Rare Earth Elements

Rare Earth Elements

• 1.
  The Rare EarthElements (REE) are a set of elements generally considered by the scientific community to consist of the lanthanides, of the ‘f’ electron orbital block of the periodic table, plus the transition metals Scandium and Yttrium. The latter elements are included for their association with the lanthanides in REE ores, though they are of little use to REE-centric geochemical studies, and are ignored hence. They are generally incompatible elements, and have strong enrichment patterns in evolved igneous rocks - though they are seldom concentrated in ore quantities - hence the origin of ‘rare’ in the REE term; this is despite their average crustal abundance being close to that of copper and tin. REE were a source of difficulty for early chemists, being extremely difficult to separate, all sharing +3 valence state possibilities, and having large ionic and atomic radii. The radii of REE undergo ‘lanthanide contraction’ due to f electron orbital oddities relating to nuclear shielding. As atomic number increases, so decreases atomic and ionic radii, albeit incrementally. This incremental decrease in radii however, is significant when it comes to the compatibility of the REE in question, with smaller ion radii elements having higher partition coefficients. REE can be separated into LREE and HREE - lanthanum to samarium and europium to lutecium. This distinction is based on the beginning of opposite spin electron pairing in the HREE. The primary pattern found in REE is that of chondritic (solar system) abundance, in which REE follow a gentle negative slope of abundance, with a saw-tooth pattern superimposed over it (the peaks forming over even proton number elements). The general trend of decreasing abundance of heavier elements explains the averaged slope, whilst the greater nuclear binding energy of even proton number isotopes has resulted in their heightened abundances during nucleosynthesis. The REE budget in evolved rocks is almost entirely contained within a specific suite of minerals, which have REE compatible sites. These include monazite, allanite, zircon, bastnäsite, apatite and xenotime, mostly phosphates or carbonate-fluorides. The extreme concentration factor of REE in these specific minerals relates to their low compatibility in most crustally voluminous silicate minerals - a result of their high ionic radii combined with their standard 3+ valence state. Until the concentration of REE in the melt becomes high enough for such rare minerals to crystallise however, they are generally crystallised in very small quantities into feldspars. This normal non-major phase compatibility means that REE are unaffected by most magmatic processes, retaining roughly chondritic ratios, increasing their interpretative power. The most important pattern in REE to geologists is that provided by partial melting and fractional crystallisation. With progressive reduction of ionic radius along the lanthanides, comes incremental increase in compatibility. This results in high concentrations of REE in partial melts, and even higher concentrations of REE in these melts if they crystallise fractionally. The jigsaw abundance pattern provided by nucleosynthesis is avoided in these abundance curves by normalisation, typically to chondritic abundances. This relative incompatibility ‘sorting’ has resulted in the upper mantle having depleted REE signatures, as well as fresh MORB, which is derived from this depleted mantle. Continental crust however, has high concentrations of REE. This is especially true of the most evolved rocks - felsic granites and rhyolites, but particularly carbonatites and peralkaline rocks. Ce/Yb tends to provide a good slope approximation. HREE all partition relatively well into the mineral garnet (X3Y2(SiO4)3), all having partition coefficients ~ equal to or greater than one. The HREE replace divalent metal cations in the X site of the garnet crystal lattice. This replacement, combined with the spinel-garnet phase transition in the upper mantle has resulted in an important tool for igneous
• 2.
  petrologists. Garnet containsthe vast majority of the HREE budget in the mantle, and so a significant HREE depletion is visible in spider diagram plots of REE in OIBs, where the melts are hypothesised to have formed at greater depths than MORB and E-MORB melts. The theory is lent greater credence by the seismic tomography mapping of low velocity seismic zone pillars below OIB hotspots, interpreted to be mantle plumes. MORB and E- MORB signatures differ in their lack of a HREE contraction, being derived from a shallower part of the mantle where spinel is dominant, and there is therefore no holding back of HREE. Garnet is not the only mineral to have an anomalous compatibilities for specific REE. europium is less incompatible than all other REE in plagioclase feldspar. This is because europium can substitute its standard 3+ valence state for a +2 valence state - replacing the usual 2+ calcium ion in the crystal lattice. Systems that have crystallised a significant volume of plagioclase will tend to have strong negative Eu anomalies. This has lead to a greater understanding of the formation of the moon. The anorthosite lunar highlands formed first, as low density plagioclase floated to the top of the post-lunar-accretion magma ocean. This happened as dense olivine and pyroxenes sank to the bottom of the newly forming lunar mantle. The remaining ‘KREEP’ melt of the former magma ocean was then sandwiched between the dense mantle, and the anorthosite crust, before eventually breaking through to form the basaltic plains of the lunar maria. These lunar basalts are heavily REE enriched, but europium depleted, because of the early formation of the plagioclase (and therefore europium) rich crustal anorthosite. The REE neodymium and samarium have become a very important tool for the dating of igneous rocks. Isochron dating can be carried out with the 147Sm-134Nd system. Combined with 87Rb-87Sr, this can further define enrichment or depletion patterns in rocks, by plotting Rb-Sr against Sm-Nd. Rb-Sr is the inverse of the Sm-Nd system, allowing for high resolution interpretation of things like crustal contamination, and upper mantle depletion. In summary, REE are concentrated in the crust of the earth due to their general pattern of incompatibility. The most incompatible elements are the lightest, due to their larger ionic radii. Individual groups or specific elements however can be held in voluminous silicate phases, which results in strong and obvious anomalies - which tell the observer a great deal about melt evolution. Even without anomalous abundances, REE can tell the observer a great deal about magmatic evolution, and can even help to describe and categorise specific rock types, like boninites.