Reading the Moon's volcanic record by ion microprobe analysis of Apollo 14 glass beads

Abstract

The application of secondary ion mass spectrometry to lunar volcanic problems is demonstrated by individually analyzing representative glass from the seven Apollo 14 pyroclastic glass bead groups (black, orange, yellow, LAP, green A, VLT, and green B) for selected trace elements (e.g., rare earth elements [REE], Ba, Sr, Zr, V, and Co). The trace element characteristics of glass beads are useful for differentiating impact from volcanic glass beads and identifying other volcanic glass types (e.g., LAP). Trace element modeling indicates that the bead groups are unrelated by low-pressure fractional crystallization to each other, to Apollo 14 crystalline basalts, or to basalts from other landing sites. A possible exception is the relation between LAP and Apollo 14 aluminous basalts. The absence of evolved basalts derived from primary magmas with volcanic glass compositions suggests either that these evolved basalts have not been sampled or that fire fountaining tapped mantle sources subtly different from crystalline mare-basalt source regions. Hybridization of mantle source regions is preferred to assimilation-fractional crystallization processes to explain the incorporation of the evolved potassium-REE-phosphorus (KREEP) component identified in these primitive magmas. Apollo 14 volcanic glass and mare basalt trace element signatures indicate that the character of the Apollo 14 mantle source region is intrinsically different from that of other sites, which suggests that large-scale mantle heterogeneities exist.