The chromium isotopic composition of Almahata Sitta

– Nine ureilitic fragments of the anomalous polymict ureilite Almahata Sitta have been analyzed for their Cr isotopic compositions. All the samples, including both nonmagnetic and magnetic portions, show essentially the same e54Cr deficit (−0.77 ± 0.10), relative to the terrestrial Cr standard. This contrasts with the variable positive 54Cr anomalies observed for carbonaceous chondrites, but agrees with the values measured for eucrites, diogenites, and mesosiderites (Trinquier et al. 2007). This implies that, contrary to previous suggestions based on O isotopes, ureilites were not derived from any known carbonaceous chondrite parent body. Instead, the Almahata Sitta parent body may have accreted in a nebular region/environment similar to that of the howardite, eucrite, and diogenite (HED) parent body. In addition, the lack of variation in e54Cr combined with variable O isotopic compositions in the meteorite fragments suggests that whatever process(es) caused the O isotopic heterogeneity of the solar system was probably not responsible for heterogeneity in e54Cr. The samples show resolvable variations in e53Cr (0.15–0.41) that are correlated with Mn/Cr ratios, suggesting that live 53Mn was present at the time of formation of Almahata Sitta. The isochron yields an initial 53Mn/55Mn value of 3.1 (±1.1) × 10−6, corresponding to an age of 4563.6 ± (2.2) Ma when related to U-Pb and Mn-Cr data for the D’Orbigny angrite. This age is consistent with the Mn-Cr and Al-Mg ages of two other polymict ureilites (Goodrich et al. 2010). Magmatic activity on the ureilites’ parent body seems to have postdated the formation of refractory inclusions by approximately 4–5 Ma.