Trace-element partition coefficients for perovskite and hibonite in meteorite compositions

Abstract

The concentrations of 28 elements were measured using an ion microprobe in perovskite, hibonite and coexisting melts, in isothermal crystallization experiments on chemical compositions similar to those of Compact 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-values. Perovskite and hibonite D's have ranges of 10^−2 for Si to 20 for Th and 3·10^−3 for Si to ∼8 for La, respectively. There are regular relationships between the ionic radius, the valence of the trace element and the partition coefficients in perovskite and hibonite. While there are differences in the D-values between perovskite and hibonite, they follow very similar trends with perovskite typically having D-values that are 5–10 times higher for the same element. Perovskite and hibonite D's are almost identical for the divalent cations Ba (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 continuously from D_La = 6 to D_Lu = 0.03 in hibonite. For perovskite, REE D's increase slightly from D_La = 10 to D_Nd = 15 and then decrease continuously to D_Lu = 1.0 and D's for trivalent cations with smaller ionic radii 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 and DSi in hibonite, the D-values for tetravalent cations and Nb, the only pentavalent element, fall within the range of D's for the REE. D_th/D_U equals 3 in perovskite and ∼ 15 in hibonite. Our data can be applied to the genesis and evolution of hibonite in refractory meteorite inclusions. For example, low Ba relative to other refractory elements, such as Hf, Zr, La, etc., in hibonite has been observed in some 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 hibonite rather than the increased volatility of Ba under oxidizing conditions during condensation. In addition, since D_La/D_Lu > 50 for hibonite. LREE/HREE ratios of 1 in hibonite in some CTA CAI from Leoville and Allende are inconsistent with hibonite equilibrating with the melts that formed these inclusions and the hibonite is relict. Similar applications are possible with our perovskite partitioning data. For example, it is likely that high-REE (500−1000 × chondritic) perovskite with Th/U of 3–4 that are found in the outer region of Type B 1 CAI have not been in equilibrium with the CAI melt that contains ∼ 20 × ch REE and a Th/U ratio of 3 and they are probably relics that survived the most recent partial melting event.

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