resources Article Sources of Extraterrestrial Rare Earth Elements: To the Moon and Beyond Claire L. McLeod 1,* and Mark. P. S. Krekeler 2 1 Department of Geology and Environmental Earth Sciences, 203 Shideler Hall, Miami University, Oxford, OH 45056, USA 2 Department of Geology and Environmental Earth Science, Miami University-Hamilton, Hamilton, OH 45011, USA;
[email protected] * Correspondence:
[email protected]; Tel.: 513-529-9662; Fax: 513-529-1542 Received: 10 June 2017; Accepted: 18 August 2017; Published: 23 August 2017 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 × 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 tranquillityite (up to 4.6 wt % yttrium, up to 0.25 wt % neodymium), however, lunar sample REE abundances are low compared to terrestrial ores.