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

Abstract

Listen

Translate article

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%./amu for Ca and up to +19%./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%. while nonlinear 50Ti anomalies in HAL and 7–971 are +15.0 ± 3.6%. (2σ) and −4.4 ± 6.5%., 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 16O relative to terrestrial with a mean value of +46.8 ± 1.6%. The three HAL-type inclusions are also enriched in 16O but are mass fractionated in their O 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.