ORTHO-PYROXENE AS A RECORDER OF LUNAR CRUST EVOLUTION - AN ION MICROPROBE INVESTIGATION OF MG-SUITE NORITES

The lunar Mg suite, which includes dunites, troctolites, and norites, make up to approximately 20-30% of the Moon's crust down to a depth of approximately 60 km. The remainder is largely anorthositic. This repo rt focuses on norites (which consist of mostly orthopyroxene and plagi oclase) because we have found that the chemical characteristics of ort hopyroxene, are effective recorders of their parental melt composition s. The values of Mg' for the parental melts calculated from those in o rthopyroxene show that most melts have values of Mg' in the range of 0 .36-0.60. This compares with a range of Mg' values for lunar volcanic picritic glass beads of 0.4-0.68 (Shearer and Papike, 1993). Therefore , although the cumulate whole-rock compositions of the Mg suite can be extremely magnesian (Mg' > 0.90), the calculated parental melts are n ot anomalously high in Mg. A chemical characteristic of the Mg-suite n orites that is more difficult to explain is the high KREEP content of the calculated parental melts. The REE contents (especially HREE) for the calculated norite parental melts match or exceed the high-K KREEP component of Warren (I 989). Therefore, the mixture of a KREEP compone nt and a picritic melt, by itself, cannot explain the high estimated R EE contents of the melts parental to norites. Advanced crystallization (>90%), especially of plagioclase, may also be required.