Neodymium, strontium and chromium isotopic studies of the LEW86010 and Angra dos Reis meteorites and the chronology of the angrite parent body

Abstract

Abstract— Neodymium, strontium, and chromium isotopic studies of the LEW86010 angrite established its absolute age and the formation interval between its crystallization and condensation of Allende CAIs from the solar nebula. Pyroxene and phosphate were found to contain ∼98% of its Sm and Nd inventory. A conventional 147 Sm‐ 143 Nd isochron yielded an age of 4.53 ± 0.04 Ga (2 σ) and ∍ 143 Nd = 0.45 ± 1.1. An 146 Sm‐ 142 Nd isochron gives initial 146 Sm/ 144 Sm = 0.0076 ± 0.0009 and ∍ 143 Nd = −2.5 ± 0.4. The Rb‐Sr analyses give initial 87 Sr/ 86 Sr (I 87 Sr ) = 0.698972 ± 8 and 0.698970 ± 18 for LEW and ADOR, respectively, relative to 87 Sr/ 86 Sr = 0.71025 for NBS987. The difference, ΔI 87 Sr , between I 87 Sr for the angrites and literature values for Allende CAIs, corresponds to ∼9 Ma of growth in a solar nebula with a CI chondrite value of 87 Rb/ 86 Sr = 0.91, or ∼5 Ma in a nebula with solar photospheric 87 Rb/ 86 Sr = 1.51. Excess 53 Cr from extinct 53 Mn (t 1/2 = 3.7 Ma) in LEW86010 corresponds to initial 53 Mn/ 55 Mn = 1.44 ± 0.07 × 10 −6 and closure to Cr isotopic homogenization 18.2 ± 1.7 Ma after formation of Allende inclusions, assuming initial 53 Mn/ 55 Mn = 4.4 ± 1.0 × 10 −5 for the inclusions as previously reported by the Paris group (Birck and Allegre, 1988). The 146 Sm/ 144 Sm value found for LEW86010 corresponds to solar system initial ( 146 Sm/ 144 Sm) o = 0.0080 ± 0.0009 for crystallization 8 Ma after Allende, the difference between Pb‐Pb ages 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 undifferentiated “chondritic” parent body formed from the solar nebula ∼2 Ma after Allende CAIs and subsequently underwent differentiation accompanied by loss of volatiles. Parent bodies with Rb/Sr similar to that of CI, CM, or CO chondrites could satisfy the Cr and Sr isotopic systematics. If the angrite parent body had Rb/Sr similar to that of CV meteorites, it would have to form slightly later, ∼2.6 Ma after the CAIs, to satisfy the Sr and Cr isotopic systematics.