Measuring Pb, Th, and U inter-element ratios in geological materials using extreme ultraviolet laser ablation and ionization mass spectrometry

Abstract

Extreme ultraviolet laser ablation and ionization time-of-flight mass spectrometry (EUV TOF), using a laser that operates at a wavelength of 46.9 nm (26.4 eV photon energy), is a relatively new analytical technique with many unexplored performance characteristics. In this work, we use the EUV TOF to directly measure the ²⁰⁶Pb/²³⁸U and ²³²Th/²³⁸U isotope ratios in different geological materials to investigate if the EUV laser is a good candidate for reducing matrix effects in geological analyses. We use the EUV TOF to analyze synthetic glasses of silicate, basalt, and phosphate as well as mineral samples ranging from iron manganese to zircons and monazites to uraninites and a thorite at a spatial scale of approximately 8 μm and at depths ≤500 nm. The results show that the EUV TOF measures ²⁰⁶Pb/²³⁸U ratios that have a systematically low bias in glass, iron manganese, zircon, and monazite matrices. This bias can be calibrated using NIST 610 as a non-matrix matched external calibration sample, resulting in ²⁰⁶Pb/²³⁸U ratios that are within analytical uncertainty (±2σ) of the expected values. Unlike ²⁰⁶Pb/²³⁸U, the ²³²Th/²³⁸U ratios measured in the glass, zircon, and monazite matrices do not show a systematic bias and are within ±2σ of the expected values from direct measurements. The ²⁰⁶Pb/²³⁸U and ²³²Th/²³⁸U ratios measured in the uraninite and thorite samples are not within analytical uncertainty of the expected values. The uncertainty in the uraninite and thorite measurements are likely a result of the high amounts of Pb, Th, and U in these samples relative to the other analyzed samples and their effect on the instrument's performance. Overall, this study shows that the EUV ionization mechanisms for Pb, Th, and U are similar in glass, iron manganese, zircon, and monazite matrices, resulting in inter-element ratios that are within analytical uncertainty. EUV laser ablation and ionization is therefore a good candidate for directly measuring the isotopic content in select geological materials at spatial scales of <10 μm.