K-CA AND RB-SR DATING OF 2 LUNAR GRANITES - RELATIVE CHRONOMETER RESETTING

K-Ca and Rb-Sr age determinations were made for lunar granite 12033,57 6. This small fragment is a subsample of soil particle 12033,507, whic h was collected subsurface at the north rim of Head Crater. The K-Ca m ineral isochron is well-defined and yields an age of 3.62 +/- 0.11 (2s igma) Ga for lambda(K-40) = 0.5543 Ga-1 and an initial Ca-40/Ca-44 of 47.160 +/- 0.006 (2sigma) (normalized to Ca-42/Ca-40 = 0.31221). The R b-Sr mineral isochron is quite disturbed and yields an imprecise young age of 2.20 +/- 0.65 Ga for lambda(Rb-87) = 0.0139 Ga-1 and a high in itial Sr-87/Sr-86 of 0.775 +/- 0.022. Assuming that the granite crysta llized approximately 4.0 Ga ago (a modeled formation age), and was sho ck-heated 0.8 Ga ago (Ar-39-Ar-40 age), the intermediate Rb-Sr and K-C a ages are interpreted as due to partial resetting by thermal diffusio n of strontium and calcium. Diffusion data determined for argon in 120 33,507 give a value of D/a2 approximately 1 X 10(-6) s-1 for a heating temperature of 700-degrees-C; at this temperature it would require a time on the order of days to totally degas argon. The required diffusi on coefficients, D, for calcium (7 approximately 9 X 10(-13) cm2 s-1) and Sr (approximately 8 X 10(-12) cm2 s-1) that are predicted from the observed age resetting are in good agreement with those determined ex perimentally in granitic melts. The K-Ca system was about a factor of ten more resistant to resetting than the Rb-Sr system, which in turn w as approximately 10 times more resistant to resetting than the K-Ar sy stem. Diffusion data indicate that isotopic exchange of calcium and st rontium was responsible for the partial resetting of the K-Ca and Rb-S r chronometers, rather than elemental exchange of potassium and rubidi um among mineral phases. Rb-Sr isotopic data also were obtained for gr anite 14303,206, a subsample of breccia clast 14303,204, which was col lected at the Fra Mauro region about 180 km from the Apollo 12 site. T he mineral isochron is quite disturbed and yields an imprecise age of 3.95 +/- 0.38 Ga for X(Rb-87) = 0.01 39 Ga-1 and initial Sr-87/Sr-86 = 0.7046 +/- 0.005 1. The Rb-Sr age is comparable to the previously det ermined K-Ca age of 4.04 +/- 0.64 Ga. The Rb-Sr and K-Ca ages of this clast are imprecise and are statistically indistinguishable from the w ell-defined Ar-39-Ar-40 degassing age of 3.83 +/- 0.03 Ga or the Rb-Sr and K-Ca model ages of 4.30 +/- 0.06 Ga and 4.25 +/- 0.12 Ga, respect ively. Because these model ages are within error of a reported zircon U-Pb age of 4.31 Ga, we assume this granitic clast crystallized -4.3 G a ago and was shock-heated approximately 3.83 Ga ago. Using the appare nt resetting of the K-Ar, Rb-Sr, and K-Ca isotopic systems, and litera ture data for relative strontium and calcium diffusivity, we estimate a shock-reheating temperature of approximately 860-degrees-C would be necessary to partially reset the Rb-Sr and K-Ca ages to their nominal values. This reheating temperature is uncertain because of the large e rrors on the ages, but would be consistent with complete resetting of the K-Ar age and partial resetting of the Rb-Sr and K-Ca ages, if the hot clast was buried under approximately 1 m of the ejecta blanket for approximately 1 day.