PXRF calibrations for volcanic rocks and the application of in-field analysis to the geosciences

Advances in portable X-ray fluorescence (pXRF) technologies have made such instruments an attractive choice for geologists who wish to collect chemical data in the field. The acceptance of pXRF data for research purposes such as, for example, petrogenetic studies by the geologic community has been hampered by a perception of poor quality. Here, we show that current-generation pXRF instruments can yield high-quality data when deployed either in the laboratory, or as a base-camp field instrument. We have developed four calibrations for major, minor and trace elements in geological materials using minimal sample preparation: one for the broad range of geologic compositions (0-100% SiO2) using rock powders, and three with an emphasis on volcanic rock samples: rock billets ranging from primitive basalt to rhyolite, basalt rock powder, and rhyolitic pumice powder. The calibrations use influence coefficients to correct for absorbance and secondary enhancement of one element by X-rays of other elements present in the sample. Nugget effects can have a significant influence over the determination of the concentration of elements such as Zr if proper care in sample homogenization is not taken. We evaluate data quality for each element in each calibration, and find that for several elements (Ti, Mn, Fe, Ca, K, P, Cr, Ni, Zn, Rb, Sr, Y, Zr, Nb), precision and accuracy approach those obtainable from full-size wavelength-dispersive XRF analysis. Maximization of data quality is achieved by matching of calibration material types and unknowns.