DEVELOPMENT OF A U-TH-PB MONAZITE DATING PROCEDURE USING EPMA AT VIRGINIA TECH

Monazite is a common accessory mineral found in igneous and metamorphic rocks, which can be dated due to its relatively high concentrations of U and Th. While monazite is typically dated using laser ablation- and ion probe-based mass spectrometry, it has also been dated successfully using electron probe microanalysis (EPMA), which is less precise but provides superior spatial resolution compared to these other methods. We have been developing a monazite dating protocol on a new JEOL JXA-iHP200F microprobe at Virginia Tech using procedures for the measurement of U, Th, Pb, and Y described previously in the literature and well-characterized monazites with known ages to constrain the likely uncertainty that users can expect. Initial analysis of three samples previously studied for use as standards and an additional low-actinide, young sample acting as a “blank” were used to build an empirical calibration curve to for uncertainty assessment.

Because EPMA is a spectroscopic method that performs elemental quantification using characteristic X-rays, treatment of elemental interferences (e.g. Y Lγ vs Pb Mβ) and background fitting (e.g. curved vs linear background models) are fundamental considerations affecting our results. Wavelength scans were conducted around the peak and background collection points for Th, U, and Pb for each standard to check for possible interferences from higher-order X-ray lines. Background correction was done through linear interpolation on each point measured. This final procedure yields results that are generally accurate to 10s of Ma, though the discrepancy is generally lower for some Phanerozoic and late Proterozoic samples. We found limited improvement to the accuracy of the method from using longer analysis times at higher beam power, which suggests that methodological uncertainty provides a first order limitation on the accuracy of monazite ages obtained using EPMA, and errors introduced by X-ray counting statistics are less significant. Ongoing work aims to optimize sample throughput and minimize analytical error to provide the SE US petrologic community with another resource for the rapid and cost-effective dating of monazite at sufficient accuracy and precision to resolve major trends.