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REVIEW 4 major objections 8 minor 10 references

Sulfur in Hydrothermal Fluids

T0 review · 4 major / 8 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read The chapter argues that in situ spectroscopy has established intermediate-valence sulfur species, especially the trisulfur radical ion $\mathrm{S}_3^{\bullet-}$, as controlling sulfur redox kinetics and metal transport in hydrothermal…

desk verdict A comprehensive and honest review of sulfur speciation in hydrothermal fluids, though the S3*- rate-controlling claim is a hypothesis, not an established result. read the letter →

arxiv 2506.05845 v1 pith:IHRRNDBB submitted 2025-06-06 physics.chem-ph

Gleb S. Pokrovski (GET) , Kalin Kouzmanov (UNIGE) , Stefansson Andri This is my paper
classification physics.chem-ph
keywords sulfurspeciationhydrothermalfluidstrisulfurradicalionoredepositformationinsituspectroscopysulfate-sulfidekineticsthermodynamicmodelingmetaltransport
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The chapter sets out to establish that the classic sulfate-versus-sulfide picture of sulfur in hydrothermal fluids is incomplete. Drawing on two decades of in situ analytical, experimental, and computational work, it argues that intermediate-valence sulfur species—polysulfides, thiosulfate, sulfite, and above all the trisulfur radical ion $\mathrm{S}_3^{\bullet-}$—are real, sometimes dominant components of deep hot fluids even though they are lost or altered in quenched samples. The practical stake is ore formation: because these species act as ligands for gold, copper, and other chalcophile metals, ignoring them mispredicts metal solubility, redox state, and where ore minerals precipitate. The chapter therefore builds and evaluates a thermodynamic speciation framework, based on HKF, Akinfiev-Diamond, density, and Ryzhenko-Bryzgalin equations of state, intended to replace simpler models for geological fluids. A sympathetic reader would take away that sulfur redox chemistry and metal transport in hydrothermal systems should be modeled with intermediate-valence species as first-order components, not as minor corrections.

What carries the argument

The central object is the trisulfur radical ion $\mathrm{S}_3^{\bullet-}$, a chain of three sulfur atoms carrying one unpaired electron, identifiable by its resonance-enhanced Raman band near 530 cm$^{-1}$ and its visible absorption in the 500–700 nm range. It does the work of linking sulfur redox kinetics to equilibrium thermodynamics: it forms from sulfate and sulfide, or from thiosulfate, at high temperature; it is the proposed rate-controlling intermediate in the eight-electron sulfate-sulfide exchange; and it complexes chalcophile metals. The argument is carried by a set of thermodynamic equations of state—HKF, Akinfiev-Diamond, density, and Ryzhenko-Bryzgalin—used to extrapolate sparse high-temperature data, together with in situ optical cells and synthetic and natural fluid inclusions that allow the species to be observed before quenching. The paper also uses the sulfidation-state concept as a compact variable tying sulfur speciation to ore-mineral stability.

What would settle it

Measure, by in situ resonance Raman or sulfur K-edge XANES, the concentration of $\mathrm{S}_3^{\bullet-}$ in natural fluid inclusions or in experimental sulfate-sulfide fluids of known temperature, pH, and total sulfur, and compare with equilibrium calculations from the chapter's recommended thermodynamic data; a systematic deficit of more than about one order of magnitude at 200–400 °C, or an isotopic exchange rate that does not track $\mathrm{S}_3^{\bullet-}$ abundance, would falsify the rate-controlling-intermediate claim.

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Extended reading notes

Core claim

On the chapter's own terms, its central claim is that progress in in situ approaches has changed the received picture of sulfur speciation in deep geological fluids and of sulfur's role in metal transport and ore formation. The load-bearing discovery is the trisulfur radical ion $\mathrm{S}_3^{\bullet-}$: detected by resonance Raman spectroscopy in experimental sulfate-sulfide and thiosulfate solutions above roughly 150–200 °C, in synthetic fluid inclusions, and in natural inclusions, this species is argued to be the rate-controlling intermediate in sulfate-sulfide redox reactions and a significant sulfur ligand at acidic to neutral pH. The chapter also argues that equilibrium among sulfur species, rather than exclusive dominance of sulfate and sulfide, governs fluid redox potential, sulfur isotope fractionation, and the solubility of metals such as gold. Where earlier kinetic models invoked thiosulfate as the controlling intermediate, the chapter presents $\mathrm{S}_3^{\bullet-}$ as the better-supported candidate, with polysulfide dianions dominating in alkaline solutions and polymeric zero-valent sulfur playing a role at low pH. In short, the authors contend that the middle oxidation states of sulfur are not transient curiosities but central actors in hydrothermal geochemistry.

