Laser probe 40Ar-39Ar dating of impact melt glasses in Lunar Breccia 15466

A laser probe has been used to measure 40Ar-39Ar and cosmic ray exposure ages of individual microgram regions of melt glass and clasts in lunar breccia 15466. The 40Ar-39Ar age, 130 +/-90 Ma, is imprecise due to a variable trapped (40Ar/36Ar) ratio, 3.1 +/- 0.4, but is indistinguishable from the cosmic-ray exposure age of 60 +/- 12 Ma. This indicates that the glass, which constitutes 90% of the breccia, was formed very recently, possibly as recently as the time indicated by the exposure age. A high trapped (38Ar/36Ar) ratio (0.21 compared to the solar wind of 0.19), together with the high trapped (40Ar/36Ar) ratio, indicates that the glass and the breccia formed from preirradiated near surface soil. The age of the breccia is a candidate age for Spur Crater. Age measurements on clasts occluded by the glass are variable and much older, indicating that they were relatively unaffected by the brief heating by the melt glass. The low ages obtained for the glass by the laser technique contrast with much older ages obtained previously from stepped heating measurements. The high stepped heating ages of the glass probably arise from the presence of small occluded clasts and are thus an artifact. This observation seems to invalidate a number of similar published stepped heating ages on lunar impact glasses.