Untitled Document
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
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