# Sources of Extraterrestrial Rare Earth Elements: To the Moon and Beyond
## 5 Lunar Resources
### Lunar REE-Bearing Minerals  
The presence of KREEP components in lunar samples is \"unequivvocally accepted\" yet a pristine urKREEP signature has not yet been found [112]. The composition of urKREEP, from which KREEP-rich lithologies derive their KREEPy signatures, was modelled in [112; 113]. Figure 6 summarizes the primitive mantle-normalized REE-signatures of (1) chondrite; (2) different geochemical reservoirs on Earth: upper, middle, lower, and bulk continental crust; (3) the REE-signatures of lunar highland  
Figure 5: (**a**) From NASA, Thorium (Th) distribution map of the lunar nearside (left) and far side (right), mapped by the Lunar Prospector. Image available at [108] and used with permission; (**b**) Gamma ray spectrometer (GRS) generated map of thorium abundances for mid-latitudes with abundances \(>\)12 ppm in isolated locations. Modified from [105]. samples (FANs), mare basalts, a lunar granite, a lunar dunite, and KREEP-rich lithologies; (4) signatures of REE-bearing phases found on the Moon so far (and discussed in more detail later); and (5) the signature of the urKREEP reservoir. As illustrated in Figure 6, and discussed earlier, the lunar highlands samples and mare basalts display complementary geochemical signatures with positive and negative Eu-anomalies respectively. From [112; 113] the degree of fractionation that would be required to form a residual liquid during LMO that has the composition of urKREEP is \"within the realm\" of silicate liquid immiscibility, whereby two melts--one K-rich and one REEP-rich--are segregated. This process has the potential to account for the presence of lunar granites (derived from the K-rich melt) and evolved phosphate phases in highland samples (see discussion later). The KREEP-rich fraction then has the potential to ascend and interact with the lunar crust, potentially acting as a metsomatic agent. As shown in Figure 6, the modeled composition of urKREEP (orange) is elevated in KREEP components relative to the KREEP-like signatures (shown in red), and is higher in KREEP components than any other lunar lithology (Granite, Apollo Basalts, Dunite and the FANs). The majority of the ITEs on the lunar surface, in rocks and soils, are associated with the PKT (Figure 5). However, the majority of lunar samples contain apatite and/or merrillite as a trace phase (Figure 6), and in samples which exhibit a KREEP signature, merrillite (H-free whitlockite) is almost always present, often co-existing with apatite [120]. Figure 7a-c shows several excellent examples of lunar fluorapatite in an Apollo 10 basalt, and lunar apatite crystals lining vugs in the Apollo 14 breccias. In both examples, (fluoro)apatite occurs as euhedral to subhedral grains. Figure 6: Rare-earth elements—Earth’s primitive mantle normalized concentrations of terrestrial and lunar reservoirs, rocks types, and REE-bearing phases. Samples and data sources (all lunar data is available in the Lunar Sample Compendium [94] unless stated otherwise): Ferroan Anorthosites 60025 and 62236; Chondrite: [114]; Dunite: [115]; BCC (Bulk), LCC (Lower), MCC (Middle), and UCC (Upper Continental Crust) from [116]; Apollo Basalts: 12011, 12075, 14053, 15016, and 70017; KREEP basalts 12013, 14073, 14076, 14078, 14303, 14310, 15382, 15386, 15405, 61156, and 72275; Granite: [117]; urKREEP from [112]; Apatite and Merrilite from [118]; and Monazite from [119]. Also shown in Figure 6 are normalized REE compositions of REE-bearing phases found throughout the lunar sample collection: apatite, merrillite, and monazite. In lunar merrillite, REE substitutes for Ca, and H is substituted by Na and K. This mineral appears to be unique to the PKT with the formula Ca\({}_{17.3}\)Y\({}_{0.4}\)(La-Lu)\({}_{0.88}\)(Mg,Mn,Fe)\({}_{2.4}\)(Na,K)\({}_{0.07}\)(P,Al,Si)\({}_{13.9}\)O\({}_{56}\) (sample 14310,123; [96]).