MINERAL CHEMISTRY AND THE PETROGENESIS OF EUCRITES .2. CUMULATE EUCRITES

We carried out an ion microprobe study of REEs and other selected trac e and minor element concentrations in orthopyroxene, augite lamellae, and plagioclase of two cumulate eucrites Moama and Moore County in ord er to reevaluate the relationships between cumulate and noncumulate eu crites and among cumulate eucrites. Pyroxene and plagioclase in these two meteorites are more homogeneous in major, minor, and trace element s than in noncumulate eucrites, and trace and minor element concentrat ions in Moore County are higher than in Moama. Although subsolidus equ ilibration erased any original igneous zoning, it did not affect the R EE patterns of these minerals. Both pigeonite and plagioclase of Moama and Moore County have higher LREE/HREE ratios than those phases in no ncumulate eucrites. This indicates that cumulate eucrites derived from melts with fractionated REE patterns and, thus are not related to non cumulate eucrites. In addition, the inconsistency between degrees of f ractional crystallization deduced from major and trace elements and th e differences of LREE/HREE ratios recorded in pigeonite and plagioclas e also suggest that Moama and Moore County did not form in the same fr actional crystallization sequence. This study confirms earlier observa tions and conclusions of Ma and Schmitt (1979) and poses problems for prevalent models. Copyright (C) 1997 Elsevier Science Ltd.