Some 183W/184W anomalies were already observed in meteorites by Qin et al. (2008) for IVB iron meteorites, or by Burkhardt et al. (2012) in Murchison leachates. They are associated to an excess of 184W interpreted as an excess of s -process component. In Tafassasset, the ε183Wε183W/184W values are correlated to the amount of silicates in the different fractions, and inversely correlated with the metal abundance. The excess in 184W can be related to the 54Cr-excess observed by G?pel et al., 2009 and G?pel et al., 2011 in Tafassasset, even if no evidence exists thus far to support a common carrier phase. Such isotopic compositions may be the MPI-0479605 signature of presolar grains ( Avila et al., 2012, Davis et al., 2009, Qin et al., 2011 and Zinner et al., 2005) condensed in AGB (Asymptotic Giant Branch) stars. The absence of W anomalies in the metal may be explained either by the metal coming from an isotopically distinct reservoir (i.e. the metal is an exogenous phase), or by dilution effect. Indeed, the W concentration is 40 times higher in the metal compared to silicates. A mass balance calculation, using a typical W concentration of 1300 ppm for SiC grains and typical 183W-anomalies of −120‰−120‰ (Avila et al., 2012), shows that a concentration of 50 ppm of presolar SiC grains in the silicates of Tafassasset can explain the observed 184W signature. This result is well within the range of 3–220 ppm of presolar grain contents observed in CR chondrites ( Floss and Stadermann, 2009, Leitner et al., 2012 and Nagashima et al., 2004). If the metal phase of Tafassasset also contains 50 ppm presolar grains carrying nucleosynthetic anomalies, tap root would result in an observed ε183Wε183W/184W of −0.02, indistinguishable from the standard within uncertainty.
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