Carbonaceous chondrites hibonite

Hibonite rich inclusions of the CM2 and oxide grains in carbonaceous chondrites... 

On Earth, the Ti(m) oxidation state is unstable. However, Ti3+-bearing minerals are well-characterized in some meteorites and Moon rocks, generally coexisting with Ti4 in such phases as calcic pyroxene, ulvospinel, hibonite and ilmenite. In hibonite, CaAlI20I9, a refractory phase in carbonaceous chondrites, EPR and optical spectral data indicate that Ti3+ ions are present (Hunger and Stolper, 1986 Live et al., 1986). The trivalent Ti ions may be stabilized in the five-coordinated trigonal bipyramidal M5 site of the hibonite structure... 

Beckett J. R. and Stolper E. (1994) The stability of hibonite, mehhte and other aluminous phases in silicate melts imphcations for the origin of hibonite-bearing inclusions from carbonaceous chondrites. Meteoritics 29, 41 -65. 

Ireland T. R. (1990) Presolar isotopic and chemical signatures in hibonite-bearing refractory inclusions from the Murchison carbonaceous chondrite. Geochim. Cosmochim. Acta 54, 3219-3237. 

Simon S. B., Davis A. M., Grossman L., and Zinner E. K. (1998) Origin of hibonite-pyroxene spherules found in carbonaceous chondrites. Meteorit. Planet. Sci. 33, 411-424. 

The existence of a canonical ( Al/ Al)o value was previously based on analyses of CAIs only from carbonaceous chondrites refractory inclusions from ordinary and enstatite chondrites are rare and often very small, and thus few had been discovered and none analyzed. There are now data for four CAIs from unequilibrated ordinary chondrites (Russell et al., 1996 Huss et al., 2001) and for 11 hibonite-bearing inclusions from enstatite chondrites (Guan et al., 2000) all are consistent with ( Al/ Al)o in the range (3.5-5.5) X 10 , except for 4 of the (very small) hibonite grains for which Mg could not be resolved. Thus, the same canonical value characterizes CAIs from all major meteorite classes. The possible meaning of this confirmation in terms of nebular chronology based on Al is not completely straightforward, however. 

Figure 5 Chondrite-normalized REE patterns for selected cal-cium-alumimmi inclusions (CAI) from the Allende carbonaceous chondrite and refractory mineral grains (perovskite, hibonite) from the Murchison carbonaceous chondrite." The highly irregular REE patterns, including anomalies for the least refractory Ce, Eu, and Yb, are indicative of localized very high temperatures leading to complex REE evaporation/condensation processes...

The isotopic anomalies detected in the iron peak elements are extremely small (Figure 4), since the anomalous material is diluted with material with a terrestrial isotopic composition. This contamination results from the fact that the meteorite inclusion is taken into solution before chemically extracting the relevant element in a form suitable for conventional TIMS analysis. However, ion microprobe mass spectrometry can be used to analyse small meteoritic inclusions in situ without the need of chemical processing. This enables single inclusions to be analysed for a variety of elements, whilst maintaining the petrographic context of the sample. The carbonaceous chondrites Murchison and Murray also contain refractory inclusions such as corundum and hibonite, but they are invariably small and difficult... 

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