A super-refractory inclusion containing nonstoichiometric spinel from the CO3.0 chondrite Yamato 81020

We studied a unique super-refractory inclusion with a core-mantle structure from CO3.0 Yamato 81020 by secondary ion mass spectrometry, electron microprobe, and scanning electron microscope techniques. The core consists largely of hibonite and nonstoichiometric Al-rich spinel indicating formation as a liquid at an exceptionally high temperature (>1900 degrees C). The mantle consists almost entirely of melilite with gehlenitic compositions (ranging from angstrom k(2) to angstrom k(25)). The oxygen- and magnesium-isotopic compositions of the core and mantle are very different; typically, Delta O-17 (equivalent to delta O-17 - 0.52 delta O-18) similar to-26 parts per thousand and integral(Mg) (mass fractionation of Mg isotopes) similar to 10 parts per thousand/amu in the core and Delta O-17 similar to-7 parts per thousand and integral(Mg) similar to 1 parts per thousand/amu in most of the mantle. The chemical and O, Mg-isotopic data indicate that the core and mantle formed in separate events, and that the melilite now in the core was formed during the mantle-melting event, probably filling preexisting voids and surficial cavities. Analyses of core and mantle phases plot along a single Al-26-Mg-26 isochron with initial (Al-26/Al-27)(0) corresponding to 4.8 +/- 1.0 (+/- 2 sigma) x 10(-5), suggesting a similar formation age to normal CAIs in chondrites.