Load-bearing premise

The load-bearing premise is that chemical equilibrium is attained among sulfur species in hydrothermal fluids; where sulfate-sulfide redox is kinetically frozen, especially at lower temperatures or alkaline pH, the predicted intermediate-valence speciation and the metal-transport conclusions built on it do not apply.

Editorial extensions

If this is right

  • Thermodynamic databases that omit $\mathrm{S}_3^{\bullet-}$ and other intermediate-valence species will miscompute sulfur redox state and mineral solubility in hydrothermal systems above roughly 200 °C.
  • Quenched samples and bulk fluid-inclusion leachates cannot be used alone to infer deep-fluid sulfur speciation, because intermediate species disproportionate or decompose on cooling; in situ measurements are required.
  • Sulfur isotope geothermometers and kinetic models based on sulfate-sulfide exchange must account for $\mathrm{S}_3^{\bullet-}$, and for polysulfides in alkaline fluids, or they will misestimate equilibration timescales.
  • Gold and other chalcophile metal transport models should include hydrosulfide, polysulfide, and radical-sulfur ligands, changing predicted ore deposition zones.
  • Fluid oxidation state and ore-forming potential are better expressed through sulfidation state than through oxygen fugacity alone.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the paper is right, sulfate-sulfide isotopic exchange rates could be used as a quantitative proxy for in situ $\mathrm{S}_3^{\bullet-}$ abundance, offering an indirect test of the thermodynamic data.
  • If the paper is right, the same speciation framework likely applies to other high-temperature aqueous environments where intermediate sulfur states are quenched, including seafloor hydrothermal plumes, subduction-zone fluids, and sulfur-rich planetary brines; the chapter hints at this when discussing radical oxysulfur carriers of oxidizing potential.
  • If the paper is right, thermochemical sulfate reduction models could be reinterpreted with $\mathrm{S}_3^{\bullet-}$ abundance as the master variable, shifting predictions of sour-gas formation and sulfur isotope signatures away from thiosulfate-based schemes.
  • If the paper is right, database builders could rewrite sulfide mineral solubility products in terms of $\mathrm{H}_2\mathrm{S}$ and $\mathrm{HS}^-$ rather than $\mathrm{S}^{2-}$, a change that would propagate through every equilibrium code that currently uses the free sulfide ion.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 8 minor

Summary. This manuscript is a book-chapter review of sulfur chemical speciation and behavior in hydrothermal fluids. It covers analytical and spectroscopic methods (sampling and wet chemistry, crush-leach fluid inclusions, LA-ICPMS, Raman, X-ray spectroscopies, UV-Vis, NMR, XPS, EPR), the kinetics and thermodynamics of aqueous sulfur species, the speciation of base, precious, and critical metals, and the application of these data to geothermal, basinal, magmatic-hydrothermal, and metamorphic fluids. The chapter's central thesis is that in situ experimental and analytical advances have produced 'paradigm changes' in understanding sulfur speciation in deep geological fluids—notably the identification and thermodynamic characterization of the trisulfur radical ion S3•−—and in its role in metal transport and ore deposit formation. The review explicitly flags many poorly constrained quantities, including the S2− dissociation constant, polythionate and persulfate Gibbs energies, and thiosulfate protonation constants, and it gives practical model recommendations (HKF, AD, RB, density model).

Significance. As a review, the chapter is comprehensive, well organized, and unusually candid about data quality: it quantifies uncertainties (e.g., >30 kJ/mol for polythionates, 1–2 log units for sulfate ion pairs), recommends writing sulfide solubility products with HS−/H2S rather than S2−, and distinguishes well-constrained thermodynamic data from speculative values. It also cites independent confirmations (Jacquemet et al. 2014; Truche et al. 2014) alongside the authors' own Raman and thermodynamic work, which reduces circularity concerns. If the main narrative is accepted, the chapter would be a useful reference for ore-deposit geochemistry and fluid-rock interaction modeling. The main weakness is that the strongest claim—that S3•− is the rate-controlling intermediate in sulfate–sulfide exchange, with paradigm-level consequences for metal transport—is built on detection and abundance arguments rather than kinetic measurements, and the manuscript itself concedes that a comprehensive kinetic model is missing.

