McLeod and Krekeler reported in 2017 that the resource budget of Earth is limited. Rare-earth elements (REEs) are used across the world by society on a daily basis yet several of these elements have <2500 years of reserves left, based on current demand, mining operations, and technologies. With an increasing population, exploration of potential extraterrestrial REE resources is inevitable, with the Earth’s Moon being a logical first target. Following lunar differentiation at ~4.50–4.45 Ga, a late-stage (after ~99% solidification) residual liquid enriched in Potassium (K), Rare-earth elements (REE), and Phosphorus (P), (or “KREEP”) formed. Today, the KREEP-rich region underlies the Oceanus Procellarum and Imbrium Basin region on the lunar near-side (the Procellarum KREEP Terrain, PKT) and has been tentatively estimated at preserving 2.2 × 108 km3 of KREEP-rich lithologies.
The majority of lunar samples (Apollo, Luna, or meteoritic samples) contain REE-bearing minerals as trace phases, e.g., apatite and/or merrillite, with merrillite potentially contributing up to 3% of the PKT. Other lunar REE-bearing lunar phases include monazite, yittrobetafite (up to 94,500 ppm yttrium), and tranquility (up to 4.6 wt % yttrium, up to 0.25 wt % neodymium), however, lunar sample REE abundances are low compared to terrestrial ores. At present, there is no geological, mineralogical, or chemical evidence to support REEs being present on the Moon in concentrations that would permit their classification as ores. However, the PKT region has not yet been mapped at high resolution, and certainly has the potential to yield higher REE concentrations at local scales (<10s of kms).
Future lunar exploration and mapping efforts may therefore reveal new REE deposits. Beyond the Moon, Mars and other extraterrestrial materials are host to REEs in apatite, chevkinite-perrierite, merrillite, whitlockite, and xenotime. These phases are relatively minor components of the meteorites studied to date, constituting <0.6% of the total sample. Nonetheless, they dominate a samples REE budget with their abundances typically 1–2 orders of magnitude enriched relative to their host rock. As with the Moon, though phases which host REEs have been identified, no extraterrestrial REE resource, or ore, has been identified yet. At present extraterrestrial materials are therefore not suitable REE-mining targets. However, they are host to other resources that will likely be fundamental to the future of space exploration and support the development of in situ resource utilization, for example: metals (Fe, Al, Mg, PGEs) and water.
Daily use of REEs by modern day society is placing an increasing demand for extraction of these elements from limited terrestrial resources. With an increasing population, a projected 8.5 billion by 2020, it is natural to consider exploration and characterization of extraterrestrial REE sources in Resources 2017, order to sustain our reliance on raw materials and energy reserves. Earth’s nearest neighbor in space, the Moon, is a logical first target both in terms of evaluating resource potential.
Following formation of the Moon as the result of a giant impact between proto-Earth and an impacting Mars-sized body c. 70–100 Million years after the onset of Solar System formation, molten material (a primordial magma ocean) began to solidify and differentiate. After ~99% crystallization a KREEP (potassium, rare-earth element, phosphorus)-rich layer concentrated incompatible elements, a geochemical signature detected in many lunar samples today. Today, KREEP-rich lithologies are associated with enrichments in thorium on the lunar near-side (in the PKT), as observed by the gamma ray spectrometer on the Lunar Prospector. The volume of KREEP-rich lithologies associated with this region of the Moon is estimated at 2.2 × 108 km3, with potential reserves of REEs at ~2.25 × 1014–4.5 × 1014 kg. The majority of lunar samples contain REE-bearing phases as minor or trace components, typically apatite and/or merrillite with KREEP-like lithologies almost always containing merrillite. The presence of merrillite has the potential to account for up to 3% of a lithologies volume in the PKT however, this does not permit the classification of REEs on the Moon as ores. While REE abundances of trace phases in lunar rocks are high, their abundances are low compared to terrestrial ores meaning there is to date, no geological evidence to support mining the Moon for REEs yet.
With the development of future technologies capable of mapping at higher resolutions, and in situ exploration by future missions, economically viable lunar resources may yet be discovered. However, other potential resources do currently exist on the Moon—including helium-3, water, oxygen, and metals (Fe, Al, Mg)—which will likely be fundamental to future space exploration. Beyond the Moon, samples from Mars and chondritic meteorites are also host to REEs, predominantly hosted in merrillite, in addition to apatite, whitlockite, xenotime, and recently-identified chevkinite-perrierite. These minor phases dominate the REE budget with abundances 1–2 orders of
magnitude higher than their bulk host rock. However, as with the Moon, REEs have not yet been found in high enough concentrations in these extraterrestrial samples to warrant their classification as an ore resource. The concentrations of REEs in martian merrilites for example are several orders of magnitude lower than lunar merrillites, a difference attributed to the planetary bodies’ crystallization history during the early evolution of the Solar System. While the relative concentrations and abundances of the REEs in the phases that have been characterized to date provide crucial insights into the differentiation history of planetary objects, current and future missions to the Moon, Mars, and other nearby objects in our Solar System may yet reveal one or more extraterrestrial REE resources that one day will be utilized.
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