Heterogeneous distribution of moderately volatile elements in the Moon determined from Apollo 15 and 17 pyroclastic glass beads

The Apollo 17 high-Ti orange (74220) and Apollo 15 low-Ti green (15426) lunar pyroclastic glasses are some of the most primitive igneous samples from the Moon and are considered critical for understanding the volatile content of the lunar interior. The orange and green glass deposits are petrologically distinct, containing both holohyaline (glassy) and crystallized beads. In this study, edge and center analyses on holohyaline beads representative of the deposits were conducted by laser ablation inductively coupled plasma mass spectrometry to constrain the distribution of moderately volatile elements (MVE: K, Cu, Zn, Cs, Ga, Ge, Rb, Cd, and Pb), and trace element images were produced of the beads in 74220. Bead edges have elevated MVE abundances compared to centers in the larger (107 µm average diameter) low-Ti Apollo 15 green glasses, likely resulting from syn -eruptive processes. Leaching experiments of 15426 bulk beads support a large fraction of Na, K, Zn, Cu, Cd and Pb on their outer surfaces. The smaller (42 µm average diameter) high-Ti Apollo 17 orange glasses have a greater extent of overlap in MVE contents between bead edges and centers. Orange and green glass bead centers offer approximations of melt MVE abundances, indicating ∼500 µg/g K, ≤20 µg/g Zn, ∼6 µg/g Cu, <4 µg/g Ga, ≤ 1 µg/g Rb, <0.1 µg/g Pb and ≤ 100 µg/g K, ≤1 µg/g Zn, ≤2.5 µg/g Cu, <2 µg/g Ga, ≤ 0.5 µg/g Rb and Pb, respectively. These estimates are as much as ten times lower than bulk bead MVE abundances within the pyroclastic glass deposits, are depleted compared to terrestrial mid-ocean ridge basalts, and are similar to, or lower than, bulk silicate Earth (BSE) concentration estimates. Partial melting estimates for the source of the pyroclastic glass beads indicate similarities with tholeiitic and komatiite lavas on Earth and between ∼10 and 30 % melting of their mantle source, consistent with high mantle potential temperatures at ∼3.5 billion years ago in the Moon. The estimated MVE composition of the orange glass bead mantle source is marginally higher than the green glass mantle source, and both are within or lower than bulk silicate Moon estimates. More shallowly derived mare basalts have been shown to be yet more MVE depleted, indicating that the lunar interior had a heterogeneous distribution of volatile elements, with a deep interior with volatile abundances ∼10 times lower than BSE sampled by the pyroclastic glass beads, volatile-poor upper magma ocean cumulates, and an incompatible volatile-enriched KREEP reservoir.