This study is aimed to evaluate the role played by the sulfur radical ions (S3− and S2−) and molecular sulfur (S0) on sulfur isotope fractionation and to investigate if these species may leave an isotope fingerprint in hydrothermal systems. For this purpose, we combined (i) experiments using a hydrothermal reactor with aqueous S3−(S2−)-S0-sulfate-sulfide fluids and pyrite across a wide range of temperatures (300–450 °C), pressures (300–800 bars), fluid acidity (4 < pH < 8) and with elevated total sulfur concentrations (0.1–1.0 mol/kg fluid) favorable for formation of those polymeric sulfur species, (ii) precise quadruple S isotope analyses of the different S-bearing aqueous species in sampled fluids and in-situ precipitated pyrite, and (iii) thermodynamic modeling of sulfur aqueous speciation and solubility. Our results quantitatively confirm both equilibrium and kinetic SO4-H2S and pyrite-H2S mass dependent fractionation (MDF) factors previously established using extensive experimental and natural data from more dilute fluids in which polymeric sulfur species are negligible. MDF signatures of S0 measured in the sampled fluids of this study reveal different S0-forming pathways such as (i) breakdown on cooling of S3− (and S2−) and other chain-like S0 polymers only stable at high temperature and being isotopically identical to H2S; (ii) cyclooctasulfur (S80, liquid or solid) precipitating by recombination of sulfate and sulfide and/or by exchange with polysulfide dianions (Sn2−) on cooling and being slightly 34S-enriched compared to H2S (by ∼2‰ of δ34S); and iii) a different type of S0 resulting from thiosulfate irreversible breakdown and being highly 34S-depleted (by ∼12‰) relative to H2S. Our data do not show any significant mass independent fractionation (MIF) of 33S and 36S, with Δ33S and Δ36S values of any S aqueous species and pyrite being within ±0.1‰ and ±1.0‰, respectively. Therefore, under the investigated experimental conditions, the radical S3− ion is unlikely to generate significant MIF in the hydrothermal fluid phase and in pyrite and native sulfur precipitated therefrom. Our study supports the existing interpretations of small Δ33S and Δ36S variations between sulfide/sulfate-bearing fluid and pyrite as MDF in terms of reaction kinetics, different reaction pathways, and mass conservation effects such as mixing of S reservoirs or Rayleigh distillation. Our data extend, across a wider range of sulfur concentration and chemical speciation, the existing multiple S isotopes models that exploit such variations as a complement to the traditional δ34S tracer to monitor the approach to equilibrium and evolution of hydrothermal fluids.

中文翻译：

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自由基离子和分子硫存在下水热系统中的多硫同位素分馏

本研究旨在评估硫自由基离子（S3-和S2-）和分子硫（S0）对硫同位素分馏所起的作用，并研究这些物质是否会在热液系统中留下同位素指纹。为此，我们将 (i) 使用水热反应器的实验与 S3-(S2-)-S0-硫酸盐-硫化物水溶液和黄铁矿相结合，在广泛的温度 (300-450 °C)、压力 (300-800条）、流体酸度（4 < pH < 8）和总硫浓度升高（0.1-1.0 mol/kg 流体）有利于形成这些聚合硫物种，(ii) 不同含硫的精确四重 S 同位素分析样品流体中的含水物质和原位沉淀黄铁矿，以及 (iii) 硫水形态和溶解度的热力学模型。我们的结果定量证实了平衡和动力学 SO4-H2S 和黄铁矿-H2S 质量相关分馏 (MDF) 因子，这些因子之前使用来自更稀的流体的广泛实验和自然数据建立，其中聚合物硫物质可以忽略不计。在本研究的采样流体中测量的 S0 的 MDF 特征揭示了不同的 S0 形成途径，例如（i）在 S3−（和 S2−）和其他链状 S0 聚合物冷却时分解，仅在高温下稳定且同位素相同到 H2S；(ii) 环八硫（S80，液体或固体）在冷却时通过硫酸盐和硫化物的重组和/或通过与多硫化物二价阴离子（Sn2-）交换而沉淀，并且与 H2S 相比略微富含 34S（δ34S 的约 2‰）；iii) 硫代硫酸盐不可逆分解产生的不同类型的 S0，相对于 H2S，34S 高度耗尽（约 12‰）。我们的数据没有显示 33S 和 36S 的任何显着的质量独立分馏 (MIF)，任何 S 水性物质和黄铁矿的 Δ33S 和 Δ36S 值分别在 ±0.1‰ 和 ±1.0‰ 以内。因此，在研究的实验条件下，自由基 S3- 离子不太可能在热液流体相和黄铁矿和从中沉淀的天然硫中产生显着的 MIF。我们的研究支持在反应动力学、不同反应途径和质量守恒效应（如 S 储层的混合或瑞利蒸馏）方面对含硫化物/硫酸盐流体和黄铁矿作为 MDF 之间的小 Δ33S 和 Δ36S 变化的现有解释。我们的数据扩展，