Most of the elements of the Periodic Table are present below 0.1 wt.% in crustal rocks. The element colors in the Perodic Table above are based on element abundances in MORB. Only the 13 elements colored yellow have average wt.% values above 0.1 wt.% in MORB's. All the rest are trace elements in MORB. In other rocks, the list of major and trace elements might be different (e.g. K is a trace element in most mantle rocks), but most elements are trace elements in most rocks.
With modern instrumentation, it is possible to measure the weight percentages of many trace elements in rocks and minerals, even though those percentages might be as low as a few parts per billion (ppb). The proportions of trace elements in igneous rocks reflect chemical reactions among minerals and magma as the rocks are formed. Petrologists use measurements of trace elements to help unravel the histories of igneous rocks. This chapter provides some background to help understand the behavoir of trace elements during igneous processes. Concepts that are developed in this chapter include:
- Distribution Coefficients
- Raleigh Fractionation
- Partial Melting
- Rare Earth Element Diagrams
- Goldschmidt's Rules