ABSTRACT

The last liquids of the lunar magma ocean, known as urKREEP, should be highly enriched in volatiles, such as water, fluorine, and chlorine. We find chlorine-rich glasses in two pristine KREEP basalts from the Moon and calculate the volatile contents of the urKREEP component, and use this to estimate the fluorine and chlorine content of the lunar magma ocean. The Cl/Nb and F/Nd of KREEP imply that the lunar magma ocean was depleted in fluorine and chlorine by an order of magnitude compared to the Earth’s mantle. The extremely dry nature of most lunar samples is simply a result of partial melting of magma ocean cumulates that had already lost their volatiles to the urKREEP layer. The volatile-rich KREEP component may have helped lower the solidus of high-temperature magma ocean cumulates that were melted to form the Mg-suite rocks of the highlands, and also aided the dissemination of the KREEP signature into the upper crust. The chlorine-rich KREEP glasses also demonstrate that the large chlorine isotope anomaly found in lunar samples is likely an early lunar signature.