Assessment of nebular versus parent body processes on presolar components present in chondrites: Evidence from osmium isotopes

We report Os isotope compositions of acid residues separated from six carbonaceous chondrites that were subjected to varying degrees of aqueous alteration on their parent bodies. The residues from three enstatite chondrites were also investigated in order to evaluate the effects of nebular and parent body processing on the survival of presolar grains in meteorites formed under redox conditions that were markedly reduced, compared to those under which carbonaceous and ordinary chondrites formed. All acid residues from CM and CR chondrites show enrichments in s-process Os isotopes relative to the Solar System average recorded in bulk chondrites. The extent of the anomaly present positively correlates with the degree of aqueous alteration of the host chondrites. This correlation was probably caused by selective destruction/modification of presolar grains carrying r-process-enriched Os during progressive aqueous alteration on the parent bodies. The r-process-enriched component was likely either presolar silicates formed in Type II supernova ejecta, or other unidentified reduced presolar phases such as metal alloys, carbides and silicides. Acid residues from enstatite chondrites have Os isotope anomalies that are much more enriched in the s-process components, relative to the residues from carbonaceous and ordinary chondrites that experienced the same grade of thermal metamorphism. This most likely reflects the selective destruction of s-process-enriched presolar phases that occurred under the more oxidized conditions experienced by carbonaceous and ordinary chondrites, either while components formed in the nebula, or on their parent bodies. To account for the uniform, terrestrial Os isotopic composition in all types of bulk chondrites, it is required that r-process or s-process-enriched Os, released from presolar phases during nebular or parent body processing, was re-incorporated into a new phase(s) which was not lost from the location where the bulk meteorites were derived. However, because parent body processing might have acted differently on other elements (e.g., open system behavior of fluid mobile elements during aqueous alteration), recent findings of isotopic heterogeneities in bulk meteorites should be evaluated not just by invoking nebular heterogeneities, but by also considering the effects of parent body processing.