TOWARDS ONE LAB’S ACCURATE RESOLUTION OF WHOLE-ROCK MAJOR-ELEMENT GEOCHEMESTRY: AN UNDERGRADUATE EFFORT

As part of an intensive summer undergraduate research program, the authors created improved standard materials for quantitative analysis through a benchtop wavelength-dispersive x-ray fluorescence (WDXRF) instrument (a Rigaku Supermini 200). Although its low fixed power and Pd tube limit versatility, instrument drift is minimal, and the compact design and ease of operation are amenable to inexperienced users. Its qualitative and semi-quantitative modes permit new operators to learn about developing precise and accurate assessments. And with proper standardization, the instrument can acquire accurate values for major elements from fusion beads.

The MSU Texas lab uses ultrapure 65.75% LiBO₄, 33.75% Lithium LiBO₂, 0.5% LiI (wetting agent) in a 5:1 flux:rock ratio. The authors labored to replace a legacy standard set made with a slightly different (and somewhat unconstrained) bead recipe. Although the legacy set produced values within Cat 2 RSD = 0.2c^0.8495 (Thompson et al., 1996, Geostandards Newsletter) for SiO₂, Al₂O₃, CaO, K₂O and P₂O₅ using a fundamental-parameters calculation, it required an additional linear offset bias using a subset of standards produced with the above recipe.

USGS and GSJ SRM were dried overnight and then fired for LOI; the fired powder was then homogenized with flux and fused into a 30 mm disc. The new standard set was evaluated through semi-quantitative analysis to confirm approximate composition. The set was then used in a new empirical analytical routine; matrix corrections employed LaChance theoretical alpha parameters. The calibration set was then evaluated as unknowns to verify re-acquisition of their values, and separate internal standards provided further verification of the routine. The new set removes the need for bias correction and increased the accuracy to Cat 2 or better RSD for the remaining major elements.