^1,2Thomas J. Barrett,^1,2,3Katharine L. Robinson,^4,5Jessica J. Barnes,⁶G. Jeffrey Taylor,⁶Kazuhide Nagashima,⁶Gary R. Huss,³Ian A. Franchi,^3,7Mahesh Anand,^1,2David A. Kring 
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.08.004]
¹Center for Lunar Science and Exploration, Lunar and Planetary Institute, USRA, Houston, TX 77058
²NASA SSERVI
³School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
⁴ARES, NASA Johnson Space Center, Houston TX 77058, USA
⁵Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
⁶Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mānoa, Honolulu, HI 96822, USA
⁷Department of Earth Sciences, Natural History Museum, London SW7 5BD, UK
Copyright Elsevier

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.