TRACE-ELEMENT FRACTIONATION BY IMPACT-INDUCED VOLATILIZATION - SIMS STUDY OF LUNAR HASP SAMPLES

Impact is a dominant process acting on the surfaces of planetary bodie s and is the major process that modifies the physical and chemical nat ure of planetary materials on atmosphere-free bodies once volcanic act ivity has ceased. Elements with relatively high volatilities are readi ly mobilized during impact. HASP (high alumina, silica poor) compositi ons are thought to be generated by impact-induced volatilization proce sses on the moon. The high Al/Si ratio of HASP is believed to be the r esult of preferential loss of SiO2 relative to Al2O3. Here we report S IMS analyses for HASP glass beads and devitrified glasses from the Apo llo 14 regolith to assess the behavior of Li, Be, B, REE, Sr, Y, and Z r during impact events. We compare the behavior of these trace element s to the behavior of major and minor elements analyzed by the electron microprobe. The HASP samples we studied fall into two compositional g roups with one group apparently derived from KREEPY lunar lithologies and the other from anorthosite. The KREEPy HASP beads have an estimate d mass loss of 28% compared to their KREEPy protolith. We estimate tha t 44% of SiO2 and 35% of FeO were lost during impact-induced volatiliz ation. The mass loss estimate for the anorthositic protolith to produc e anorthositic HASP is more uncertain because of the uncertainity of t he protolith composition. However, an estimate of similar to 25% appea rs reasonable with 38% loss of silica. Clearly, impact processes, in t he early stages of planetary evolution, are important in the modificat ion of geochemical signatures, and ordinarily geochemically coherent e lements may be decoupled because of different volatilities.