A chemical and isotopic study of hi~nite-~eh refractory inclusions in p~mitive meteorites

Abstract-The ion microprobe has been used to determine the concentrations of a number oftrace elements (including all 14 stable rare earth elements) and the isotopic compositions of Ca and Mg in seven hibonite-rich refractory inclusions. Hibonite in every inclusion analyzed shows the effect of RAZE partitioning with other solid or liquid phases, in which hibonite is enriched in light over heavy REE compared to other existing phases. This partitioning causes Cl chondrite-normalized enrichments to drop by factors of 4 to 22 from lanthanum to lutetium. Fractionations due to gas-solid partitioning of REE are sometimes uperimposed on the pattern produced by partitioning between condensed phases. Two inclusions, HAL and DH-H 1, show evidence for formation in extremely oxidizing environments and their Ca is mass fm~ionat ~ in favor of the heavy isotopes by 7 and 12~/~u, reape&vely. Two ~~ndurn-hi~nite inclu~ons show evidence of extremely high temperature gas-solid fractionation, such that the more volatile tight REE are depleted as a group relative to the more refractory heavy REE. These inciusions both have Ca that is mass fractionated in favor of the light isotopes. Hibonite preserves the largest nuclear isotope anomalies inTi found to date (FAHEY ef al., 198Sa; HINTON et al., 1987b), but there does not seem to be a simple relationship between chemical composition, isotopic mass fractionation and the sign and magnitude of nuclear isotopic anomalies in hibonite-rich inclusions. One possibility for producing a variety of chemical compositions and degrem of mass fractionation while preserving nuclear isotope anomalies is by multiple cycles of condensation and evaporation in a turbulent protoplanetary accretion disk (MORL~LL