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

Reger PM, Roebbert Y, Neumann W, Gannoun A, Regelous M, Schwarz WH, Ludwig T, Trieloff M, Weyer S, Bouvier A (2023)

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Publication Type: Journal article

Publication year: 2023

Journal

Book Volume: 343

Pages Range: 33-48

DOI: 10.1016/j.gca.2022.12.025

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 short-lived 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 (2σ). The weighted mean of the Pb-Pb ages of seven SIMS analyses of merrillites are 4564.3 ± 5.2 Ma (2σ). 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-grained and four plagioclase fractions defines an initial ²⁶Al/²⁷Al ratio of [8.89 ± 0.79] × 10⁻⁶ corresponding to a formation age of 1.83 ± 0.12 Ma after CAIs ([5.23 ± 0.13] × 10⁻⁵; 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. The concordance of the Pb-Pb and ²⁶Al-²⁶Mg ages of ungrouped CC achondrites when anchored to EC 002 suggest that ²⁶Al was homogeneously distributed between the NC and CC reservoirs at the time of their parent body accretion. Furthermore, the presence of internal U isotope heterogeneities found between mineral and whole-rock samples of EC 002 supports the need of U isotope analysis of meteoritic samples dated using the Pb-Pb chronometer.

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APA:

Reger, P.M., Roebbert, Y., Neumann, W., Gannoun, A., Regelous, M., Schwarz, W.H.,... Bouvier, A. (2023). Al-Mg and U-Pb chronological records of Erg Chech 002 ungrouped achondrite meteorite. Geochimica Et Cosmochimica Acta, 343, 33-48. https://dx.doi.org/10.1016/j.gca.2022.12.025

MLA:

Reger, Philip M., et al. "Al-Mg and U-Pb chronological records of Erg Chech 002 ungrouped achondrite meteorite." Geochimica Et Cosmochimica Acta 343 (2023): 33-48.

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