Simultaneous determination of mass-dependent Mg isotopic variations and radiogenic 26Mg by laser ablation-MC-ICP-MS and implications for the formation of chondrules

1Zhengbin Deng,1Marc Chaussidon,1,2,3Denton S.Ebel,4Johan Villeneuve,1Julien Moureau,1Frédéric Moynier
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.01.015]
1Université de Paris, Institut de physique du globe de Paris, CNRS, UMR 7154, Paris 75005, France
2Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA
3Department of Earth and Environmental Sciences, Columbia University, New York, USA
4Centre de Recherches Pétrographiques et Géochimiques, Université de Lorraine, CNRS 7358, Vandoeuve-lès-Nancy, France
Copyright Elsevier

Improvements in our understanding of the formation of chondrules requires a better knowledge of the thermal histories and the nature of their solid precursors. We present an in situ nanosecond laser ablation multi-collector inductively-coupled-plasma mass-spectrometry (LA-MC-ICP-MS) technique to measure simultaneously mass-dependent Mg isotopic fractionations and radiogenic 26Mg in chondritic components, thus allowing us to investigate within a chronological framework the thermal processes redistributing Mg in chondrules and their precursors. The internal 26Al-26Mg isochrons provide initial 26Al/27Al ratios from 5.46 (± 0.38) × 10−5 to 6.14 (± 0.92) × 10−5 for amoeboid olivine aggregates (AOAs) and Ca-, Al-rich inclusions (CAIs), and from 0.16 (± 0.08) × 10−5 to 1.87 (± 0.92) × 10−5 for chondrules from Allende and Leoville chondrites, which are consistent with the previously reported values. The combination of these values with up to 2.5‰ variation of the 25Mg/24Mg ratio within the studied chondrules shows that: (i) AOAs and the precursors of chondrules were likely formed via condensation of rapid-cooling gas reservoirs, and (ii) Mg stable isotopes are probably at disequilibrium between olivines and mesostases in some chondrules, likely due to Mg loss by vaporization during chondrule formation. We use these new observations to propose that Mg isotopes can likely serve as a tracer for the thermal histories of chondrules. We present here a scenario taking into account Mg loss by vaporization from chondrule melt and Mg gain into the melt by olivine dissolution. The existing Mg isotopic observations in chondrule melts and olivines can be explained in a scenario with a homogeneous distribution of Mg isotopes and initial 26Al in the accretion disk, provided that chondrule precursors have been heated up to sufficiently high peak temperatures (up to 2123 K) and stayed above 1800 K for several tens of minutes to allow for significant Mg evaporation. These conditions are most consistent with a shock wave model for the origin of chondrules.

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