Mineralogical and isotopic constraints on early, high-temperature events and reservoirs recorded in the interior of a Type B Ca-Al-rich inclusion from the reduced CV3 chondrite Vigarano

Abstract

A coordinated mineralogical and oxygen and Al-Mg isotopic study of a Type B Ca-Al-rich inclusion (CAI) from the reduced CV3 chondrite Vigarano was carried out using electron microprobe analysis, transmission electron microscopy, and secondary ion mass spectrometry. This CAI, a once molten igneous object, shows heterogeneous oxygen isotopic compositions among its constituent mineral phases. Spinel is uniformly ¹⁶O-rich with Δ¹⁷O ≤ −23 ‰. The Δ¹⁷O values of Al,Ti-diopside in the CAI core range from −11 ‰ to −18 ‰, which are positively correlated with its TiO₂ contents, whereas diopside in the Wark–Lovering (WL) rim is ¹⁶O-rich with Δ¹⁷O = −23 ‰. Melilite in the CAI mantle shows Δ¹⁷O values ≤ −10 ‰ only when Åk_<15 but becomes more ¹⁶O-poor (Δ¹⁷O = −3 ‰ to −9 ‰) when Åk_>15. These correlated chemical and oxygen isotopic variations were likely established during crystallization of melilite and Al,Ti-diopside from a partial melt, while spinel preserves the original ¹⁶O-rich composition of the CAI precursor. We infer that a partial melt was isotopically evolving from ¹⁶O-rich to ¹⁶O-poor during melilite crystallization, then back to ¹⁶O-rich during Al,Ti-diopside crystallization via exchange with different gas reservoirs of ¹⁶O-poor and ¹⁶O-rich compositions during heating event(s). Our Al-Mg isotopic measurements of the CAI core and mantle define a single isochron with an inferred initial ²⁶Al/²⁷Al ratio of (4.93 ± 0.18) × 10⁻⁵, indistinguishable from that of the WL rim. This indicates that multiple high-temperature events and oxygen isotope exchange with isotopically distinct gas reservoirs occurred rapidly during the CAI formation.