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Early Inner Solar System Origin for Anomalous Sulfur Isotopes in Differentiated Protoplanets

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Early Inner Solar System Origin for Anomalous Sulfur Isotopes in Differentiated Protoplanets Early inner solar system origin for anomalous sulfur isotopes in differentiated protoplanets Michael A. Antonellia,1,2, Sang-Tae Kimb, Marc Petersa, Jabrane Labidic, Pierre Cartignyc, Richard J. Walkera, James R. Lyonsd, Joost Hoeka, and James Farquhara,e aDepartment of Geology, University of Maryland, College Park, MD 20742; bSchool of Geography and Earth Sciences, McMaster University, Hamilton, ON L8S 4K1, Canada; cLaboratoire de Géochimie des Isotopes Stables, Institut de Physique du Globe de Paris, UMR 7154 CNRS, Universite Paris Denis-Diderot, Sorbonne Paris Cite, 75005 Paris, France; dSchool of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287; and eEarth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742 Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved November 5, 2014 (received for review September 30, 2014) Achondrite meteorites have anomalous enrichments in 33S, rela- been classified as magmatic (1, 2). Our data for group IAB tive to chondrites, which have been attributed to photochemistry includes five chemically (and possibly genetically) distinct sub- in the solar nebula. However, the putative photochemical reac- groups (2). Collectively, all of the iron meteorites examined have tions remain elusive, and predicted accompanying 33S depletions overlapping δ34S values consistent with those reported by pre- have not previously been found, which could indicate an errone- vious studies (9, 10), ranging from −1.41‰ to +1.29‰ with an ous assumption regarding the origins of the 33S anomalies, or of average of −0.01 ± 0.81‰ (2σ SD, 2s.d.) (Fig. 1). We have the bulk solar system S-isotope composition. Here, we report well- chosen to report uncertainties either as 2σ SEs (2SE) where resolved anomalous 33S depletions in IIIF iron meteorites (<−0.02 appropriate (for averages derived from groups with more than per mil), and 33S enrichments in other magmatic iron meteorite eight analyzed members), or as 2s.d. long-term external re- groups. The 33S depletions support the idea that differentiated producibility for individual data points and for groups where the planetesimals inherited sulfur that was photochemically derived number of analyzed samples is small (±0.3‰, ±0.008‰,and from gases in the early inner solar system (<∼2 AU), and that bulk ±0.3‰ for δ34S, Δ33S, and Δ36S, respectively; see SI Text, Fig. inner solar system S-isotope composition was chondritic (consis- S2, and Tables S3 and S4). There is no distinction between tent with IAB iron meteorites, Earth, Moon, and Mars). The range magmatic and nonmagmatic irons in δ34S, and most individual of mass-independent sulfur isotope compositions may reflect spa- groups have average δ34S values within 2s.d. of zero (±0.3‰). tial or temporal changes influenced by photochemical processes. A In contrast to δ34S, the new results show that Δ33S values are tentative correlation between S isotopes and Hf-W core segrega- uniform within iron meteorite groups, but that there are re- tion ages suggests that the two systems may be influenced by solvable Δ33S differences between different groups. Values of common factors, such as nebular location and volatile content. Δ33S range from −0.031‰ to +0.031‰ [normalized to troilite from the Canyon Diablo IAB iron meteorite (CDT)], with dif- iron meteorites | sulfur isotopes | protoplanetary disk | solar system | ferent iron meteorite groups forming discernible clusters in Δ33S photochemistry space (Fig. 1). The nonmagmatic iron meteorites examined show 33 EARTH, ATMOSPHERIC, a limited range in Δ S compared with magmatic irons: Group AND PLANETARY SCIENCES f all of the extraterrestrial materials found on Earth, iron IAB (n = 23), including samples from the main group (including Ometeorites have always been the most conspicuous. So- CDT), three subgroups, two grouplets, and several ungrouped 33 called “magmatic” iron meteorites are likely to be samples from IAB iron meteorites, has an average Δ Sof+0.004 ± 0.002‰ the cores of magmatically differentiated protoplanetary parent bodies (1), whereas “nonmagmatic” iron meteorites are commonly Significance suggested to sample solidified melt pockets that formed via im- pacts onto nondifferentiated (chondritic) parent bodies (2). In- This investigation focuses on the sulfur isotopic compositions of dividual members from an iron meteorite group are assumed to magmatically differentiated meteorites, the oldest igneous rocks derive from a common parent body, based on shared chemical and in our solar system. We present evidence of anomalous 33S isotopic characteristics (1–4). Chemical and isotopic differences depletions in a group of differentiated iron meteorites, along among the different iron groups provide convincing evidence that with 33S enrichments in several other groups. The complemen- different individual parent body planetesimals incorporated ge- tary positive and negative compositions, along with observed