ANATEXIS OF LUNAR CUMULATE MANTLE IN TIME AND SPACE - CLUES FROM TRACE-ELEMENT, STRONTIUM, AND NEODYMIUM ISOTOPIC CHEMISTRY OF PARENTAL APOLLO-12 BASALTS

In an effort to elucidate the processes of lunar mantle melting, and t he magma evolution of mare basalts within Oceanus Procellarum on the w estern lunar near-side, we have analyzed seven fine-grained to vitroph yric Apollo 12 basalts for trace-elements; five of these also have bee n analyzed for Nd and Sr isotopic compositions. These samples represen t all three main groups identified among the Apollo 12 mare basalts an d have been proposed as parental melts to their respective groups, i.e ., olivine-, pigeonite-, and ilmenite-basalts. The sources for these l ow Ti mare basalts are postulated to have formed from crystallization of a global magma ocean. Li-Be systematics, combined with REE data, in dicate that the specific sources for the Apollo 12 low-Ti mare basalts were generated after 82-94% crystallization of this lunar magma ocean . In fact, it seems that all mare basalts analyzed from the Apollo col lections were generated from cumulates precipitated in the last 20% of the magma ocean. Chemical compositions of fine-grained pigeonite and olivine basalts are consistent with 7-9% nonmodal (in proportions not defined by experimental petrology and phase equilibria) melting of a s ource consisting of 48% olivine, 30% calcic clinopyroxene, and 22% pig eonite (as per Neal et al., 1994b). Sm-Nd and Rb-Sr abundance data als o suggest that the pigeonite- and olivine-basalt source contained from 0.3 to 0.5% trapped residual liquid from the magma ocean. The composi tions of the two fine-grained ilmenite basalts are consistent with 5-7 % partial melting of a source with subequal proportions of olivine (45 .5%) and pigeonite (42.5%) and lesser amounts of clinopyroxene (11.5%) and entrained plagioclase (0.5%). Furthermore. the ilmenite source wa s nearly devoid of trapped liquid (<0.15%). A few of these samples do indicate minor post-extrusive fractionation, but most of the samples a re considered to be unfractionated, primitive magmas that are parental to the other mare basalts. Isotopic systematics of the Apollo 12, fin e-grained, parental basalts are consistent with their derivation from two distinct mantle source regions. Both of these sources were LREE de pleted for extended periods of time: up to 600 million years for the i lmenite-basalt source and up to 900 million years for the pigeonite- a nd olivine basalt source. Due in part to the relatively small proporti on of low Sm/Nd, trapped, residual magma-ocean liquid in the source (< 0.15%), the Nd isotopic compositions of the ilmenite basalts are among the most radiogenic ever analyzed from the Moon (epsilon(Nd) = +10.5 to +11.2, at 3.2 Ga). The mantle source for the olivine and pigeonite basalts contained a higher proportion of trapped, residual, magma-ocea n liquid (0.3 to 0.5%), thus yielding less radiogenic Nd isotopic sign atures (epsilon(Nd) = +4.3 to +4.7, at 3.2 Ga). By integrating this in formation on parental, low-Ti, Apollo 12 basalts with mare basalt and picritic glass data from other landing sites, as well as telescopic an d remote-sensing data, we propose a model for melting of the lunar int erior. The upper 400-500 km of the lunar mantle is a consequence of in cipient melting of the Moon and formation of a global magma ocean. Thi s magma ocean became progressively enriched in incompatible elements a s it precipitated the cumulate upper mantle. This incompatible-element enriched liquid was also trapped in varying proportions in the differ entiated cumulates. The earliest, extensive mare magmas (high-Ti mare basalts) were generated at shallow depths in the mantle from cumulate source-regions that had trapped relatively large proportions of this i ncompatible-element enriched, residual, magma ocean liquid. These sour ce regions also contained the late-crystallizing phase ilmenite and, t hus, generated high-Ti magmas. The trapped liquid component contained elevated abundances of heat-producing elements (K, U, and Th), increas ing the fertility of associated source-regions. With time, melting mov ed progressively deeper in the mantle to source-regions with less of t he residual, heat-producing, magma ocean liquid. Due to density contra sts, the ilmenite-bearing upper portions of the lunar mantle sank into the cumulate pile, possibly carrying more fertile material with it an d allowing melting of more Mg-enriched source regions (to form the hig h-Ti picritic glass beads). Thus, the major controlling factors in the melting of the lunar interior could be the proportion of trapped, mag ma-ocean liquid in the cumulate source and the sinking of more fertile , ilmenite-bearing material into the lower mantle. Copyright (C) 1997 Elsevier Science Ltd.