Inter- and Intracrystalline Isotopic Disequilibria: Techniques and Applications

Abstract

In the last ~20 years, we have seen a significant expansion of techniques related to geochemical and isotopic microsampling of materials in the earth sciences. From constraining pre-eruptive histories of flood basalt magmas to identifying the natal rivers of origin of anadromous fishes, these techniques have had significant impacts in a wide variety of scientific fields. Nowhere has the impact been greater than in identifying the sources, processes, and timing of processes involved in igneous magmatic systems. Both technique refinements and the development of new technologies have aided in advancing microsampling applications, thus allowing for a better understanding of the sources and mechanisms responsible for changing geochemical and isotopic signatures in natural systems. In this chapter, we focus on the techniques and technologies associated with radiogenic isotope microsampling and review applications of these techniques as utilized in scientific investigations. Isotope microsamping is a logical extension of earlier studies that evaluated individual components of magmas and magma systems, including melts and minerals. From the use of petrographic microscopes and the later introduction of the electron microprobe, the focus on internal chemical variations in melts and minerals is critical to assessing the petrogenetic histories of igneous rocks. Even today, these technologies are used to ensure that further trace element and isotopic analyses are undertaken in a textural and major element context. For trace elements and isotopes, early studies (e.g., Cortini and van Calsteren 1985) confirmed variations in the melt and mineral components of many igneous rocks but focused on mineral or glass separates. Potential information associated with isotopic variations retained by individual crystals or internal variations within individual crystals was lost. Later studies such as Geist et al. (1988) focused on isotope variations within single megacrysts to constrain mixing scenarios between basaltic, andesitic, and rhyolitic magmas, and Davidson et al. (1990) used …