Zoning in granitoid accessory minerals as revealed by backscattered electron imagery

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Abstract
Accessory minerals are often difficult to investigate with light optics as the mineral grains tend to be small and the refractive indices high. Textural features due to variations in composition are well displayed in such minerals by backscattered electron imagery under circumstances designed to select only the composition contribution to electron backscattering and displayed as atomic number (Z)-contrast imagery (ZCI). It is shown by this technique that compositional zonation patterns are very common and sector zoning in titanite is described for the first time. The compositional basis for zonation of titanites in this study is shown to be controlled by coupled substitutions involving the REE. The technique is particularly good at revealing rounded cores to zircon grains which are normally taken to be refractory grains from the magma source region, and ZCI studies may improve targeting of grains for U-Pb geochronological investigations. Several examples are presented of applications of the technique to accessory minerals encountered in polished thin sections of granitoids in the Caledonian of Scotland. The consequences of ZCI studies for trace element modelling of REE in granitoid petrogenesis are discussed.