K-CA CHRONOLOGY OF LUNAR GRANITES

K-Ca age determinations were undertaken for three lunar granitic sampl es (14321,1062, 14303,206, and 12013,141 ). K-Ca isotopic analyses of bulk samples of seven pristine lunar igneous rocks including four mare basalts ( 100 1 7, 15555, 14321,1394, and 14305,304,371), one norite (15445,17), one anorthosite (60025), one KREEP basalt (72275,543), and seven meteorites (Juvinas, Pasamonte, Moore County, Stannern, Y75011, 84B, Zagami, and Shergotty) were also performed. The K-Ca mineral isoc hron for lunar granitic clast 14321,1062 yields an age of 4.06 +/- 0.0 7 Ga for lambda(K-40) = 0.5543 Ga-1 and an initial Ca-40/Ca-44 of 47.1 41 -/+ 0.010 (normalized to Ca-42/Ca-44 = 0.31221). The K-Ca age is in excellent agreement with the Rb-Sr and Sm-Nd ages and is slightly old er than the average Ar-39-Ar-40 and U-Pb zircon ages. Low precision K- Ca ages of 4.04 +/- 0.64 Ga and 3.76 +/- 0.72 Ga were obtained for lun ar granites 14303,206 and 12013,141, respectively, due to their lower K/Ca ratios. Initial Ca-40/Ca-44 values for seven lunar and seven mete oritic bulk samples are similar and yield an average initial Ca-40/Ca- 44 of 47.136 +/- 0.002 (2sigma(m)) and +/- 0.006 (2sigma(p)). Assuming this value as the lunar initial Ca-40/Ca-44, a ten- to twenty-fold fr actionation for K/Ca during the formation of granite 14321 is calculat ed for a two-stage model. The large K/Ca enrichment is consistent with a granite genesis model involving a silicate-liquid immiscibility pro cess. Literature data for lunar granites studied so far exhibit a wide range of ages from 3.9 to 4.4 Ga. Thus, lunar granites probably repre sent samples from about eight different intrusive bodies. The positive K and K/Ca correlations of lunar rocks strongly supports an early glo bal lunar differentiation. However, most granites were probably not di rectly produced from this early differentiation event.