Sm-Nd systematics of lunar ferroan anorthositic suite rocks: Constraints on lunar crust formation

摘要

We have measured Sm-Nd systematics, including the short-lived Sm-146-Nd-142 chronometer, in lunar ferroan anorthositic suite (FAS) whole rocks (15415, 62236, 62255, 65315, 60025). At least some members of the suite are thought to be primary crystallization products formed by plagioclase flotation during crystallization of the lunar magma ocean (LMO). Most of these samples, except 62236, have not been exposed to galactic cosmic rays for a long period and thus require minimal correction to their Nd-142 isotope composition. These samples all have measured deficits in Nd-142 relative to the JNdi-1 terrestrial standard in the range -45 to -21 ppm. The range is -45 to -15 ppm once the 62236 Nd-142/Nd-144 ratio is corrected from neutron-capture effects. Analyzed FAS samples do not define a single isochron in either Sm-146-Nd-142 or Sm-147-Nd-143 systematics, suggesting that they either do not have the same crystallization age, come from different sources, or have suffered isotopic disturbance. Because the age is not known for some samples, we explore the implications of their initial isotopic compositions for crystallization ages in the first 400 Ma of solar system history, a timing interval that covers all the ages determined for the ferroan anorthositic suite whole rocks as well as different estimates for the crystallization of the LMO.62255 has the largest deficit in initial Nd-142 and does not appear to have followed the same differentiation path as the other FAS samples. The large deficit in Nd-142 of FAN 62255 may suggest a crystallization age around 60-125 Ma after the beginning of solar system accretion. This result provides essential information about the age of the giant impact forming the Moon. The initial Nd isotopic compositions of FAS samples can be matched either with a bulk-Moon with chondritic Sm/Nd ratio but enstatite-chondrite-like initial Nd-142/Nd-144 (e.g. 10 ppm below modern terrestrial), or a bulk-Moon with superchondritic Sm/Nd ratio and initial Nd-142/Nd-144 similar to ordinary chondrites. (C) 2014 Elsevier Ltd. All rights reserved.