High-accuracy analyses of key minor and trace elements in zircon by electron probe microanalysis

Abstract

Beyond its geochronological potential, zircon geochemistry is increasingly used not only for estimating formation temperature or identifying rock type and origin, but also for distinguishing magmatic, metamorphic and mineralization processes. Minor and trace elements (e.g., Al, P, Ti, Y, Yb, Lu, Hf, Th, and U) in zircon are informative, but high spatial resolution microanalysis techniques are urgently needed to address the limited size of zircon grains. Here, we developed a new EPMA method to determine minor and trace elements in zircon with high precision and accuracy. Detection limits and precision can be improved to several ppm level (e.g., Ti, 9 μg g⁻¹, 3σ) by using high acceleration voltage and high beam current combined with long counting time. The use of matrix-matched reference materials (GJ-1, Tanz, and Qinghu zircon) with well-characterized trace elements of interest is important for improving and monitoring the analytical accuracy. Careful background offsets and background regression models need to be obtained via high-sensitivity WDS scan on each target element. An exponential background regression model was applied to Ti, Al, Th, and U, whereas other elements required linear background regression. In addition, adjusting the calibration standard for Al and suppressing spectral interferences (e.g., P, Y, and Yb) enabled highly accurate EPMA measurements of trace elements below 1000 μg g⁻¹. The spatial resolution of EPMA in zircon analysis, even under extreme conditions (20 kV and 500 nA), remains below 3 μm, surpassing that of laser ablation inductively coupled plasma mass spectrometry. Using our protocols, we have successfully measured the contents of minor and trace elements in zircons from the Chang'E-6 lunar anorthosite sample. Overall, this improved EPMA method could broaden the applications of zircon composition to geological evolution processes of the Earth and the Moon.