¹Georgy V.Makhatadze(Георгий В. Махатадзе),¹Martin Schiller,¹Martin Bizzarro
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.12.020]
¹Centre for Star and Planet Formation (StarPlan), Globe Institute, Faculty of Health and Medical Sciences, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen K, Denmark
Coypright Elsevier

Various chemical elements including nickel (Ni) exhibit mass-independent isotope heterogeneity on a bulk meteorite level, which is generally accepted to reflect the heterogeneous distribution of presolar carrier(s) from different nucleosynthetic sources. Thus, understanding the nature of the carriers can help decipher the origin of the observed nucleosynthetic variability, which remains elusive. In this study, we present the first high precision measurements of mass-independent and mass-dependent Ni isotope compositions for step-leaches of the CI chondrite Ivuna and Efremovka CAIs supplemented by bulk chondrite measurements. Step-leaches record highly anomalous Ni isotope signatures that can be attributed to at least four diverse nucleosynthetic sources. The most anomalous leachates show either large deficits (up to 0.1 %) in the neutron-poor 58Ni and 60Ni nuclides (L11, thought to contain mainly s-process derived Ni) or minor enrichments and deficits (∼100 ppm) in 60Ni or 64Ni (L6, L8, L9 and L10, all thought to derive mainly from supernovae). Pristine CAIs record Ni isotope compositions typified by enrichments in 58Ni of up to 400 ppm. Our new data for bulk chondrites agree with earlier work and emphasize the appropriateness of using the 62Ni/61Ni ratio for internal normalization. Based on the compositional relations between the step-leaches data, CAIs, and bulk meteorites, we show that 60Ni variability is consistent with being of nucleosynthetic origin as opposed to reflecting variable Fe/Ni ratios in the presence of live 60Fe. Finally, we infer that the observed Ni nucleosynthetic disk variability is predominantly driven by a combination of processes separating different nucleosynthetic carriers in the disk from each other, including thermal processing and size-based sorting.

¹Philip M.Reger,²Yvonne Roebbert,³Wladimir Neumann,⁴Abdelmouhcine Gannoun,⁵Marcel Regelous,³Winfried H.Schwarz,³Thomas Ludwig,³Mario Trieloff,²Stefan Weyer,^6,1Audrey Bouvier
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.12.025]
¹Department of Earth Sciences, Institute of Earth and Space Exploration, University of Western Ontario, N6A 5B7 London, Ontario, Canada
²Institut für Mineralogie, Leibniz-Universität Hannover, 30167 Hannover, Germany
³Institut für Geowissenschaften, Klaus-Tschira-Labor für Kosmochemie, Universität Heidelberg, 69120 Heidelberg, Germany
⁴Laboratoire Magmas et Volcans, Université Clermont-Auvergne, F-63000 Clermont-Ferrand, France
⁵GeoZentrum Nordbayern, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91054 Erlangen, Germany
⁶Bayerisches Geoinstitut, Universität Bayreuth, 95447 Bayreuth, Germany
Copyright Elsevier

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.