Characterization of the noble gases and CRE age of the D’Orbigny angrite

Introduction: The D’Orbigny angrite, a 16.55 kg stone, was found 1979 in Argentina [1]. Mineralogy and chemistry of this meteorite were characterized in detail [2-6]. A Pb-U-Th age of 4.559 Ga was obtained for pyroxenes by Jagoutz et al. [7]. Here we report results on the noble gas isotopic composition and, in particular, on the cosmic-ray exposure (CRE) age of D’Orbigny. Cosmogenic component: The nuclides He, Ne, Ar, Kr, and Xe are mainly of cosmogenic origin. CRE ages were calculated from the stable isotopes mentioned above, using production rates from Eugster and Michel [8]; the shielding was corrected as for eucrites. The Kr-Kr age was derived using a mean life τ81=0.33 Ma [9] and P81/P83 = 0.44[(Kr/Kr)c+(Kr/Kr)c] [10]. The results are given in Table 1. The average CRE age is 12.3+0.9 Ma. It differs from that of the other five angrites: Angra dos Reis (55.5+1.2 Ma [11]), LEW 86010 (17.6+1.0 Ma [12)], LEW 87051 (>0.2 Ma [12]), Asuka 881371 (5.4+0.7 Ma [13]), and Sahara 99555 (6.6+0.8 Ma [14]). These differing ejection ages suggest that at least five different sites of the parent body were sampled.