Electron Probe Microanalysis in Mineralogy

Electron probe microanalysis (EPMA) is a technique used to determine the chemical composition of materials at the micrometer scale. The instrument (“electron-probe,” or often simply known as the “probe”) is a variant of a scanning electron microscope (SEM). The detection of characteristic X-rays, generated by inner shell ionization of the material, permits both quantification of elemental concentrations and illustration of their distribution across the sample by the production of X-ray maps. Mineralogy has played an important role in the development of EPMA since its development in the 1950s and the technique is now routinely employed in hundreds of Earth Science research laboratories around the world. A key component of the EPMA is the wavelength dispersive spectrometer (WDS) which displays much better X-ray resolution than the cheaper and more accessible energy dispersive detector (EDS) commonplace on SEMs. WDS permits high precision analysis of major and minor elements and in some instances it can accurately measure trace elements to < 10 ppm. Chemical age dating of monazite and other minerals can be achieved with WDS using EPMA. An important beneficiary of the technique is the high-pressure experimental petrology community where the grain sizes of run products are often much smaller than those found in nature, requiring the highest resolution analysis possible with modern field emission gun instruments (FEG-EPMA).