major comments (4)
  1. [§4.3.2] The claim that S3•− 'may therefore act as the rate controlling intermediate' is not established by the cited evidence. In situ Raman detection above 150–200 °C and an abundance maximum at pH 4–6 demonstrate presence and abundance, but a species present in a reacting mixture may be a spectator or side product whose formation is not rate limiting. The chapter itself states that 'more accurate knowledge of the abundances of such intermediate-valence S-S-type species ... would be necessary to build a comprehensive kinetic model' and that thiosulfate (acidic pH) and polysulfide dianions (alkaline pH) remain viable alternatives. The text should either label the rate-control statement explicitly as a hypothesis with falsifiable kinetic tests, or present rate measurements of S3•− formation/consumption relative to the sulfate–sulfide exchange rate. As written, the statement overstates the evidence and is then used to support the stronger 'paradigm change' claim of the abstract.
  2. [§4.3.2 / §4.3.4] The contrast drawn in §4.3.2 between detection of S3•− and non-detection of thiosulfate or polysulfide dianions in Raman experiments is weakened by the detection limits acknowledged in §4.3.4 ('~0.01–0.1 m for Raman spectroscopy at such conditions'). Non-detection at those concentrations is consistent with thiosulfate or polysulfide abundances below Raman sensitivity but potentially relevant to natural fluids. The argument would need to show that the expected concentrations of these species under the experimental conditions exceeded the detection limits, or it should avoid using 'in contrast' as a disconfirmation of alternative intermediates.
  3. [§4.3.4 (Sulfide species, HS− ion)] There is a quantitative inconsistency in the discussion of H2S dissociation at high pressure. The text states that 'only a few percent of H2S will be ionized at 500 °C and 5 kbar in an aqueous fluid of neutral pH (because pK24 is ~2 log units lower than the pH of water neutrality point, Fig. 4.11)'. The Henderson–Hasselbalch relation gives [HS−]/[H2S] = 10^(pH − pK24); if pK24 is two log units lower than pH, the ratio is about 100, meaning H2S would be nearly fully deprotonated, not 'only a few percent' ionized. Either the sign of the offset or the numerical conclusion is wrong. Since HS− is identified elsewhere in the chapter as the key ligand for chalcophile metals, this inconsistency should be corrected and checked against Fig. 4.11 before it propagates into the metal-transport discussion.
  4. [Abstract / §4.1] The abstract and introduction state that recent progress 'led to paradigm changes about sulfur chemical speciation in deep geological fluids and its role in metal transport and ore deposit formation.' In view of the caveats acknowledged in §4.3.2 (equilibrium is assumed, comprehensive kinetic model missing) and §4.3.4 (large thermodynamic uncertainties for many intermediate-valence species), the paradigm-change claim is too strong as currently worded. The authors should distinguish between well-supported advances (in situ characterization of S3•− and improved thermodynamic data for sulfate/sulfide species) and hypotheses that remain to be tested (rate control by S3•−, quantitative control of metal transport by intermediate-valence sulfur species).
minor comments (8)
  1. [§4.2.4] Several typos in this section: 'Forth' should be 'Fourth', 'strenching bands' should be 'stretching bands', 'assymetry' should be 'asymmetry', and 'dymanics' should be 'dynamics'.
  2. [§4.2.4 and §4.3.4] The citation 'Fansang and Zajacz 2025' appears in §4.2.4, while 'Farsang and Zajacz 2025' is used in §4.3.4; please unify the spelling and check the reference-list entry.
  3. [§4.2.5] The author name 'Alfonso-Mori' appears in §4.2.5 while 'Alonso-Mori' is used later in the same section; please verify the correct spelling and years (2009, 2010) in the reference list.
  4. [§4.3.4] In the persulfate subsection, 'Williamson and Rimsdidt (1992)' is a typo for 'Rimstidt'; the correct spelling is used elsewhere in the chapter.
  5. [§4.3.4] In the sulfite/disulfite subsections, the citation 'Beyard et al. 2014' is inconsistent with 'Beyad et al. 2014'; please unify and verify the correct form.
  6. [§4.3.4 (Polysulfides)] The misspelling 'Kamishny' appears in the polysulfide subsection; the correct name is 'Kamyshny'.
  7. [§4.3.4 (Sulfate and Sulfide subsections)] Minor typographical issues include 'The pressure exerts an opposit trend' (should be 'opposite trend') and 'Helg eson and Shock (1988)' (likely 'Shock and Helgeson 1988').
  8. [Glossary] The glossary lists '(FP)MD: (first principles) modecular dynamics'; 'modecular' should be 'molecular'.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: this is a review chapter whose claims rest on cited experimental and computational measurements, including independent confirmations of the authors' own prior work.

