Rapid effects of terrestrial alteration on highly siderophile elements in the Sutter's Mill meteorite.

Abstract

The 187Re-187Os isotopic systematics of many bulk chondrites plot well beyond analytical uncertainties of a primordial isochron. Limited variations in 187Os/188Os, coupled with large variations in Re/Os ratios among chondrites, suggest that this apparently open-system behavior is a result of the comparatively recent gain or loss of Re and/or Os. In order to assess whether or not rapid alteration in the terrestrial environment could be responsible for open-system behavior in chondrites, four pieces of the Sutter's Mill meteorite were examined for Os isotopic systematics and abundances of highly siderophile elements. Pieces SM1 and SM2 were collected prior to a rain event, within 2 days of the fall. Pieces SM51 and SM53 were collected after a rain event. There are significant but minor relative and absolute variations in the abundances of the highly siderophile elements, as well as 187Os/188Os among the four pieces. Rhenium-Os isotopic data for SM1 and SM2 plot within analytical uncertainties of a primordial isochron, while powders made from SM51 and SM53 do not. These results suggest that interactions with rain caused some redistribution of Re, and to a lesser extent Os, within small pieces of the meteorite. Thus, Re-Os isotopic systematics of <dm-size pieces of chondrites must be considered susceptible to modification after only a short time on the surface, where exposed to rain.