KCa chronology of lunar granites

Abstract

K-Ca age determinations were undertaken for three lunar granitic samples (14321, 1062, 14303,206, and 12013,141). KCa isotopic analyses of bulk samples of seven pristine lunar igneous rocks including four mare basalts (10017, 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 isochron for lunar granitic clast 14321,1062 yields an age of 4.06 ± 0.07 Ga for λ( 40K) = 0.5543 Ga −1 and an initial 40Ca 44Ca of 47.141 ∓ 0.010 (normalized to 42Ca 44Ca = 0.31221 ). The K-Ca age is in excellent agreement with the Rb-Sr and Sm-Nd ages and is slightly older than the average 39Ar- 40Ar 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 lunar granites 14303,206 and 12013,141, respectively, due to their lower K Ca ratios. Initial 40Ca 44Ca values for seven lunar and seven meteoritic bulk samples are similar and yield an average initial 40Ca 44Ca of 47.136 ± 0.002 (2 σ m ) and ±0.006 (2 σ p ). Assuming this value as the lunar initial 40Ca 44Ca , a ten- to twenty-fold fractionation for K Ca during the formation of granite 14321 is calculated for a two-stage model. The large K Ca enrichment is consistent with a granite genesis model involving a silicate-liquid immiscibility process. 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 represent samples from about eight different intrusive bodies. The positive K and K Ca correlations of lunar rocks strongly supports an early global lunar differentiation. However, most granites were probably not directly produced from this early differentiation event.