MUCH OF OUR PRESENT QUANTITATIVE UNDERSTANDING OF THE IMPACT HISTORY OF THE INNER SOLAR SYSTEM -- A MAJOR DOMAIN OF INTEREST FOR THE SOLAR SYSTEM WORKINGS PROGRAM -- DERIVES FROM 40AR/39AR GEOCHRONOLOGY OF LUNAR IMPACT MELT MATERIALS IN RETURNED SAMPLES AND METEORITES. MOST PUBLISHED DATES WERE OBTAINED THROUGH INCREMENTAL HEATING EXPERIMENTS PERFORMED ON POLYMINERALIC (OR MINERALS + MELT) ROCK FRAGMENTS OF SUCH MATERIAL. UNFORTUNATELY THE INTERPRETATION OF AGE SPECTRA PRODUCED FROM SUCH EXPERIMENTS IS NOT ALWAYS STRAIGHTFORWARD. IT HAS BEEN SHOWN RECENTLY THAT 40AR/39AR ULTRAVIOLET LASER ABLATION MICROPROBE (UVLAMP) STUDIES OF SUCH COMPLEX MATERIALS WITH HIGH SPATIAL RESOLUTION CAN PROVIDE VALUABLE AGE INFORMATION AND AID IN THE INTERPRETATION OF INCREMENTAL HEATING DATASETS (E.G. MERCER ET AL. 2015 SCIENCE ADVANCES). ENCOURAGED BY PRELIMINARY RESULTS FOR APOLLO 15 SAMPLE 15455 WE PROPOSE TO USE UVLAMP TECHNOLOGY TO QUANTIFY THE DISTRIBUTION OF RADIOGENIC 40AR IN ANCIENT (PRESUMABLY PRE-4.0 GA) PLUTONIC PLAGIOCLASE XENOCRYSTS FOUND IN APOLLO 15 AND 16 IMPACT MELT BRECCIAS AND TO LEVERAGE THAT INFORMATION TO CONSTRAIN THE AGES AND THERMAL HISTORIES OF THE IMPACT MELTS IN WHICH THEY ARE FOUND. SIMILAR ANALYSES OF PLAGIOCLASE CRYSTALS IN LARGER XENOLITHS AND COMPARISON WITH THE XENOCRYST RESULTS MAY PROVIDE ADDITIONAL INSIGHTS REGARDING THE THERMAL HISTORIES OF THE SOURCE REGIONS OF THOSE LITHIC CLASTS. OUR INTENT IS TO ADD SUBSTANTIVELY TO OUR UNDERSTANDING OF THE SPECTRUM OF AGES AND EXTENTS OF IMPACT MELTS AT VARIOUS APOLLO COLLECTION SITES. GOALS OF THE PROPOSED WORK INCLUDE: 1) DEMONSTRATING THE UTILITY OF LASER MICROPROBE 40AR/39AR DATING OF ENTRAINED PLAGIOCLASE XENOCRYSTS FOR ESTABLISHING PRECISE IMPACT MELT AGES; 2) ESTABLISHING THE AGES OF IMPACT MELTING EVENTS RESPONSIBLE FOR THE MELT BRECCIAS COLLECTED AT THE APOLLO 15 AND 16 LANDING SITES; 3) ADDING TO OUR UNDERSTANDING OF VARIATIONS IN THE DIFFUSIVITY OF AR ISOTOPES IN LUNAR PLAGIOCLASES; 4) QUANTIFYING 40AR/39AR APPARENT AGE GRADIENTS - OR LACK OF GRADIENTS - IN COARSE PLAGIOCLASE XENOCRYSTS AND COARSE PLAGIOCLASE CRYSTALS IN XENOLITHS; 5) CONSTRAINING THE PLAUSIBLE MAXIMUM TEMPERATURE-DURATION RELATIONSHIPS OF THE EVENTS RESPONSIBLE FOR DEVELOPMENT OF THE STUDIED BRECCIAS THROUGH THERMAL-KINETIC MODELING OF THE ACQUIRED UVLAMP DATA; AND 6) EVALUATING THE EXTENT TO WHICH 40AR/39AR GEOCHRONOLOGY OF APOLLO 15 AND 16 IMPACT MELT SAMPLES CAN BE EXPECTED TO PROVIDE ACCURATE AND PRECISE CONSTRAINTS ON THE AGE OF THE IMBRIUM IMPACT.