EVOLUTION OF THE LUNAR CRUST - SIMS STUDY OF PLAGIOCLASE FROM FERROANANORTHOSITES

The lunar crust, down to a depth of similar to 65 km, is composed of o lder (>4.5 Ga) ferroan anorthosites and younger (4.43-4.17 Ga) Mg-suit e lithologies which include dunites, troctolites, and norites. The ano rthosites are generally inferred to represent floating cumulates in a lunar magma ocean (possible depth 800 km, moon's radius similar to 1,7 38 km). The cumulates that are inferred to be located near the base of the magma ocean are dominantly olivine and pyroxene. The last dregs o f the magma ocean are enriched in incompatible elements and have been named KREEP (K, rare earth elements, P). KREEP, formed in this manner, is probably concentrated near the crust/mantle boundary at similar to 70 km depth. We are attempting to characterize melts parental to ferr oan anorthosites and Mg-suite norites by analyzing REEs (La, Ce, Nd, S m, Eu, Dy, Er, Yb) and Ba, Sr, and Y in their cumulus plagioclase. If the cumulus grains have not been compromised by postcrystallization ef fects and if we know the relevant mineral/melt partition coefficients (Ds) we can invert the trace element data for plagioclase to parental melt compositions. Melts parental to ferroan anorthosites are estimate d to contain REE at concentrations ten to fifty times chondrites. Melt s parental to the earlier crystallizing anorthosites (lower REE) have virtually no Eu anomaly, while melts parental to later crystallizing a northosites (higher REE) have small negative Eu anomalies. This is qua litatively consistent with the fractionation of Eu relative to other R EE by crystallization of plagioclase with large positive Eu anomalies. Melts parental to the Mg-suite have much higher total REE and very la rge negative Eu anomalies. The characteristics of Mg-suite norite pare ntal magmas may have been derived by the assimilation of KREEP (near t he crust/mantle boundary) by Mg-rich basaltic melts formed deeper in t he cumulate pile or near the contact between the lower cumulate horizo ns of the magma ocean and primitive, unprocessed lunar mantle (similar to 800 km). Copyright (C) 1997 Elsevier Science Ltd.