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1. Isotopic evidence of long-lived volcanism on Io

   https://doi.org/10.1126/science.adj0625  

   de_Kleer, Katherine; Hughes, Ery C; Nimmo, Francis; Eiler, John; Hofmann, Amy E; Luszcz-Cook, Statia; Mandt, Kathy (May 2024, Science)

   Jupiter’s moon Io hosts extensive volcanism, driven by tidal heating. The isotopic composition of Io’s inventory of volatile chemical elements, including sulfur and chlorine, reflects its outgassing and mass-loss history and thus records information about its evolution. We used submillimeter observations of Io’s atmosphere to measure sulfur isotopes in gaseous sulfur dioxide and sulfur monoxide, and chlorine isotopes in gaseous sodium chloride and potassium chloride. We find³⁴S/³²S = 0.0595 ± 0.0038 (equivalent to δ³⁴S = +347 ± 86‰), which is highly enriched compared to average Solar System values and indicates that Io has lost 94 to 99% of its available sulfur. Our measurement of³⁷Cl/³⁵Cl = 0.403 ± 0.028 (δ³⁷Cl = +263 ± 88‰) shows that chlorine is similarly enriched. These results indicate that Io has been volcanically active for most (or all) of its history, with potentially higher outgassing and mass-loss rates at earlier times. 

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2. Tropospheric carbonyl sulfide mass balance based on direct measurements of sulfur isotopes

   https://doi.org/10.1073/pnas.2020060118  

   Davidson, Chen; Amrani, Alon; Angert, Alon (February 2021, Proceedings of the National Academy of Sciences)

   null (Ed.)

   Robust estimates for the rates and trends in terrestrial gross primary production (GPP; plant CO 2 uptake) are needed. Carbonyl sulfide (COS) is the major long-lived sulfur-bearing gas in the atmosphere and a promising proxy for GPP. Large uncertainties in estimating the relative magnitude of the COS sources and sinks limit this approach. Sulfur isotope measurements ( 34 S/ 32 S; δ 34 S) have been suggested as a useful tool to constrain COS sources. Yet such measurements are currently scarce for the atmosphere and absent for the marine source and the plant sink, which are two main fluxes. Here we present sulfur isotopes measurements of marine and atmospheric COS, and of plant-uptake fractionation experiments. These measurements resulted in a complete data-based tropospheric COS isotopic mass balance, which allows improved partition of the sources. We found an isotopic (δ 34 S ± SE) value of 13.9 ± 0.1‰ for the troposphere, with an isotopic seasonal cycle driven by plant uptake. This seasonality agrees with a fractionation of −1.9 ± 0.3‰ which we measured in plant-chamber experiments. Air samples with strong anthropogenic influence indicated an anthropogenic COS isotopic value of 8 ± 1‰. Samples of seawater-equilibrated-air indicate that the marine COS source has an isotopic value of 14.7 ± 1‰. Using our data-based mass balance, we constrained the relative contribution of the two main tropospheric COS sources resulting in 40 ± 17% for the anthropogenic source and 60 ± 20% for the oceanic source. This constraint is important for a better understanding of the global COS budget and its improved use for GPP determination. 

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3. Secular Evolution of Mantle Sulfur Isotopes Since the Archean

   https://doi.org/10.1029/2025GC012858  

   Eguchi, J; Black, B A; Nisbet, E G; Moussallam, Y; Georgeais, G (May 2026, Geochemistry, Geophysics, Geosystems)

