40AR/39AR AGE SPECTRA OF APOLLO-15 IMPACT MELT ROCKS BY LASER STEP-HEATING AND THEIR BEARING ON THE HISTORY OF LUNAR BASIN FORMATION

We have obtained 26 high-resolution (16-51 steps) 40Ar/39Ar age spectra using a continuous laser system on submilligram fragments of recrystallized melt and single-crystal plagioclase clasts from 12 Apollo 15 impact melt rocks collected at the Apennine Front where the Imbrium and Serenitatis basins intersect. These melt rocks represent a wide range of compositions and at least half a dozen different impacts. Six of the melt rocks have reproducible, intermediate-temperature plateaus over 40% or more of the 39Ar released; the plateaus are interpreted as crystallization (impact) ages and much of the non-plateau behavior is attributable to recoil. Samples 15294,6,21, 15304,7,69, 15314,26,156, 15357,15, and 15359,12 have mean 40Ar/39Ar plateau ages that are statistically indistinguishable and fall within the narrow range 3852 +/- 14 (2sigma) Ma to 3870 +/- 12 Ma with a weighted mean of 3865 +/- 5 Ma. Sample 15356,9 has a mean plateau age of 3836 +/- 11 Ma and may represent a distinctly younger impact. A seventh sample (15314,30,158) has a peculiar but reproducible double plateau; a low-T one at 3873 +/- 9 Ma, which we think records the crystallization age, and a high-T one of 3831 +/- 10 Ma, which we interpret as an experimental (39Ar recoil) artifact. Four of the remaining melt rocks (15308,9, 15414,2,37, 15436,2, 15445,253) have complex 40Ar/39Ar age spectra that indicate that they either formed in or were disturbed by impacts that occurred less-than-or-equal-to 3850 Ma but did not completely reset the K-Ar isotopic system. Sample 15414,3,36 is different. Its spectrum may represent release mainly from clasts that were not well degassed in a melt event at 3870 Ma; the melt phase contains little potassium. Because most of the Apennine Front material must be coeval with or predate formation of the Imbrium Basin, it seems likely that the Imbrium impact is no older than 3870 Ma and probably no older than 3836 Ma. So far there is no convincing evidence in the lunar record for melt-producing impacts, such as basin formation, older than about 3.9 Ga.