full rationale

This manuscript is a review/overview of sulfur speciation in hydrothermal fluids, not a paper that derives new predictions from fitted inputs. Its load-bearing assertions—such as the stability of S3•− at elevated temperature and its possible role as a rate-controlling intermediate—are supported by cited in situ Raman measurements (Pokrovski and Dubessy 2015; Jacquemet et al. 2014; Truche et al. 2014; Barré et al. 2017) and by thermodynamic regressions from those experiments. Although the authors cite their own prior work in several places, those citations are not load-bearing in a circular sense: the key S3•− results are independently confirmed by other groups, and the review itself repeatedly flags open questions (e.g., the absence of a comprehensive kinetic model for sulfate–sulfide reactions, the equilibrium assumption, and the uncertain stability of S2−). The 'rate-controlling intermediate' claim is presented as a hypothesis with the word 'may', and the text explicitly notes that alternative mechanisms remain viable. No equation is defined in terms of a claimed prediction, no fitted parameter is relabeled as a prediction, and no uniqueness theorem from the authors is invoked to force a conclusion. This is therefore a normal non-circular literature review; any concerns about the strength of the kinetic evidence belong to scientific correctness, not circularity.

Assumptions & free parameters 3 free parameters · 4 assumptions · 0 invented entities

The review does not introduce new free parameters or invented entities. The scientific narrative rests on fitted thermodynamic parameters from the cited literature, primarily HKF and AD model coefficients, and on domain assumptions about equilibrium and representativeness. No machine-checked proofs or code are provided.

free parameters (3)
  • HKF model coefficients for aqueous sulfur species and metal complexes = Various values from SUPCRT92, SUPCRTBL, and cited regressions
    Used throughout Section 4.3.4 to predict the stability and distribution of sulfate, sulfide, sulfite, polysulfide, and S3*- species at elevated temperature and pressure. These coefficients are empirical fits to experimental data from the cited literature.
  • S3*- formation constant parameters = log K versus T polynomial or HKF coefficients from Pokrovski and Dubessy (2015)
    The chapter's emphasis on S3*- as a key intermediate relies on thermodynamic data fitted to Raman spectroscopic measurements in model solutions.
  • AD model parameters for H2S and SO2 = Three adjustable parameters per species from Akinfiev and Diamond (2003)
    Used to describe vapor-liquid partitioning of volatile sulfur species in hydrothermal fluids, as discussed in Section 4.3.3.
assumptions (4)
  • domain assumption Chemical equilibrium is attained in hydrothermal fluids for the thermodynamic calculations.
    Stated in Section 4.3.2 as a critical assumption. The authors acknowledge it can fail at lower temperatures and alkaline pH.
  • domain assumption Thermodynamic model extrapolations with the HKF and AD models remain valid beyond the fitted temperature-pressure range.
    Sections 4.3.3 and 4.3.4 rely on HKF and AD predictions to 500-800 C and high pressures while noting limited experimental validation above 300 C for many species.
  • domain assumption Spectroscopic detection of sulfur species in model solutions and fluid inclusions is representative of natural hydrothermal fluids.
    Section 4.2.4 notes Raman detection limits of hundreds of ppm sulfur, so S3*- detection in experiments required high total sulfur concentrations that are atypical for many natural fluids.
  • standard math Standard thermodynamic definitions of Gibbs energy, equilibrium constants, and the Born solvation model apply.
    Equations 4.2 to 4.6 and 4.17 to 4.18 are used without derivation as background formalism.

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Cite this review

Pith. "Pith review of Sulfur in Hydrothermal Fluids." pith.science (2026). https://pith.science/paper/IHRRNDBB

@misc{pith2026250605845,
  author       = {Pith},
  title        = {Pith review of: Sulfur in Hydrothermal Fluids},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/IHRRNDBB}},
  note         = {Machine review of arXiv:2506.05845}
}
read the original abstract

This chapter overviews sulfur chemical speciation and behavior in different types of hydrothermal fluids across the lithosphere, spanning from active geothermal systems to basinal brines, to metamorphic and magmatic-hydrothermal fluids. The information on sulfur in these fluids stems from a wide range of recent analytical, experimental and computational methods whose advantages and limitations are discussed. A special emphasis is given to in situ approaches that have enabled unprecedented insights into the fascinating sulfur chemistry in hydrothermal fluids. These insights motivated a critical review of the chemical speciation of key base, precious, and critical metals whose behavior is intimately linked to sulfur. The results allow the role played by the different sulfur ligands on hydrothermal transport of metals and ore deposit formation to be evaluated. An outline of major challenges and emerging perspectives for sulfur-related research in geological fluids ends up this chapter.

