EVIDENCE FOR DISTILLATION IN THE FORMATION OF HAL AND RELATED HIBONITE INCLUSIONS

Four hibonite-bearing refractory inclusions, HAL from Allende, DH-H1 from the Dhajala H3 chondrite, 7-404 and 7-971 from the Murchison CM2 chondrite, have related chemical and isotopic systematics: they exhibit large Ce and V depletions and very low Mg and Ti concentrations compared to other meteoritic hibonites and have mass-fractionated Ca- and Ti-isotopic compositions with enrichments in the heavy isotopes by up to +13 parts per thousand/amu for Ca and up to +19 parts per thousand/amu for Ti. There is no correlation between Ca- and Ti-isotopic mass fractionations, but the Ti-isotopic mass fractionation is inversely correlated with the Ti concentration as expected for Rayleigh-law mass fractionation. The inclusions also have nonlinear 48Ca anomalies within error of -5 parts per thousand while nonlinear Ti-50 anomalies in HAL and 7-971 are +15.0 +/- 3.6 parts per thousand (2-sigma) and -4.4 +/- 6.5 parts per thousand, respectively. Oxygen-isotopic compositions were measured in HAL, 7-404, and 7-971, as well as a range of morphologically different refractory inclusions from Murchison. The Murchison refractory inclusions all show enrichments in O-16 relative to terrestrial with a mean value of +46.8 +/- 1.6 parts per thousand. The three HAL-type inclusions are also enriched in O-16 but are mass fractionated in their 0 isotopes in favor of the heavy isotopes with variable degrees of mixing with isotopically normal O. The scatter of the data from 7-404 and 7-971 in terms of mass fractionation exceeds the reproducibility predicted from measurements of the Burma-spinel standard and indicates the presence of intrinsic O-isotopic heterogeneities in these hibonites. These chemical and isotopic characteristics are consistent with the formation of HAL-type inclusions as distillation residues. A distillation origin is supported by chemical and isotopic measurements of a hibonite-bearing distillation residue produced in the laboratory by evaporating terrestrial kaersutite. The residue from this experiment has large Ce and V depletions, no detectable Mg, and mass fractionated Ca- and Ti-isotopic compositions. However, while the HAL-type refactory inclusions owe their chemical and isotopic characteristics to a distillation origin, subsequent thermal events are required to explain certain petrographic and chemical characteristics.