Geology, Geochemistry and Natural Abundances

Abstract Rare earth elements (REE) are important trace elements for evaluating geological processes and are strategic metals with applications in high technology. REE are lithophile, electropositive, mostly trivalent, and refractory; their importance to geochemistry results from systematic variations in ionic radius and complexing, cerium (Ce 3+/4+ ) and europium (Eu 2+/3+ ) redox states, and slight variations in condensation temperatures. Influence from the tetrad effect remains controversial. In most common rock‐forming minerals, REE are incompatible and concentrated in magmas and late crystallizing minerals. They are essential constituents in about 200 minerals, most being rare in nature. Economical concentrations are found mostly in association with carbonatite magmatism, heavy mineral placers, and several types of regolith. Type CI chondritic meteorites have relative REE abundances identical to the solar photosphere and are taken to represent the REE content of the solar nebula. The silicate fraction of terrestrial planets and the Moon also have chondritic‐relative REE distributions, but absolute abundances differ depending on volatile content and amount of iron partitioned into metal cores. Refractory inclusions in some carbonaceous chondrites display erratic REE patterns resulting from fractionation of most from least refractory REE during localized evaporation/condensation. On Earth, REE partition into silicate mantle and crust and are excluded from the metal core. Oceanic crust is depleted in light REE (LREE), paralleling the pattern of “depleted” mantle, which in turn largely reflects long‐term growth of LREE‐enriched continental crust by partial melting of the upper mantle above subduction zones. Continental crust is further separated into lower and upper crust. Very uniform sedimentary REE patterns are representative of upper continental crust. The pattern is notable for europium depletion due to partitioning of Eu II into residual plagioclase during partial melting. Since plagioclase is stable only to 40 km depth on Earth, the anomaly points to shallow intracrustal partial melting processes. Such patterns are observed back to the late Archean, at which time, the upper crust does not display anomalous europium on average and thus formed by different processes. Natural waters are characterized by extremely low but variable REE concentrations due to low solubility and tendency to adsorb onto particle surfaces. Seawater is characterized by severe cerium depletion, resulting from oxidation to Ce IV and separation from other REE by preferential uptake onto Mn IV ‐oxide particles. Enrichment of heavy REE in seawater relative to the crust results from greater stability of heavy rare earth element (HREE) complexes. REE residence times in seawater are short, ranging from 50 (Ce) to about 2500 (HREE) years.