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

Abstract

The isotopic composition of oxygen as well as ²⁶Al-²⁶Mg and ³⁶Cl-³⁶S systematics were studied in Curious Marie, an aqueously altered Allende CAI characterized by a Group II REE pattern and a large ²³⁵U excess produced by the decay of short-lived ²⁴⁷Cm. Oxygen isotopic compositions in the secondary minerals of Curious Marie follow a mass-dependent fractionation line with a relatively homogenous depletion in ¹⁶O (Δ¹⁷O of −8‰) compared to unaltered minerals of CAI components. Both Mg and S show large excesses of radiogenic isotopes (²⁶Mg^∗ and ³⁶S^∗) that are uniformly distributed within the CAI, independent of parent/daughter ratio. A model initial ²⁶Al/²⁷Al ratio [(6.2 ± 0.9) × 10⁻⁵], calculated using the bulk Al/Mg ratio and the uniform δ²⁶Mg^∗ ∼ +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 ²⁶Al/²⁷Al ratio of this CAI implies not only the early condensation of the CAI precursor but also that aqueous alteration occurred early, when ²⁶Al 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 ²⁴⁷Cm/²³⁵U 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 ³⁶S excesses (Δ³⁶S^∗ ∼ +97‰) detected in the secondary phases of Curious Marie are attributed to ³⁶Cl decay (t<inf>1/2</inf> = 0.3 Myr) that was introduced by Cl-rich fluids during the aqueous alteration event that led to sodalite formation. A model ³⁶Cl/³⁵Cl ratio of (2.3 ± 0.6) × 10⁻⁵ is calculated at the time of aqueous alteration, translating into an initial ³⁶Cl/³⁵Cl ratio of ∼1.7–3 × 10⁻⁵ at solar system birth. The Mg and S radiogenic excesses suggest that ²⁶Al and ³⁶Cl co-existed in the early solar nebula, raising the possibility that, in addition to an irradiation origin, ³⁶Cl could have also been derived from a stellar source.