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

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 epsilon W-182 and 6.5 epsilon W-183 in the initial leachate (acetic acid) to -60 epsilon W-182 and -40 epsilon W-183 in the leachate residue. The observed variations are readily explained by variable mixing of s-process depleted and s-process enriched components. One Ws-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 W-182 (epsilon W-182(corrected) = epsilon W-182(measured) - (1.41 + 0.05) x epsilon W-183(measured); epsilon W-182(corrected) = epsilon W-182(measured) - (-0.12 + 0.06) x epsilon W-184(measured)), a prerequisite for the successful application of the Hf-W chronometer to samples with nucleosynthetic anomalies. The overall magnitude of theWisotope 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 pattern. 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. (C) 2015 Elsevier Ltd. All rights reserved.