The University of Arizona

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

C. L. V. CAILLET KOMOROWSKI, E. K. ZINNER, K. D. MCKEEGAN, R. HERVIG, P. R. BUSECK

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.

Keywords


Calcium-aluminum-rich inclusions (CAIs);CV carbonaceous chondrite;Meteorites;wollastonite;petrological;isotopical and trace element study

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