
handle: 1885/30221
Abstract Precise U–Pb ages, determined with double spike ( 202 Pb– 205 Pb) thermal ionization m1ass 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 206 Pb/ 204 Pb ratios in these fractions are between 6300 and 14,100 for AdoR, 1160–4500 for LEW, and 608–8500 for D’Orbigny. Blank-corrected 206 Pb/ 204 Pb ratios for all three meteorites vary from 2160 to over 100,000. These fractions yielded precise and reproducible 207 Pb ∗ / 206 Pb ∗ 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-isotopic ages 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 53 Mn– 53 Cr “absolute” ages by 0.7 Ma. Third, a precise age of D’Orbigny allows consistent linking of the 53 Mn– 53 Cr and 26 Al– 26 Mg extinct nuclide chronometers to the absolute lime scale.
pyroxene, uranium-lead dating, Keywords: isotopic composition, meteorite, mass spectrometry
pyroxene, uranium-lead dating, Keywords: isotopic composition, meteorite, mass spectrometry
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