Early degassing of lunar urKREEP by crust-breaching impact(s)

1Jessica J. Barnes, 1,2Romain Tartèse, 1,3Mahesh Anand, 4Francis M. McCubbin, 5Clive R. Neal, 1Ian A. Franchi
1Planetary & Space Sciences, The Open University, Walton Hall, MK7 6AA, UK
2Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Muséum National d’Histoire Naturelle, Sorbonne Universités, CNRS, UPMC & IRD, 75005 Paris, France
3Earth Sciences department, Natural History Museum, London, SW7 5BD, UK
4NASA Johnson Space Center, Mailcode XI2, 2101 NASA Parkway, Houston, TX 77058, USA
5Department of Civil & Environmental Engineering & Earth Science, University of Notre Dame, IN 46556, USA

Current models for the Moon’s formation have yet to fully account for the thermal evolution of the Moon in the presence of H2O and other volatiles. Of particular importance is chlorine, since most lunar samples are characterised by unique heavy δ37Cl values, significantly deviating from those of other planetary materials, including Earth, for which δ37Cl values cluster around ∼0‰. In order to unravel the cause(s) of the Moon’s unique chlorine isotope signature, we performed a comprehensive study of high-precision in situ Cl isotope measurements of apatite from a suite of Apollo samples with a range of geochemical characteristics and petrologic types. The Cl-isotopic compositions measured in lunar apatite in the studied samples display a wide range of δ37Cl values (reaching a maximum value of +36‰), which are positively correlated with the amount of potassium (K), Rare Earth Element (REE) and phosphorous (P) (KREEP) component in each sample. Using these new data, integrated with existing H-isotope data obtained for the same samples, we are able to place these findings in the context of the canonical lunar magma ocean (LMO) model. The results are consistent with the urKREEP reservoir being characterised by a δ37Cl ∼+30‰. Such a heavy Cl isotope signature requires metal-chloride degassing from a Cl-enriched urKREEP LMO residue, a process likely to have been triggered by at least one large crust-breaching impact event that facilitated the transport and exposure of urKREEP liquid to the lunar surface.

Reference
Barnes JJ, Tartèse R, Anand M, McCubbin FM, Neal CR, Franchi IA (2016) Early degassing of lunar urKREEP by crust-breaching impact(s). Earth and Planetary Science Letters 447, 84–94
Link to Article [doi:10.1016/j.epsl.2016.04.036]
Copyright Elsevier

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