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Geochemists recognize the utility of the rare earth elements (REE) as especially powerful tools for tracing geochemical processes within the earth. Interest in the REEs reflects, in large part, their unique and chemically coherent properties that arise as a result of their trivalent charge and the similar ionic radii of all 14 naturally occurring rare earths. For example, owing to their trivalent charge, the REEs are chemically fractionated from divalent Ba and tetravalent Hf, their nearest neighbors in the Periodic Table of the Elements. Furthermore, because the ionic radii of the REEs vary systematically across the REE series (ie, ionic radius decreases with increasing atomic number, which is referred to as the “lanthanide contraction”), individual REEs are also subject to chemical fractionation within the series itself. As the ionic radii decrease systematically across the series with increasing atomic number, geochemical …