First evidence of the trisulfur radical ion S3− and other sulfur polymers in natural fluid inclusions
Introduction
Sulfur speciation and concentration in deep basinal brines play a crucial role in the formation of sour (H2S-bearing) gas field (Orr, 1974, Worden and Smalley, 1996, Machel, 2001, Walters et al., 2015, Cai et al., 2016) and the genesis of many sedimentary-hosted ore deposits (Heydari and Moore, 1989, Basuki et al., 2008, Thom and Anderson, 2008). One of the most important reactions controlling the sulfur behavior in deep sedimentary environments is the abiogenic reduction of sulfate to sulfide coupled with the oxidation of hydrocarbons, which is termed thermochemical sulfate reduction (TSR). This reaction occurs at temperature above 100–140 °C and is under strong kinetic control (Kiyosu and Krouse, 1993, Goldhaber and Orr, 1995, Cross et al., 2004, Thom and Anderson, 2008, Zhang et al., 2008, Truche et al., 2009, Yuan et al., 2013). The sulfur speciation has also a major impact on the rate and extent of TSR, because both the reactivity of dissolved sulfate compounds (e.g. SO42 −, HSO4−, CaSO40, MgSO40) and the relative stability of intermediate valence sulfur species (e.g. thiosulfate, polysulfides, labile organic sulfur compounds) drive the electron transfer from S+ 6 to S2 − (Goldstein and Aizenshtat, 1994, Cai et al., 2003, Amrani et al., 2008, Ma et al., 2008, Gvirtzman et al., 2015). Most of our knowledge of sulfur speciation in saline brines from deep burial diagenetic environments relies on measurements made on quenched fluids from surface springs, water/oil production wells, and fluid inclusions entrapped in minerals. Sulfate and sulfide are the dominant dissolved sulfur species in these environments, with concentrations up to 0.5 mol/kgH2O (Boiron et al., 1999, Horita et al., 2002, Worden et al., 2003). Other intermediate valence sulfur species such as sulfite (SO3−), thiosulfate (S2O32 −), polythionates (SnO62 −), polysulfides (Sn2 −) or dissolved elemental sulfur (S0) have been found in surface hydrothermal springs, and sulfide rich water well (Boulègue, 1978, Takano, 1987, Webster, 1987, Veldeman et al., 1991, Takano et al., 1994, Xu et al., 1998, Kamyshny et al., 2008, Nordstrom et al., 2009, Kaasalainen and Stefánsson, 2011). However, their speciation and concentration may not reflect the true sulfur chemistry in deep geological fluids that is known to be very sensitive to temperature (T), pressure (P), redox, and pH conditions (Giggenbach, 1974, Ohmoto and Lasaga, 1982, Barnes, 1997).
The recent discovery of the trisulfur ion S3− in aqueous S-rich fluids from laboratory experiments using in-situ Raman spectroscopy at T > 200 °C, acidic-to-neutral pH (2–6), and redox condition enabling coexistence of sulfate and sulfide has challenged our interpretation of sulfur behavior in hydrothermal fluids (Pokrovski and Dubrovinsky, 2011, Chivers and Elder, 2013, Jacquemet et al., 2014, Truche et al., 2014, Pokrovski and Dubessy, 2015). Nevertheless, little attention has been given to the sulfur speciation in natural geological fluids, and the concentration of intermediate sulfur species at relevant T-P is unknown. To our knowledge there is no study on sulfur speciation in natural fluid inclusions at elevated T.
Here, we performed quantitative in-situ analysis of sulfur speciation in well-preserved natural fluid inclusions from a geological setting where TSR occurred. We used in-situ Raman spectroscopy coupled to a heating stage to analyze samples under temperatures representative of their entrapment conditions.
Section snippets
Geological setting
For the purpose of this study, rock samples were selected from the Carnian evaporites (upper Triassic), named “gypse nappe” formation, outcropping in the Arc Valley (French Alps), between Modane and Sollières-l'Envers (Fig. 1). This geological formation exposes to the surface a world-class evidence of TSR occurrence in previously deeply buried evaporites, with coexistence of sulfate (anhydrite), sulfide (pyrite), and native sulfur at both macro (Fig. 2) and microscopic scale (not shown). The
Choice of the host mineral
The natural fluid inclusions studied here are hosted in quartz, fluorite or anhydrite. Quartz and fluorite minerals are present as small crystals (100 μm to 1 cm of diameter) embedded in the anhydrite matrix. Fluorite is the best host mineral to study sulfur speciation by Raman spectroscopy, because of the absence of peaks overlap between fluorite and sulfur species. However, thermal dilatation of fluorite may slightly change the liquid-vapor ratio in the fluid inclusion (Bodnar and Bethke, 1984)
Fluid inclusions characterization
At 25 °C, all studied fluid inclusions contain several phases: aqueous liquid, gas, spherules of native sulfur, and crystals of halite. All fluid inclusions display similar salinities (25 ± 4 wt% eq. NaCl, with three times more NaCl than CaCl2) and fluid composition (Supplementary Tables S1 and S2). Th values ranged from 98 to 355 °C, with a quasi-Gaussian distribution centered at 176 °C (Fig. 3).
Fluid inclusions composition at room temperature (25 ± 2 °C) was analyzed by Raman spectroscopy for each
Representativeness and preservation of the fluid inclusions
Here, all the studied fluid inclusions display the same textural and chemical features irrespective of their host minerals: i) same irregular shape, ii) same liquid-vapor ratio (≈ 8% ± 4%) and chemistry, iii) same salinities, and iv) same sulfur speciation. The fluid inclusions contain H2 and CH4, demonstrating that the reducing conditions have been preserved. This observation does not contradict with the presence of sulfate in the geological fluid because its reduction is under strong kinetic
Conclusions and perspectives
The main conclusions from this study are:
- 1.
We reveal for the first time the formation of the trisulfur S3− ion and other polymeric S species (Sn2− ± Sn0) at T > 100 °C in natural fluids from typical deeply buried sedimentary environment where TSR occurred.
- 2.
The concentration of S3− ion can reach 2800 ppm at 300 °C in fluid inclusions containing sulfate-sulfide concentration above 0.1 mol/kgH2O.
- 3.
This work confirms the previous findings that S3− is a major stable intermediate valence S species involved in the
Acknowledgements
This work was funded by LABEX ANR-10-LABX-21-01 Ressources21 (Strategic metal resources of the 21st Century) and the French Ministry of Higher Education and Research. The authors are extremely grateful to M.C. Jodin-Caumon and P. Robert for technical assistance during Raman spectroscopic analysis and O. Barrès during Infrared spectroscopic analysis. The paper benefited greatly from the review provided by G. Pokrovski, H. Ohmoto and one anonymous reviewer. We thank M. Böttcher for editorial
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