Deciphering the origin(s) of H and Cl in Apollo 15 quartz monzodiorites: evidence for multiple processes and reservoirs

Barrett, Thomas J.; Robinson, Katharine L.; Barnes, Jessica J.; Jeffrey Taylor, G.; Nagashima, Kazuhide; Huss, Gary R.; Franchi, Ian A.; Anand, Mahesh and Kring, David A. (2023). Deciphering the origin(s) of H and Cl in Apollo 15 quartz monzodiorites: evidence for multiple processes and reservoirs. Geochimica et Cosmochimica Acta (Early Access).

DOI: https://doi.org/10.1016/j.gca.2023.08.004

Abstract

Apollo 15 quartz monzodiorites (QMDs) are reported to contain some of the most deuterium-depleted apatite found in lunar samples. In this study, apatite from six Apollo 15 QMDs, including three samples from 15405 not previously investigated, were analyzed for their H and Cl isotopes. Apatite in 15405 are extremely 2H (or D)-poor, with δD values ranging from – 658 ± 53 to − 378 ± 113 ‰, comparable to apatite data from related samples 15403 and 15404. In addition to new H isotope data, the first Cl-isotope data for lunar QMDs are presented. Apatite in 15405 and related samples are enriched in 37Cl with respect to Earth, with measured δ37Cl values ranging from + 13 to + 37 ‰. These values are within the reported δ37Cl range for KREEP-rich samples. The fact that the Cl isotopic composition of apatite in QMDs are similar to those in other lunar lithologies, but the H isotopic data are distinct and unique, provides possible further evidence for the existence of a D-poor reservoir in the lunar interior. Raman spectroscopy of the silica polymorph in sample 15405 reveals it to be a mixture of quartz and cristobalite. Based on available experimental data on the stability of various silica phases over a range of pressure and temperature regime, a deep-seated origin in the crust for QMDs may be possible which would support an endogenous origin of the H-Cl isotope systematics of the QMDs. The role of impact-induced transformation of silica phases and its contributing towards low D/H ratio in apatite, however, cannot be ruled out.

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