Origin of isotopic heterogeneity in the solar nebula by thermal processing and mixing of nebular dust

Abstract

We have investigated Mo and W isotope compositions in acid leachates and an insoluble residue from the Murchison carbonaceous chondrite. The new data reveal variable enrichments of s- and r-process isotopes and demonstrate that several isotopically diverse presolar components are present in Murchison. The insoluble residue is enriched in s-process Mo and W, evidently due to the enrichment of presolar SiC grains. In contrast, Mo and W released by leaching with weak acids are depleted in s-process isotopes, most likely reflecting the isotope composition of the homogenized portion of the protosolar nebula. The Mo and W isotope compositions of the different leach steps are broadly correlated as expected from s-process nucleosynthesis theory, indicating that Mo and W are presumably hosted in the same carriers. However, at the bulk meteorite scale, no nucleosynthetic W isotope anomalies have been identified (except for IVB iron meteorites) in spite of large Mo isotope heterogeneities among the same samples. This decoupling of Mo and W isotopes in bulk meteorites may reflect physical mixing of varying proportions of isotopically diverse presolar components with a "normal" solar nebula component. Due to the high W/Mo ratio and near-terrestrial W isotope composition of the latter, such mixing has no measurable effect on W isotopes, but results in large Mo isotope variations. Alternatively, thermal processes within the solar nebula imparted Mo isotope heterogeneity on an initially homogeneous mixture of presolar dust, while W was not affected. Removal of volatile Mo oxides during the thermal destruction of fragile presolar components would have created isotopically heterogeneous reservoirs of nebular dust. Accretion of meteorite parent bodies from such variably processed dust would thus result in Mo isotope heterogeneities at the bulk meteorite scale. Other elements such as Os and W were not or only slightly affected because they were more refractory during thermal processing and, therefore, remained isotopically homogeneous. Thermal processing of presolar dust within the solar nebula can thus account for both isotope heterogeneities observed for some elements and the lack of such isotopic heterogeneity for other elements. © 2012 Elsevier B.V.