Uses of Strontium Isotopes in Petrogenesis

Abstract

The isotopic composition of strontium is not only a useful indicator of the ages of rocks and minerals, but it also contains information about the origin of igneous rocks and about the geologic processes that have affected their chemical compositions. We showed in the previous chapter that the Rb-Sr isochron method of dating leads not only to the determination of the age of a suite of comagmatic igneous rocks, but also indicates the “initial ratio,” which is the ⁸⁷Sr/⁸⁶Sr ratio of the magma from which the rocks crystallized. Its value depends on the previous history of the strontium and reflects particularly the Rb/Sr ratios of systems in which the strontium had previously resided. In general there is no way of knowing, in a particular case, in how many systems the strontium previously resided, what the Rb/Sr ratios were in those systems, or how long the strontium resided in each system. We can deal rigorously only with single-stage histories by assuming that the strontium was part of only one previous system for a specified period of time before being incorporated into a particular magma. Although these assumptions undoubtedly oversimplify the real situation in many instances, the approach has shed light on the origin of igneous rocks.