minerals Article Rare Earth Elements in Planetary Crusts: Insights from Chemically Evolved Igneous Suites on Earth and the Moon Claire L. McLeod 1,* and Barry J. Shaulis 2 1 Department of Geology and Environmental Earth Sciences, 203 Shideler Hall, Miami University, Oxford, OH 45056, USA 2 Department of Geosciences, Trace Element and Radiogenic Isotope Lab (TRaIL), University of Arkansas, Fayetteville, AR 72701, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-513-529-9662 Received: 5 July 2018; Accepted: 8 October 2018; Published: 16 October 2018 Abstract: The abundance of the rare earth elements (REEs) in Earth’s crust has become the intense focus of study in recent years due to the increasing societal demand for REEs, their increasing utilization in modern-day technology, and the geopolitics associated with their global distribution. Within the context of chemically evolved igneous suites, 122 REE deposits have been identified as being associated with intrusive dike, granitic pegmatites, carbonatites, and alkaline igneous rocks, including A-type granites and undersaturated rocks. These REE resource minerals are not unlimited and with a 5–10% growth in global demand for REEs per annum, consideration of other potential REE sources and their geological and chemical associations is warranted. The Earth’s moon is a planetary object that underwent silicate-metal differentiation early during its history. Following ~99% solidification of a primordial lunar magma ocean, residual liquids were enriched in potassium, REE, and phosphorus (KREEP). While this reservoir has not been directly sampled, its chemical signature has been identified in several lunar lithologies and the Procellarum KREEP Terrane (PKT) on the lunar nearside has an estimated volume of KREEP-rich lithologies at depth of 2.2 × 108 km3. This reservoir therefore offers a prospective location for future lunar REE exploration. Within the context of chemically evolved lithologies, lunar granites are rare with only 22 samples currently classified as granitic. 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. Keywords: rare earth elements; igneous; granite; Moon; lunar; resources 1. Introduction Ore deposits associated with chemically evolved igneous rock suites have been the intense focus of research over the past several decades [1–14]. Magmatic-associated mineral deposits are mineralogically and chemically diverse, occur in a range of geological settings, and have long been the target of the mineral industry. They host a wide array of precious and semi-precious metals including tin (Sn), tungsten (W), lithium (Li), molybdenum (Mo), bismuth (Bi), iron (Fe), copper (Cu), and silver (Ag), and the rare earth elements (REEs) which includes the lanthanide series (Lanthanum (La) to lutetium (Lu) in addition to scandium (Sc) and yttrium (Y)). The occurrence, distribution, and potential for economic extraction of the REEs have become the intense focus of world markets in recent years [15,16]. This has been due to the extensive use Minerals 2018, 8, 455; doi:10.3390/min8100455 www.mdpi.com/journal/minerals Minerals 2018, 8, 455 2 of 24 Minerals 2018, 8, x FOR PEER REVIEW 2 of 23 ofelements these elementsin modern in day modern technology, day technology, their critica theirlity criticalityin supporting in supporting research throughout research throughout medicine, medicine,military-based military-based technologies, technologies, and communication, and communication, and their and role their in the role development in the development of clean of energy clean energytechnologies technologies [17–20]. [17 These–20]. elements These elements do not dooccur not naturally occur naturally as metals as metals like copper like copper (Cu) or (Cu) lead or (Pb), lead (Pb),instead instead being being complexed complexed in a inwide a wide array array of differen of differentt mineral mineral types types from from carbonates carbonates to tosilicates silicates to tophosphates. phosphates. According According to the to theUnited United States States Geologic Geologicalal Survey Survey (USGS) (USGS) [21], [ 21there], there are currently are currently 799 799major major REE REE mineral-bearing mineral-bearing deposits deposits identified identified globally, globally, of of which which 122 122 are are associated associated with alkaline igneousigneous activityactivity (not(not includingincluding carbonatites,carbonatites, of which there areare 149149 [[22]).22]). These deposits are oftenoften specificallyspecifically associatedassociated withwith chemicallychemically