APPLICATION OF PXRF TO RAPIDLY IDENTIFY METASOMATIC EFFECTS: AN EXAMPLE FROM THE NEOPROTEROZOIC CATOCTIN FORMATION IN CENTRAL VIRGINIA

The application of portable X-Ray Fluorescence (pXRF) for in-field lithologic analysis has been expanding over the past decade. Using pXRF, semi-quantitative whole rock geochemical data may be acquired for each sample in ~120 seconds. This rapid analytical method has some limitations, which are instrument and element specific. The pXRF data were collected using a Thermo Niton XL3t GOLDD+ for both powder and whole rock samples.

The first goal was to determine which specific elements could be used with validity from whole rock pXRF data. This was accomplished by comparing collected pXRF data to XRF, ICP-MS, and INAA data for known United States Geological Survey (USGS) and National Bureau of Standards (NBS) standard reference materials. In addition, Catoctin Formation metabasalt samples with major and trace element geochemistry were utilized as working standards. Major oxides SiO₂, TiO₂, Fe₂O₃, MnO, CaO, and K₂O and trace elements Cu, Rb, Sr, V, and Zr were statistically valid and used in the following applications. Specific elements with a ± 20% deviation from USGS or NBS reported values were considered invalid for analytical interpretations. Based on this criterion, major element oxides Al₂O₃ and MgO and trace elements Ba, Sc, Y, and Zn were excluded.

The second goal was to apply pXRF analytical techniques to field samples collected from Neoproterozoic Catoctin Formation metabasalt in the Blue Ridge anticlinorium. The Catoctin Formation contains metavolcanic and metasedimentary rocks at middle to upper greenschist facies. The metamorphic mineral assemblage for the metabasalt rocks includes albite + chlorite + epidote + sphene + ilmenite ± actinolite ± quartz ± calcite. Whole rock pXRF data from metabasalt rocks, compared to average N-MORB data, were plotted using geochemical interpretation diagrams including isocon (Grant, 1986) and spider plots (Sun and McDonough, 1989). Major element oxides CaO, SiO₂, P₂O₅, and TiO₂ and trace elements Rb and Sr are mobilized through hydration reaction equilibrium. Isocon and spider diagrams show the degree of metasomatism when an altered sample is compared to an unaltered sample (average N-MORB). To test the validity of the pXRF results related to mobility, the samples were also analyzed for whole rock major, trace, and RE element by methods comparable to USGS and NBS standards.