U-Pb ages of angrites

Precise U-Pb ages, determined with double spike (Pb-202-Pb-205) thermal ionization m l ass spectrometry, are reported for angrites Angra dos Reis (AdoR), Lewis Cliff 86010 (LEW), and D'Orbigny. Nineteen of 23 acid-washed pyroxene fractions from these meteorites and whole rock fractions from D'Orbigny contain between 0.5 and 1.3 pg of total common Pb, indistinguishable from analytical blank. Measured Pb-206/Pb-204 ratios in these fractions are between 6300 and 14,100 for AdoR, 1160-4500 for LEW, and 608-8500 for D'Orbigny. Blank-corrected 206Pb/204Pb ratios for all three meteorites vary from 2160 to over 100,000. These fractions yielded precise and reproducible Pb-207(*)/Pb-206(*) dates with the average values of 4557.65 +/- 0.13 Ma for AdoR, 4558.55 +/- 0.15 Ma for LEW, and 4564.42 +/- 0.12 Ma for D'Orbigny. Pb-Pb isochrons including data with slightly elevated common Pb, and U-Pb upper concordia intercepts, yield similar dates. The implications of these new Pb-isotopicages of angrites are threefold. First, they demonstrate that AdoR and LEW are not coeval, and the group of slowly cooled angrites is therefore genetically diverse. Second, the new age of LEW suggests an upward revision of 53Mn-Cr-53 absolute ages by 0.7 Ma. Third, a precise age of D'Orbigny allows consistent linking of the Mn-53-Cr-53 and Al-26-Mg-26 extinct nuclide chronometers to the absolute lime scale. (c) 2007 Published by Elsevier Ltd.