³⁶Cl,²⁶Al, and O isotopes in an Allende type B2 CAI: Implications for multiple secondary alteration events in the early solar system

Abstract

We measured ³⁶Cl-36³⁶S and ²⁶Al-²⁶Mg systematics and O isotopes of secondary phases in a moderately altered type B2 CAI (CAI#2) from the Allende CV3 chondrite. CAI#2 has two distinct alteration domains: the anorthite-grossular (An-Grs) domain that mostly consists of anorthite and grossular, and the Na-rich domain that mostly consists of sodalite, anorthite, and Fe-bearing phases. Large ³⁶S excesses (up to ∼400‰) corresponding to an initial ³⁶Cl/³⁵Cl ratio of (1.4 ± 0.3) × 10⁻⁶ were observed in sodalite of the Na-rich domain, but no resolvable ²⁶Mg excesses were observed in anorthite and sodalite of the Na-rich domain (the initial ²⁶Al/²⁷Al ratio < 4.4 × 10⁻⁷). If we assume that the ³⁶Cl-³⁶S and the ²⁶Al-²⁶Mg systematics were closed simultaneously, the ³⁶Cl/³⁵Cl ratio would have to be on the order of ∼10⁻² when CAIs were formed. In contrast to sodalite in Na-rich domain, significant ²⁶Mg excesses (up to ∼35‰) corresponding to an initial ²⁶Al/²⁷Al ratio of (1.2 ± 0.2) × 10⁻⁵ were identified in anorthite of the An-Grs domain. The ²⁶Al-²⁶Mg systematics of secondary phases in CAI#2 suggest that CAIs experienced multiple alteration events. Some of the alteration processes occurred while ³⁶Cl (half-life is 0.3 Myr) and ²⁶Al (half-life is 0.72 Myr) were still alive, whereas others took place much later. Assuming that ²⁶Al was homogeneously distributed in the solar nebula, our study implies that alteration of CAIs occurred as early as within 1.5 Myr of CAI formation and as late as 5.7 Myr after.

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