A 33S enrichment in ureilite meteorites:: Evidence for a nebular sulfur component

Acid volatile sulfur extracted from ureilite meteorites carries a small S-33 enrichment relative to carbonaceous chondrites, enstatite chondrites, ordinary chondrites, and troilite from iron meteorites: Delta(33)S (=delta(33)S - 1,000 x (1 delta(34)S/1,000)(0.515) - 1) = 0.042 parts per thousand +/- 0.007 parts per thousand (standard error of 22 analyses). In situ production of sulfur by cosmic-ray spallation reactions involving Fe is unlikely to cause the enrichment because the ureilites have short cosmic-ray exposure ages, low Fe/S relative to the only documented phases that contain spallogenic sulfur (the metal phase in iron meteorites), and no corresponding S-36 enrichment. Sulfur derived from cosmic-ray spallation has been documented in the metal phase in iron meteorites, and it is characterized by Delta(36)S/Delta(33)S similar to 8, inconsistent with present observations. We argue that this enrichment derives from heterogeneity in the presolar nebula. A S-33 enrichment in the presolar reservoir may derive from mixing among diverse nucleosynthetic sources or from mass-independent fractionations caused by gas-phase chemistry. In addition, several gas-phase reactions have been shown to produce mass-independent compositions for sulfur isotopes. One that both matches fractionations for all sulfur isotopes and is relevant to the presolar nebula has yet to be identified. An appropriate additive nucleosynthetic component has also not been identified. Copyright (C) 2000 Elsevier Science Ltd.