Lecture 5 Nomenclature For Trace Element Classification

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Lecture 5Nomenclature for Trace Element ClassificationWe have already grouped elements into two classes, major elements and traceelements (Figure 1). In addition, there are many ways of classifying trace elements.Examples are:1) Siderophile elements: elements that concentrate in metallic iron such as Ni,Co, Os, IrChalcophile elements: elements that concentrate in sulfides, such as Se, As,Zn, CdLithophile clements: elements that concentrate in silicate phases, such as Rb,Sr, Ba, Nb, Ta, Th, U, REEAtmophile elements: naturally occurring gaseous elements such as N and raregasesThese definitions are useful when considering formation of major earthreservoirs, i.e., core, mantle, crust and atmosphere.2) Compatible and Incompatible elements: In an equilibrium system composedof minerals and coexisting silicate melt, compatible elements arepreferentially partitioned into the solid phases and incompatible elements arepreferentially partitioned into the melt. This terminology is commonly used inthe context of partial melting of mantle rocks to form basaltic magma. Uppermantle rocks are rich in olivine and pyroxenes and the elements Sc, Cr, Coand Ni are compatible (enriched) in these phases relative to a silicate melt.However, a key aspect of this definition is to specify what mineral phases are1

being considered, e.g. Sc is compatible in clinopyroxene but not in olivine, Zris compatible in zircon and P in apatite but neither is compatible in olivine orpyroxene.Incompatible elements are preferentially partitioned into silicate melts ratherthan coexisting minerals. Incompatible elements can be sub-divided based onthe ratio of charge/ionic radius (Z/r) (see Figure 10).3) Large Ion Lithophile – the term large-ion lithophile element (or LILE) isfrequently used, but poorly defined in the geochemical literature.Someauthors use LILE as a synonym for incompatible trace elements, whereasothers use LILE to refer to a specific subset of incompatible trace elementscharacterized by large ionic radius to ionic charge ratio (or low ”fieldstrength” elements). The term was first used by Gast (1972) to encompass thecations K, Rb, Sr, Cs, Ba, REE, Th and U. Gast also included Li as a LILE,since it has a large radius to charge ratio, even though it is small (IR 0.82).Because of the confusion in the literature regarding usage of LILE, it isrecommended that the term be restricted to lithophile trace elements having alarge ionic radius to charge ratio that have ionic radii greater than those ofCa2 and Na1 , the largest cations common to rock forming minerals. By thisdefinition, LILE are K, Rb, Cs, Sr, Ba, Pb and Eu 2 (Figure 10).2

CsLarge IonLithophile Elementsz/r 1.8Rb1.6Ionic radius (10 -10 m)K2 .02.0BaPb1.4Na1.2SrEuLaNdY1.0EuLuThUHigh FieldStrength ElementsCeLi0.8ZrHfNbTaTiU0.60.401234Ionic charge (z)567Figure 10. Ionic radii versus ionic charge for elements that are incompatible in mafic minerals.Elements with Z/r 2 are classed as high field strength elements; they are incompatible becauseof the difficulty in achieving charge balance when these ions with Z 3 to 6 substitute for majorelement cations with Z 2. In contrast, elements with relatively high ionic ratios, hence low Z/r,are classed as large-ion lithophile elements. Ionic radii are for CN6 (for Li and ionswith 4, 5, 6 charge) and CN8 (for all other alkalic metals, and ions with 2 and 3 charge) arefrom Whittaker and Muntus (1970).3

4) High Field Strength ElementsAlthough these elements do not have large IR, because of their high charge andthe consequent difficulty in achieving charge balance, they are typicallyincompatible. Usually HFSE is used to designate Ti, Zr, Hf, Nb and Ta allwith Z/r 2 (Figure 10); however, it is important to realize that these elementsdo not always partition in a similar manner.4

MIT OpenCourseWarehttp://ocw.mit.edu12.479 Trace-Element GeochemistrySpring 2013For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.

Nomenclature for Trace Element Classification . We have already grouped elements into two classes, major elements and trace elements (Figure 1). In addition, there are many ways of classifying trace elements. Examples are: 1) Siderophile elements: elements that concentrate in metallic iron such as Ni, Co, Os, Ir. Chalcophile elements

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