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The return of igneous textured lunar samples during Apollo highlighted the importance of impact melts and the impact process. Distinguishing such melts from pristine (volcanic) rocks proved difficult. Traditionally, such distinctions required time-consuming experiments or destructive analyses to determine major element chemistry (does this composition represent a realistic liquid?) or highly siderophile element contents (do the abundances indicate meteorite contamination?) of the samples. We present a non-destructive method that shows great potential to make such distinctions using quantitative petrography. Currently, we are focusing on plagioclase and olivine. Crystal Size Distributions (CSDs) are used to quantify population densities of a given mineral phase. For plagioclase, the CSD slope and intercept were calculated in the size range 0.1-1mm and <0.3 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. The implications of this technique are profound for the SPA sample return mission (along with samples returned from other impact basins) because the sample sizes will probably not allow highly siderophile element abundances to be determined. 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, as well as adding to our knowledge of mare basalt eruption ages.
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