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.

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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 ^{174,176,177,178,179,180,182}Hf, 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.

^{182,183,184,186}W 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 (N_r = N_Θ N_s) 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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