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Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved November 5, 2014 (received for review September 30, 2014)
Significance
This investigation focuses on the sulfur isotopic compositions of magmatically differentiated meteorites, the oldest igneous rocks in our solar system. We present evidence of anomalous 33S depletions in a group of differentiated iron meteorites, along with 33S enrichments in several other groups. The complementary positive and negative compositions, along with observed covariations in 36S and 33S, are explained by Lyman-α photolysis of gaseous H2S in the solar nebula. Confirmation of photochemically predicted 33S depletions implies that the starting composition of inner solar system sulfur was chondritic, consistent with the Earth, Moon, Mars, and nonmagmatic iron meteorites. Differentiated protoplanets, however, appear to have accreted from materials processed under conditions where sulfur was volatile and UV radiation was present (<∼2 AU).
Abstract
Achondrite meteorites have anomalous enrichments in 33S, relative to chondrites, which have been attributed to photochemistry in the solar nebula. However, the putative photochemical reactions remain elusive, and predicted accompanying 33S depletions have not previously been found, which could indicate an erroneous assumption regarding the origins of the 33S anomalies, or of the bulk solar system S-isotope composition. Here, we report well-resolved anomalous 33S depletions in IIIF iron meteorites (<−0.02 per mil), and 33S enrichments in other magmatic iron meteorite groups. The 33S depletions support the idea that differentiated planetesimals inherited sulfur that was photochemically derived from gases in the early inner solar system (<∼2 AU), and that bulk inner solar system S-isotope composition was chondritic (consistent with IAB iron meteorites, Earth, Moon, and Mars). The range of mass-independent sulfur isotope compositions may reflect spatial or temporal changes influenced by photochemical processes. A tentative correlation between S isotopes and Hf-W core segregation ages suggests that the two systems may be influenced by common factors, such as nebular location and volatile content.
Footnotes
- ↵1To whom correspondence should be addressed. Email: mantonelli{at}berkeley.edu.
↵2Present address: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720.
Author contributions: M.A.A. and J.F. designed research; M.A.A., S.-T.K., M.P., J.L., J.H., 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.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1418907111/-/DCSupplemental.
Inner solar system origin for anomalous S isotopes
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- Physical Sciences
- Earth, Atmospheric, and Planetary Sciences
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