{"id":"a34d4a2b-49b3-4e33-a754-1cf177ec76a1","arxiv_id":"2506.05845","paper_version":1,"verdict":"UNVERDICTED","confidence":"MODERATE","novelty_score":3.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"A book chapter reviewing sulfur speciation in hydrothermal fluids, its measurement, thermodynamic modeling, and its role in metal transport and ore deposit formation.","lead":"This preprint is a book chapter that reviews how sulfur behaves in hot fluids underground, from geothermal systems to magma-related fluids, and how that behavior controls whether metals like gold and copper form ore deposits. It weighs the analytical methods and thermodynamic models used to measure sulfur speciation, and identifies where the data are solid and where they are not.","discovery_kind":"review","skeptic_critique":{"model":"deepseek-v4-flash","headline":"S3*- is advanced as rate-controlling intermediate based on in situ detection, not on kinetic measurements; the paradigm-shift claim rests on an unverified assignment.","rationale":"The reader correctly identified that equilibrium attainment is a necessary assumption for the thermodynamic predictions, but the more specific and more load-bearing weakness is the kinetic role of S3*- itself. The chapter's paradigm-shift claim depends on S3*- being the rate-controlling intermediate; yet the evidence is presence-based and the chapter itself flags the missing kinetic model. This is a concrete scientific gap, not merely a caveat about equilibrium. However, because the chapter already uses tentative language ('may therefore act') and explicitly calls for more data, the reader's UNVERDICTED verdict remains appropriate: the review is honest about its limitations, but the central claim is not verified. The paper would be strengthened by either direct kinetic evidence or a clear statement that the S3*- mechanism is a working hypothesis. No change to the verdict is needed, as UNVERDICTED already conveys that the central claim is not established.","tokens_in":50639,"tokens_out":3833,"duration_ms":38087,"concrete_test":"Re-analyze the time-resolved datasets underlying the cited S3*- studies (e.g., Pokrovski and Dubessy 2015; Truche et al. 2014; Jacquemet et al. 2014): extract simultaneous concentrations of S3*-, sulfate, sulfide, and thiosulfate during sulfate-sulfide equilibration at 300°C and pH 5. Test whether the observed sulfate-sulfide exchange rate follows a rate law based on S3*- abundance (rate = k[S3*-]^n, with the independently measured S3*- formation constant). If the predicted rate deviates by more than an order of magnitude from the measured rate, or if a model excluding S3*- fits equally well, the rate-controlling role is falsified. Alternatively, conduct an isotopic tracer experiment using 34S-labeled sulfide: if S3*- is rate-controlling, the label should appear in S3*- before sulfate, with a transient enrichment consistent with its steady-state abundance.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The chapter's strongest claim (Section 4.3.2) is that intermediate-valence sulfur species, specifically the trisulfur radical ion S3*-, control sulfur redox chemistry and thereby metal transport in hydrothermal fluids. The evidence offered is that S3*- is observed by Raman spectroscopy in sulfate-sulfide solutions at >150-200°C and that its abundance maximum lies at pH 4-6, where the sulfate-sulfide exchange is moderately fast. Neither observation establishes rate control. A species present in a reacting mixture may be a spectator, a side product, or an intermediate whose formation is not rate-limiting. The chapter itself concedes that a comprehensive kinetic model is missing ('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 alternative mechanisms (thiosulfate at acidic pH, polysulfide dianions at alkaline pH) remain viable. Consequently, the central assertion that S3*- 'may therefore act as the rate controlling intermediate' is a hypothesis, not a demonstrated result. If S3*- is not rate-controlling, the paradigm-change claim about sulfur speciation controlling metal transport is unsupported.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","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).","tokens_in":50811,"tokens_out":12947,"duration_ms":108021,"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":[{"comment":"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.","section":"§4.3.2"},{"comment":"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.","section":"§4.3.2 / §4.3.4"},{"comment":"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.","section":"§4.3.4 (Sulfide species, HS− ion)"},{"comment":"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).","section":"Abstract / §4.1"}],"minor_comments":[{"comment":"Several typos in this section: 'Forth' should be 'Fourth', 'strenching bands' should be 'stretching bands', 'assymetry' should be 'asymmetry', and 'dymanics' should be 'dynamics'.","section":"§4.2.4"},{"comment":"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.","section":"§4.2.4 and §4.3.4"},{"comment":"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.","section":"§4.2.5"},{"comment":"In the persulfate subsection, 'Williamson and Rimsdidt (1992)' is a typo for 'Rimstidt'; the correct spelling is used elsewhere in the chapter.","section":"§4.3.4"},{"comment":"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.","section":"§4.3.4"},{"comment":"The misspelling 'Kamishny' appears in the polysulfide subsection; the correct name is 'Kamyshny'.","section":"§4.3.4 (Polysulfides)"},{"comment":"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').","section":"§4.3.4 (Sulfate and Sulfide subsections)"},{"comment":"The glossary lists '(FP)MD: (first principles) modecular dynamics'; 'modecular' should be 'molecular'.","section":"Glossary"}],"recommendation":"major_revision","confidential_remarks":"The chapter is a strong review and is broadly suitable for the volume, but the rate-control claim for S3•− in Section 4.3.2 is the authors' own hypothesis and should receive the same critical distance applied to other species; the abstract should not present it as established. The HS−/pK inconsistency in Section 4.3.4 is a concrete numerical error that should be checked carefully. Note also that the version provided for review was truncated after Section 4.3.4, so Sections 4.4–4.7 (metals, geological fluids, perspectives) could not be fully assessed; the complete manuscript should be available for the final decision."