Intrinsic W nucleosynthetic isotope variations in carbonaceous chondrites: Implications for W nucleosynthesis and nebular vs. parent body processing of presolar materials

21/06/2015 10:08 · by karmaka · in carbonaceous chondrite, CI, CM, CV, presolar grains, radiometric dating

Christoph Burkhardt, Maria Schönbächler

Icarus
In Press, Accepted Manuscript, Available online 20 June 2015

The progressive dissolution of the carbonaceous chondrites Orgueil (CI1), Murchison (CM2) and Allende (CV3) with acids of increasing strength reveals correlated W isotope variations ranging from 3.5 ε182W and 6.5 ε183W in the initial leachate (acetic acid) to –60 ε182W and –40 ε183W in the leachate residue. The observed variations are readily explained by variable mixing of s-process depleted and s-process enriched components. One W s-process carrier is SiC, however, the observed anomaly patterns and mass-balance considerations require at least on additional s-process carrier, possibly a silicate or sulfide. The data reveal well-defined correlations, which provide a test for s-process nucleosynthesis models. The correlations demonstrate that current models need to be revised and highlight the need for more precise W isotope data of SiC grains. Furthermore the correlations provide a mean to disentangle nucleosynthetic and radiogenic contributions to 182W (ε182Wcorrected= ε182Wmeasured –(1.41±0.05) × ε183Wmeasured; ε182Wcorrected= ε182Wmeasured –(–0.12±0.06) × ε184Wmeasured), a prerequisite for the successful application of the Hf-W chronometer to samples with nucleosynthetic anomalies.

The overall magnitude of the W isotope variations decreases in the order CI1>CM2>CV3. This can be interpreted as the progressive thermal destruction of an initially homogeneous mixture of presolar grains by parent-body processing. However, not only the magnitude but also the W anomaly patterns of the three chondrites are different. In particular leach step 2, that employs nitric acid, reveals a s-deficit signature for Murchison, but a s-excess for Orgueil and Allende. This could be the result of redistribution of anomalous W into a new phase by parent-body alteration, or, the fingerprint of dust processing in the solar nebula. Given that the thermal and aqueous alteration of Murchison is between the CI and CV3 chondrites, parent-body processing is probably not the sole cause for creating the different patters. Small-scale nebular redistribution of anomalous W may have played a role as well. Similar nebular processes possibly acted differently on specific carrier phases and elements, resulting in the diverse nucleosynthetic signatures observed in planetary materials today.