[S-GC38] Solid Earth Geochemistry, Cosmochemistry

[SGC38-P01] Primary issues in major element characterization of ultramafic rocks: revisiting the matrix effect in XRF analysis and reference values of igneous reference materials

*Kazuto Mikuni^1,2, Akira Ishikawa^1,3, Ryu Sato¹ (1.Department of Earth and Planetary Sciences, Institute of Science Tokyo, 2.Geological Survey of Japan, AIST, 3.JAMSTEC)

Keywords:XRF, Ultramafic rock, Peridotite, Matrix effect, Geochemical reference material

The dynamics of the Earth's mantle, such as partial melting and deformation, have been directly inferred and unraveled by examining peridotites. Among the various types of information that can be extracted from mantle-derived rocks, whole-rock chemical composition is one of the most fundamental and important information to characterize their origin. Whole-rock major and selected minor elements are routinely quantified through X-ray fluorescence (XRF) analysis, and the quantification of elemental concentrations in igneous samples by XRF is primarily conducted using the calibration curve method with multiple reference materials. While analytical precision and sensitivity largely depend on the instrument, the accuracy of the quantification, which is fundamental to the results, depends on the accuracy of the calibration curve. The accuracy of the calibration curve, in turn, is influenced by the certified values of the utilized reference materials and the matrix effects. Due to the limited availability of ultramafic reference materials with certified major element values (e.g., JP-1), it is necessary to include other igneous rock reference materials in the calibration. Meanwhile, the reference values and the matrix effects have not been thoroughly considered, which could be problematic for the accurate quantification of whole-rock geochemical compositions of peridotites.

Here, we focused on (1) the matrix effect observed in X-ray fluorescence of total iron (T-Fe₂O₃) of ultramafic rocks compared to other igneous rock matrices and (2) the selection of appropriate reference materials and associated certified values to optimize the major elements, Cr, and Ni analysis for ultramafic rocks by XRF. The issue (1) has not been widely recognized, likely because even if ultramafic rocks exhibit unique matrix effects, the number of ultramafic reference materials included in the calibration curve has been very limited (often only JP-1). As a result, these effects may have either been overlooked as negligible or unintentionally corrected through whole-element matrix collection. Furthermore, regarding issue (2), which has a primary and direct impact on the accuracy of the calibration curve as well, minor elements such as Cr and Ni require careful selection of appropriate values, as multiple data exist for certain reference materials. We use the four certified and an in-house ultramafic reference materials: JP-1, JP-2, OKUM, UB-N, and GP13 to investigate aforementioned issues.