Exploring the lunar mantle with secondary ion mass spectrometry: a comparison of lunar picritic glass beads from the Apollo 14 and Apollo 17 sites

Abstract

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 (TiO 2, Al 2O 3) 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.