U–Th isotopic microanalysis of zircon reference materials and KBSI working standards

Abstract

Abstract Background The 238U–230Th disequilibrium dating of the mineral zircon (ZrSiO4) provides an efficient tool for investigating the time scales of Quaternary magmatic processes. In situ mass spectrometric U–Th microanalysis of zircon requires careful calibration and correction of the measured isotope data, particularly for the instrumental fractionation of U and Th isotopes. Findings For the selection of suitable calibration materials for U–Th isotopic analysis using a laser ablation multiple collector inductively coupled plasma mass spectrometer (LA-MCICPMS), we estimated the homogeneity of four reference zircons (91500, TEMORA 2, FC1, and Plešovice) and two zircon working standards (LKZ-1 and BRZ-1) in terms of their 232Th/238U ratios, based on the Pb isotopic compositions measured by a sensitive high-resolution ion microprobe (SHRIMP). The measured LA-MCICPMS 232Th/238U ratios of the zircons were calibrated externally using the SHRIMP 208Pb/206Pb-based average value of the 91500 zircon, 232Th/238U = 0.351 ± 0.035 (error corresponds to 1 standard deviation). The molecular interference-corrected 230Th/232Th ratios of the zircons were calibrated based on the assumption that the Plešovice zircon is in 238U–230Th secular equilibrium. After the calibration and correction, the activity ratios of 230Th/232Th and 238U/232Th for all reference zircons and working standards were plotted on the equiline. Conclusions This study confirms that the 91500 zircon is relatively homogeneous in terms of U/Th ratios (relative standard deviation = ~ 10%) and does not support a recent claim that the Plešovice zircon is not in 238U–230Th radioactive equilibrium. The working standards LKZ-1 and BRZ-1 can be used to check the reliability of U–Th isotopic analyses for Quaternary zircons.

Keywords

• Zircon standard
• U–Th isotopes
• Uranium-series disequilibrium dating
• Laser ablation-ICPMS