EXPERIMENTAL STUDIES OF TRACE-ELEMENT PARTITIONING IN CA, AL-RICH COMPOSITIONS - ANORTHITE AND PEROVSKITE

Using electron probe and ion probe techniques, experimental crystal-li quid partition coefficients (D) have been measured in meteoritic Ca,Al -rich inclusion (CAI) compositions for Mg, Zr, Y, and REEs in anorthit e and perovskite. Partitioning data for Ti in anorthite and Al in pero vskite are also reported. Where cross-comparisons are possible between electron and ion probe data, agreement is good. Concentration variati ons in anorthite for many elements are beyond what can be explained by fractional crystallization, but show well-defined interelement correl ations which could reflect the temperature dependence of the D values or could result from liquid boundary layers. For anorthite our D value s for REEs are lower, and they decrease more rapidly with decreasing i onic radius than most in the literature. Most of the older REE D patte rns for plagioclase in the literature are too flat because of insuffic ient purity of mineral separates or because of analytical problems. Ne w ion probe data for minor and trace elements in anorthite from type B CAIs permit detailed comparisons with fractional crystallization mode ls based on the measured D values. For most comparisons, models and ob servations do not agree for amounts of crystallization less than 90%. It is possible that anorthite does not appear until after 95% crystall ization, compared to about 40% which would be expected from isothermal equilibrium crystallization experiments. The LREEs are highly compati ble elements in perovskite, but D values drop sharply for the HREEs, Y , and Zr. D values for REEs increase strongly from air to highly reduc ing conditions as coupled REE-Ti+3 substitutions become possible. Mode l calculations show that REE patterns in igneous perovskites from CAIs will reflect the D pattern, and the models can explain some REE patte rns from compact type A CAIs. However, there are other sets of trace e lement data for perovskite in CAIs that cannot easily be explained by igneous processes.