The Use of Trace Elements in Igneous Petrology
Trace elements are those which occur in very low concentrations in common rocks
(usually < 0.1 % by weight). Their concentrations are therefore commonly expressed in
parts per million (ppm; 1 ppm = 10
weight%). Unlike major elements, trace elements
tend to concentrate in fewer minerals, and are therefore more useful in formulating
models for magmatic differentiation, and in some cases, in predicting the source of a
particular magma. Trace elements most commonly used for the interpretation of the
petrogenesis of igneous rocks include: Ni, Cr, Sc, V, Rb, Ba, Sr, Zr, Y, Nb and the rare
earth elements (La to Lu). Keep in mind that the concentration of trace elements will
vary with the rock type; whereas Ni and Cr show higher concentrations in mafic and
ultramafic rocks, Zr and Rb are more concentrated in acidic rocks. Accordingly, some
major or minor elements as K and P, which occur in very low concentrations in basalts
(approaching trace levels), are just as useful in petrogenetic interpretations as some
trace elements (in the case of basalts only, .
. of course!).
The incorporation of a trace element in the crystal structure of one or more minerals
depends largely on its charge and radius, but also on the electronegativity of this
element, and crystal field effects. Accordingly, a trace element will either substitute for
a major element in the structure of a crystallizing mineral, or remain in the liquid.
Types of trace elements:
In addition to this simple classification of trace elements into compatibles and
incompatibles, trace elements are perhaps better classified on the basis of their
geochemical characteristics (which will naturally influence whether the element
becomes compatible or incompatible).
1) Large ion lithophile elements (LILE): These elements are characterized by large