H and Cl isotope systematics of apatite in brecciated lunar meteorites Northwest Africa 4472, Northwest Africa 773, Sayh al Uhaymir 169, and Kalahari 009

摘要

We have investigated the H and Cl systematics in apatite from four brecciated lunar meteorites. In Northwest Africa (NWA) 4472, most of the apatites contain ~2000-6000 ppm H_2O with δD between-200 and 0‰, except for one grain isolated in the matrix, which contains ~6000 ppm H_2O with δD of ~500-900‰. This low-δD apatite contains ~2500-7500 ppm Cl associated with δ~(37)Cl of ~15-20‰, while the high-dD grain contains ~2500 ppm Cl with δ~(37)Cl of ~7-15&. In NWA 773, apatites in a first group contain ~700-2500 ppm H_2O with dD values averaging around ~0 ± 100‰, while apatites in a second group contain ~5500-16500 ppm H_2O with δD ~250 ± 50‰. In Sayh al Uhaymir (SaU) 169 and Kalahari (Kal) 009, apatites are similar in terms of their H_2O contents (~600-3000 ppm) and δD values (-100 to 200‰). In SaU 169, apatites contain ~6000-10,000 ppm Cl, characterized by δ~(37)Cl of ~5-12‰. Overall, most of the analyzed apatite grains have δD within the range reported for carbonaceous chondrites, similar to apatite analyzed in ancient (>3.9 Ga) lunar magmatic. One grain in NWA 4472 has H and Cl isotope compositions similar to apatite from mare basalts. With an age of 4.35 Ga, this grain could be a representative of the oldest known lunar volcanic activity. Finally, since numerous evolved clasts in NWA 773 formed through silicate liquid immiscibility, the apatite grains with extremely high H_2O contents, reaching pure hydroxylapatite composition, could provide insights into the effects of such process on the evolution of volatiles in lunar magmas.