Al-Mg and U-Pb chronological records of Erg Chech 002 ungrouped achondrite meteorite - Archive ouverte HAL Access content directly
Journal Articles Geochimica et Cosmochimica Acta Year : 2023

Al-Mg and U-Pb chronological records of Erg Chech 002 ungrouped achondrite meteorite

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Abstract

Achondrite meteorites are remnants of the earliest planetary differentiation processes in the Solar System. They have been used to anchor short-lived radiochronometers to absolute ages determined from long-lived radiochronometers. More specifically, when comparing the isotopic systematics of the shortlived 26Al-26Mg chronometer anchored to absolute U-corrected Pb-Pb ages, inferences about the distribution of 26Al (half-life of 717 000 yr) in the protoplanetary disk can be evaluated. The ungrouped achondrite Erg Chech (EC) 002 has a distinct mineralogy and more evolved elemental composition compared to basaltic achondrites. In situ and solution 26Al-26Mg chronometry and 53Mn-53Cr chronometry suggest that EC 002 formed within 0.7 to 2.2 Ma after the formation of Ca-Al-rich inclusions (CAIs), making it the oldest known sample of igneous crust in the Solar System (Barrat et al., 2021; Anand et al., 2022; Zhu et al., 2022; Fang et al., 2022). Here we present the U-corrected Pb-Pb age and 26Al-26Mg age obtained by MC-ICPMS solution analysis of the same mineral separate samples of EC 002. In addition, six merrillite grains were analyzed by in-situ SIMS to determine their Pb-Pb individual ages. The U isotope composition of EC 002 exhibits internal heterogeneities between leached pyroxene (238U/235U = 137.766 ± 0.027) and the bulk rock (238U/235U = 137.8190 ± 0.0074). The Pb isotope composition of progressively leached pyroxenes are characterized by radiogenic 206Pb/204Pb ratios (ranging from 41 to 23487). Using the U isotope composition of the leached pyroxenes, the resulting age of the 207Pb/206Pb-204Pb/206Pb isochron is 4565.87 ± 0.30 Ma (2r). The weighted mean of the Pb-Pb ages of seven SIMS analyses of merrillites are 4564.3 ± 5.2 Ma (2r). These similar ages (within uncertainty) indicate rapid cooling and the absence of significant thermal events after 4559 Ma on the parent body of EC 002. The 26Al-26Mg isochron through a bulk rock, pyroxene, fine-rained and four plagioclase fractions defines an initial 26Al/27Al ratio of [8.89 ± 0.79] 106 corresponding to a formation age of 1.83 ± 0.12 Ma after CAIs ([5.23 ± 0.13] 105; Jacobsen et al., 2008). The initial 26Al abundance is consistent with previous MC-ICP-MS 26Al-26Mg reported systematics for EC 002 (Fang et al., 2022), but 0.46 ± 0.13 Myr older than the in situ SIMS 26Al-26Mg age previously reported by Barrat et al. (2021).When anchored to the absolute Pb-Pb age of CV3 CAIs (4567.30 ± 0.16 Ma; Connelly et al., 2012), the Al-Mg model age of EC 002 is 4565.47 ± 0.20 Ma, slightly younger than its U-corrected Pb-Pb age.
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hal-03924686 , version 1 (05-01-2023)

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Philip M Reger, Yvonne Roebbert, Wladimir Neumann, Abdelmouhcine Gannoun, Marcel Regelous, et al.. Al-Mg and U-Pb chronological records of Erg Chech 002 ungrouped achondrite meteorite. Geochimica et Cosmochimica Acta, In press, 343, pp.33-48. ⟨10.1016/j.gca.2022.12.025⟩. ⟨hal-03924686⟩
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