Igneous petrology

The composition of igneous rocks and minerals can be determined via a variety of methods of varying ease, cost, and complexity. The simplest method is observation of hand samples with the naked eye and/or with a hand lens. This can be used to gauge the general mineralogical composition of the rock, which gives an insight into the composition. A more precise but still relatively inexpensive way to identify minerals (and thereby the bulk chemical composition of the rock) with a petrographic microscope. These microscopes have polarizing plates, filters, and a conoscopic lens that allow the user to measure a variety of crystallographic properties. Another method for determining mineralogy is to use X-ray diffraction, in which a powdered sample is bombarded by X-rays, and the resultant spectrum of crystallographic orientations is compared to a set of standards. One of the most precise ways of determining chemical composition is by the use of an electron microprobe, in which tiny spots of materials are sampled. Electron microprobe analyses can detect both bulk composition and trace element composition.

The dating of igneous rocks determines when magma solidified into rock. Radiogenic isotopes are frequently used to determine the age of igneous rocks.

In this dating method the amount of ⁴⁰Ar trapped in a rock is compared to the amount of ⁴⁰K in the rock to calculate the amount of time ⁴⁰K must have been decaying in the solid rock to produce all ⁴⁰Ar that would have otherwise not have been present there.

The rubidium–strontium dating is based on the natural decay of ⁸⁷Rb to ⁸⁷Sr and the different behaviour of these elements during fractional crystallization of magma. Both Sr and Rb are found in most magmas; however, as fractional crystallization occurs, Sr will tend to be concentrated in plagioclase[1] crystals while Rb will remain in the melt for a longer time. ⁸⁷Rb decays in magma and elsewhere so that every 1.42×10¹¹ years half of the amount has been converted into ⁸⁷Sr. Knowing the decay constant and the amount of ⁸⁷Rb and ⁸⁷Sr in a rock it is possible to calculate the time that the ⁸⁷Rb must have needed before the rock reached closure temperature to produce all ⁸⁷Sr, yet considering that there was an initial ⁸⁷Sr amount not produced by ⁸⁷Rb in the magmatic body. Initial values of ⁸⁷Sr, when the magma started fractional crystallization, might be estimated by knowing the amounts of ⁸⁷Rb and ⁸⁷Sr of two igneous rocks produced at different times by the same magmatic body.

Stratigraphic principles may be useful to determine the relative age of volcanic rocks. Tephrochronology is the most common application of stratigraphic dating on volcanic rocks.

In petrology the mineral clinopyroxene is used for temperature and pressure calculations of the magma that produced igneous rock containing this mineral. Clinopyroxene thermobarometry is one of several geothermobarometers. Two things make this method especially useful: first, clinopyroxene is a common phenocryst in igneous rocks easy to identify; and secondly, the crystallization of the jadeite component of clinopyroxene implies a growth in molar volume being thus a good indicator of pressure.