TRACE AND MINOR ELEMENTS IN MINERALS OF NAKHLITES AND CHASSIGNY - CLUES TO THEIR PETROGENESIS

Ion microprobe measurements of REE (rare earth element) and other sele cted trace and minor element concentrations were made in clinopyroxene , olivine, feldspar, and chlorapatite in Chassigny and the three nakhl ites (Nakhla, Governador Valadares, and Lafayette). In Chassigny, we h ave found pigeonite occurring in a poikilitic habit; it is the first r eported occurrence of this mineral in this meteorite. The most signifi cant implication of this finding is that estimates of the major elemen t composition of the Chassigny parent melt will have to accommodate pi geonite as a primary mineral crystallizing before plagioclase. Chlorap atite contains the highest REE abundances in all meteorites studied he re. The REE patterns of chlorapatite, plagioclase, and augite in nakhl ites and Chassigny are similar (although the downward trends of REE pa tterns are somewhat steeper for these minerals in Chassigny than in th e nakhlites). Pyroxenes in nakhlites and Chassigny are extensively zon ed in REEs and other trace and minor elements, such as Y, Zr, Ti, and Al, which appear to be ''resistant'' to diffusive reequilibration. Thu s, primary igneous zoning of these elements has been preserved in the pyroxenes of these meteorites and can be used to infer their crystalli zation histories. The calculated REE patterns of the parent melts of n akhlites and Chassigny are LREE enriched and parallel to their whole-r ock REE patterns, indicating that, once crystal accumulation occurred, the intercumulus trapped melt fractionally crystallized in an essenti ally closed-system and that infiltration (i.e., metasomatism) by LREE- enriched magmas need not be invoked to explain the LREE enrichment in bulk samples of these meteorites. Fractional crystallization model cal culations for Nakhla also confirm these conclusions. The distributions of trace and minor elements in augites in nakhlites and Chassigny sug gest that all three nakhlites could be samples from different horizons of the same lithologic unit but that Chassigny was not comagmatic wit h the nakhlites.