Trace-element fractionation by impact-induced volatilization; SIMS study of lunar HASP samples

Abstract

Impact is a dominant process acting on the surfaces of planetary bodies and is the major process that modifies the physical and chemical nature of planetary materials on atmosphere-free bodies once volcanic activity has ceased. Elements with relatively high volatilities are readily mobilized during impact. HASP (high alumina, silica poor) compositions are thought to be generated by impact-induced volatilization processes on the moon. The high Al/Si ratio of HASP is believed to be the result of preferential loss of SiO 2 relative to Al 2 O 3 . Here we report SIMS analyses for HASP glass beads and devitrified glasses from the Apollo 14 regolith to assess the behavior of Li, Be, B, REE, Sr, Y, and Zr during impact events. We compare the behavior of these trace elements to the behavior of major and minor elements analyzed by the electron microprobe. The HASP samples we studied fall into two compositional groups with one group apparently derived from KREEPy lunar lithologies and the other from anorthosite. The KREEPy HASP beads have an estimated mass loss of 28% compared to their KREEPy protolith. We estimate that 44% of SiO 2 and 35% of FeO were lost during impact-induced volatilization. The mass loss estimate for the anorthositic protolith to produce anorthositic HASP is more uncertain because of the uncertainty of the protolith composition. However, an estimate of approximately 25% appears reasonable with 38% loss of silica. Clearly, impact processes, in the early stages of planetary evolution, are important in the modification of geochemical signatures, and ordinarily geochemically coherent elements may be decoupled because of different volatilities.