evolved,evolved, intrusiveintrusive dike,dike, graniticgranitic pegmatite,pegmatite, and/orand/or alkalinealkaline igneousigneous complexes,complexes, andand areare thethe primaryprimary focusfocus ofof thisthis review.review. The mineralogy of of Earth’s Earth’s crust crust is isdominated dominated by bycommon common rock-forming rock-forming silicate silicate minerals, minerals, e.g., e.g.,quartz, quartz, feldspars, feldspars, ferromagnesium ferromagnesium minerals, minerals, amphibol amphiboles,es, and micas. and The micas. elemental The elemental budget of budgetEarth’s ofcrust Earth’s is therefore crust is thereforecharacterized characterized by the cations by the cationswhich are which integral are integral structural structural components components to these to thesephases phases (Figure (Figure 1). Combined,1). Combined, the themajor major rock-formi rock-formingng minerals minerals account account for for 99% 99% of Earth’s upperupper crustal elementelement budgetbudget withwith onlyonly 2.16%2.16% of the remainingremaining 1%1% beingbeing comprised ofof thethe REEsREEs (Figure(Figure1 1).). Figure 1. Summary of Earth’s upper crustal element budget [23]. Silicon and O account for 76%, Figure 1. Summary of Earth’s upper crustal element budget [23]. Silicon and O account for 76%, with with Al at 8%, total alkalis (Na, Ca, K) at ~8%, and Fe and Mg contributing 6.5% combined. From the Al at 8%, total alkalis (Na, Ca, K) at ~8%, and Fe and Mg contributing 6.5% combined. From the remaining 1% (these elements are often referred to as trace elements), the REEs contribute 2.16% with remaining 1% (these elements are often referred to as trace elements), the REEs contribute 2.16% with Ce, Nd, Y, and La the most abundant at 32%, 17%, and 13% (Y and La) respectively of the total REE Ce, Nd, Y, and La the most abundant at 32%, 17%, and 13% (Y and La) respectively of the total REE budget. Yb, Ho, Eu, Er, Tm, and Lu collectively comprise 3.2% of the REE budget (not shown above). budget. Yb, Ho, Eu, Er, Tm, and Lu collectively comprise 3.2% of the REE budget (not shown above). Despite comprising <0.022% of the elemental budget of Earth’s upper crust (Figure1), the REEs are oneDespite of modern-day comprising societies <0.022% most of th criticale elemental commodities. budget of TheyEarth’s are upper fundamental crust (Figure to today’s 1), the world REEs economyare one of and modern-day with a global societies shift towards most critical establishing commodities. clean and They green are energyfundamental sources, to the today’s demand world for REEseconomy will and only with continue a global to increase shift towards [16,20]. establishing clean and green energy sources, the demand for REEsIt is thereforewill only continue the objective to increase of this paper[16,20]. to (1) consider and evaluate the occurrence of Earth’s REE resourcesIt is therefore associated the objective with chemically of this paper evolved to (1) igneous consider alkaline and evaluate suites; (2)the compareoccurrence and of contrast Earth’s theseREE resources with the associated association with of REE-mineral chemically evolved bearing igneous granitic alkaline lithologies suites; on (2) another compare planetary and contrast body whichthese with underwent the association metal-silicate of REE-mineral differentiation, bearing the Earth’sgranitic moon; lithologies and (3) on evaluate another theplanetary potential body for economicallywhich underwent justifiable metal-silicate extraction differentiation, of lunar REEs. the With Earth’s our increasingmoon; and reliance(3) evaluate on natural the potential resources, for andeconomically the need forjustifiable REEs to extraction sustain the of research lunar REEs. and development With our increasing of future reliance technologies, on natural the exploitation resources, ofand space-based the need resourcesfor REEs isto an sustain inevitable the reality research of humanities and development future mineral of future exploration technologies, [19,24]. the exploitation of space-based resources is an inevitable reality of humanities future mineral exploration 2.[19,24]. Geological and Mineralogical Occurrences The REE deposits associated with evolved magmatic systems share a suite of common 2. Geological and Mineralogical Occurrences characteristics [25]: The REE deposits associated with evolved magmatic systems share a suite of common (1) Predominantly associated with alkaline environments; characteristics [25]: (1) Predominantly associated with alkaline environments; Minerals 2018, 8, 455 3 of 24 Minerals 2018,