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KREEP is a lunar material having very high concentrations of incompatible elements; its name is an acronym for the incompatibles K, rare‐earth elements (REE), and P. Although a few pristine (endogenously igneous) KREEPy samples were returned from the Apollo 15 and 17 sites, most KREEPy samples are polymict breccias. Most models of KREEP petrogenesis have been based on partial melting of a variety of sources. Such models fail to explain the veritable absence of variations in incompatible element patterns over the sampled portion of the moon. We have defined a KREEP component based on the average composition of Apollo 14 breccias having extremely high concentrations of incompatible elements. Normalization of accurate incompatible data for KREEPy samples from the Apollo 12, 14, 15, 16, and 17 sites to this component virtually always shows no resolvable fractionation (e.g., <10% variation in the La/Lu ratio), whereas partial melting models typically produce larger fractionations (±20–25% in La/Lu) from a factor of 2 difference in degree of partial melting. Required is a single major source that could provide KREEP to widely separated locations on the nearside of the moon. The anorthositic crust of the moon is commonly attributed to the flotation of plagioclase on a deep, moon‐wide magma ocean. Fractional crystallization of this magma ocean would have produced large enrichments of incompatibles in a residual liquid. No other plausible major source of incompatibles has been proposed. We borrow the German prefix ur—meaning primeval and designate this residual liquid ‘urKREEP.’ We propose that all KREEPy rocks originated by dilution of urKREEP with crustal or mantle materials during assimilation, or zone‐refining (pristine samples), or impact‐induced brecciation (breccias and melt rocks). The formation of urKREEP cannot be dated precisely. Correction of breccia Rb‐Sr model ages for Rb loss or gain during the early intense bombardments yields ages that cluster in the range 4.4–4.5 Gy. This implies that crystallization of the magma ocean was essentially complete at this time and is in general agreement with U‐Pb evidence indicating crustal formation at 4.4 Gy. Assuming that the moon had the composition of an H‐group chondrite depleted in Fe‐Ni and FeS and that half the incompatibles fractionated into materials other than urKREEP, the thickness of a moon‐wide urKREEP layer was <2 km. Thorium concentrations determined by gamma ray spectroscopy indicate that about 4% of the incompatibles in an H chondritic moon are now in the outermost kilometer.

Hydrogen isotope ratios in lunar rocks and the delivery of cometary water to the MoonWater plays a critical role in the evolution of planetary bodies 1 , and determination of the amount and sources of lunar water has profound implications for our understanding of the history of the Earth-Moon system. During the Apollo program, the lunar samples were found to be devoid of indigeneous water 2,3 . The severe depletion of lunar volatiles 4 , including water, has long been seen as strong support for the giant-impact origin of the Moon 5 . Recent studies have found water in lunar volcanic glasses 6 and in lunar apatite 7-9 , but the sources of lunar water have not been determined. Here we report ion microprobe measurements of water and hydrogen isotopes in the hydrous mineral apatite, found in crystalline lunar mare basalts and highlands rocks collected during the Apollo missions. We find significant water in apatite from both mare and highlands rocks, indicating a role for water during all phases of the Moon's magmatic history. Variations of hydrogen isotope ratios in apatite suggest the lunar mantle, solar wind protons, and comets as possible sources for water in lunar rocks and imply a significant delivery of cometary water to the Earth-Moon system shortly after the Moon-forming impact.

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