Sources of Extraterrestrial Rare Earth Elements: To the Moon and Beyond

Abstract

Abstract: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 × 10 8 km 3 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 tranquillityite (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.

Introduction to a Resources Special Issue on Criticality of the Rare Earth Elements: Current and Future Sources and Recycling

“…This indicates that it is currently unlikely that the Moon contains economically relevant abundances of the REE [17]. However, the authors suggest that this may be a result of a lack of information about the Procellarum KREEP Terrane, an area of concentrated KREEP magmatism that may yield locally elevated REE concentrations [17]. This suggests that future lunar exploration and mapping may reveal areas containing elevated concentrations of the REE.…”

“…Each of the sets of samples we have from the Moon from the Apollo and Luna missions as well as from the lunar meteorite catalogue contain accessory REE minerals such as apatite, merrillite, monazite, yttrobetafite, and tranquillityite, although lunar REE abundances are low compared to similar terrestrial samples. This indicates that it is currently unlikely that the Moon contains economically relevant abundances of the REE [17]. However, the authors suggest that this may be a result of a lack of information about the Procellarum KREEP Terrane, an area of concentrated KREEP magmatism that may yield locally elevated REE concentrations [17].…”

“…This suggests that future lunar exploration and mapping may reveal areas containing elevated concentrations of the REE. McLeod and Krekeler [17] also state that Mars and other extraterrestrial materials contain REE-bearing minerals, albeit at low modal abundances. This indicates that these materials may potentially be sources for the REE as a by-product of the production of other commodities vital to space exploration and utilization [17].…”

“…McLeod and Krekeler [17] move the focus of this special issue to the Moon and beyond, focusing on potential extraterrestrial sources of the REE. Late-stage lunar magmatism generated residual melts that were enriched in K, the REE, and P (i.e., KREEP).…”

Rare Earth Elements in Planetary Crusts: Insights from Chemically Evolved Igneous Suites on Earth and the Moon

“…However, these extraterrestrial granites exhibit chemical affinities to terrestrial A-type granites. On Earth, these anorogenic magmatic systems are hosts to U-Th-REE-ore deposits and while to date only U-Th regions of enrichment on the lunar surface have been identified, future exploration of the lunar surface and interior may yet reveal U-Th-REE regions associated with the distribution of these chemically distinct, evolved lithologies.Minerals 2018, 8, 455 2 of 24 of these elements in modern day technology, their criticality in supporting research throughout medicine, military-based technologies, and communication, and their role in the development of clean energy technologies [17][18][19][20]. These elements do not occur naturally as metals like copper (Cu) or lead (Pb), instead being complexed in a wide array of different mineral types from carbonates to silicates to phosphates.…”