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

Abstract

Ion microprobe measurements of REE (rare earth element) and other selected trace and minor element concentrations were made in clinopyroxene, olivine, feldspar, and chlorapatite in Chassigny and the three nakhlites (Nakhla, Governador Valadares, and Lafayette). In Chassigny, we have found pigeonite occurring in a poikilitic habit; it is the first reported occurrence of this mineral in this meteorite. The most significant implication of this finding is that estimates of the major element composition of the Chassigny parent melt will have to accommodate pigeonite as a primary mineral crystallizing before plagioclase.Chlorapatite contains the highest REE abundances in all meteorites studied here. The REE patterns of chlorapatite, plagioclase, and augite in nakhlites and Chassigny are similar (although the downward trends of REE patterns are somewhat steeper for these minerals in Chassigny than in the nakhlites). Pyroxenes in nakhlites and Chassigny are extensively zoned in REEs and other trace and minor elements, such as Y, Zr, Ti, and Al, which appear to be ''resistant'' to diffusive reequilibration. Thus, primary igneous zoning of these elements has been preserved in the pyroxenes of these meteorites and can be used to infer their crystallization histories. The calculated REE patterns of the parent melts of nakhlites and Chassigny are LREE enriched and parallel to their whole-rock REE patterns, indicating that, once crystal accumulation occurred, the intercumulus trapped melt fractionally crystallized in an essentially 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 calculations for Nakhla also confirm these conclusions. The distributions of trace and minor elements in augites in nakhlites and Chassigny suggest that all three nakhlites could be samples from different horizons of the same lithologic unit but that Chassigny was not comagmatic with the nakhlites.