In situ isotopic studies of the U-depleted Allende CAI Curious Marie: Pre-accretionary alteration and the co-existence of ^(26)Al and ^(36)Cl in the early solar nebula

Abstract

The isotopic composition of oxygen as well as ^(26)Al-^(26)Mg and ^(36)Cl-^(36)S systematics were studied in Curious Marie, an aqueously altered Allende CAI characterized by a Group II REEpattern and a large ^(235)U excess produced by the decay of short-lived ^(247)Cm. Oxygen isotopic compositions in the secondary minerals of Curious Marie follow a mass-dependent fractionation line with a relatively homogenous depletion in ^(16)O (Δ^(17)O of −8‰) compared to unaltered minerals of CAI components. Both Mg and S show large excesses of radiogenic isotopes (^(26)Mg∗ and ^(36)S∗) that are uniformly distributed within the CAI, independent of parent/daughter ratio. A model initial ^(26)Al/^(27)Al ratio [(6.2 ± 0.9) × 10^(−5)], calculated using the bulk Al/Mg ratio and the uniform δ^(26)Mg∗ ∼ +43‰, is similar to the canonical initial solar system value within error. The exceptionally high bulk Al/Mg ratio of this CAI (∼95) compared to other inclusions is presumably due to Mg mobilization by fluids. Therefore, the model initial ^(26)Al/^(27)Al ratio of this CAI implies not only the early condensation of the CAI precursor but also that aqueous alteration occurred early, when ^(26)Al was still at or near the canonical value. This alteration event is most likely responsible for the U depletion in Curious Marie and occurred at most 50 kyr after CAI formation, leading to a revised estimate of the early solar system ^(247)Cm/^(235)U ratio of (5.6 ± 0.3) × 10^(−5). The Mg isotopic composition in Curious Marie was subsequently homogenized by closed-system thermal processing without contamination by chondritic Mg. The large, homogeneous ^(36)S excesses (Δ^(36)S∗ ∼ +97‰) detected in the secondary phases of Curious Marie are attributed to ^(36)Cl decay (t_(1/2) = 0.3 Myr) that was introduced by Cl-rich fluids during the aqueous alteration event that led to sodalite formation. A model ^(36)Cl/^(35)Cl ratio of (2.3 ± 0.6) × 10^(−5) is calculated at the time of aqueous alteration, translating into an initial ^(36)Cl/^(35)Cl ratio of ∼1.7–3 × 10^(−5) at solar system birth. The Mg and S radiogenic excesses suggest that ^(26)Al and ^(36)Cl co-existed in the early solar nebula, raising the possibility that, in addition to an irradiation origin, ^(36)Cl could have also been derived from a stellar source.

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