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A seawater-sulfate origin for early Earth’s volcanic sulfur


Mass-independent fractionation of sulfur isotopes (MIF-S)—as recorded primarily in pre-2.5 billion years ago (Ga) sedimentary rocks—has been interpreted as evidence of photolysis of volcanic SO2 in an anoxic troposphere. Here, I present thermodynamic and kinetic calculations, combined with data on the geology, mineralogy and chemical and isotopic compositions of modern and Archaean (3.8–2.5 Ga) aged volcanic samples from different tectonic settings, to examine early Earth’s sulfur cycle. Based partly on the similarities between submarine hydrothermal deposits and arc volcanic rocks in pyrite (FeS2) abundances and sulfur isotopic compositions (for example, the presence of both positive and negative δ34S values), I conclude that degassing of sulfur (mostly as SO2) into the atmosphere has been carried out primarily by subaerial eruptions of oxidized, arc-like magmas since at least 3.5 Ga. The generation of volcanic SO2 requires plate tectonics and the involvement of sulfate-rich seawater, which requires large exposed lands and an oxygenated atmosphere. I propose that the MIF-S signatures in sedimentary rocks were created by ultraviolet photochemical reactions between SO2 from explosive volcanic eruptions and O2 in the stratosphere, above an oxygen-rich troposphere, or by high-temperature reactions between organic compounds and sulfate in the oceans.

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Fig. 1: Chemical compositions of volcanic gases (theoretical).
Fig. 2: Isotopic and chemical compositions of volcanic gases and igneous rocks (measured).
Fig. 3: Sulfur isotopic compositions of submarine hydrothermal deposits of Archaean ages.
Fig. 4: Sulfur chemistry of hydrothermal fluids (theoretical).
Fig. 5: The Archaean sulfur cycles.

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Data availability

Methods, Extended Data and Supplementary Information are available in the online version of the paper. The data used for construction of figures and tables in the main body and in the Extended Data are presented in the Supplementary Information. Source data are provided with this paper.


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I thank A. Chorney, Y. Watanabe and A. Ishida for sulfur isotope analyses of samples from the Panorama and Big Stubby deposits, and Z.-K. Liu and T. Ikeshima for suggestions regarding thermochemical calculations. I am grateful to H. Barnes, D. Cole, D. Eggler, K. Furlong, Z.-K. Liu, H. Naraoka, S. Ono, S. Poulson, E. Ripley, A. Rose, B. Voight, Y. Watanabe and B. Scailler for comments on an earlier manuscript, and to A. Whitfield, K. Spangler and A. Dilla for proofreading the manuscript. I greatly benefited from discussions with A. Hickman, F. Albaréde, J. Brainard, J. Korenaga, T. Kakegawa, K. Yamaguchi, T. Otake, M. Schoonen and S. Ono on various topics in this paper. Years of research support from the NASA Astrobiology Institute, the National Science Foundation, the Japanese Ministry of Science, Education and Sports and The Pennsylvania State University are gratefully acknowledged.

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Extended data

Extended Data Fig. 1 A phase diagram for aqueous sulfur species (H2S(aq), HSO4 and SO2(aq)).

The conditions are: ΣS = 0.01 mol/kg H2O, pH = 2 and T = 25–350 °C. It shows that SO2(aq) dissociates into H2S(aq) and HSO4 when cooled to < ~300 °C and forms native sulphur (S(s, l)) at T < ~200 °C. At 25 °C, the concentration of SO2(aq) in the fluids will be reduced to < ~10−10 m; the partial pressure of SO2(g) in equilibrium with this solution would be < ~10−10 bar (modified after Drummond14). See Supplementary Table 1 in Supplementary Information for the pertinent thermodynamic data.

Extended Data Fig. 2 Examples of exposed land surfaces and subaerial volcanism during the Archaean.

a, A photo showing: (i) the world’s oldest erosional unconformity with paleosols that developed during the 3,425–3,350 Ma period on top of a deep-submarine volcanic sequence; (ii) the overlying shallow-water sediments of the Strelley Pool Formation (conglomerates, sandstones, carbonates and cherts); and (iii) the subaerial Euro Basalt. The photo was taken on the Steer Ridge in East Pilbara, Western Australia (738939E, 7655261N, looking NW). b, A generalized stratigraphic column of the Pilbara Super Group, Western Australia, showing the two long periods of subaerial erosion (modified after Hickman68 and van Kranendonk et al.69).

Extended Data Fig. 3 Physical and chemical processes in magmas.

a, The changes in volume and mechanical energy released during the second-boiling reaction: H2O-saturated melt → (crystals+’vapor’). Values of ΔVr and PΔV are for the complete crystallization of a granodiorite magma with an initial H2O content of 2.7 wt%. (Modified after Burnham & Ohmoto20). b, Relationship between Fe3+/ΣFe ratios and logfO2 (relative to the FMQ buffer) values of igneous rocks based on the equation by Berry et al.21. c, Relationships between logfO2 and SO42−/ΣS values of volcanic rocks according to Carrol & Webster22.

