The stable tungsten isotope composition of modern igneous reservoirs

Tungsten is a highly incompatible and fluid mobile element in the Earth's crust and mantle, but its geochemical cycle in magmatic systems still remains poorly understood. Tungsten stable isotopes represent a novel tool with the potential to better constrain this cycle, because of possible W isotope fractionation during changes in redox-state (valence states +4 and +6) and coordination (tetrahedral and octahedral). By employing a W-180-W-183 double spike our analytical protocol for stable W isotope measurements yields an external reproducibility of +/- 0.018 parts per thousand (2 s.d.) in .delta(186/184) W and enables the resolution of small scale stable W isotope variations between different igneous reservoirs. Here, we present the first stable W isotope data for representative volcanic rocks from different igneous settings including mid-ocean ridge basalts (MORBs), ocean island basalts (OIBs) and basalts to dacites from various subduction-related settings. The delta W-186/184 values of MORB samples from the Mid-Atlantic Ridge (+0.088 +/- 0.017 parts per thousand, n = 8) and OIB samples from the Canary Islands and Reunion Island (+0.084 +/- 0.019; 2 s.d.; n = 17) show a narrow range and are analytically indistinguishable. This calls for a homogeneous stable W isotope composition in the mantle. Subduction-related rocks are more variable in their delta W-186/184 values (-0.009 to +0.195 parts per thousand). Samples from the Troodos ophiolite complex in Cyprus show elevated W/Th and mainly high delta W-186/184 values (+0.101 to +0.195 parts per thousand, n = 16) due to relative W enrichment from a subducted sediment component with heavy W isotope composition. In support of this model, Eastern Mediterranean sediments are isotopically heavier than mantlederived rocks (+0.085 to +0.144 parts per thousand in delta W-186/184, n = 5). Lavas from the Sunda arc show regional variability with a tendency towards lower delta W-186/184 values in the Central/East Java region (as low as -0.009 parts per thousand). However, sediments from this region show uniformly high delta W-186/184 values of up to +0.301 parts per thousand and thus fail to explain the light compositions of the lavas. Hence, we rather propose that either the local subduction of an oceanic basement relief (Roo Rise) or source overprint by partial melts from deeply subducted crust accounts for low delta W-186/184 values in respective lavas. In arc settings, where the trace element budget is controlled by fluid-like components (New Britain) or melt-like components (Papua New Guinea) derived from shallowly subducted oceanic crust, we observe mantle-like W/Th and a narrow range in delta W-186/184 values (+0.109 +/- 0.027 parts per thousand, n = 19), which is only slightly elevated compared to MORBs and OIBs. This might hint towards the preferential release of isotopically heavy W during shallow fluid-induced melting events. Our results demonstrate that the stable W isotope composition of igneous rocks represents a novel tool to trace processes controlling the geochemical cycle of W in the silicate Earth. (C) 2019 The Author(s). Published by Elsevier Ltd.