TRACE-ELEMENT PARTITION-COEFFICIENTS FOR PEROVSKITE AND HIBONITE IN METEORITE COMPOSITIONS

The concentrations of 28 elements were measured using an ion microprob e in perovskite, hibonite and coexisting melts, in isothermal crystall ization experiments on chemical compositions similar to those of Compa ct Type A (CTA) Ca-, Al-rich inclusions (CAI) and to a hibonite-glass microspherule. The mineral/melt partition coefficients (D) calculated from the measured concentrations for both minerals define reliable D-v alues. Perovskite and hibonite D's have ranges of 10(-2) for Si to 20 for Th and 3.10(-3) for Si to similar to 8 for La, respectively. There are regular relationships between the ionic radius, the valence of th e trace element and the partition coefficients in perovskite and hibon ite. While there are differences in the D-values between perovskite an d hibonite, they follow very similar trends with perovskite typically having D-values that are 5-10 times higher for the same element. Perov skite and hibonite D's are almost identical for the divalent cations B a (0.02 and 0.03, respectively) and Sr (1.1 and 0.8, respectively) in our experiments. D-Mg for perovskite is low, 0.03, when compared with the value for hibonite, 0.5. Mineral/melt D's for the REE decrease con tinuously from D-La=6 to D-La=0.03 in hibonite. For perovskite, REE D' s increase slightly from D-La=10 to D-Nd=15 and then decrease continuo usly to D-Lu=1.0 and D's for trivalent cations with smaller ionic radi i than the REE are lower, with D-Al=0.08 and D-Sc=0.15 lower than D-Cr =0.8 and D-V=1.0. With the exception of D-Th and D-Si in perovskite an d D-Si in hibonite, the D-values for tetravalent cations and Nb, the o nly pentavalent element, fall within the range of D's for the REE. D-T h/D-U equals 3 in perovskite and similar to 15 in hibonite. Our data c an be applied to the genesis and evolution of hibonite in refractory m eteorite inclusions. For example, low Ba relative to other refractory elements, such as Hf, Zr, La, etc., in hibonite has been observed in s ome hibonite-bearing inclusions. Since D-Ba<<D-Hf<<D-Zr and <<D-La in our experiments low Ba may result from the incompatibility of Ba in hi bonite rather than the increased volatility of Ba under oxidizing cond itions during condensation. In addition, since D-La/D-Lu>50 for hiboni te, LREE/HREE ratios of I in hibonite in some CTA CAI from Leoville an d Allende are inconsistent with hibonite equilibrating with the melts that formed these inclusions and the hibonite is relict. Similar appli cations are possible with our perovskite partitioning data. For exampl e, it is likely that high-REE (500-1000Xchondritic) perovskite with Th /U of 3-4 that are found in the outer region of Type BI CAI have not b een in equilibrium with the CAI melt that contains similar to 20Xch RE E and a Th/U ratio of 3 and they are probably relies that survived the most recent partial melting event.