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Tungsten isotopic compositions in stardust SiC grains from the Murchison meteorite: Constraints on the s-process in the Hf-Ta-W-Re-Os region

We report the first tungsten isotopic measurements in stardust silicon carbide (SiC) grains recovered from the Murchison carbonaceous chondrite. The isotopes 182W, 183W, 184W, 186W and 179Hf, 180Hf were measured on both an aggregate (KJB fraction) and single stardust SiC grains (LS+LU fraction) believed to have condensed in the outflows of low-mass carbon-rich asymptotic giant branch (AGB) stars with close-to-solar metallicity. The SiC aggregate shows small deviations from terrestrial (=solar) composition in the 182W/184W and 183W/184W ratios, with deficits in 182W and 183W with respect to 184W. The 186W/184W ratio, however, shows no apparent deviation from the solar value. Tungsten isotopic measurements in single mainstream stardust SiC grains revealed lower than solar 182W/184W, 183W/184W, and 186W/184W ratios. We have compared the SiC data with theoretical predictions of the evolution of W isotopic ratios in the envelopes of AGB stars. These ratios are affected by the slow neutron-capture process and match the SiC data regarding their 182W/184W, 183W/184W, and 179Hf/180Hf isotopic compositions, although a small adjustment in the s-process production of 183W is needed in order to have a better agreement between the SiC data and model predictions. The models cannot explain the 186W/184W ratios observed in the SiC grains, even when the current 185W neutron-capture cross section is increased by a factor of two. Further study is required to better assess how model uncertainties (e.g., the formation of the 13C neutron source, the mass-loss law, the modelling of the third dredge-up, and the efficiency of the 22Ne neutron source) may affect current s-process predictions.

grain LU-41 has deviations from the solar isotopic ratios that are larger than 2σ. No obvious 217 correlations between W isotopic compositions and C-, N-, and Si-isotopic compositions were 218 found. 219 Additionally, we provide information on the 179 Hf/ 180 Hf ratios ( Table 2). The weighted mean which has the largest deficit in 182 W, 183 W, and 186 W with respect to 184 W, also has a significant 226 deficit in 179 Hf with respect to 180 Hf (δ 179 Hf/ 180 Hf = −637 ± 121 ‰). All single grains and KJB 227 spot analyses show lower 180 Hf/ 184 W ratios than current predictions from s-process 228 nucleosynthesis calculations (see section 4). 229 We shall now consider some factors that may affect the present determinations. First, there is 230 a potential problem of contamination with foreign materials and minerals, originating either 231 from the meteorite itself or as a result of sample preparation. Tungsten, unfortunately, may be 232 a contaminant from the sample preparation. Sodium polytungstate was used for density 233 separation of graphite from SiC (Amari et al. 1994). The density separation was followed by 234 several washing procedures in order to remove any contaminants. Based on the behaviour of the isotopic ratios as a function of acquisition time, there is no evidence from our analyses to 236 suggest that the W isotopic compositions are not intrinsic to the grains or result from surface 237 contamination. Furthermore, we did not detect W in the Au-foil, onto which both the KJB and 238 LS+LU fractions had been deposited. Nonetheless, three out of five single grains show no 239 apparent deviation from the solar isotopic ratios (for both W and Hf), and contamination with 240 solar material cannot be completely ruled out. Kashiv (2004) found very high W abundance in 241 stardust SiC grains (W enrichment factor, relative to solar W/Si ratio, of ~ 300 to 8700), much 242 higher than expected based on AGB stellar models and thermodynamic condensation 243 calculations. In contrast to Kashiv (2004), our W concentration measurements are semi-244 quantitative only. Nevertheless, we found for the KJB fraction a W enrichment factor ~ 10, 245 and for the single grains from the LS+LU fraction an enrichment factor < 3, which are much 246 lower than the values found by Kashiv (2004). This author also suggested that W 247 contamination could be from heavy metal alloy holders used during analytical procedures.

248
Contamination from the Murchison meteorite is another possibility. In order to minimize any 249 residual surface contamination, all spots and grains analysed were initially rastered across an 250 area slightly larger than the analytical pit by the O 2 − primary beam for ~ 60 s before data 251 acquisition.

252
Another complication encountered during ion microprobe analysis of SiC grains is the lack of 253 suitable SiC standards. The accuracy and precision of ion microprobe measurements is known 254 to be highly dependent on the availability of suitable standards because of the variability of 255 ion emission produced by sputtering of solid geological and cosmochemical materials.

256
Standards that are compositionally and structurally similar to the analytical target, i.e., matrix-257 matched, are therefore highly desirable. Furthermore, assessment of molecular interferences 258 produced by combinations of isotopes of the major elements is particularly important for the   The models presented here have been selected for C/O > 1 in the envelope so that the 394 condition for the formation of SiC is satisfied.

395
In spite of all the differences between the two evolutionary codes, which employ a variety of  The KJB average measurement is also well explained as the average of different stellar 411 masses, except that the 183 W/ 184 W ratio is below the average of the stellar models (Fig. 7b, e).

412
Also, the 183 W/ 184 W ratio observed in the most extreme grain, LU-41, is slightly lower than the 413 lowest predicted value (but note that the error bars are 1σ).

414
On the other hand, most models predict lower 186 W/ 184 W ratios than those measured in the 415 grains (Fig. 6b, d and Fig. 7c, f)  a higher s-process contribution to 182 W and a lower 182 W r-residual (see Table 13 of