Distributions of heavy p-process isotopes 180W and 174Hf in bulk solar system materials

Seminar Date: 
05 Dec 2013 - 15:00
Stefan T.M. Peters
University of Cologne, Germany

The abundance of the rare p-process isotope 180W is variable between different iron meteorite classes. This variability was previously interpreted to dominantly reflect heterogeneity of a p-process component in the solar system at the time at which the iron meteorite parent bodies formed (Schulz et al., 2013). However, in two cases 180W appears variable between different irons from the same meteorite groups. Also, for one particular meteorite, 180W appears variable at the specimens scale. Although neutron capture effects caused by the exposure to cosmic radiation may account for some of this variability, resolvable within-group variations appear to exist for weakly-exposed meteorites. Hafnium and Pt have similarly high condensation temperatures to W and presolar carrier phases of these elements are therefore expected to have been affected likewise during thermal processing in the solar nebula. High-precision MC-ICPMS data indicate that 174Hf was uniformly distributed at the time at which the chondrite and achondrite parent bodies formed. Preliminary data on 190Pt in iron meteorites suggest that also p-process Pt does not show evidence for nucleosynthetic heterogeneity.
The uniform solar system distributions of 174Hf and possibly 190Pt are difficult to reconcile with a nucleosynthetic origin of the 180W heterogeneities. Combined Os-W concentration and W isotope measurements demonstrate that instead, the 180W heterogeneities are better explained by alpha decay of long-lived 184Os. An isochron for the iron meteorite parent bodies provides the first empirical evidence for alpha instability of 184Os. The calculated half life is ~1013 yr and is in good agreement with theoretical nuclear models. Furthermore, terrestrial silicates show a deficit in 180W relative to chondrites that is consistent with Os/W fractionation due to core formation ~4.5 Ga ago. There will be a brief outlook on the potential of the 184Os-180W decay system as a tracer in geo- and cosmochemistry.