Origin of hibonite-pyroxene spherules found in carbonaceous chondrites

Abstract

We have studied both of the known glass-free, hibonite-pyroxene spherules: MYSM3, from Murray (CM2), and Y17-6, from Yamato 791717 (CO3). They consist of hibonite plates (~ wt% TiOtot2) enclosed in Al-richpyroxene that has such high amounts of CaTs (CaAl2SiO6) component, up to ~80mol%, that is must have crystallized metastably. Within the pyroxene, abundances of MgO and SiO2 are strongly correlated with each other and are anticorrelated with those of Al2O3, posssibly reflecting different relative distribution coefficients for Ti3+ and Ti4+ in the aluminous pyroxene of the spherules from those found for fassaite in Type B inclusions. Prevously described hibonite-silicate spherules have 26Mg deficits but the present samples do not. Furthermore, the pyroxene in Y17-6 has excess 26Mg, while the hibonite it encloses does not, indicating that the two phases either had different initial 26Al27Al ratios or different initial 26Mg/24Mg ratios. The Ti isotpic compositions of the present samples are highly unusual: δ50Ti=103.4±5.2‰ in MYSM3 and -61.4±4.1‰ in Y17-6, which are among the largest 50Ti anoamlies reported for any refractory inclusion. The textures suggest that hibonite crystallized first; but based on the calculated bulk compositions of both spherules, it is not the liquidus phase in either sample, which suggests that the hibonite in both samples is relict. The presence of ragged hibonite grains in MYSM3 and rounded hibonite grains in Y17-6 and a lck of isotopic equilibrium between pyroxene and hibonite support this conclusion. The spherules crystallized from liquid droplets that probably formed as a result of the melting of solid precursor grains that included hibonite. The heating events were too short and/or not hot enough to melt all the hibonite. The droplets cooled quickly enough that CaTs-rich pyroxene crystallized instead of anorthite. Based on the observed differences in istopic composition, it is unlikely that the precursors of the present samples formed in the same reservoir as each other or as the previously described hibonite-silicate spherules, providing further evidence of the isotopic heterogeneity of the early solar nebula.