Th/U variability in Allende chondrules
Résumé
14 Lead isotope compositions were measured on both single and combined chondrules from the 15 CV3 carbonaceous chondrite Allende with the goal of determining the range of Th/U implied by 16 the radiogenic 208Pb*/206Pb* values. All samples were aggressively acid step-leached to 17 separate radiogenic from primordial lead. It is found that apparent Th/U varies both between 18 individual chondrules and between the different leaching fractions of each chondrule or group of 19 chondrules. Specifically, the apparent Th/U ratio deviates from the planetary value (3.876), 20 varying spectacularly from 0.65 to 14.6. Variations between leachates and residues disclose the 21 existence of internal heterogeneities, while inter-chondrule variations reveal the presence of 22 external heterogeneities. Three main explanations for the observed Th-U fractionation that are 23 not mutually exclusive prevail: (1) uranium species, notably UO and UO 2 , coexisted in the 24 nebular gas at high temperature, whereas Th existed exclusively as ThO 2 ; (2) chondrules 25 *Manuscript 2 interacted with an exotic oxidized vapor; and (3) chondrules represent melt of dust of different 26 origins, a hypothesis dictated by the evidence of internal heterogeneity. The extent to which the 27 measured apparent Th/U variability is due to each of these particular processes is difficult to 28 assess, but the existence of substantial Th/U heterogeneity, especially within, but also among, 29 single (or pooled) chondrules from the same chondrite calls for caution when Pb-Pb linear 30 arrays, or mixing lines, are assigned isochronous significance. 31 32
Domaines
Géochimie
Origine :
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