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Since construction of the first SHRIMP ion microprobe in the late 1970s demonstrated that it was possible technically to achieve high mass resolution and high sensitivity simultaneously by using a large secondary ion mass analyzer (magnet radius 1 meter), studies using ion microprobe analyses have revolutionized geology. The biggest impact has been on geochronology. The ability of large ion probes to measure the isotopic composition of trace quantities of Pb in scarce, fine grained accessory minerals such as zircon, monazite, titanite and xenotime has made it possible, not only to measure the formation ages of rocks with high precision and accuracy, but also to study their origins and thermal histories. This vital information is preserved in many ways for example, in mixed populations of grains of different ages in single rocks, in the multiple growth layers within single crystals, and in the thin overgrowths of new minerals that form during hydrothermal events. Ion probes have also had a major impact on studies requiring high precision, high spatial resolution, analyses of stable isotopes. The pioneering research on S isotopes by Eldridge and his co-workers using SHRIMP I demonstrated for the first time the enormous range of S isotopic compositions that could be present within different minerals in a single rock or within crystals of the same mineral grown at different times.