A highly depleted moon or a non-magma ocean origin for the lunar crust?

Coupled Sm-146,Sm-147-Nd-142,Nd-143 systematics were analyzed on lunar samples that span the range of source compositions created ill the early lunar differentiation event (ferroan anorthosites, high-Mg suite norite, KREEP and low-and high-Ti basalts). If fit with a single isochron, the data agree with previous suggestions of a similar to 250 Ma duration for closure of the Sm-Nd system in the lunar interior. The two crustal rocks, 60025 and 78236, among oldest samples found on the lunar surface, have high initial Nd-142/Nd-144 ratios (epsilon Nd-142 =0.24-0.27 relative to O-chondrites), which indicates that they derive from a source strongly depleted in light rare earth elements (LREE) (Sm-147/Nd-144 similar to 0.23). This result suggests either that the Moon formed with a strongly superchondritic Sm/Nd ratio or that the lunar crust formed as a product of a post-magma ocean remelting of LREE depleted cumulates in the lunar interior. The basalt data measured here add support to the idea that the lunar interior evolved with a superchondritic Sm/Nd, similar to that of the early Earth. Addition of differentiation steps adds complexity, and inherit non-uniqueness, to the interpretation of the lunar Nd isotope data, but our preferred model has the first stage of lunar evolution (4.567 to 4.44 Ga) occurring on an already differentiated Earth, with the Moon forming shortly before 4.45 Ga and undergoing rapid magma ocean evolution to form the crust, the high Sm/Nd source reservoirs of many mare basalt types, and KREEP by 4.44 Ga. Continued differentiation of the lunar interior through the increasing depletion of the source of high-Ti mare basalts then continued until similar to 4320 Ma. (C) 2007 Elsevier B.V. All rights reserved.