Exploring the lunar mantle with secondary ion mass spectrometry: a comparison of lunar picritic glass beads from the Apollo 14 and Apollo 17 sites
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The major element characteristics of lunar picritic beads indicate that their composition approaches that of primary basaltic liquids and therefore may be our best geochemical-mineralogical probes of the lunar interior. The picritic magmas appear to represent partial melting of a variety of mantle mineral assemblages. Secondary ion mass spectrometry (SIMS) analysis of individual glass beads collected from the Apollo 14(A-14) and Apollo 17(A-17) landing sites shows that although there is substantial major, minor (TiO2, Al2O3) and trace element variation at each site, the glasses from each site have distinguishable trace element signatures (Ba/Sr, LREE/HREE). The incompatible element characteristics of glasses of nearly identical major element chemistry are strikingly different between sites. The contrast between the A-14 and A-17 glasses appears to be the result of a higher KREEP component in all the A-14 glasses. The incorporation of evolved KREEP component into high-Mg magmas is not a result of either assimilation or magma mixing but incorporation during the partial melting episode that produced the picritic magmas. This is a more thermally efficient mechanism for incorporation of the KREEP component into a picritic magma. Calculated mantle chemistries based upon glass compositions, low degrees of partial melting and mineralogical components used in previous models suggest that the difference between A-14 and A-17 mantle sources is a 0.1-1.4% KREEP intercumulate melt. This difference will increase with an increase in the degree of partial melting in the model calculations. This intercumulate melt may interstitially reside in an evolved, high-Ti “cumulate” component. The juxtaposition of evolved (high-Ti cumulates +KREEP) and primitive (low-Ti cumulates) mantle components has been attributed to original mantle inhomogeneities, density contrasts during magma ocean crystallization and sinking of high-density evolved cumulates into less dense, primitive cumulates within the context of either magma ocean- or serial magmatism-type models. Regardless of the model, the trace element signatures of glasses (and the basalts?) indicate that the character of the A-14 mantle source is intrinsically different from that of the A-17 mantle source. This indicates large-scale lateral (and vertical) inhomogeneities in the lunar mantle.