Isotopic and chemical investigations on Angra dos Reis

Abstract

We report on extensive isotopic studies of Pb, Sr and Xe and on chemical abundance measurements of K, Rb, Sr, Ba, Nd, Sm, U and Th for total meteorite and mineral separates of the Angra dos Reis achondrite. U-Pb, Th-Pb and Pb-Pb ages are concordant at 4.54 AE for the total meteorite and for high-purity whitlockite in Angra dos Reis. This establishes Angra dos Reis as an early planetary differentiate which has not been disturbed for these systems since 4.54 AE ago. Measured ^(87)Sr/^(86)Sr in pyroxene and whitlockite for Angra dos Reis (ADOR) are distinctly below BABI by two parts in 10^4 and only one part in 10^4 above the lowest ^(87)Sr/^(86)Sr (ALL) measured in an Allende inclusion. The difference in ADOR-ALL corresponds to an interval of condensation in the solar nebula of ∼3 m.y. If ^(26)Al was the heat source for the magmatism on the parent planets of Angra dos Reis and the basaltic achondrites (BABI) then the relatively large difference in ^(87)Sr/^(86)Sr, BABI - ALL, must be the result of planetary evolution rather than condensation over ∼10 m.y. Xe isotopic measurements confirm the presence of large amounts of ^(244)Pu-produced fission Xe and show that ^(244)Pu was enriched in the whitlockite relative to the pyroxene by a factor of ∼18. We present chemical element enrichment factors between the whitlockite and the fassaitic pyroxene in Angra dos Reis. The enrichment factors demonstrate close analogy between the rare earth elements and their actinide analogs. The enrichment factor for Pu is intermediate to the enrichment factors of Nd and Sm.

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