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

Acid volatile sulfur extracted from ureilite meteorites carries a small S-3 3 enrichment relative to carbonaceous chondrites, enstatite chondrites, ord inary 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 producti on 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 spalloge nic sulfur (the metal phase in iron meteorites), and no corresponding S-36 enrichment. Sulfur derived from cosmic-ray spallation has been documented i n 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 di verse nucleosynthetic sources or from mass-independent fractionations cause d by gas-phase chemistry. In addition, several gas-phase reactions have bee n shown to produce mass-independent compositions for sulfur isotopes. One t hat both matches fractionations for all sulfur isotopes and is relevant to the presolar nebula has yet to be identified. An appropriate additive nucle osynthetic component has also not been identified. Copyright (C) 2000 Elsev ier Science Ltd.