NEODYMIUM, STRONTIUM AND CHROMIUM ISOTOPIC STUDIES OF THE LEW86010 AND ANGRA-DOS-REIS METEORITES AND THE CHRONOLOGY OF THE ANGRITE PARENT BODY

Neodymium, strontium, and chromium isotopic studies of the LEW86010 an grite established its absolute age and the formation interval between its crystallization and condensation of Allende CAIs from the solar ne bula. Pyroxene and phosphate were found to contain similar to 98% of i ts Sm and Nd inventory. A conventional Sm-147-Nd-143 isochron yielded an age of 4.53 +/- 0.04 Ga (2 sigma) and epsilon(Nd)(143) = 0.45 +/- 1 .1. An Sm-146-Nd-142 isochron gives initial Sm-146/Sm-144 = 0.0076 +/- 0.0009 and epsilon(Nd)(142) = -2.5 +/- 0.4. The Rb-Sr analyses give i nitial Sr-87/Sr-86 (I-Sr(87)) = 0.698972 +/- 8 and 0.698970 +/- 18 for LEW and ADOR, respectively, relative to Sr-87/Sr-86 = 0.71025 for NBS 987. The difference, Delta I-Sr(87), between I-Sr(87) for the angrites and literature values for Allende CAIs, corresponds to similar to 9 M a of growth in a solar nebula with a CI chondrite value of Rb-87/S-86 = 0.91, or similar to 5 Ma in a nebula with solar photospheric Rb-87/S r-86 = 1.51. Excess Cr-53 from extinct Mn-53 (t(1/2) = 3.7 Ma) in LEW8 6010 corresponds to initial Mn-53/Mn-55 = 1.44 +/- 0.07 x 10(-6) and c losure to Cr isotopic homogenization 18.2 +/- 1.7 Ma after formation o f Allende inclusions, assuming initial Mn-53/Mn-55 = 4.4 +/- 1.0 x 10( -5) for the inclusions as previously reported by the Paris group (Birc k and Allegre, 1988). The Sm-146/Sm-144 value found for LEW86010 corre sponds to solar system initial (Sm-146/Sm-144)(0) = 0.0080 +/- 0.0009 for crystallization 8 Ma after Allende, the difference between Pb-Pb a ges of angrites and Allende, or 0.0086 +/- 0.0009 for crystallization 18 Ma after Allende, using the Mn-Cr formation interval. The isotopic data are discussed in the context of a model in which an undifferentia ted ''chondritic'' parent body formed from the solar nebula similar to 2 Ma after Allende CAIs and subsequently underwent differentiation ac companied by loss of volatiles. Parent bodies with Rb/Sr similar to th at of CI, CM, or CO chondrites could satisfy the Cr and Sr isotopic sy stematics. If the angrite parent body had Rb/Sr similar to that of CV meteorites, it would have to form slightly later, similar to 2.6 Ma af ter the CAIs, to satisfy the Sr and Cr isotopic systematics.