Evidence for distillation in the formation of HAL and related hibonite inclusions

Four hibonite-bearing refractory inclusions, HAL from Allende, DH-Hl from the Dhajala H3 chondrite, 7-404 and 7-97 1 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 Caand Ti-isotopic compositions with emichments in the heavy isotopes by up to + 13G/amu for Ca and up to + 19k/amu for Ti. There is no correlation between Caand 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% while nonlinear “Ti anomalies in HAL and 7-97 1 are + 15.0 f 3.6%0 (2~) and -4.4 f 6.5%0, 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 I60 relative to terrestrial with a mean value of +46.8 f 1.6%0. The three HAL-type inclusions are also enriched in I60 but are mass fractionated in their 0 isotopes in favor of the heavy isotopes with variable degrees of mixing with isotopically normal 0. The scatter of the data from 7-404 and 7-97 1 in terms of mass fractionation exceeds the reproducibility predicted from measurements of the Burma-spine1 standard and indicates the presence of intrinsic O-isotopic heterogeneities in these hibonites. These chemical and isotopic characteristics are consistent with the formation of HALtype 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 Caand 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.