ARGON-40 ARGON-39 AGE SPECTRA OF APOLLO-17 HIGHLANDS BRECCIA SAMPLES BY LASER STEP HEATING AND THE AGE OF THE SERENITATIS BASIN/

We have obtained high-resolution (21-63 steps) Ar-40/Ar-39 age spectra using a continuous laser system on 19 submilligram samples of melt ro cks and clasts from Apollo 17 samples collected from the pre-Imbrian h ighlands in the easternmost part of the Serenitatis basin. The samples include poikilitic melt rocks inferred to have been formed in the Ser enitatis basin-forming impact, aphanitic melt rock whose compositions vary and whose provenance is uncertain, and granulite, gabbro, and mel t clasts. Three of the poikilitic melts have similar age spectrum plat eau ages (72395,96, 3893 +/- 16 Ma {2 sigma}; 72535,7, 3887 +/- 16 Ma; 76315, 150, 3900 +/- 16 Ma) with a weighted mean age of 3893 +/- 9 Ma , which we interpret as the best age for the Serenitatis basin-forming impact. Published Ar-40/Ar-39 age spectrum ages of Apollo 17 poikilit ic melts are consistent with our new age but are much less precise. Tw o poikilitic melts did not give plateaus and the maxima in their age s pectra indicate ages of greater than or equal to 3869 Ma (72558,7) and greater than or equal to 3743 Ma (77135,178). Plateau ages of two poi kilitic melts and two gabbro clasts from 73155 range from 3854 +/- 16 Ma to 3937 +/- 16 Ma and have probably been affected by the ubiquitous (older?) clasts and by post-formation heating (impact) events. Platea u ages from two of the aphanitic melt ''blobs'' and two granulites in sample 72255 fall in the narrow range of 3850 +/- 16 Ma to 3869 +/- 16 Ma with a weighted mean of 3862 +/- 8 Ma. Two of the aphanitic melt b lobs from 72255 have ages of 3883 +/- 16 Ma and greater than or equal to 3894 Ma, whereas a poikilitic melt clast (of different composition from the ''Serenitatis'' melts) has an age of 3835 +/- 16 Ma, which is the upper limit for the accretion of 72255. These data suggest that e ither the aphanitic melts vary in age, as is also suggested by their v arying chemical compositions, or they formed in the 72255 accretionary event about 3.84-3.85 Ga and older relict material is responsible for the dispersion of ages. In any case the aphanitic melts do not appear to be Serenitatis products. Our age for the Serenitatis impact shows, on the basis of the isotopic age evidence alone, that Serenitatis is >20-25 Ma and probably >55-60 Ma older than Imbrium (less than or equa l to 3870 Ma and probably less than or equal to 3836 Ma [Dalrymple and Ryder, 1993]). Noritic granulite sample 78527 has a plateau age of 41 46 +/- 17 Ma, representing a minimum age for cooling of this sample in the early lunar crust. So far there is no convincing evidence in the lunar melt rock record for basin-forming impacts significantly older t han 3.9 Ga.