Variable distribution of s-process Hf and W isotope carriers in chondritic meteorites – evidence from 174Hf and 180W

1,2Bo-Magnus Elfers, 1,2Peter Sprung, 1,2,3Markus Pfeifer, 1,2Frank Wombacher, 4Stefan T.M.Peters, 1,2CarstenMünker
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.08.009]
1Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Straße 49b, 50674 Köln, Germany
2Steinmann-Institut, Poppelsdorfer Schloss, Meckenheimer Allee 169, 53115 Bonn, Germany
3School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ, United Kingdom
4Geowissenschaftliches Zentrum der Georg-August-Universität Göttingen, Goldschmidtstraße 1, 37077 Göttingen, Germany
Copyright Elsevier

The stepwise acid digestion of primitive chondritic meteorites allows the identification of nucleosynthetic isotope anomalies that are otherwise hidden on the bulk rock scale. Here, we present combined Hf and W isotope data for acid leachates, residues, and bulk rock aliquots of several primitive chondrites that include highly precise analyses of the heavy p-process isotopes 174Hf and 180W. Including data for these two p-process isotopes enables, for the first time, the clear-cut discrimination between s- and r-process contributions to the Hf and W isotope inventory. Our analyses reveal Hf and W isotopic homogeneity at the bulk rock scale, but significant Hf and W isotope anomalies that are complementary between acid leachates and residues. Since both r- to p-process isotope ratios are invariant in leachates and residues, the observed anomalies can unambiguously be tied to variable contributions of carrier phases enriched in s-process nuclides, as previously inferred for, i.e., Mo and Ru in leaching experiments. Hafnium and W isotope anomalies co-vary in leachate and residue fractions from CM chondrites, whereas CO and CV chondrites are characterized by distinctly larger Hf isotope anomalies compared to W. This observation is most likely explained by more efficient homogenization of s-process W carrier(s) or, alternatively, by local redistribution of anomalous W into secondary less resistant phases during parent body and/or nebular processing. This implies the presence of different s-nuclide carrier phases for Hf and W. Several carriers of s-process-material appear to have been selectively dissolved by our leaching protocol, while contributions from r- and p-process Hf and W carrier phases appear invariant, possibly due to the generally more labile nature of their carrier phases during solar nebula and/or parent body processing.

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