For many years it has been possible to measure accurately the small-scale variations in the major and minor element chemistry of minerals by the electron microprobe technique, in which the sample surface is bombarded with an electron beam and the X-ray signal is measured (Chapter 2). Abundances of isotopes, trace and ultra-light elements (e.g. H and Li) which cannot be determined by this method can, however, be measured by secondary ion mass spectrometry (SIMS; Shimizu, Semet and Allègre, 1978; Shimizu and Hart, 1982; Reed, 1989; Zinner, 1989). The ion microprobe is a SIMS instrument with a focused primary ion beam, which permits in situ microanalysis of minerals in samples prepared as standard polished sections. Nearly all elements from H to U can be detected and many can be analysed quantitatively down to part-per-million levels, or below.
