major elements

major elements A major element is often defined as an element that occurs in the Earth's crust in an average concentration greater than 1.0 wt %. This definition restricts the number of elements to eight: oxygen, silicon, aluminium, iron, calcium, magnesium, sodium, and potassium in decreasing abundance. Other elements are, however, also fairly abundant; and the definition therefore is commonly extended to include titanium, manganese, hydrogen, and phosphorus.

Major element data are useful in several ways, ranging from simple characterization of rock types to quantitative assessment of geochemical processes. One of the principal purposes for obtaining major element data is the classification of rocks, particularly of igneous rocks. The characterization of volcanic rocks is often based on whole-rock major element data, whereas the nature and abundance of the minerals present in a given rock provide the basis for a widely used nomenclature of plutonic rocks. This difference is due to the fact that most volcanic rocks are microcrystalline or partly glassy, and thus it is more difficult to classify them by their mineralogical composition. In order to characterize volcanic rocks, major element data are used in a variety of classification schemes that use discrimination diagrams based on oxide or cation contents (in weight% or mole% respectively), or on the so-called norm calculations, which yield, in conjunction with several assumptions, a theoretical mineral content derived from the chemical analysis of the rock. Major element data are of only limited importance in the classification of sedimentary rocks (e.g. some clastic sediments), but may help in determining the original nature of a metamorphic rock by comparing its composition to that of present-day igneous or sedimentary rocks. This approach is, however, valid only if the metamorphic rock has not been altered or weathered, and if it can be shown that the metamorphism was isochemical. It may be difficult to demonstrate this, because several major elements (particularly potassium, sodium, and hydrogen) have been shown to be easily removed from, or added to, a rock during metamorphism (e.g. dehydration of sedimentary rocks or metasomatism). In both igneous and metamorphic rocks the major element content plays an essential role in constraining the possible mineral assemblages as well as the mineral compositions at a given pressure and temperature. Major element data are commonly displayed graphically in variation diagrams, which are invaluable in unravelling possible correlations and trends between different elements in a data set. From existing interrelationships it is often possible to infer and model quantitatively geochemical processes such as the mixing of two or more components, fractional crystallization, partial melting, assimilation of country rocks by an ascending melt, or element mobility. The use of variation diagrams is not restricted to major elements. They are particularly powerful tools when both major and trace element data are included.

Oxygen and hydrogen play an important role in modern geochemistry, because their stable isotopes may be fractionated as a consequence of mass differences between the isotopes. Determination of isotope ratios (¹⁸O/¹⁶O, D/H) in minerals and rocks thus provides insight into a variety of processes including reaction mechanisms, diffusion, evaporation, and fluid–rock interaction. In addition, the stable isotopes of oxygen and hydrogen are used as palaeothermometers and as tracers to determine the source of an element. Only one major element, potassium, is used for radiometric dating of rocks (the K–Ar method).

Major element concentrations (traditionally expressed as oxide wt%; O not analysed) are commonly determined by X-ray fluorescence analysis. The determination of the hydrogen (or H₂O) content and the distinction between FeO and Fe₂O₃ require other analytical methods, such as conventional wet-chemical, colorimetric, or spectroscopic techniques.

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