Procedures for accurate U and Th isotope measurements by high precision MC-ICPMS

Abstract

We present multi-collector (MC) inductively coupled plasma mass spectrometry (ICPMS) protocols developed to obtain high precision, accurate determinations of U and Th isotope ratios that are applicable to a wide range of geological materials. MC-ICPMS provides a means to make high precision measurements but a recent laboratory inter-comparison, Regular European Inter-laboratory Measurement Evaluation Programme (REIMEP)-18, indicates that accurate results for U isotope ratios are not currently achieved by all facilities using MC-ICPMS. We detail a suite of protocols that can be used for a wide variety of U and Th isotope ratios and total loads. Particular attention is devoted to instrument optimisation, instrumental backgrounds, stability and memory effects, multiplier nonlinearity and yield determinations. Our results indicate that the extent of mass fractionation of U and Th analyses run under similar instrumental conditions is 0.48% per amu and 0.45% per amu, respectively, but cannot be distinguished at per mil precision levels. However, we note that multiplier–Faraday cup gain can be significantly different for U and Th by 1% and thus a U standard should not be used to correct Th measurements. For this reason, a combination of thermal ionisation mass spectrometry (TIMS) and MC-ICPMS methods have been used to determine the isotopic ratio of an in-house Th standard (TEDDi). As part of our methods, TEDDi and the U standard NBL-112a are used as bracketing standards for Th and U samples, respectively. While the in-house Th standard has 229Th– 230Th– 232Th composition specific for bracketing low 232Th analyses, the methods have been also successful for silicates with 230Th/ 232Th <10 −5. Using NBL-112a, TEDDi and a gravimetrically calibrated mixed 229Th– 236U spike, we demonstrate secular equilibrium in natural materials such as Table Mountain Latite and a Long Valley Glass Mountain sample with a reproducibility of ±3.8 per mil for 230Th/ 238U and ±2.8 per mil for 234U/ 238U. We also present results for a variety of U and Th certified reference materials as well as carbonate and silicate standards such as ANU coral (AC-1) and Basalt Columbia River (BCR-2). Furthermore, we find the 230Th– 238U ratio of Harwell uraninite slightly greater than unity. This is important because many laboratories use only uraninite for the calibration of their spikes.