   Abstract Sulfur isotopes in mantle plume‐derived magmas show heterogeneities attributed to recycling of material from the Earth's surface. The sedimentary sulfur isotope record exhibits dramatic temporal variations over Earth history, raising the question of whether its secular evolution is echoed in mantle plume‐derived magmas. We present new secondary ion mass spectrometry and X‐ray absorption near‐edge structure measurements of δ³⁴S and Fe³⁺/ΣFe in naturally glassy melt inclusions from the 2.7 Ga Belingwe komatiites. We find that δ³⁴S of Belingwe melt inclusions is relatively homogeneous (+3.2 ± 0.9) and elevated relative to the depleted upper mantle. We evaluate explanations for elevated magmatic δ³⁴S including degassing, sulfide fractionation, assimilation, subduction‐like processes, and mantle recycling. We find no evidence for significant sulfur isotope fractionation via degassing or sulfide saturation. The elevated δ³⁴S in the Belingwe komatiites could reflect late‐stage assimilation of sediments or seawater. Alternatively, Belingwe komatiites may have formed in a subduction‐like setting, as modern arcs display a bias toward positive δ³⁴S. However, we find these scenarios less favorable. Instead, our preferred interpretation is that elevated δ³⁴S was supplied to the komatiite mantle source via recycled lithologies such as sediments, altered oceanic crust, and/or pyroxenite. Combined with δ³⁴S data from mantle plume‐derived magmas spanning a wide age range, our results resemble the secular evolution of δ³⁴S in surface reservoirs. We suggest that the δ³⁴S record in mantle plume‐derived magmas may echo secular evolution in the surficial sulfur reservoir with a delay of several hundred million years, linking Earth's surface and interior sulfur cycles. 

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4. Oxidized Sulfur Species in Slab Fluids as a Source of Enriched Sulfur Isotope Signatures in Arcs

   https://doi.org/10.1029/2024GC011542  

   Beaudry, Patrick; Sverjensky, Dimitri A (July 2024, Geochemistry, Geophysics, Geosystems)

   Abstract Recycling of oxidized sulfur from subducting slabs to the mantle wedge provides simultaneous explanations for the elevated oxygen fugacity (fO2) in subduction zones, their high hydrothermal and magmatic sulfur outputs, and the enriched sulfur isotopic signatures (i.e., δ34S > 0‰) of these outputs. However, a quantitative understanding of the abundance and speciation of sulfur in slab fluids consistent with high pressure experiments is lacking. Here we analyze published experimental data for anhydrite solubility in H₂O‐NaCl solutions to calibrate a high‐pressure aqueous speciation model of sulfur within the framework of the deep earth water model. We characterize aqueous complexes, required to account for the high experimental anhydrite solubilities. We then use this framework to predict the speciation and solubility of sulfur in chemically complex fluids in equilibrium with model subducting mafic and ultramafic lithologies, from 2 to 3 GPa and 400 to 800°C at logfO₂from FMQ‐2 to FMQ+4. We show that sulfate complexes of calcium and sodium markedly enhance the stability of sulfate in moderately oxidized fluids in equilibrium with pyrite atfO₂conditions of FMQ+1 to +2, causing large sulfur isotope fractionations up to 10‰ in the fluid relative to the slab. Such fluids could impart oxidized, sulfur‐rich and high δ³⁴S signatures to the mantle wedge that are ultimately transferred to arc magmas, without the need to invoke³⁴S‐rich subducted lithologies. 

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5. Endogenous, yet Exotic, Sulfur in the Lunar Mantle

   https://doi.org/10.1029/2024JE008834  

   Dottin, J W; Simon, S B; Shearer, C K; Benson, J; Fu, H; Boesenberg, J S; Monteleone, B (September 2025, Journal of Geophysical Research: Planets)

   Abstract In situ sulfur isotope analyses of sulfides found in regolith particles from the recently opened Apollo 17 drive tube 73001/2 reveal variable³⁴S/³²S and³³S/³²S that range from that of typical lunar basalts to strongly³⁴S‐ and³³S‐depleted values (δ³⁴S = −4.1 ± 0.35 to +1.5 ± 0.029; Δ³³S = −2.8 ± 0.48 to −0.1 ± 0.27 (2σ)). All Δ³³S and δ³⁴S data are positively correlated. The correlation indicates mixing between at least two distinct sources of sulfur in the lunar mantle, one of which is associated with photochemically processed sulfur from a gaseous environment (strongly negative Δ³³S). The gaseous precursor and its preservation in endogenous sulfides require that either the sulfur was delivered to the lunar mantle early in its history through unknown processes or it is a remnant of the giant Moon‐forming impactor. 

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