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 the magma evolution of mare basalts within Oceanus Procellarum on the western lunar near-side, we have analyzed seven fine-grained to vitrophyric Apollo 12 basalts for trace-elements; five of these also have been analyzed for Nd an...

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Published in:Geochimica et cosmochimica acta 1997-07, Vol.61 (13), p.2731-2747
Main Authors: Snyder, Gregory A., Neal, Clive R., Taylor, Lawrence A., Halliday, Alex N.
Format: Article
Language:English
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Summary:In an effort to elucidate the processes of lunar mantle melting, and the magma evolution of mare basalts within Oceanus Procellarum on the western lunar near-side, we have analyzed seven fine-grained to vitrophyric Apollo 12 basalts for trace-elements; five of these also have been analyzed for Nd and Sr isotopic compositions. These samples represent all three main groups identified among the Apollo 12 mare basalts and have been proposed as parental melts to their respective groups, i.e., olivine-, pigeonite-, and ilmenite-basalts. The sources for these low-Ti mare basalts are postulated to have formed from crystallization of a global magma ocean. LiBe systematics, combined with REE data, indicate 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 collections 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 source consisting of 48% olivine, 30% calcic clinopyroxene, and 22% pigeonite (as per Neal et al., 1994b). SmNd and RbSr abundance data also suggest that the pigeonite- and olivine-basalt source contained from 0.3 to 0.5% trapped residual liquid from the magma ocean. The compositions 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 was nearly devoid of trapped liquid (
ISSN:0016-7037
1872-9533
DOI:10.1016/S0016-7037(97)00082-3