The Trans-Atlantic Geotraverse (TAG) active mound is a seafloor hydrothermal deposit that has been sampled by drilling to the base of its alteration zone, providing a rare opportunity for in situ investigation of the effects of water-rock interactions. The δS signature of sulfide minerals in seafloor hydrothermal settings is commonly explained by invoking isotopic mixing and fractionation based endmember models. Here, we present textural evidence of the replacement reactions that underlie the fractionation model endmember, found in core samples recovered from the TAG active mound during ODP expedition 158. The samples were obtained at the TAG-1 area, at the transition between shallower pyrite-silica (± anhydrite) breccias and deeper pyrite-silica breccias. The shallower breccias show porous domains associated with anhydrite, quartz and pyrite. Similar textures were also observed in the deeper samples, in which anhydrite is absent. Our results suggest that at stockwork depths, anhydrite is removed during its interaction with hydrothermal fluids, leaving behind pores lined by quartz crystals and partially filled by pyrite. The textural evolution suggests that pyrite grows at the expense of anhydrite, leading to δS values in pyrite controlled by the isotopic fractionation factor between SO and HS. Our petrographic observations are supported by geochemical modelling at in situ temperatures, estimated from analysis of fluid inclusions in quartz. Our interpretation is in agreement with previously published δS data from the TAG active mound. Both the highest δS signatures in disseminated pyrite and the lowest δS signatures in vein-related mineralization, as well as the progressive S-enrichment with depth recorded by the isotopic stratification at the TAG active mound can be explained based on the temperature at which water-rock interactions occurred. This research provides direct evidence of the mineral replacement processes occurring within the active TAG sulfide mound and their link to established models of isotopic fractionation. The textural evidence for these processes is characterized in detail and can aid the interpretation of the evolution of other seafloor hydrothermal deposits.

中文翻译：

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球状黄铁矿角砾岩中的硬石膏置换反应及其对大西洋中脊 TAG 热液丘中黄铁矿 δ34S 特征的地球化学控制

跨大西洋地质穿越（TAG）活动丘是一个海底热液沉积物，通过钻探到其蚀变带底部进行取样，为现场研究水-岩石相互作用的影响提供了难得的机会。海底热液环境中硫化物矿物的 δS 特征通常通过调用基于同位素混合和分馏的端元模型来解释。在这里，我们提供了分馏模型端元基础上的替代反应的结构证据，这些证据是在 ODP 探险 158 期间从 TAG 活性土丘中回收的岩心样本中发现的。这些样本是在 TAG-1 区域获得的，位于较浅的黄铁矿-之间的过渡处。硅质（±硬石膏）角砾岩和更深的黄铁矿硅质角砾岩。较浅的角砾岩显示出与硬石膏、石英和黄铁矿相关的多孔区域。在较深的样品中也观察到类似的纹理，其中不存在硬石膏。我们的结果表明，在网状结构深度，硬石膏在与热液相互作用的过程中被去除，留下由石英晶体排列并部分被黄铁矿填充的孔隙。结构演化表明，黄铁矿以硬石膏为代价生长，导致黄铁矿中的 δS 值受 SO 和 HS 之间的同位素分馏因子控制。我们的岩相观测得到了原位温度地球化学模型的支持，该模型是根据石英中流体包裹体的分析估算得出的。我们的解释与之前发布的来自 TAG 活动土丘的 δS 数据一致。浸染状黄铁矿中的最高 δS 特征和与矿脉相关的矿化中的最低 δS 特征，以及 TAG 活动丘同位素分层记录的随深度逐渐富集的硫，都可以根据水的温度来解释。岩石发生相互作用。这项研究提供了活跃 TAG 硫化物丘内发生的矿物替代过程及其与已建立的同位素分馏模型的联系的直接证据。这些过程的结构证据得到了详细的表征，可以帮助解释其他海底热液矿床的演化。