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

作者：Kurzweil Florian^*; Muenker Carsten; Tusch Jonas; Schoenberg Ronny

来源：Chemical Geology, 2018, 476: 407-417.

DOI：10.1016/j.chemgeo.2017.11.037

• Tungsten
• Stable W isotopes, double-spike
• Reference material

Tungsten is a moderately siderophile element and, thus, enriched in the Earth's core. Moreover, W behaves incompatibly during partial melting, causing relative enrichment in the Earth's crust compared to the mantle. However, little is known about the geochemical cycle of the redox-sensitive element W in the crust-mantle system and in modern to ancient low-temperature environments. High resolution stable W isotope measurements of rock samples from different geochemical reservoirs might be a powerful tool to better constrain this cycle. So far, low relative mass differences between the different W isotopes and analytical challenges hampered such high-resolution measurements. Notably, some pioneering studies on the stable W isotope composition of geological reference material show inconsistent results, calling for further verification of the true compositions of these materials.
This study presents an analytical protocol for stable W isotope measurements including the calibration of a W-180-W-183 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 delta(186)/W-184; 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 delta W-186/184 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 delta(186)/W-184 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 delta(186)/W-184 values. Magmatic reference materials range in d(186)/W-184 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.

• 出版日期2018-1-5