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Tungsten isotopic compositions of presolar silicon carbide grains: implications for 182hf-182w and 187re-187os chronometers

J. N. Avila1, T. R. Ireland1, F. Gyngard2, S. Amari2, E. Zinner2

1 Research School of Earth Sciences and Planetary Science Institute, The Australian National University, Canberra ACT 0200, Australia.
2 Laboratory for Space Sciences and Physics Department, Washington University, St. Louis, MO 63130, USA.

Image caption

The s-process nucleosynthesis in the Hf-Ta-W-Re-Os path has received considerable attention lately (Fig. 01). New neutron capture cross-sections for 174,176,177,178,179,180,182Hf, 185W and 186,187,188Os have been reported (Sonnabend et al., 2003; Mosconi et al., 2006; Wisshak et al., 2006; Vockenhuber et al., 2007), and small anomalies in W and Os isotopes have been observed in primitive meteorites (Brandon et al., 2005; Yokoyama et al., 2007; Qin et al., 2008). However, as suggested by Vockenhuber et al. (2007) and Sonnabend et al. (2003), model calculations for s-processes nucleosynthesis appear to underestimate 182W and overestimate 186Os, and this may have implications for the 182Hf-182W and 187Re-187Os chronometers. Tungsten isotopes are particularly important because they are affected by several branching points (182Ta, 181,182Hf, and 185W), which also affect Re and Os isotopes. Here we report W isotopic measurements in presolar SiC grains in order to provide additional constraints on s-process nucleosynthesis.

182,183,184,186W and 180Hf were measured with SHRIMP RG at ANU in an aggregate of presolar SiC grains (KJB fraction) extracted from the Murchison meteorite (Amari et al., 1994). Tungsten isotopes were measured as WO+ ions, which have a higher yield than the atomic species (WO+/W+ ~ 3). An O- primary ion beam of 5 nA was focused to sputter an area of 20 mm in diameter. SHRIMP RG was operated at a mass resolving power of m/Δm= 5000 (at 1% peak). At this level, isobaric interferences were well resolved from the WO+ species. NIST silicate glasses and synthetic SiC were used to monitor instrumental mass fractionation and isobaric interferences.

The W isotopic compositions are anomalous in comparison to those observed in normal solar system materials. The SiC grains appear to be enriched in 182W and 184W relative to 183W, as expected for s-process nucleosynthesis in AGB stars (e.g. Qin et al., 2008). However, an unexpected enrichment in 186W is observed. The low 180Hf/183W ratios determined here imply a low contribution from radiogenic 182W after SiC condensation, otherwise the 182W excesses would be even higher. The enrichment in 182W appears to be plausible, given the observation that the calculated r-process residue of 182W (Nr = NΘ Ns) has a significant positive deviation from the smooth r-abundance distribution (Wisshak et al., 2006; Vockenhuber et al., 2007). A lower r-process component of solar 182W would imply a shorter time interval between the last r-process contribution to solar 182Hf and the formation of solid parent bodies (Vockenhuber et al., 2007).

As discussed by Sonnabend et al. (2003), the stellar s-process model shows a 20% overproduction of 186Os, and consequently it underestimates the s-process contribution to 187Re. The enrichment observed in 186W requires the activation of the 185W branching point during AGB thermal pulses, when marginal activation of the 22Ne(a,n)25Mg source produces neutron densities as high as Nn= 5 x 109 neutrons cm-3 (Lugaro et al., 2003), bypassing 186Os. This result is in disagreement with 96Zr depletions in SiC grains, which indicate that the 22Ne(a,n)25Mg source was weak in their parent stars (Nicolussi et al., 2003). However, the overabundance of 186Os could also be the result of uncertainties in the nuclear physics data.

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