Wavelength-dispersive spectral analysis and quantification of monazites by electron-probe microanalysis

The problem of determining the chemical composition of monazite grains through electron probe microanalysis is studied, by using a scanning electron microscope with a wavelength dispersive spectrometer. A careful qualitative analysis is performed with the purpose of determining all the elements present in the samples, the lines to be used in the quantifications trying to minimize interferences, the angular positions and the acquisition times for the measurement of peak and background intensities and the crystals to be used. Particular emphasis is devoted to the analysis of Th, U and Pb, which are used to determine the age of the rock by means of the U–Th–Pb method, commonly used in geochronology. Quantitative determinations of the chemical composition of monazite grains are performed, optimizing the experimental conditions on the basis of the qualitative analysis. The determinations were made under two different criteria of quantification of oxygen, and the dissimilar results obtained are discussed.