¹Elizabeth A. Heiny,¹Edward M. Stolper,¹John M. Eiler
Proceedings of the National Academy of Sciences of the United States of America (PNAS) 122, e2418198122 Link to Article [https://doi.org/10.1073/pnas.2418198122]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125

The isotope anomalies of noncarbonaceous (NC) and carbonaceous (CC) extraterrestrial materials provide a framework for tracing the distribution and accretion of matter in the early solar system. Here, we extend this framework to sulfur (S)—one of six “life-essential” volatile elements [TC ~ 664 K]—via the mass-independent S-isotope compositions of differentiated meteorites. We observe that on average, NC and CC iron meteorites are characterized by distinct Δ33S (Δ33SNC = 0.013 ± 0.003‰; Δ33SCC = −0.021 ± 0.009‰; 2 SE). The average Δ36S of NC and CC irons are less well resolved (Δ36SNC = −0.006 ± 0.039‰; Δ36SCC = −0.101 ± 0.114‰; 2 SE), but the Δ36S values of the CC irons are concentrated in the lower half of the range of those observed for iron meteorites. A lack of CC achondrite S-isotope analyses prevents direct comparison of the Δ33S and Δ36S of NC and CC achondrites, but the average Δ33S and Δ36S of NC achondrites (Δ33S = 0.02 ± 0.008; Δ36S = −0.019 ± 0.064‰; 2 SE) overlap with those of the NC irons. The average Δ33S values of NC achondrite groups also correlate with nucleosynthetic anomalies of other elements (e.g., Cr) previously used to define isotopic heterogeneity within the NC reservoir. The position of the Earth in Δ33S-Δ36S composition space implies that ~24% of terrestrial S derives from CC materials, while the majority (~76%) was delivered by NC materials.

¹Nathan Asset, ¹Marc Chaussidon, ²Guillaume Lombardi, ³Johan Villeneuve, ⁴Romain Tartèse, ⁵Smail Mostefaoui, ⁵François Robert
Proceedings of the National Academy of Sciences of the United States of America (PNAS) 122, e2426711122 Link to Article [https://doi.org/10.1073/pnas.2426711122]
¹Universite Paris-Cite, Institut de Physique du Globe de Paris, CNRS, Paris F-75005, France
²Laboratoire des Sciences des Procédés et des Matériaux (LSPM—CNRS), Université Sorbonne Paris Nord, Villetaneuse F-93430, France
³Centre de Recherches Pétrographiques et Géochimiques, CNRS, Université de Lorraine, UMR 7358, Vandœuvre-lès-Nancy 54501, France
⁴Department of Earth and Environmental Sciences, The University of Manchester, Manchester M13 9PL, United Kingdom
⁵Institut Origine et Evolution, Muséum National d’Histoire Naturelle, Sorbonne Université, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie – UMR 7590 CNRS, Paris 75005, France

Contrary to all terrestrial rocks, planets and meteorites exhibit oxygen isotope variations decorrelated with the mass difference of their atomic nuclei. It has been proposed that, in the protosolar nebula (PSN), these variations could result from mass independent isotopic fractionation (MIF) either during specific chemical reactions similar to those responsible for the formation of ozone in the Earth’s atmosphere or during ultraviolet (UV)-photolysis of carbon monoxide (CO) gas in the PSN. However, these potential chemical MIF reactions (Chem-MIFs) are not identified in conditions close to the PSN, and there is no experimental demonstration that large MIF signature can be transferred to solids forming in the PSN. Here, we show that MIFs, up to 60‰ depletion in 16O, are produced by high-temperature reactions in a plasma during the condensation of carbonaceous solids from a gas containing two of the most abundant PSN molecular species (H2O and CH4). This effect is attributed to the formation in the plasma of the activated complex H2O2* followed by its stabilization by reactions with CHx• radicals. Although it is premature to assert that this reaction represents the main process resulting in MIF of oxygen isotopes in the solar system, our result demonstrates the potential importance of a Chem-MIF effect in a PSN where plasma zones develop.