Chemical Properties and Isotope Systematics

Abstract

The origin of igneous rocks is now understood in considerable detail based on interpretations of mineralogical, chemical, and isotopic data. The insights that have been gained about the petrogenesis of different kinds of rocks have enhanced our understanding of how the Earth works on a global scale, and how it has evolved since it formed about 4.5 × 10⁹ years ago. The evidence that has provided these insights has come from many sources, including the concentrations of trace elements and the isotope compositions of strontium (Sr), neodymium (Nd), lead (Pb), hafnium (Hf), and osmium (Os) all of which have radiogenic isotopes. In addition, a complete listing of all sources of isotopic data includes certain elements of low atomic number (e.g. H, C, O, N, and S) whose stable isotopes are fractionated by physical, chemical, and biological processes. The resulting theory explains igneous activity as a manifestation of the complex dynamic processes that occur in the mantle of the Earth. The objective of this book is to use the isotope compositions of Sr, Nd, and Pb in volcanic and plutonic rocks to demonstrate how isotopic data have been used to explain the origin of igneous rocks. This chapter contains brief summaries of relevant facts and theories that will facilitate the scientific journey we are about to undertake. Additional information is available in a textbook by Faure (1986).