- © The Mineralogical Society Of America
Secondary ion mass spectrometry (SIMS) is a versatile technique for measuring the chemical and isotopic composition of solid materials on a scale of a few microns. A beam of high-energy primary ions is focused onto the polished target surface, sputtering (ablating) atoms and molecules, and in the process ionizing a small fraction. These secondary ions, which reflect the target composition, are analyzed by mass spectrometry. The SIMS instrument most common in geoscience is the ion microprobe, which uses a focused primary ion beam in either static or scanning mode to sample target areas usually 10- to 50-μm diameter. Total sampling depth is typically less than 5 μm and the sampled mass only a few nanograms, making the analysis for most samples effectively non-destructive. Coupled with surface imaging techniques such as backscattered electron (BSE) and cathodoluminescence (CL), SIMS enables finely targeted chemical and isotopic analysis of specific domains exposed on a crystal surface.
Zircon is particularly suitable for SIMS U–Th–Pb geochronology. It is a physically and chemically robust mineral that crystallizes under a range of geological conditions, incorporating trace U and Th, but little or no Pb. Zircon grains are commonly composite, having survived and grown during several geological events. This growth record is sometimes visible under an optical microscope, but is best revealed by CL and BSE imaging of polished sectioned grains (Fig. 1⇓). The U–Th–Pb closure temperature of unaltered zircon is very high (>900°C), and the growth domains commonly can preserve an isotopic record of thermal events spanning tens to thousands of millions of years. This record of provenance, and igneous and metamorphic history, is accessible only to microanalytical techniques such as SIMS.