History - First Generation

In the mid 1970s a revolution was taking place whereby chemical separation of rocks and minerals was being replaced by in situ analysis. Electron microprobes had been developed that allowed chemical compositions to be determined in minerals through the measurement of the intensity of chemical specific, characteristic X-rays. The electron probe performs well for major elements, but background radiation restricted the measurement of trace elements. The logical extension was the adoption of Secondary Ion Mass Spectrometry (SIMS) to allow measurement of low abundance elements.

The cornerstone of SIMS is the sputtering of material with a primary ion beam to produce a secondary ion beam. This ion beam can be analysed in a mass spectrometer for ionic abundance. Elemental abundances can be determined limited only by counting statistics. However, isotopic compositions can also be obtained. Decay of elements such as U and Th to Pb isotopes can be related to geological time.

The first ion microscopes were small instruments that had low sensitivity and low mass resolution. In order to carry out geochronological analyses, the abundance of 204Pb has to be accurately measured and the abundance of this isotope can be very low (attogram levels). The smaller ion microscopes could not carry out this measurement.