Application of μXRF analysis on the Upper Cretaceous Mancos Shale: A comparison with ICP-OES/MS

This study investigates the elemental signature of fine-grained sedimentary rock (mudstone) obtained using energy-dispersive micro-X-ray fluorescence (μXRF). A short section (28.5 cm) of Upper Cretaceous Mancos Shale was analysed with an Itrax μXRF core scanner at 0.2 mm intervals for comparison to elemental and petrographic composition. Fifteen subsamples were collected at 2 cm intervals for ICP-OES/MS analysis to obtain absolute concentrations of elements and oxides, and thin sections were used for petrographic and microfacies analysis. The data obtained from the Itrax core scanner show a high level of reproducibility between separate scans and moderate to strong Spearman-Rho correlation (R2 > 0.6) to ICP-derived concentrations for several oxides: Fe2O3, SiO2, Al2O3, K2O, TiO2, and MnO. Common elemental ratios used in palaeoenvironmental studies (e.g., Zr/Rb, Ca/Ti, Mn/Ti) also showed good correlation to ICP-derived data when the trends were compared. The elemental signature obtained from the Itrax also showed good relationships to the petrography and lithology, with the Si/Ti ratio accurately identifying quartz-rich layers, and the Ca/Ti and Mn/Ti ratios corresponding to increased carbonate content, which are of importance in the planning of hydraulic fracturing for unconventional resources. At core scale, the elemental trends in Zr/Rb, Ti, and Si were useful for detecting short-term, rapid patterns of sedimentary events, likely resulting from storm activity (e.g., tempestites, wave-enhanced sediment gravity flows). The high-sampling resolution data from μXRF core scanners can provide advantages over traditional geochemical techniques since data aliasing is minimized. Analysing these elements and ratios in longer core sections from the Mancos Shale may serve as a proxy for determining high-frequency climatic changes in the Cretaceous Interior Seaway and allow for improved correlations of intra- and inter-basinal events in other Cretaceous basins.