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Lunar impact melts and pristine basalts exhibit similar textures. Distinguishing between them is difficult, usually requiring sample destruction to quantify siderophile element abundances. We have applied “quantitative petrography” to distinguish between lunar impact melts or pristine basalts; plagioclase and olivine crystal size distributions (CSDs) show distinct differences between the two melt types. CSDs of plagioclase and olivine from the A-14 high-Al basalts were examined in comparison with plagioclase CSDs from impact melts 14310, 14073 and 14276 and with olivine from the A-14 olivine vitrophyres. This work is incorporating A-16 aluminous impact melts and the A-12 olivine mare basalts to examine the crystallization regimes of plagioclase and olivine and test the preliminary conclusions made from the A-14 study.
Results: impact melts have shallower plagioclase and steeper olivine CSD slopes than their pristine counterparts. This suggests that in impact melts crystallization nuclei for plagioclase were fewer than in plagioclase-rich mare basalts (the opposite to the conclusion of previous experimental work). The reverse situation is seen for olivine. Our study strongly suggests that quantitative petrography can be used to distinguish between pristine and impact melts.
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