持在远高于蛇纹石-水镁石平衡的水平。相反，二氧化硅活性会聚在接近水镁石和橄榄石之间的亚稳态平衡的水平上。