A new carbonate-specific quantification procedure for determining elemental concentrations from portable energy-dispersive X-ray fluorescence (PXRF) data

Abstract

Portable energy-dispersive X-ray fluorescence (PXRF) analyzers are routinely used to generate large elemental datasets in sedimentary strata. These data provide the basis for assessing stratigraphic changes in geochemistry, bulk mineralogy, and paleo-redox conditions. The manufacturer provided procedure used to calculate elemental concentrations from PXRF photon counts that was developed for siliciclastic mudrocks. This quantification is, however, routinely applied to other sedimentary rocks, such as carbonates, which often have very different elemental concentrations and rock textures. The current study reports elemental concentration data from 57 limestone and dolomite rock samples measured by both PXRF and conventional geochemical analyses including inductively coupled plasma-mass spectrometry (ICP-MS), inductively coupled plasma - optical emission spectrometry (ICP-OES), and wavelength-dispersive X-ray fluorescence (WD-XRF). Carbonate samples were subdivided into two groups. The calibration set (N = 43) is used to establish a carbonate-specific PXRF quantification procedure, and to investigate the effects of different sample preparation techniques. The validation set (N = 14) is used to evaluate the applicability of both the newly developed carbonate-specific quantification procedure and the existing mudrock quantification procedure for the carbonate rock suite.PXRF measured photon counts in the calibration set exhibit strong positive linear correlations (R2 = 1–0.76) with the elemental concentrations independently measured from ICP-MS and/or ICP-OES, and WD-XRF. These linear relationships were used to calculate elemental concentrations in the validation set, which were then statistically compared with the measured elemental concentrations. Results of the carbonate-specific quantification procedure show that 13 elements (Mg, Al, Si, P, K, Ca, Ti, Mn, Fe, Rb, Sr, Y, and Zr) in the carbonate validation set are quantified with PXRF at the definitive data quality level. The mudrock-based quantification procedure, in contrast, quantified only 4 elements at the definite data quality level (Si, K, Ca, and Zr), suggesting that carbonate-specific calibration should be used to ensure higher data quality. The results from the sample preparation experiments exhibit different linear regressions for whole rock, loose powders, pressed powder pellets, and fused discs.The results of this study show that (i) PXRF can reliably identify and accurately quantify the concentrations of 13 common rock-forming elements in carbonate rocks; (ii) the carbonate-specific quantification procedure provides a higher level of data quality for carbonate rocks compared to the mudrock-based quantification; (iii) carbonate rock samples are inherently heterogeneous and thus require more intensive sample preparation and advanced statistical methods to account for this in the quantification procedure; and (iv) the matrix effect that results from different sample preparations must be accounted for in the quantification procedure.