The White Angel: A unique wollastonite-bearing, mass-fractionated refractory inclusion from the Leoville CV3 carbonaceous chondrite

Abstract

We report the study of an unusual compact type A refractory inclusion, named the White Angel, from the Leoville CV3 meteorite. The petrologic, mineral chemical, isotopic, and traceelement signatures of this once-molten Ca-Al-rich inclusion (CAI), which contains large, equant wollastonite crystals, indicate a short multistage history that occurred very early, before substantial decay of 26Al. Magnesium in the inclusion is isotopically heavy, with FMg reaching 18/amu, in the range of fractionated and with unidentified nuclear effects (FUN) inclusions. However, the absence of any nuclear anomalies in Ca and Ti and an inferred 26Al/27Al ratio of (5.5 +/- 0.9) 10^(5) indicate that the White Angel belongs to the F inclusions. Silicon and oxygen are also mass fractionated in favor of the heavy isotopes, but to a lesser extent. The O isotopes show a range in 16O excesses. On an O three-isotope plot, data points lie on a line parallel and to the right of the carbonaceous chondrite anhydrous mineral mixing line, with wollastonite being the most 16O-rich phase. The chondritenormalized rare earth and trace-element pattern of the whole inclusion is the complement of an ultrarefractory pattern indicating that precursor phases of the CAI must have condensed in an Al-, heavy rare earth element (HREE)-depleted reservoir. Melting of those precursor phases in an 16O-rich environment and evaporation led to mass-dependent isotopic fractionation of Mg, Si, and O. Partial isotopic exchange with a reservoir containing unfractionated Mg took place at a later stage but before any measurable decay of 26Al. Some minerals (melilite and perovskite) in the White Angel equilibrated oxygen isotopes with a relatively 16O-poor reservoir that was also mass-fractionated toward the heavy isotopes, different from that with which the normal or FUN inclusions interacted.