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 Q17O <-23 %o. The Q17O values of Al,Ti-diopside in the CAI core range from-11 %o to-18 %o, which are positively correlated with its TiO2 contents, whereas diopside in the Wark-Lovering (WL) rim is 16O-rich with Q17O =-23 %o. Melilite in the CAI mantle shows Q17O values <-10 %o only when & Aring;k<15 but becomes more 16O-poor (Q17O =-3 %o to-9 %o) when & Aring;k>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 f 0.18) x 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.