In basaltic volcanic ash recovered from a seamount at 3000 m water depth, we discovered marcasite and pyrite precipitation within cavities that formed by partial to complete dissolution of olivine. In places, these cavities are reminiscent of negative crystal shapes; elsewhere they apparently continue along cracks. In strong contrast, adjacent volcanic glass shows little, if any, evidence for dissolution. The FeS2 precipitates were commonly found to be conjoined and planar aggregates, occurring in the center of the voids. Their maximum volume fraction in relation to the void space as determined by 2D and 3D imaging techniques corresponds to the amount of iron released by olivine dissolution. Almost all occurrences of FeS2 precipitation are related to Cr-spinel inclusions in the former olivine. We propose that rapid olivine dissolution was initiated by reduced H2S-bearing fluids at olivine grain boundaries or surfaces exposed by cracks. Many of these cracks are connected to spinel grains, where the iron liberated from olivine is mineralized as FeS2, initially facilitated by heterogeneous nucleation. Subsequent pyrite and/or marcasite precipitation occurred as overgrowths on existing FeS2 aggregates. The particular chemical environment of low-pH, hydrogen sulfide-bearing fluids may have enhanced olivine dissolution by (1) keeping Fe in solution and (2) sequestering important quantities of Fe as FeS2. The in situ oxidation of ferrous Fe and precipitation of ferric hydroxides at the olivine surface commonly observed in oxic environments were obviously impeded. It would have slowed down olivine dissolution to rates more similar to the dissolution of basaltic glass. We have no direct indication that the process of rapid olivine dissolution was aided by subseafloor life. However, the presence of fibrous structures with small sulfide particles could indicate late colonization of sulfate-reducing bacteria that may add an additional path of iron fixation.

中文翻译：

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海底风化作用增强：海底火山灰中的快速橄榄石溶解和硫化铁形成

在从 3000 m 水深的海山中回收的玄武质火山灰中，我们发现了由橄榄石部分至完全溶解形成的空腔内的白铁矿和黄铁矿沉淀。在某些地方，这些空洞让人联想到负晶体形状；在其他地方，它们显然沿着裂缝继续。与此形成强烈对比的是，相邻的火山玻璃几乎没有溶解的迹象（如果有的话）。FeS2 沉淀物通常被发现是连体和平面聚集体，出现在空隙的中心。通过 2D 和 3D 成像技术确定的与空隙空间相关的最大体积分数对应于橄榄石溶解释放的铁量。几乎所有发生的 FeS2 沉淀都与前橄榄石中的 Cr-尖晶石包裹体有关。我们提出，橄榄石的快速溶解是由在橄榄石晶界或裂缝暴露的表面处减少的含 H2S 流体引发的。这些裂缝中的许多都与尖晶石晶粒相连，其中从橄榄石中释放的铁矿化为 FeS2，最初是由异质成核促进的。随后的黄铁矿和/或白铁矿沉淀作为现有 FeS2 聚集体的过度生长而发生。低 pH 值、含硫化氢流体的特定化学环境可能通过 (1) 将 Fe 保持在溶液中和 (2) 以 FeS2 形式隔离大量的 Fe，从而增强了橄榄石的溶解。在有氧环境中常见的橄榄石表面的铁亚铁原位氧化和氢氧化铁沉淀明显受到阻碍。它会减慢橄榄石的溶解速度，使其更类似于玄武岩玻璃的溶解速度。我们没有直接的迹象表明，橄榄石的快速溶解过程受到海底生物的帮助。然而，具有小硫化物颗粒的纤维结构的存在可能表明硫酸盐还原细菌的晚期定植可能增加铁固定的额外途径。