New constraints on early Solar System chronology from Al-Mg and U-Pb isotope systematics in the unique basaltic achondrite Northwest Africa 2976

Audrey Bouvier, Lev J. Spivak-Birndorf, Gregory A. Brennecka, Meenakshi Wadhwa

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Abstract

We report elemental abundances and the isotopic systematics of the short-lived 26Al-26Mg (half-life of ~0.73Ma) and long-lived U-Pb radiochronometers in the ungrouped basaltic meteorite Northwest Africa (NWA) 2976. The bulk geochemical composition of NWA 2976 is clearly distinct from that of the eucrites and angrites, but shows broad similarities to that of the paired NWA 001 and 2400 ungrouped achondrites indicating that it is likely to also be paired with these two samples. The major and trace element abundances in NWA 2976 further indicate that it formed by extensive melting and magmatic fractionation processes on its parent body. The Al-Mg and Pb-Pb isotope systematics indicate that this meteorite represents the earliest stages of crust formation on a differentiated parent body in the early Solar System. The absolute Pb-Pb internal isochron age of NWA 2976, obtained from acid leaching residues of three whole-rock samples and two pyroxene separates, is 4562.89±0.59Ma (MSWD=0.02). This Pb-Pb age is calculated using the measured 238U/235U ratio of a NWA 2976 whole-rock of 137.751±0.018 (2σ) which was determined relative to the recently revised value of 137.840±0.008 for the SRM 950a U isotope standard. The Al-Mg systematics reveal the presence of 26Mg isotopic anomalies produced by the decay of 26Al with an (26Al/27Al)0 of (3.94±0.16)×10-7, and indicate a time of formation of 0.26±0.18Ma after the D'Orbigny angrite. Using the revised Pb-Pb age of 4563.36±0.34Ma for the D'Orbigny anchor (corrected for its U isotopic composition), we deduce an Al-Mg model age of 4563.10±0.38Ma for NWA 2976, which is consistent with its Pb-Pb internal isochron age.The concordance of the Pb-Pb and Al-Mg chronometers, when taking into account the differences in the U isotopic compositions of the D'Orbigny and NWA 2976 achondrites (whose parent bodies likely formed in distinct regions of early Solar System as indicated by their different oxygen isotopic compositions), implies that 26Al was homogeneously distributed in the early Solar System. It also suggests that igneous processes on planetesimals, as represented by the formation of various basaltic meteorite groups that likely originated on distinct parent bodies (e.g., eucrites and angrites, as well as ungrouped achondrites), were widespread throughout the protoplanetary disk within the first ~5Ma of the history of the Solar System.

Original languageEnglish (US)
Pages (from-to)5310-5323
Number of pages14
JournalGeochimica et Cosmochimica Acta
Volume75
Issue number18
DOIs
StatePublished - Sep 15 2011

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ASJC Scopus subject areas

  • Geochemistry and Petrology

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