Isotopic studies of ferroan anorthosite 62236: A young lunar crustal rock from a light rare-earth-element-depleted source

Isotopic analyses of mineral fractions and whole rocks from the ferroan anorthosite 62236 yield a Sm-Nd isochron with an age of 4.29 +/- 0.06 Ga and an initial epsilon(Nd)(143) value of +3.1 +/- 0.9. We have also measured epsilon(Nd)(142) anomalies of +0.25 on two fractions of 62236. These values are higher than the value of -0.1 predicted if 62236 was derived from a chondritic source at 4.29 Ga, but are consistent with the positive initial epsilon(Nd)(143) value. The Sm-Nd isotopic composition of 62236 has been modified by the capture of thermal neutrons such that the Sm-147/Nd-144, Nd-143/Nd-144, and Nd-142/Nd-144 ratios measured on the mineral fractions and whole rocks must be corrected. The corrections do not significantly alter the Sm-Nd isotopic results determined on 62236, Despite the fact that the Ar-Ar and Rb-Sr isotopic systematics of 62236 have been reset by impact metamorphism at 3.93 +/- 0.04 Ga, the Sm-Nd systematics appear to have been unaffected. The Sm-Nd isotopic systematics of 62236 provide several constrains on models of lunar crustal differentiation provided they have not been reset since crystallization. First, the relatively young age of 62236, as well as the old ages determined on several crustal plutonic rocks of the Mg-suite, require multiple sources of magmas on the Moon very early in its history. Second, positive epsilon(Nd)(143) values determined on all analyzed ferroan anorthosites suggest that they were derived from sources depleted in light rare earth elements. And third, models based on initial epsilon(Nd)(143) and present-day epsilon(Nd)(143) values suggest that the source of 62236 was depleted in light rare earth elements at similar to 4.46 Ga. In order to reconcile these observations with the lunar magma ocean model (1) the magma ocean must have existed for a very short period of time, and may have had a sub-chondritic Nd/Sm ratio, and (2) the youngest ferroan anorthosites, such as 62236, cannot be cumulates from the magma ocean, but must form by other processes. Copyright (C) 1999 Elsevier Science Ltd.