Inter- and intra-group compositional variations in Apollo 15 pyroclastic green glass:An electron- and ion-microprobe study

Abstract

Representative samples of the five compositionally distinct groups (A-E) of Apollo 15 pyroclastic green glass were systematically analyzed for major- and trace-element abundances by electron-and ion-microprobe techniques. These primary magma compositions define a high-Co trend (groups A and D) and a low-Co trend (groups B and C) on Sr, Zr, Ba, and Nd versus Co variation diagrams. Group E glasses are significantly enriched in Sr relative to the glass groups of the two trends. Pearce element ratio analyses and Sr/Ca-Ba/Ca systematics indicate that olivine, orthopyroxene, and clinopyroxene controlled green glass compositions. Simple crystal-liquid igneous fractionation processes involving such an assemblage, however, cannot explain the observed inter- and intra-group trace-element variation trends. The magmas represented by the high- and low-Co trends are interpreted to have been erupted from two distinct but compositionally variable source regions. A source mixing model involving differentiated cumulate, trapped residual liquid, and extremely Fe-rich components can successfully account for the compositional ranges of the two sources required to produce the observed chemical variations upon partial melting. Group E glasses appear to represent a magma erupted from a source with a more limited chemical variability. The Apollo 15 green glasses represent multiple eruptive events from three chemically distinct but compositionally variable source regions assuming that this mixing model is correct.