Uranium Isotope Ratios of Twenty-nine Geological Rock Reference Materials Measured by MC-ICP-MS

Abstract

Uranium isotope ratios are reported for twenty-nine geological reference materials (RMs) including igneous, metamorphic and sedimentary rocks, as well as sediments. Measurements were conducted on a multi-collector inductively coupled plasma-mass spectrometer (MC-ICP-MS) using the ²³³U-²³⁶U double spike technique. These RMs possess a wide range of matrices with U mass fractions ranging from 0.11 μg g^-1 to 18.7 μg g^-1. The δ^234/238U values (relative to the n(²³⁴U)/n(²³⁸U) ratio at secular equilibrium) show minor fluctuations around the secular equilibrium value except for two Pleistocene speleothem RMs. The δ^238/235U values (relative to the value for NBL CRM 112a (NBS SRM 960)) of igneous rocks and metamorphic rocks are within the range of the continental crust value (-0.29 ± 0.03‰, 95% CI). The δ^238/235U value for sedimentary rock and sediment RMs spans a wide range of 0.98‰, suggesting significant ²³⁸U/²³⁵U isotopic fractionation in low temperature environments. The range of δ^238/235U RMs reported in this study (-0.63‰ to 0.35‰) exceeds that of previous RMs (-0.61‰ to 0.17‰), allowing inter-laboratory calibration of wider ranges of geological and environmental samples.