Extended Data Fig. 4 Chemical compositions of volcanic gases (measured).

Comparisons of the H2/H2O ratios versus: a, the SO2/H2S ratios and b, HCl/HF ratios (See Supplementary Tables 1 and 2 in S.I. for the data). Note that the high-T volcanic gases from arc volcanoes are more oxidized (log(H2/H2O)=<−1.5), SO2-rich (SO2/H2O>1) and HCl-rich (HCl/HF>1) compared to those from ocean island volcanoes.

Extended Data Fig. 5 Sulfur isotope systematics of the Phanerozoic submarine hydrothermal systems.

a, A schematic illustration of S isotopic characteristics of volcanogenic massive sulphide (VMS) deposits. H2S(HTF) is comprised of H2S(I) from leaching of FeS in submarine volcanic rocks and H2S(II) from thermochemical reduction of seawater sulfate. b & c, Modeling the δ34S values of H2S(HTF) in submarine hydrothermal fluids based on a mixing model (see Methods). The yellow box represents the common ranges for the δ34SH2S(HTF) (+5 ± 3‰), F (0.15-0.5), mH2S(HTF) (10-3±1 mol/kg H2O) and the minimum ΣSO42− concentrations of the contemporaneous seawaters (10-2.5±1.2 mol/kg H2O) during the Phanerozoic.

Extended Data Fig. 6 Sulfur isotope systematics of the Archaean submarine hydrothermal systems (I).

a & b, Modeling the δ34S values of H2S(HTF) in submarine hydrothermal fluids based on a mixing model (see Methods) suggests the common ranges for the δ34SH2S(HTF) (+1 ± 2‰), F (0 - 0.1) and mH2S(HTF) (10-2±1 mol/kg H2O) in the submarine hydrothermal fluids and the minimum ΣSO42− concentrations of the contemporaneous seawaters (10-2±1 mol/kg H2O. (b) illustrate the method of estimating the minimum ΣSO42− of Archaean seawater from a Δ33Ssulphide value. (c) An analysis by the author of S isotope data on the 2.7 Ga Kidd Creek VMS data by Jamieson et al.60 (see Methods).

Extended Data Fig. 7 Sulfur isotope systematics of the Archaean submarine hydrothermal systems (II).

δ34S and Δ33S values of sulphides (mostly pyrite) and barite from submarine hydrothermal deposits of Archaean ages, showing that the H2S in the ore-forming fluids were mostly mixtures of H2S(I) from leaching of igneous FeS and H2S(II) from inorganic thermochemical reduction of seawater ΣSO42− with δ34S= +5 ± 3 ‰ and Δ33S = −1 ± 0.5 ‰. (See Supplementary Information Table 5 for the data). The H2S(III) in (d) was possibly created by thermochemical reduction of seawater sulphates by organic matter under submarine hydrothermal conditions.

Extended Data Fig. 8 A logfS2—logfO2 diagram for major Fe-S-O bearing minerals at T = 600 °C and fH2O=15 kb.

Fa: fayalite, Po: pyrrhotite, py: pyrite, M: magnetite, H: haematite, S(l): liquid sulphur. Green circle: Normal mantle conditions, blue circle: Po + Py + M coexistence, red circle and red line: Py + M ( ± H) coexistence, common conditions for the fluids from hydrothermally-altered oceanic crust. Note that the fH2S values depend on fH2O, increasing with increasing fH2O, but the fSO2 values are independent of fH2O.

Extended Data Fig. 9 Geochemical signatures in submarine basalts of O2- and U-rich modern deep-sea water.

Depth profiles of Fe3+/ΣFe ratios a, and U/Th ratios b, of oceanic basalts in ODP drill holes. (see Supplementary Information Table 5 for the data).

Extended Data Fig. 10 Geochemical signatures in submarine basalts of O2- and U-rich Archaean deep-sea water.

a, U/Th ratios and Ce anomalies in submarine volcanic rocks associated with the 2.8 Ga Lady Asima VMS deposits the 2.6 Ga Hutti VMS deposits in India. b, U/Th and Fe3+/ΣFe ratios of magnetite-series granitic batholiths and plutons (3.1–3.45 Ga in ages) from the Kaapvaal Craton, South Africa. (see Supplementary Information Table 6 for the data).

Supplementary information

Supplementary Information

Supplementary Discussions.

Supplementary Table 1

Therodynamic data used in this study.

Supplementary Table 2

Chemical compositions of volcanic gases.

Supplementary Table 3

Sulfur isotopic compositions of volcanic gases.

Supplementary Table 4

Contents and isotopic compositions of sulfur in volcanic rocks.

Supplementary Table 5

Sulfur isotopic compositions of submarine hydrothermal deposits of Archaean ages.

Supplementary Table 6

Redox element geochemistry of Archaean igneous rocks.

Source data

Source Data Fig. 2

Chemical and isotopic compositions of volcanic gases.

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Ohmoto, H. A seawater-sulfate origin for early Earth’s volcanic sulfur. Nat. Geosci. 13, 576–583 (2020).

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