Sulfur isotope evidence for surface-derived sulfur in Eoarchean TTGs

The radiogenic Pb isotope compositions of basalts from the Samoan hotspot suggest various mantle endmembers contribute compositionally distinct material to lavas erupted at different islands [1]. Basalts from the Samoan islands sample contributions from all of the classical mantle endmembers, including extreme EM II and high 3He/4He components, as well as dilute contributions from the HIMU, EM I, and DM components. Here, we present multiple sulfur isotope data on sulfide extracted from subaerial and submarine whole rocks associated with several Samoan volcanoes&#8212;Malumalu, Malutut, Upolu, Savaii, and Tutuila&#8212;that sample the full range of geochemical heterogeneity at Samoa and allow for an assessment of the S-isotope compositions associated with the different mantle components sampled by the Samoan hotspot. We observe variable S concentrations (10-1000 ppm) and &#948;34S values (-0.29&#8240; to +4.84&#8240; &#177; 0.3, 2&#963;). The variable S concentrations likely reflect weathering, sulfide segregation and degassing processes. The range in &#948;34S reflects mixing between the primitive mantle and recycled components, and isotope fractionations associated with degassing. The majority of samples reveal &#916;33S within uncertainty of &#916;33S=0 &#8240; &#177; 0.008, suggesting &#916;33S is relatively well mixed within the Samoan mantle plume. Important exceptions to this observation include: (1) a negative &#916;33S (-0.018&#8240; &#177;0.008, 2&#963;) from a rejuvenated basalt on Upolu island (associated with a diluted EM I component) and (2) a previously documented small (but resolvable) &#916;33S values (up to +0.027&#177;0.016) associated with the Vai Trend (associated with a diluted HIMU component) [2]. The variability we observed in &#916;33S is interpreted to reflect contributions of sulfur of different origins and likely multiple crustal protoliths. &#916;36S vs. &#916;33S relationships suggest all recycled S is of post-Archean origin. The heterogeneous S isotope values and distinct isotopic compositions associated with the various compositional trends confirms a prior hypothesis; unique crustal materials are heterogeneously delivered to the Samoan mantle plume and compositionally influence the individual groups of islands.[1] Jackson et al. (2014),&#160;Nature;&#160;[2] Dottin et al. (2020),&#160;EPSL

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Five-S-isotope evidence of two distinct mass-independent sulfur isotope effects and implications for the modern and Archean atmospheres

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1803420115 ◽

2018 ◽

Vol 115 (34) ◽

pp. 8541-8546 ◽

Cited By ~ 13

Author(s):

Mang Lin ◽

Xiaolin Zhang ◽

Menghan Li ◽

Yilun Xu ◽

Zhisheng Zhang ◽

...

Keyword(s):

Sulfur Isotope ◽

Sulfur Isotopes ◽

Isotope Effects ◽

Chemical Species ◽

Ice Cores ◽

High Sensitivity ◽

Thermal Reactions ◽

Mass Independent Fractionation ◽

Species Analysis ◽

Isotope Evidence

The signature of mass-independent fractionation of quadruple sulfur stable isotopes (S-MIF) in Archean rocks, ice cores, and Martian meteorites provides a unique probe of the oxygen and sulfur cycles in the terrestrial and Martian paleoatmospheres. Its mechanistic origin, however, contains some uncertainties. Even for the modern atmosphere, the primary mechanism responsible for the S-MIF observed in nearly all tropospheric sulfates has not been identified. Here we present high-sensitivity measurements of a fifth sulfur isotope, stratospherically produced radiosulfur, along with all four stable sulfur isotopes in the same sulfate aerosols and a suite of chemical species to define sources and mechanisms on a field observational basis. The five-sulfur-isotope and multiple chemical species analysis approach provides strong evidence that S-MIF signatures in tropospheric sulfates are concomitantly affected by two distinct processes: an altitude-dependent positive 33S anomaly, likely linked to stratospheric SO2 photolysis, and a negative 36S anomaly mainly associated with combustion. Our quadruple stable sulfur isotopic measurements in varying coal samples (formed in the Carboniferous, Permian, and Triassic periods) and in SO2 emitted from combustion display normal 33S and 36S, indicating that the observed negative 36S anomalies originate from a previously unknown S-MIF mechanism during combustion (likely recombination reactions) instead of coal itself. The basic chemical physics of S-MIF in both photolytic and thermal reactions and their interplay, which were not explored together in the past, may be another ingredient for providing deeper understanding of the evolution of Earth’s atmosphere and life’s origin.

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