Multiple sulfur isotope fractionation in hydrothermal systems in the presence of radical ions and molecular sulfur

Abstract

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.