Lithophile elements partitioning

In summary, a key aspect to the utility of U-series isotopes in the study of arc lavas is that whereas Th and Pa are observed and predicted to behave as relatively immobile high field strength elements (HFSE), Ra and (under oxidizing conditions) U behave like large ion lithophile elements (LILE) and are significantly mobilized in aqueous fluids. Fluid-wedge interaction will only serve to increase these fractionations. Just how robust the experimental partition coefficients are remains to be established by future experiments. 

Although the rare-earth elements (REEs) have similar geochemical behavior, since they are all large-ion lithophile elements and most of them partition among melts and mineral phases as a smooth function of ionic radius (with the exception of europium, which, commonly being... 

RSEs comprise two groups of metals the HSEs—osmium, rhenium, ruthenium, iridium, platinum, and rhodium with metal/silicate partition coefficients >10" —and the two moderately siderophile elements—molybdenum and tungsten (Table 2). As the major fractions of these elements are in the core of the Earth, it is not possible to establish independently whether the iDulk Earth has chondritic ratios of RLE to RSE, i.e., whether ratios such as Ir/Sc or W/Hf are chondritic in the bulk Earth. Support for the similar behavior of RLE and RSE in chondritic meteorites is provided by Figure 9. The ratio of the RSE, Ir, to the nonrefractory siderophile element, Au, is plotted against the ratio of the RLE, Al, to the nonrefractory lithophile element, Si. Figure 9 demonstrates that RLEs and RSEs are correlated... 

White J. C. (2003) Trace element partitioning between alkali feldspar and peraUcalic quartz trachyte to rhyolite magma Part 11. Empirical equations for calculating trace element partition coefficients of large-ion lithophile, high field strength and rare-earth elements. Am. Mineral. 88, 330-337. 

The issue of niobium in the core is of particular interest for the chondritic model of the bulk Earth. Niobium has always been thought to be refractory and hthophile, yet it is depleted in the upper mantle relative to other refractory and lithophile elements. Failure to locate hidden niobium-rich reservoirs in the lower mantle or the core would lead to serious problems for the well-established chondritic model. Recently, Wade and Wood (2001) studied partitioning of niobium between hquid metal and liquid sihcate under high pressure and temperature. They found that niobium becomes more siderophile with increasing pressure, hence opening up the possibility of storing niobium in the core. 

Dalpe C. and Baker D. R. (2000) Experimental investigation of large-ion-lithophile-element-, high-field-strength-element-and rare-earth-element-partitioning between calcic amphi-bole and basaltic melt the effects of pressure and oxygen fugacity. Contrib. Mineral. Petrol. 140, 233-250. 

Lithophile elements are those which have a preference for a silicate host, whereas chalcophile elements have an affinity for sulfur and so will most frequently be found in sulfides. Siderophile elements are those which will partition preferentially into a metallic iron phase and so are enriched in the Earth s core and in iron meteorites. Atmophile elements prefer the gaseous phases of the Earth atmosphere. This classification is discussed more fully in Chapter 2, Section 2.3.2. 

Nevertheless, we can make some general statements about the geochemistry of differentiated planets. The planetesimals from which they accreted had compositions determined largely by element volatility. Once assembled into a planet and heated, the partitioning of elements into cores and mantles was governed by their siderophile or lithophile affinities. Further differentiation of mantles to form crusts was controlled by the compatible or incompatible behavior of elements. 

Differentiation of terrestrial planets includes separation of a metallic core and possible later fractionation of mineral phases within either a solid or molten mantle (Figure 1). Lithophile and siderophile elements can be used to understand these two different physical processes, and ascertain whether they operated in the early Earth. The distribution of elements in planets can be understood by measuring the partition coefficient, D (ratio of concentrations of an element in different phases (minerals, metals, or melts)). 

Figure 16 Comparison of observed (open) and calculated (solid) depletions of phosphorus, tungsten, cobalt, nickel, molybdenum, and rhenium (circles) together with those for gallium, tin, and copper (inverted triangles) (sources Righter and Drake, 1997, 1999, 2000). The calculated depletions utilize the partitioning expressions of Righter and Drake (1999) for conditions of 2,250 ( 300) K (1,973 °C), 27 ( 6) GPa, AIW = — 0.4 ( 0.3) between a hydrous peridotite (NBO/t = 2.65) magma ocean and metallic liquid. The observed depletions are those of McDonough and Sun (1995), but volatility corrected as described by Newsom and Sims (1991), where the correction is made based on comparisons to trends of lithophile volatile element depletions.

REEs are classified as lithophiles and are partitioned into the earth s crust and mantle. The name rare earths originated over a century ago when the elements were first identified in minerals that, at the time, were rare. The elements are actually distributed widely over the earth and relatively accessible on the earth s surface. For a comprehensive description of REE geology, geochemistry, and natural abundances, see Geology, Geochemistry, and Natural Abundances of the Rare Earth Elements. In 2010, the United States Geological Survey (USGS) estimated that there were REE reserves of 110 million metric tons (mt). The static depletion index, the ratio of reserves to present-day production, for REEs is approximately 870 years. Thus, the primary immediate consideration is whether REE production can match demand, and particularly whether it will be possible to increase the use of dysprosium and neodymium in wind turbines and the batteries of electric vehicles. 

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