Determination of mass-dependent variations in tungsten stable isotope compositions of geological reference materials by double-spike and MC-ICPMS

Abstract

We present a double-spike technique for precise determination of mass-dependent fractionation of tungsten (W) stable isotopes. Instrumental mass bias effects and isotope fractionation during W separation are corrected for by means of a ¹⁸⁰W–¹⁸³W double-spike added prior to sample dissolution. The separation of W from the matrix is achieved via three-step anion-exchange chromatography. Tungsten stable isotopic compositions were determined by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Results are expressed relative to the NIST SRM 3136 W standard as the permil difference in ¹⁸⁶W/¹⁸⁴W (δ¹⁸⁶W). The external reproducibility of the NIST SRM 3163 W standard is ±0.05‰ with an average composition of −0.01‰ on ¹⁸⁶W/¹⁸⁴W (2s.d., n = 171). Tungsten stable isotope compositions for USGS standard reference materials are as follows: AGV-2 (andesite; δ¹⁸⁶W = 0.18 ± 0.05‰, 2s.d., n = 6), SDC-1 (mica schist; δ¹⁸⁶W = 0.36 ± 0.05‰, 2s.d., n = 6), SDO-1 (shale; δ¹⁸⁶W = 0.26 ± 0.06‰, 2s.d., n = 6), and manganese nodules NOD-A-1 (δ¹⁸⁶W = 0.07 ± 0.05‰, 2s.d., n = 10) and NOD-P-1 (δ¹⁸⁶W = 0.31 ± 0.04‰, 2s.d., n = 7). The long-term external reproducibility (2s.d.) for these rock standards is ±0.05‰.