netically distinct precursor materials (1–4). covariations in 36Sand33S, are explained by Lyman-α photolysis Recent observations that several achondrite meteorite groups of gaseous H2S in the solar nebula. Confirmation of photo- possess small, mass-independent 33S enrichments relative to chemically predicted 33S depletions implies that the starting chondrites (5) have led to the conclusion that sulfur isotopes composition of inner solar system sulfur was chondritic, consis- were heterogeneously distributed among the materials that ac- tent with the Earth, Moon, Mars, and nonmagmatic iron mete- creted to form early solar system planetesimals. This observa- orites. Differentiated protoplanets, however, appear to have tion, coupled with the ancient 182Hf-182W ages (within 1–3Myof accreted from materials processed under conditions where sulfur <∼ solar system formation) of magmatic irons (6–8), provides im- was volatile and UV radiation was present ( 2 AU). petus to search for systematic variations in mass-independent Author contributions: M.A.A. and J.F. designed research; M.A.A., S.-T.K., M.P., J.L., J.H., sulfur isotope compositions among the iron groups. and J.F. performed research; M.A.A., S.-T.K., M.P., J.L., P.C., R.J.W., J.R.L., and J.F. analyzed data; and M.A.A., P.C., R.J.W., J.R.L., and J.F. wrote the paper. Results The authors declare no conflict of interest. δ34 Δ33 Δ36 Δ33 = We report high-precision S, S, and S data (where S This article is a PNAS Direct Submission. × δ33 – δ34 + 0.515 − Δ36 = × δ36 – 1000 { S [( S 1) 1]} and S 1000 { S 1To whom correspondence should be addressed. Email: [email protected]. [(δ34S + 1)1.9 − 1]}) for 61 troilite (FeS) nodules extracted from 58 2Present address: Department of Earth and Planetary Science, University of California, different iron meteorites selected from eight different groups Berkeley, CA 94720. (IAB, IC, IIAB, IIE, IIIAB, IIIF, IVA, and IVB) (Fig. 1, Fig. S1, This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. and Tables S1 and S2). All groups except for IAB and IIE have 1073/pnas.1418907111/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1418907111 PNAS | December 16, 2014 | vol. 111 | no. 50 | 17749–17754 Downloaded by guest on September 30, 2021 Average Δ36S measurements within their uncertainties, like those for δ34S, also overlap for most iron meteorite groups, ranging from −0.34‰ to +0.28‰ (Fig. 1B). However, the nonmagmatic irons appear to have slightly positive Δ36S, while the magmatic irons tend to have more negative values. Two-tailed hetero- scedastic t tests were performed between groups to probe the statistical significance of our Δ36S isotope measurements (Table S5). These tests demarcate the probability that two sets of mea- surements come from the same population, with the output vari- able (the P value) representing the fractional probability that the two sets are identical. When comparing the Δ36Sofmagmatic groups IIAB, IIIAB, and IVA to group IAB, P values returned were lower than ∼0.02, indicating a >98% chance that they rep- resent different populations. Although the relationship is poorly defined, there appears to be a negative correlation between Δ36S and Δ33S[Δ36S/Δ33S ∼−7 ± 4 (2s.d.)] that is similar to that calcu- lated from available bulk achondrite analyses (5) [Δ36S/Δ33S ∼−6 ± 6 (2s.d.)] (Fig. S3). Fractionation Mechanisms That Affect Δ33S. While a number of mechanisms exist that can fractionate sulfur isotopes, only a few are known to produce the types of variations in Δ33S and Δ36S seen in our data, and many others can be ruled out. For instance, magmatic processes produce small-magnitude, mass-dependent fractionations because they occur at high temperature and are associated with equilibrium isotope partitioning. Furthermore, kinetic processes that could potentially generate larger fraction- ations do not provide a simple explanation for the variations of Δ33S and Δ36S, considering the small range of observed δ34S. Recent studies of magnetic isotope effects (11) have demonstrated that Δ33S can be modified by processes related to interactions between nuclear and electronic spin of radical intermediates; however, these effects do not produce variations for Δ36S and are not considered to be viable candidates to account for our obser- vations. Another recent study regarding fractionation of gases in imposed low-temperature thermal gradients (of ∼100 °C) has also demonstrated the existence of small anomalous isotope effects capable of modifying Δ33S (12), but the authors could not resolve an effect for Δ36S. Below, we discuss several mechanisms that have been shown to produce small variations of Δ33S related to mixing, distillation, and evaporation, and evaluate whether they can ex- Fig. 1. Sulfur isotope measurements in iron meteorite troilite. Measure- plain the iron meteorite data. ments of (A) Δ33S vs. δ34S and (B) Δ36S vs. Δ33S, for acid-volatile sulfur frac- Mixing of sulfur pools with different mass-dependent tions (n = 72) of troilite from 58 iron meteorites belonging to the groups S-isotopic compositions yields small deviations from the mass- IAB, IC, IIAB, IIE, IIIAB, IIIF, IVA, and IVB.
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