A method of quantitative analysis of trace elements in silicate rocks by energy-dispersive X-ray fluorescence spectroscopy

Abstract

Fourteen rock samples were analyzed to evaluate the precision and accuracy of an automated energy-dispersive x-ray fluorescence analytical method. The samples were prepared as loose powders, then analyzed for 14 trace elements (Rb, Sr, Ba, Zr, Nb, La, Ce, Y, Cr, Ni, Cu, Mo, Sn, Zn). Each sample was analyzed 10 times. Intensities for Ni, Cu, Zn and Sn were determined by integrating the net peak area; intensities for the remaining elements were determined using a gaussian-curve fitting routine. Resulting intensities were ratioed to either Compton or Compton and Rayleigh scatter intensity. Elemental concentrations were calculated by a least-squares fitting method using simple linear regressions derived from a variety of standard rock samples. Overall analytical precision is estimated at + 2% relative for elemental concentrations above 100 ppm. Our results are generally in agreement with data obtained by methods of analysis and differ by 100 ppm. Estimates of the precision and relative agreement with methods indicate that routine trace element analysis by energy-dispersive X-ray fluorescence is fast, reliable, and efficient. Introduction Many geologic studies rely on large-scale reconnaissance sampling. An example of this approach is the effort to assess the potential for economic concentrations of mineral deposits in large areas of the United States. Reconnaissance geochemical sampling in such studies requires analytical methods capable of producing multi-element analyses of large numbers of samples quickly and reliably. Energy dispersive x-ray fluorescence spectroscopy (EDXRF) is ideally suited for such projects because in this method of analysis the time required from sample preparation to production of data is relatively short. In addition, this method is non-destructive, and has been shown to be relatively precise and accurate for quantitative analysis of geologic samples (Johnson 1984, 1987; Terashima 1987). EDXRF results can thus be used as a preliminary screening step to select samples for more detailed analytical work. This report evaluates the precision of an automated trace-element analysis routine implemented on an energy dispersive x-ray fluorescence spectrometer and analyzer system (Kevex 0700/8000) , at the Branch of Eastern Mineral Resources, Reston, Virginia. In this report, an analytical method is presented which is fast, efficient, and precise for screening economically and petrologically important elements from a wide variety of felsic rock compositions such as would be obtained during reconnaissance geochemical sampling. Comparison of this automated trace element technique demonstrates good agreement with analytical determinations by techniques. Summary of Sample Preparation and Analytical Method Fourteen rock samples representing a diverse group of felsic rock types were selected to demonstrate that the method is widely applicable (table 1, see page 4). Each sample was analyzed 10 times for 14 elements. These data, along with the means and standard deviations, are shown in Appendix A. Also shown in Appendix A under the heading other methods, are the analytical results for these samples as reported by several