ION MICROPROBE INVESTIGATION OF PLAGIOCLASE AND ORTHO-PYROXENE FROM LUNAR MG-SUITE NORITES - IMPLICATIONS FOR CALCULATING PARENTAL MELT REECONCENTRATIONS AND FOR ASSESSING POSTCRYSTALLIZATION REE REDISTRIBUTION

The lunar Mg-suite, which includes dunites, troctolites, and norites, makes up to 20-30% of the Moon's crust down to a depth of similar to 6 0 km. The remainder is largely anorthosite. This report focuses on nor ites (which consist mostly of orthopyroxene and plagioclase) because w e have found that both phases are effective recorders of their parenta l melt compositions. In an earlier report, we analyzed orthopyroxene f rom twelve samples (three from Apollo 14, two from A-15, and seven fro m A-17) by SIMS for eight REE (La, Ce, Nd, Sm, Eu, Dy, Er, Yb). Invers ion of these data to estimated melt compositions yielded extremely hig h REE concentrations similar to KREEP. In this study, we report SIMS R EE data for plagioclase from these same twelve samples. The major obje ctive of this study is to estimate parental REE concentrations from bo th orthopyroxene and plagioclase data to see if both data inversions p roduce concordant melt compositions and thus better constrain the comp osition of melts parental to Mg-suite norites. The estimated REE conce ntrations from both phases show some evidence of slight postcrystalliz ation REE redistribution. Comparison of the observed ratio of REE for pyroxene/plagioclase to the ratio of the Ds for pyroxene/plagioclase i s consistent with REE redistribution which involves LREE diffusing fro m pyroxene into plagioclase and HREE diffusing from plagioclase into p yroxene. However, apparently these postcrystallization exchanges have not seriously affected our ability to estimate melt REE concentrations . The estimated melt REE concentrations using both plagioclase and ort hopyroxene, are similar to high-K KREEP especially for melts parental to A-15 norites. This study provides support for mineral trace element inversion estimates for parental melts. Even though these lunar crust al lithologies experienced high temperature annealing, apparently the cores of plagioclase and pyroxene continue to carry a record of their igneous crystallization history.