Th/U variability in Allende chondrules

1,2Janne Blichert-Toft,3Christa Göpel,3Marc Chaussidon,1,2F.Albarède
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.04.006]
1Laboratoire de Géologie de Lyon, École Normale Supérieure de Lyon, CNRS UMR 5276, Université de Lyon, 46 Allée d’Italie, 69007 Lyon, France
2Department of Earth, Environmental and Planetary Sciences, Rice University, 6100 Main Street, Houston, TX 77005, USA
3Université de Paris, Institut de Physique du Globe de Paris, CNRS, 1 Rue Jussieu, 75005 Paris, France
Copyright Elsevier

Lead isotope compositions were measured on both single and combined chondrules from the CV3 carbonaceous chondrite Allende with the goal of determining the range of Th/U implied by the radiogenic 208Pb*/206Pb* values. All samples were aggressively acid step-leached to separate radiogenic from primordial lead. It is found that apparent Th/U varies both between individual chondrules and between the different leaching fractions of each chondrule or group of chondrules. Specifically, the apparent Th/U ratio deviates from the planetary value (3.876), varying spectacularly from 0.65 to 14.6. Variations between leachates and residues disclose the existence of internal heterogeneities, while inter-chondrule variations reveal the presence of external heterogeneities. Three main explanations for the observed Th-U fractionation that are not mutually exclusive prevail: (1) uranium species, notably UO and UO2, coexisted in the nebular gas at high temperature, whereas Th existed exclusively as ThO2; (2) chondrules interacted with an exotic oxidized vapor; and (3) chondrules represent melt of dust of different origins, a hypothesis dictated by the evidence of internal heterogeneity. The extent to which the measured apparent Th/U variability is due to each of these particular processes is difficult to assess, but the existence of substantial Th/U heterogeneity, especially within, but also among, single (or pooled) chondrules from the same chondrite calls for caution when Pb-Pb linear arrays, or mixing lines, are assigned isochronous significance.

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