The quantification and application of handheld energy-dispersive x-ray fluorescence (ED-XRF) in mudrock chemostratigraphy and geochemistry

Abstract

Traditionally, analytical techniques such as wavelength-dispersive x-ray fluorescence (WD-XRF), inductively-coupled plasma (ICP) and mass spectrometry (ICP-MS), instrumental neutron activation analysis (INAA), or stationary (bench-top) energy-dispersive x-ray fluorescence (ED-XRF) have been used to generate quantitative geochemical results. However, a more efficient means of data collection using portable ED-XRF instrumentation now allows the investigator to acquire rapid, non-destructive, quantitative measurements on drill core and clean, flat rock surfaces, in addition to pressed powder pellets typically used in WD-XRF analysis. Similar to traditional XRF methods, quantification using the handheld ED-XRF requires a matrix-specific calibration. Unfortunately, very few internationally-accepted mudrock or shale reference materials exist, and their elemental ranges provide inadequate coverage for the geochemical diversity of mudrocks. In order to return reliable, calibrated results, a unique set of reference materials has been developed that incorporates a wide range of mudrock elemental compositions. The current method provides elemental calibrations for major elements heavier than sodium, and the following trace elements: Ba, V, Cr, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Mo, Th, and U.A comparison of handheld energy-dispersive and wavelength-dispersive x-ray fluorescence (ED-XRF and WD-XRF) results from pressed powder pellets of Mississippian-age Barnett Shale of North-Central Texas, USA, is presented in order to evaluate the reliability of the reference calibration and the quantification of unknown samples using two different instrument platforms. It will be demonstrated that calibrated results from the handheld ED-XRF effectively define chemostratigraphic changes in real-time. As a consequence, quantified results can be used immediately to assess changes in bulk mineralogy, paleo-redox conditions, and to link down-core geochemical changes to stratigraphic, sedimentological, and paleoenvironmental observations.