Figures

Figures reproduced from arXiv: 2506.05845 by the authors.

Figure 4
Figure 4. [PITH_FULL_IMAGE:figures/full_fig_p044_4.png] view at source ↗
Figure 4
Figure 4. a [PITH_FULL_IMAGE:figures/full_fig_p045_4.png] view at source ↗

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Reference graph

Works this paper leans on

10 extracted references · 10 canonical work pages

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    eochim Cosmochim Acta 72:5232–5242 Tremaine PR, LeBlanc JC (1980) The solubility of magnetite and the hydrolysis and oxidation of Fe2+ in water to 300 °C. J Soln Chem 9:415–442 Trigub AL, Tagirov BR, Kvashnina KO, Lafuerza S, Filimonova ON, Nickolsky MS (2017) Experimental determination of gold speciation in sulfide -rich hydrothermal fluids under a wide ...

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    45); ( b) oxygen fugacity corresponding to common redox buffers, QFM – quartz-fayalite-magnetite, NNO – nickel-nickel oxide, PPM – pyrite-pyrrhotite-magnetite, and HM – hematite-magnetite; (c) fluid acidity, pH = –log a(H+) at fO2 buffered by sulfide -sulfate equilibrium (H2S:SO4 = 4:1); (d) in equilibrium with gold metal as a function of pH at the condit...

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    that were used in its deviation (see Pokrovski and Dubessy 2015 for in- depth discussion of errors related to the use of other models and datasources). Chapter04, Pokrovski - Kouzmanov - Stefansson, Sulfur in Hydrothermal Fluids, submitted version for “The Role of Sulfur in Planetary Processes: from Cores to Atmospheres” D. Harlov and G.S. Pokrovski (eds....

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    Concentrations of aqueous sulfur species in hydrothermal saline fluid (KCl+NaCl, 10 wt% NaCl eq.) are shown as a function of temperature, along a typical pressure gradient (from 10 bar at 100 °C to 1000 bar at 600 °C) of hydrothermal ore deposit formation, in equilibrium with ( a) quar tz-faylite-magnetite- pyrrhotite, (b) pyrite-pyrrhotite-magnetite, and...

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    Ha34 = Hamer (1934), Da52 = Davis et al. (1952), Li61 = Lietzke et al. (1961), Ry64 = Ryzhenko (1964), MJ66 = Marshall and Jones (1966), Wa66 = Wallace (1966), RC69 = Readnour and Cobble (1969), MO88 = Matsushima and Okuwaki (1988), Os88 = Oscarson et al. (1988), Di90 = Dickson et al. (1990), Ru96 = Rudolph (1996), Hnedkovsky et al. (2005). Note that the ...

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    The Role of Sulfur in Planetary Processes: from Cores to Atmospheres

    and the AD model for H 2S (Akinfiev and Diamond 2003). WM32 = Wright and Maass (1932), EM67 = Ellis and Millestone (1967), EG71 = Ellis and Giggenbach (1971), Ts76 = Tsonopoulos et al. (1976), Sr77 = Sretenskaya (1977), KS78 = Kryukov and Starostina (1978), Barbero et al. (1982), SS97 = Suleimenov and Seward (1997). Note that both mode ls give almost undi...

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    The Role of Sulfur in Planetary Processes: from Cores to Atmospheres

    and the DEW model (Sverjensky et al. 2014), which yield differences of a few to 10s kJ/mol for the Gibbs energy value of OH– in particular near the water isochore of 0.4±0.05 g/cm3. b The updated version of the original SUPCRT database (2007) based on a series of subsequent pa...

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    The Role of Sulfur in Planetary Processes: from Cores to Atmospheres

    and various geothermal fields in Iceland (Kaasalanen and Stefánsson 2012; Markússon and Stefánsson 2011; Björke et al. 2015; Stefánsson et al. 2016). Alkaline hot springs represent boiled reservoir hydrothermal water at surface and are characterized by similar SO 42–/Cl– ratio...

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