# Sources of Extraterrestrial Rare Earth Elements: To the Moon and Beyond
## 5 Lunar Resources
### REEs beyond the Moon
Other elements detected included potassium, chromium, manganese, phosphorus, sulfur and chlorine [171]. To date, only one of the meteorites--a polymict regolith breccia (sample NWA 7034) and its paired stones--is considered representative of the modern-day martian surface, with the other 106 Martian samples being igneous in nature (shergottites, orthopyroxenite, clinopyroxenite, dunite) and thus providing insights into the evolution of the Martian interior [172; 173; 174; 175; 156]. Figure 8a illustrates and summarizes the REE characteristics of these different classes of martian meteorites (including NWA 7034), along with the patterns of individual REE-bearing phases within NWA 7034 recently sampled by [176]. As shown, NWA 7034 exhibits LREE enrichment in comparison to other martian rock types with absolute concentrations of La and Lu of 13.7 and 4.3 ppm, respectively. Throughout the igneous lithologies, the enriched basaltic shergottite exhibits the highest concentrations of REEs with 4.4 ppm La, 8.4 ppm Nd, 4.7 ppm Gd, and 0.43 ppm Lu (sample NWA 7257) while the orthopyroxenite and the depleted olivine-phyric shergottite exhibit the lowest LREE abundances (La at 0.15 and 0.29 ppm respectively, Ce at 0.43 and 1.07 ppm respectively). The orthopyroxenite and dunite samples exhibit the lowest HREE abundances (0.255 and 0.11 ppm Yb respectively, and 0.037 and 0.015 ppm Lu respectively). The most common REE-bearing phase throughout the martian meteorite collection is merrillite ((Ca\({}_{3}\))(PO\({}_{4}\))\({}_{2}\)), a common accessory phases in extraterrestrial samples (as is apatite, see earlier discussion of the Moon). Throughout the martian sample collection, merrillite has been identified as the dominant host of the REEs [178]. Merrillite has been found in the shergottites (olivine-phyric, basaltic, Iberzolitic), the orthopyroxenite, and the recently discovered regolith breccia (Figure 8a). As shown in Figure 8b, the abundances of the REEs in martian merrillites are significantly lower than in lunar merrillites. Broadly, the REEs substitute into the Ca site in the merrillite crystal structure, a substitution that is demonstrably more pronounced in lunar merrillites (Figure 8b). This observed difference is almost two orders of magnitude with respect to the LREEs (Figure 8b) and has been attributed to differences in their source melt composition, with the relatively depleted LREE patterns associated with derivation from depleted martian mantle sources [178], and the relatively enriched LREE patterns (La\({}_{\text{N}}\) up to 699, where \"N\" refers to the primitive-mantle normalized value) associated with LREE enrichment (e.g., [179]). This enrichment has been hypothesized to be associated with post martian  
Figure 8: **(a)** Summary of the REE characteristics of martian samples including the recently discovered regolith breccia, NWA 7034 and its REE-bearing phases. All data is available through the Martian Meteorite Compendium [177]. NWA 7034 data from [176]; **(b)** Comparison of lunar and martian merrillite. Data sources: [116; 178]. mantle differentiation, potentially as a result of small degrees of partial melting which concentrated a LREE component which was assimilated by a parental magma (e.g., [180; 181]). Alternatively, this LREE component has been suggested to have been derived from the Martian crust (e.g., [182]) or a KREEP-like component associated with late-stage martian magma ocean crystallization and similar to that which has been identified in lunar samples (see earlier, e.g., Borg and [63; 183]). While the origin of this LREE-enriched component remains poorly constrained (e.g., [184]), the differences between lunar and martian merrillites indicate that Mars experienced a different petrogenic history than the Moon (Figure 8b; e.g., [120]).