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

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 delta Cl-37 values, significantly deviating from those of other planetary materials, including Earth, for which delta Cl-37 values cluster around similar to 0 parts per thousand. 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 delta Cl-37 values (reaching a maximum value of +36 parts per thousand), which are positively correlated with the amount of potassium (K), Rare Earth Element (REE) and phosphorous (P) (CREEP) 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 delta Cl-37 similar to+30%o. 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. (C) 2016 The Authors. Published by Elsevier B.V.