Earth dating by ion probe



The ion microprobe is an instrument that is finally coming into its own in isotope and trace-element analysis of particulate mineral samples. The idealized ion microprobe would be able to analyze sample volumes of less than one cubic micrometer. The analysis would include major-element bulk composition and the chemical formula of the mineral being analyzed. More essential, the analysis would also contain trace-element composition and isotopic abundances because the ion microprobe employs a mass spectrograph as its analytical device. Ideally then, an investigator would be able to obtain major-, minor-, and trace-element data and be able to date geologically a small portion of a mineral crystal instantaneously and simultaneously. That this ideal is being realized today is evident in the recent results of W. Compston and colleagues at the Australian National University. Compston et al. have dated zircon crystals from Mount Narrayer in western Australia at 4.2 billion years, the oldest date of a terrestrial sample (New Scientist May 19, 1983). How these dates will stand up vis a vis the oldest whole-rock dates (3.8 billion years for samples from West Greenland) remains to be seen. The Mount Narrayer date is for single grains extracted from a sedimentary rock. The Greenland whole-rock dates were obtained by more conventional, though nonetheless state-of-the-art, mass-spectrometry techniques.