NON-DESTRUCTIVE IDENTIFICATION OF LUNAR IMPACT MELTS

The return of igneous textured samples from the Moon during Apollo highlighted the importance of impact melts and the impact process. Distinguishing such melts from pristine (volcanic) rocks continues to be difficult. Traditionally, such distinctions required time consuming experiments to be performed or destructive analyses to determine the major element chemistry (does this composition represent a realistic liquid?) or the highly siderophile element (HSE) contents (do the abundances indicate meteorite contamination?) of the samples. We present a new, non-destructive method that shows great potential to make such distinctions using quantitative petrography. Crystal Size Distributions (CSDs) of plagioclase and olivine are used to quantify population densities within particular size bins of a given mineral phase. For plagioclase, the CSD slope and intercept were calculated in the size range 0.1-1mm for plagioclase and <0.35 mm for olivine. By plotting the slope vs. intercept, distinctions can be made between impact melts and pristine basalts. Interestingly, Apollo 12 basalt 12038 appears to be an impact melt, yet contains low HSE abundances. Simply using a thin section could allow an easy distinction between impact melts and pristine basalts, thus allowing a more credible test of the lunar cataclysm hypothesis using future targeted sample returns from different impact basins.