Sm–Nd systematics of lunar ferroan anorthositic suite rocks: Constraints on lunar crust formation. (1st January 2015)

• Record Type: Journal Article
• Authors: Boyet, Maud; Carlson, Richard W.; Borg, Lars E.; Horan, Mary
• Is Part Of: Geochimica et cosmochimica acta. Volume 148(2015:Jan. 01)
• Page Start: 203
• Page End: 218
• Publication Date: 2015-01-01
• Subjects: Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2014.09.021
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: We have measured Sm–Nd systematics, including the short-lived 146 Sm– 142 Nd 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 142 Nd isotope composition. These samples all have measured deficits in 142 Nd relative to the JNdi-1 terrestrial standard in the range −45 to −21 ppm. The range is −45 to −15 ppm once the 62236 142 Nd/ 144 Nd ratio is corrected from neutron-capture effects. Analyzed FAS samples do not define a single isochron in either 146 Sm– 142 Nd or 147 Sm– 143 Nd 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 142 Nd and does not appear to have followed the same differentiation path as the other FAS samples. The large deficit in 142 Nd 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 142 Nd/ 144 Nd (e.g. 10 ppm below modern terrestrial), or a bulk-Moon with superchondritic Sm/Nd ratio and initial 142 Nd/ 144 Nd similar to ordinary chondrites.