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Here's my take for your files. This is a review chapter, not a primary paper, so don't look for new data. What it does well is critical synthesis: it covers analytical methods, thermodynamic models, and metal speciation with a clear eye for data quality. It repeatedly flags unreliable constants—the S2– dissociation constant that spans ten orders of magnitude, polythionate Gibbs energies with >30 kJ/mol spread, even duplicated nonsense values for protonated persulfates in standard compilations. The recommendation to exclude S2– from speciation models and to express sulfide mineral solubility via H2S/HS– rather than S2– is pragmatic and well argued. This is the kind of chapter that deserves to be on the shelf of anyone modeling hydrothermal systems.\n\nThe soft spot is the S3*- business. In section 4.3.2 the authors write that S3*- 'may therefore act as the rate controlling intermediate' in sulfate-sulfide reactions, based on its Raman detection above 150–200°C and its abundance maximum at pH 4–6. That is presence in the right place, not rate control. A species can be a spectator or a side product. The authors themselves admit that a comprehensive kinetic model is missing and that thiosulfate (acidic) and polysulfide dianions (alkaline) remain viable alternatives. So the paradigm-change claim in the introduction is stronger than the evidence, and that sentence should be softened to 'candidate intermediate' until kinetic measurements settle it. This is not a fatal flaw—the chapter is otherwise careful and self-critical—but it is a real overreach.\n\nAll in all, it's a valuable review. I'd send it to peer review; the geochemistry community needs this kind of critical assessment. My recommendation is to accept it with minor revisions, specifically to reframe the S3*- claim as a hypothesis and to cut the 'paradigm change' rhetoric down to what the data support. Who is this for? Grad students and researchers entering hydrothermal geochemistry, as well as experimentalists; the literature coverage is current and the data-quality flags are a genuine service.","headline":"A comprehensive and honest review of sulfur speciation in hydrothermal fluids, though the S3*- rate-controlling claim is a hypothesis, not an established result.","tokens_in":51375,"tokens_out":2592,"would_cite":true,"duration_ms":27282,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"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…","keywords":["sulfur speciation","hydrothermal fluids","trisulfur radical ion","ore deposit formation","in situ spectroscopy","sulfate-sulfide kinetics","thermodynamic modeling","metal transport"],"falsifier":"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.","tokens_in":50408,"feed_emoji":"🌋","tokens_out":11774,"duration_ms":109701,"temperature":0.7,"pith_summary":"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.","feed_headline":"Sulfur's hidden middle states control how ore deposits form","feed_subtitle":"In situ spectroscopy suggests the trisulfur radical ion S3•−, not just sulfate and sulfide, carries metals in deep fluids.","key_machinery":"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.","core_discovery":"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.","pith_inferences":["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."],"forward_implications":["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."],"supporting_citations":[{"why":"It provides the in situ resonance-Raman detection of S3•− and the thermodynamic properties used for the radical and related species.","marker":"Pokrovski and Dubessy 2015"},{"why":"It shows S3•− forming in synthetic fluid inclusions at high temperature and pressure, establishing its stability in confined hydrothermal fluids.","marker":"Jacquemet et al. 2014"},{"why":"It reports the first direct Raman detection of S3•− in natural fluid inclusions and supplies thermodynamic data for polysulfide ions.","marker":"Barré et al. 2017"},{"why":"It defines the classic thiosulfate-mediated kinetic model of sulfate-sulfide exchange that the chapter revises.","marker":"Ohmoto and Lasaga 1982"},{"why":"It offers the alternative polysulfide-mediated kinetic model against which the radical-ion mechanism is compared.","marker":"Chu et al. 2004"},{"why":"It detects S3•− in thermochemical sulfate reduction experiments, extending the radical's role to organic-bearing systems.","marker":"Truche et al. 2014"},{"why":"It supplies revised thermodynamic properties for polysulfide dianions used in the chapter's speciation calculations.","marker":"Kamyshny et al. 2007"},{"why":"It shows that S2− is effectively absent even in concentrated alkaline solutions, supporting speciation schemes written in terms of H2S and HS−.","marker":"May et al. 2018"},{"why":"It constrains high-temperature thiosulfate decomposition rates and sulfur isotope behavior, grounding the kinetic pathway that produces S3•−.","marker":"Kokh et al. 2020"}],"fun_headline_variants":["Trisulfur radical ion steers metal transport in deep fluids","S3•−: the reactive sulfur species behind ore deposits","Middle sulfur states, not sulfate alone, carry metals in fluids","In situ Raman reveals S3•− as key hydrothermal ligand","Sulfur's trisulfur radical redefines deep fluid transport"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"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.","fun_headline_variants_meta":{"raw":{"variants":["Trisulfur radical ion steers metal transport in deep fluids","S3•−: the reactive sulfur species behind ore deposits","Middle sulfur states, not sulfate alone, carry metals in fluids","In situ Raman reveals S3•− as key hydrothermal ligand","Sulfur's trisulfur radical redefines deep fluid transport"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000373,"raw_usage":{"total_tokens":1976,"prompt_tokens":913,"completion_tokens":1063,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":529,"completion_tokens_details":{"reasoning_tokens":977}},"tokens_in":529,"tokens_out":1063,"duration_ms":11053,"temperature":1.0,"reasoning_tokens":977,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-07T10:13:02.281109+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"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.","supporting_citations":[{"cited_title":"The Role of Sulfur in Planetary Processes: from Cores to Atmospheres","cited_arxiv_id":null,"evidence_quote":"It provides the in situ resonance-Raman detection of S3•− and the thermodynamic properties used for the radical and related species."}],"review_version":1}