# Sources of Extraterrestrial Rare Earth Elements: To the Moon and Beyond
## 5 Lunar Resources  
### The KREEP Source  
Following study of the returned Apollo samples, an easily identifiable, and geochemically distinguishable, component within the lunar sample collection was apparent. This unique signature is associated with elevated REE, K and P (or, KREEP) [102]. Historically, the origin of the KREEP component in lunar lithologies has been debated with two principal theories regarding its petrogenesis proposed throughout the literature: (1) Partial melting of the solidified lunar interior following LMO crystallization and (2) extreme fractional crystallization of the primordial LMO (Figure 3b). In [103], partial melting was concluded as not being responsible for the observed abundances of ITEs in the KREEP-rich lithologies. This was due to inconsistencies in the behavior of elements in partial melting models, most notably the large fractionations between La and Lu (which is not observed). Instead, covariation of ~20 ITEs which cover a 10-fold range of abundances in samples which are derived from spatially distinct sample sites throughout the Apollo collection, further support their derivation from a common lunar geochemical reservoir ([103]; Figure 3b). The uniformity in ITE enrichment, consistent LREE/HREE ratios (light rare-earth elements/heavy rare-earth elements), and similarity  
Figure 4: (**a**) Near side of the Earth’s Moon with Apollo landing sites shown. The majority of landing sites targeted the lunar mare (dark regions); (**b**) Left hand image shows a hand sample of 60025, a Ferroan Anorthosite. Right hand image shows a thin section photomicrograph of 60025, characterized by 70–98% plagioclase feldspar [85; 97; 98]. (**c**) Left hand image shows a hand sample of 15016, a vesiculated olivine-normative basalt. Right hand image shows a thin section photomicrograph of 15016, characterized by olivine (6–10%), pyroxene (59–67%), plagioclase (21–27%), ilmenite (6%), and <1% total of chromite, ulv\(\ddot{\text{o}}\)spiniel, and mesostasis [99; 100; 101]. All images are available from the NASA Lunar Sample Atlas [102].  
in \({}^{143}\)Nd/\({}^{144}\)Nd isotopic signatures is therefore inconsistent with local-scale partial melting and subsequent fractional crystallization, but consistent with derivation from a reservoir formed during LMO differentiation (urKREEP) [104].  
Today, KREEP-rich lithologies are demonstrably associated with relative enrichments in thorium (Th) and uranium (U) (Figure 5).  
Evaluation of the distribution of KREEP-rich lithologies is therefore possible through gamma ray mapping [105; 106]. Results from the gamma ray spectrometer onboard the orbiting Lunar Prospector orbiting spacecraft revealed a concentrated distribution of Th(and KREEP)-rich lithologies in the northern hemisphere of the near-side of the Moon (Figure 5). This distribution is also associated with the region known as Oceanus Procellarum, and the Imbrium Basin, and is now widely referred to as the Procellarum KREEP Terrain (or the PKT) [107]. The concentration of KREEP-rich lithologies on the near-side of the Moon in the PKT (Figure 5b) is not only associated with late stage LMO crystallization, but also later exhumation following an impact which formed the Imbrium Basin during a period of intense bombardment during the Moon's early history.  
The volume of KREEP lithologies underlying Oceanus Procellarum has been estimated at \(2.2\times 10^{8}\) km\({}^{3}\), based on the concentration of radioactive (heat-producing) elements and the area mapped as \"high-Th\" (Figure 3a, [109; 110; 111]). From [109], potential REE reserves in these underlying KREEP rocks has been estimated at \(\sim\)2.25 \(\times\) 10\({}^{14}\)-4.5 \(\times\) 10\({}^{14}\) kg.