The geological microprobe: The first 25 years of dating zircons

Andersen and Hinthorne reported in situ Pb isotope ages for lunar minerals in the early 1970s. 'Peak stripping' was used to subtract isobaric interferences but it worked only for old, U-rich targets. Jim Long entered the race for in situ mineral dating using high resolution to separate isobars. Despite low sensitivity, he achieved ²⁰⁷ Pb/ ²⁰⁶ Pb dating of zircons in the late 1970s. In 1980 we finished the large instrument SHRIMP I based on the magnetic analyser design of Matsuda that delivered both high resolution and high sensitivity. A year later we found zircon grains 4200 million years old, far older than any previous terrestrial age. Certain in situ isotope studies remained inaccessible, such as Sr isotopes in Ca-rich minerals. All need 20,000 R with high sensitivity to accumulate enough counts for worthwhile precision. In 1990, Matsuda published new reverse-geometry designs that have still higher resolution, one of which we completed in 1997. We achieved 20,000 R but so far only at reduced sensitivity. Its full potential will be realised after further testing including computer simulations.