Accurate in situ238U–234U–232Th–230Th analysis of silicate glasses and iron oxides by laser-ablation MC-ICP-MS

Abstract

U-series microanalysis and dating is providing new insights into the timing and rates of formation of different minerals, including opals, authigenic iron oxides, zircons, and pedogenic carbonates, during the late Pleistocene. However, the low natural abundance of U and its decay chain isotopes, makes in situ U-series isotope analysis difficult. Using a state-of-the art excimer laser-ablation system coupled to a second generation MC-ICP-MS (Finnigan Neptune), we demonstrate the ability to measure [234U/238U] and [230Th/238U] activity ratios with in-run precision of 1‰ and 3‰, respectively, at a spatial resolution of 90 μm in materials containing 100–500 μg g−1 U. To verify the accuracy and precision of our approach we have prepared and thoroughly assessed two new ‘in-house’ reference materials (Al–Ca–Si glass and α-Fe2O3) for U-series isotopic composition and micro-scale homogeneity. Together with NIST 610, these reference materials allow us to monitor and correct for Th/U elemental fractionation and to accurately determine 230Th/U isotopic ratios. We show that calibration using silicate glasses imparts a small but significant (∼3%) matrix bias for U-series analysis of iron oxides, and highlights the need for matrix matched materials to obtain accurate U-series chronologies by laser-ablation MC-ICP-MS.