53Mn-53Cr and 26Al-26Mg ages of a feldspathic lithology in polymict ureilites

We report Mn-53-Cr-53 and Al-26-Mg-26 isotopic data, obtained by in-situ SIMS analysis, for feldspathic clasts in polymict ureilites DaG 319 and DaG 165. The analyzed clasts belong to the albitic lithology, the most abundant population of indigenous feldspathic materials in polymict ureilites, and are highly fractionated igneous assemblages of albitic plagioclase. Fe-rich pyroxenes, phosphates, ilmenite, silica, and Fe(Mn, K, P, Ti)-enriched glass. Glass in DaG 165 clast 19 has extremely high and variable Mn-55/Cr-52 ratios (500-58,000) and shows correlated Cr-53 excesses up to similar to 1500 parts per thousand, clearly indicating the presence of live Mn-53 at the time of formation. The slope of the well-correlated isochron defined by glass and pyroxenes from this clast corresponds to (Mn-53/Mn-55)=(2.84 +/- 0.10) x 10(-6) (2 sigma). Data for less Mn-55/Cr-52-enriched glasses from DaG 319 clast B1, as well as phosphates from several other clasts, are consistent with this isochron. The 53Mn/55Mn ratio obtained from the isochron implies that these clasts are 0.70 +/- 0.18 Ma younger than the D'Orbigny angrite, corresponding to the absolute age of 4563.72 +/- 0.22 Ma. Plagioclase in DaG 319 clast B1 has a fairly constant Al-27/Mg-24 ratio of similar to 900 and shows resolvable Mg-26 excesses of similar to 2 parts per thousand. The slope of the isochron defined by pyroxene and plagioclase in this clast is (3.0 +/- 1.1) x 10(-7) (2 sigma), corresponding to a time difference of 5.4 (-0.3/+0.5) Ma after CAI (assuming the canonical initial Al-26/Al-27 ratio of 5 x 10(-5)) and an age 0.5 (-0.3/+0.5) Ma younger than D'Orbigny. Its absolute age (relative to D'Orbigny) is 4563.9 (+0.4/-0.5) Ma, in agreement with the Mn-53-Cr-53 age from clast 19. These data provide the first high-precision age date, similar to 5.4 Ma after CAl, for ureilites, giving a minimum estimate for the age of differentiation of their parent asteroid. Interpretation of this age for the thermal and physical history of that asteroid depends on a number of currently unknown or model-dependent parameters, including its size, bulk composition, and oxidation state, and the petrologic relationship between the feldspathic clasts and main group ureilites. (C) 2010 Elsevier B.V. All rights reserved.