Rare earth element and other trace element microdistributions in two unusual extraterrestrial igneous systems: The enstatite achondrite (aubrite) meteorites and the lunar ferroan anorthosites

This thesis reports the results of rare earth element (REE) and other trace element measurements from two unusual extraterrestrial igneous systems: the highly reduced aubritic meteorites, which consist largely of enstatite and a host of rare trace minerals, and the nearly monomineralic lunar ferroan anorthosites. REE abundances were determined because these elements are the most sensitive indicators of the evolution of a magmatic body. A relatively new technique, secondary ion mass spectrometry (SIMS), allows in situ measurement of the REE in individual mineral grains, a significant advance over previous whole rock or mineral separates analyses. Contamination problems are reduced and the data may be evaluated in a petrographic context.; The aubrites are poorly studied and the limited REE data have been confusing. Using SIMS, I characterized the nature and distribution of the REE in their constituent minerals. Oldhamite (CaS) is the main REE host in the aubrites; most other minerals are extremely REE depleted. Oldhamite displays a number of distinct REE patterns which cannot be related through crystallization from a single magma. Comparison with similar oldhamite REE patterns from the related unequilibrated enstatite chondrites suggests that some CaS survived the melting event that produced the aubrites, and therefore cannot provide any information on their igneous evolution. Other minerals and elements must be studied to understand the petrogenesis of these meteorites.; The lunar ferroan anorthosites have been extensively studied, but it is still not clear whether they formed by crystallization of an essentially moon-wide magma ocean, or are the result of a series of magmatic events. By measuring the REE in plagioclase and pyroxene grains from samples of four recently defined subgroups within the ferroan anorthosite suite, I identified inter- and intra-group variations in REE abundances, which show that relationships between these rocks are complex. Members of the "typical" ferroan anorthosites have REE abundances that are consistent with simple crystallization from an evolving magma, but other trace element abundances do not support this interpretation. Samples from the other subgroups cannot be easily related to the "typical" anorthosites nor to each other.