Abstract Silicon (Si) isotopic compositions of authigenic Si-bearing materials can be powerful tracers of geological Si cycling and reconstruction of Earth’s climate. Interpretations of Si isotope data, however, are complicated by an inadequate understanding of natural Si isotopic fractionation pathways. This study investigated Si isotope fractionation between hydrothermal fluids and amorphous silica sinters of Cistern Spring and Deerbone Spring at Yellowstone National Park, USA, with a particular focus on spatial variations along spring flowpaths. Modeling of geochemical data indicate that the fluids at Cistern Spring were supersaturated with respect to amorphous silica whereas the fluids at Deerbone Spring were undersaturated due to lower SiO2 content and a more alkaline pH. The δ30Si values of the spring waters for both sites exhibit small variations, whereas the associated amorphous silica sinters displayed a considerable range and generally decreased downflow (from − 2.25 to −5.04‰ for Cistern and from −0.11 to −0.81‰ for Deerbone). The magnitude of apparent isotopic fractionation between solid and aqueous silica (Δ30Sisolid-fluid) was significantly larger for the samples from Cistern Spring than for Deerbone Spring or similar hot spring systems at Geysir geothermal field in Iceland. The Δ30Sisolid-fluid values for Cistern Spring ranged from −2.40 to −5.49‰ along the flowpath and correlated inversely with the rate of silica precipitation, indicating a process dominated by kinetic isotope fractionation. In contrast, the magnitude of Si isotopic fractionation was significantly smaller at Deerbone Spring (+0.21 to −0.64‰). A portion of the opaline sinters at Deerbone may have formed cryogenically under freezing winter conditions that altered silica saturation and formation pathway. Rapid isotope exchange with silica undersaturated hydrothermal fluids may have erased initial kinetic isotope effects. Re-equilibration between opal-A and fluids was strongly affected by isotopic fractionation between aqueous silica species and fluid ionic strengths. These results document both kinetic and equilibrium Si isotope fractionation from field observations and highlight the pivotal role of fluid-rock interaction on the preservation of Si isotope signatures in the rock record.

中文翻译：

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美国怀俄明州黄石国家公园热液系统中的硅同位素变化

摘要 自生含硅材料的硅（Si）同位素组成可以作为地质Si循环和地球气候重建的有力示踪剂。然而，由于对天然硅同位素分馏途径的理解不足，硅同位素数据的解释变得复杂。本研究调查了美国黄石国家公园 Cistern Spring 和 Deerbone Spring 的热液流体与无定形二氧化硅烧结体之间的 Si 同位素分馏，特别关注沿泉流路径的空间变化。地球化学数据建模表明，Cistern Spring 的流体相对于无定形二氧化硅是过饱和的，而 Deerbone Spring 的流体由于 SiO2 含量较低且 pH 值偏碱性而欠饱和。两个站点的泉水的 δ30Si 值变化很小，而相关的无定形二氧化硅烧结体显示出相当大的范围，并且通常降低了下流（Cistern 从 - 2.25 到 -5.04‰，Deerbone 从 -0.11 到 -0.81‰）。与冰岛 Geysir 地热田的 Deerbone Spring 或类似温泉系统相比，Cistern Spring 样品的固体和水性二氧化硅（Δ30Sisolid-fluid）之间的表观同位素分馏幅度明显更大。Cistern Spring 的 Δ30Sisolid-fluid 值在沿流路的范围从 -2.40 到 -5.49‰ 并与二氧化硅沉淀速率成反比，表明该过程以动力学同位素分馏为主。相比之下，Deerbone Spring 的 Si 同位素分馏幅度要小得多（+0.21 至 -0. 64‰）。Deerbone 的一部分蛋白石烧结矿可能是在寒冷的冬季条件下低温形成的，这改变了二氧化硅饱和度和形成途径。与二氧化硅不饱和热液的快速同位素交换可能消除了最初的动力学同位素效应。蛋白石-A 和流体之间的重新平衡受到水性二氧化硅物质和流体离子强度之间的同位素分馏的强烈影响。这些结果记录了现场观测的动力学和平衡 Si 同位素分馏，并突出了流体-岩石相互作用对保存岩石记录中 Si 同位素特征的关键作用。与二氧化硅不饱和热液的快速同位素交换可能消除了最初的动力学同位素效应。蛋白石-A 和流体之间的重新平衡受到水性二氧化硅物质和流体离子强度之间的同位素分馏的强烈影响。这些结果记录了现场观测的动力学和平衡 Si 同位素分馏，并突出了流体-岩石相互作用对保存岩石记录中 Si 同位素特征的关键作用。与二氧化硅不饱和热液的快速同位素交换可能消除了最初的动力学同位素效应。蛋白石-A 和流体之间的重新平衡受到水性二氧化硅物质和流体离子强度之间的同位素分馏的强烈影响。这些结果记录了现场观测的动力学和平衡 Si 同位素分馏，并突出了流体-岩石相互作用对保存岩石记录中 Si 同位素特征的关键作用。