The Cl isotope composition and halogen contents of Apollo-return samples

1,2Anthony Gargano,1,2Zachary Sharp,3Charles Shearer,4Justin I. Simon,5Alex Halliday,6Wayne Buckley
Proceedings of the National Academy of Sciences of the United States of America (in Press) Link to Article [DOI:https://doi.org/10.1073/pnas.2014503117]
1Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131-0001;
2Center for Stable Isotopes, University of New Mexico, Albuquerque, NM 87131-0001;
3Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131-0001;
4Center for Isotope Cosmochemistry and Geochronology, Astromaterials Research and Exploration Science Division, The Lyndon B. Johnson Space Center, National Aeronautics and Space Administration, Houston, TX 77058;
5The Earth Institute, Columbia University, New York, NY 10025;
6Jacob–Johnson Space Center Engineering, Technology and Science Contract, The Lyndon B. Johnson Space Center, National Aeronautics and Space Administration, Houston, TX 77058

Lunar mare basalts are depleted in F and Cl by approximately an order of magnitude relative to mid-ocean ridge basalts and contain two Cl-bearing components with elevated isotopic compositions relative to the bulk-Earth value of ∼0‰. The first is a water-soluble chloride constituting 65 ± 10% of total Cl with δ37Cl values averaging 3.0 ± 4.3‰. The second is structurally bound chloride with δ37Cl values averaging 7.3 ± 3.5‰. These high and distinctly different isotopic values are inconsistent with equilibrium fractionation processes and instead suggest early and extensive degassing of an isotopically light vapor. No relationship is observed between F/Cl ratios and δ37Cl values, which suggests that lunar halogen depletion largely resulted from the Moon-forming Giant Impact. The δ37Cl values of apatite are generally higher than the structurally bound Cl, and ubiquitously higher than the calculated bulk δ37Cl values of 4.1 ± 4.0‰. The apatite grains are not representative of the bulk rock, and instead record localized degassing during the final stages of lunar magma ocean (LMO) or later melt crystallization. The large variability in the δ37Cl values of apatite within individual thin sections further supports this conclusion. While urKREEP (primeval KREEP [potassium/rare-earth elements/phosphorus]) has been proposed to be the source of the Moon’s high Cl isotope values, the ferroan anorthosites (FANs) have the highest δ37Cl values and have a positive correlation with Cl content, and yet do not contain apatite, nor evidence of a KREEP component. The high δ37Cl values in this lithology are explained by the incorporation of a >30‰ HCl vapor from a highly evolved LMO.

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