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A chemical and isotopic study of hibonite-rich refractory inclusions in primitive meteorites

Geochimica et Cosmochimica Acta
Publication Date
DOI: 10.1016/0016-7037(88)90028-2
  • Chemistry


Abstract The ion microprobe has been used to determine the concentrations of a number of trace 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 REE partitioning with other solid or liquid phases, in which hibonite is enriched in light over heavy REE compared to other coexisting phases. This partitioning causes Cl chondritenormalized enrichments to drop by factors of 4 to 22 from lanthanum to lutetium. Fractionations due to gas-solid partitioning of REE are sometimes superimposed on the pattern produced by partitioning between condensed phases. Two inclusions, HAL and DH-H1, show evidence for formation in extremely oxidizing environments and their Ca is mass fractionated in favor of the heavy isotopes by 7 and 12%./amu, respectively. Two corundum-hibonite inclusions show evidence of extremely high temperature gas-solid fractionation, such that the more volatile light REE are depleted as a group relative to the more refractory heavy REE. These inclusions both have Ca that is mass fractionated in favor of the light isotopes. Hibonite preserves the largest nuclear isotope anomalies in Ti found to date ( Fahey et al. 1985a; Hinton el 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 degrees of mass fractionation while preserving nuclear isotope anomalies is by multiple cycles of condensation and evaporation in a turbulent protoplanetary accretion disk ( Morfill, 1983; Morfill and R. N. Clayton, 1986a,b). These cycles could also occur in a presolar nebula with large non-axisymmetric temperature and pressure distributions ( Boss, 1988). Nuclear anomalies are most likely to be preserved in hibonite because hibonite is the most refractory major phase to occur commonly in meteorites.

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