Monazite Dating of Lunar Meteorite Nwa

Introduction. According to their textural and geochemical similarity it was suggested previously that the lunar meteorite NWA 4734 could be launch paired with Antarctic Mare basalts (LAP 02205/224/226/436-03632-04841) themselves possibly paired with NWA 032/470 lunar meteorites despite their different textures (1-5). A possible clue about the pairing issue is evidently their age. Method : After localization on a polished section of NWA 4734, four grains of monazite 2 4 μm were analyzed at 15 kV and 50 nA by electron microprobe with special attention to Th, U and Pb ; their measurements allow to calculate an age (6). Results : U and Th are not constant among the grains, they vary from 0 to 0.16 and from 0.12 to 2.89 % respectively The Pb-Th correlation is in agreement with a negligible initial Pb content. Age deduced from empa is 3190 Ma (±190 2σ). Discussion : An obvious question is the significance of this age. It can be either a shock age or a crystallization age. Indeed, NWA 4734 like LAP series samples exhibit significant shock features (partial maskelynitization of plagioclase, melt pockets ...). The closure temperature of monazite for the U/Pb system is given at 950 °C (7). According to Montel et al (1996) the closure for Th/Pb should even be higher and the effect of metamictization of old monazites appears negligible for crystals of less than 50 micrometers. NWA 032/470 is an impact melt with few phenocrysts and an abundant matrix finely crystallized with a quench texture. Its age of 2900 Ma (Ar/Ar, Fagan et al. 2002) is the youngest among Mare basalts and should reflect a crystallization event after a shock. Thus, it could be easily regarded as a shock age. SHRIMP U-Pb dating on apatites from LAP 02205 (2) yielded an age of 3000 Ma (±150 2σ). Rb/Sr and Sm/Nd ages are 3000 and 3200 Ma respectively (9 -10). As already mentioned, the youngest ages could be due to partial resetting upon impact.