Pb-U-Th isotopic evolution of the D`Orbigny angrite

Details Pb-U-Th isotopic evolution of the D`Orbigny angrite Pb-U-Th isotopic evolution of the D`Orbigny angrite

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....8802j&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #8802

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Scientific paper

In order to understand the early evolution of our solar system high resolution dating of meteorites is necessary. Since most of the meteorite classes were formed in the first 50 Ma (1%) of our solar system's history, high precision dating is necessary. In particular, relative dating using extinct isotopes like the Mn-Cr system hinges on an absolute calibration. Only the Pb-Pb system can serve this purpose, but few such high precisions Pb-Pb ages have been reported (1-3). Some of them have been reported with an age precision of 0,01%, which requires a measurement precision of 0.03% for the 206Pb/207Pb ratio-reached only by using the faraday collector and a tight control of the mass fractionation. In the D'Orbigny meteorite for which we have obtained Pb-Pb ages, such a high precision has not yet been achieved because the Pb concentration in the dated pyroxene is extremely low (between 7 and 17 ppb). Four pure mineral separates of clean pyroxenes (of about 15 mg each) yielded only 100 to 250 pg of Pb that was too little to run on the faraday collector. Instead, measurement was made with an ion counter giving a typical error for the 206Pb/207Pb ratio of 0,1 to 0,2% and translating into an age error of 2 to 4 my. D'Orbigny is the sixth angrite to be identified and the largest of this rare meteorite class. Until now high-precision Pb-Pb data only has existed only for Angra dos Reis and LEW 86010 (1). Two of the D'Orbigny pyroxenes are from the groundmass and the other two are idiomorphic and grew inside druses-like cavities. The latter are considered to be 100% pure while the former may contain some inclusions. Also, we analysed one sample of anorthite, which had a Pb isotopic composition close to that of terrestrial Pb, and may be dominated by terrestrial Pb. The Pb-Pb age of the two matrix pyroxenes are 4549 ± 2 Ma and 4557 ± 2 Ma; however, the U-Pb ages are distinctly disconcordant for the younger pyroxenes. The druses pyroxenes gave Pb-Pb age of 4555.4 ± 1.9 Ma and 4556 ± 4 Ma, with 206Pb/204Pb ratios of 522 and ˜2000, respectively. Furthermore, the U-Pb ages for both these pyroxenes are, within the uncertainties of the correction parameters, concordant, as is a Th-Pb age for the second analysis. Unfortunately, the relatively large error of its Pb-Pb age reflects the small amount of Pb on the filament. Refereces, 1) Lugmair and Galer (1992) GCA 56, 1673-1694. 2)Göpel et al. (1994) EPSL 121 153. 3) Amelin Y. et al. (2002) LPSC XXXIII 1151.

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