APPLICATIONS OF THE PT-190-OS-186 ISOTOPE SYSTEM TO GEOCHEMISTRY AND COSMOCHEMISTRY

Platinum is fractionated from osmium primarily as a consequence of pro cesses involving sulfide and metal crystallization. Consequently, the Pt-190-Os-186 isotope system (Pt-190-->Os-186 + alpha) shows promise f or dating some types of magmatic sulfide ores and evolved iron meteori tes. The first Pt-190-Os-186 isochrons are presented here for ores fro m the ca. 251 Ma Noril'sk, Siberia plume, and for group IIAB magmatic iron meteorites. Given the known age of the Noril'sk system, a decay c onstant for Pt-190 is determined to be 1.542 x 10(-12)a(-1), with +/-1 % uncertainty. The isochron generated for the IIAB irons is consistent with this decay constant and the known age of the group. The Os-186/O s-188 ratios of presumably young, mantle-derived osmiridiums and also the carbonaceous chondrite Allende were measured to high-precision to constrain the composition of the modern upper mantle. These compositio ns overlap, indicating that the upper mantle is chondritic within the level of resolution now available. Our best estimate for this Os-186/O s-188 ratio is 0.119834 +/- 2 (2 sigma(M)). The Pt-190/Os-186 ratios d etermined for six enstatite chondrites average 0.001659 +/- 75, which is very similar to published values for carbonaceous chondrites. Using this ratio and the presumed composition of the modern upper mantle an d chondrites, a solar system initial Os-186/Os-188 ratio of 0.119820 i s calculated. In comparison to the modern upper mantle composition, th e Os-186/Os-188 ratio of the Noril'sk plume was approximately 0.012% e nriched in Os-186. Possible reasons for this heterogeneity include the recycling of Pt-rich crust into the mantle source of the plume and de rivation of the osmium from the outer core. Derivation of the osmium f rom the outer core is our favored model. Copyright (C) 1997 Elsevier S cience Ltd.