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

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 U-233-U-236 double spike technique. These RMs possess a wide range of matrices with U mass fractions ranging from 0.11 mu g g(-1) to 18.7 mu g g(-1). The delta U-234/238 values (relative to the n(U-234)/n (U-238) ratio at secular equilibrium) show minor fluctuations around the secular equilibrium value except for two Pleistocene speleothem RMs. The delta U-238/235 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 delta U-238/235 value for sedimentary rock and sediment RMs spans a wide range of 0.98%, suggesting significant U-238/U-235 isotopic fractionation in low temperature environments. The range of delta U-238/235 RMs reported in this study (-0.63 parts per thousand to 0.35 parts per thousand) exceeds that of previous RMs (-0.61 parts per thousand to 0.17 parts per thousand), allowing inter-laboratory calibration of wider ranges of geological and environmental samples.