Chronologic implications for slow cooling of troctolite 76535 and temporal relationships between the Mg-suite and the ferroan anorthosite suite

Abstract

Ages have been obtained using the 87Rb–87Sr, 147Sm–143Nd, and 146Sm–142Nd isotopic systems for one of the most slowly cooled lunar rocks, Apollo 17Mg-suite troctolite 76535. The 147Sm–143Nd, 146Sm–142Nd, and Rb–Sr ages derived from plagioclase, olivine, and pyroxene mineral isochrons yield concordant ages of 4307±11Ma, 4299+29/−35 Ma, and 4279±52Ma, respectively. These ages are slightly younger than the age determined on ferroan anorthosite suite (FAS) rock 60025 and are therefore consistent with the traditional magma ocean model of lunar differentiation in which the Mg-suite is intruded into the anorthositic crust. However, the Sm–Nd ages record when the rock passed below the closing temperature of the Sm–Nd system in this rock at ∼825°C, whereas the Rb–Sr age likely records the closure temperature of ∼650°C. A cooling rate of 3.9°C/Ma is determined using the ages reported here and in the literature and calculated closure temperatures for the Ar–Ar, Pb–Pb, Rb–Sr, and Sm–Nd systems. This cooling rate is in good agreement with cooling rates estimated from petrographic observations. Slow cooling can lower apparent Sm–Nd crystallization ages by up to ∼80Ma in the slowest cooled rocks like 76535, and likely accounts for some of the variation of ages reported for lunar crustal rocks. Nevertheless, slow cooling cannot account for the overlap in FAS and Mg-suite rock ages. Instead, this overlap appears to reflect the concordance of Mg-suite and FAS magmatism in the lunar crust as indicated by ages calculated for the solidus temperature of 76535 and 60025 of 4384±24Ma and 4383±17, respectively. Not only are the solidus ages of 76535 and 60025 nearly concordant, but the Sm–Nd isotopic systematics suggest they are derived from reservoirs that were minimally differentiated prior to ∼4.38Ga. Although the Sr isotopic composition of 60025 indicates its source was minimally differentiated, the Sr isotopic composition of 76535 indicates it underwent fractionation just prior to solidification of the 76535. These observations are consistent with both a magma ocean or a serial magmatism model of lunar differentiation. In either model, differentiation of lunar source regions must occur near the solidification age of thee samples. Perhaps the best estimate for the formation age of lunar source regions is the Rb–Sr model age of the 76535 source region age of 4401±32Ma. This is in good agreement with Sm–Nd model ages for the formation of ur-KREEP and suggests that differentiation of a least part of the Moon could not have occurred prior to ∼4.43Ga.