Evidence for homogeneous distribution of osmium in the protosolar nebula

Separate s-, r-, and possibly p-process enriched and depleted components have been shown to host Os in low metamorphic grade chondrites, although no measureable Os isotopic anomalies have yet been discovered for bulk chondrites. Here, iron meteorites from groups IAB, IIAB, IIIAB, IVA and IVB, as well as the main group pallasites are examined. Many of these meteorites show well-resolved anomalies in ε190Os, ε189Os and ε186Osi. The anomalies, however, differ from those observed in chemically extracted components from chondrites, and are interpreted to reflect long-term exposure of the meteorites to cosmic rays, rather than nucleosynthetic effects. A neutron capture model is presented that can well account for observed isotopic variations in 190Os and 189Os. The same model predicts greater enrichment in 186Osi than is observed for at least one iron, suggesting as yet unaccounted for effects, or failings of the model. Despite the variable anomalies resulting from cosmic ray exposure, each of the major meteorite groups examined contains at one member with normal Os isotopic compositions that are unresolved from chondritic compositions. This indicates that some domains within these meteorites were little affected by cosmic rays. These domains are excellent candidates for application of the 182Hf–182W system for dating metal–silicate segregation on their parent bodies. The normal Os also implies that Os was homogeneously distributed throughout the protosolar nebula on the scale of planetesimal accretion, within the current level of analytical resolution. The homogeneity in Os contrasts with isotopic heterogeneity present for other siderophile elements, including Mo, Ru and W. The contrast in the scale of anomalies may reflect a late stage-injection of s- and p- process rich material into the coalescing nebula. Alternately, nebular thermal processing and destruction of some presolar host phases of Mo, Ru and W may also be responsible.

► Some of the irons are characterized by 186Osi, 189Os and 190Os anomalies.
► Osmium isotopic anomalies were most likely caused by cosmic ray exposure.
► Major iron and pallasite groups show no evidence for nucleosynthetic anomalies.
► Osmium was homogeneously distributed within the protosolar nebula.
► Lack of Os anomalies contrasts with other siderophile elements.