Accurate stable tungsten isotope measurements of natural samples using a 180W-183W double-spike

This study presents an analytical protocol for stable W isotope measurements including the calibration of a 180W-183W double-spike as well as W isotope and W concentration data of several geological reference materials (BHVO-2, AGV-2, SDC-1, W-2a, ScO-2, NOD-A-1, NOD-P-1). The reproducibility of stable W isotope measurements (± 0.018‰ in δ186/184W; 2 s.d.) is significantly improved compared to previous studies, which allows resolving between the stable W isotope compositions of various rock reservoirs on Earth. Relative to the NIST SRM 3163 standard, the highest δ186/184W value was observed for the Pacific Mn crust NOD-P-1 (+ 0.154 ± 0.013‰; 2 s.d.; n = 6), which is significantly different from the δ186/184W value of the Atlantic Mn crust NOD-A-1 (+ 0.029 ± 0.014‰; 2 s.d.; n = 6). Considering equilibrium fractionation between seawater WO42 − and slowly growing Mn oxides, this indicates an isotopically heterogeneous distribution of W in the modern oceans. Igneous rocks also show a resolvable range in δ186/184W values. Magmatic reference materials range in δ186/184W between + 0.016 ± 0.032‰ (andesite AGV-2; 2 s.d.; n = 5) and + 0.082 ± 0.010‰ (basalt BHVO-2; 2 s.d.; n = 5) showing relative enrichment of light isotopes in more evolved magmatic rocks. These isotopic differences might result from isotope fractionation during magmatic differentiation. Alternatively, the mobilization of W by hydrothermal and/or magmatic fluids might be accompanied by isotope fractionation.