The stable tungsten isotope composition of modern igneous reservoirs

Abstract

Abstract 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 180W-183W double spike our analytical protocol for stable W isotope measurements yields an external reproducibility of ±0.018‰ (2 s.d.) in δ186/184W 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 δ186/184W values of MORB samples from the Mid-Atlantic Ridge (+0.088 ± 0.017‰, 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 δ186/184W values (−0.009 to +0.195‰). Samples from the Troodos ophiolite complex in Cyprus show elevated W/Th and mainly high δ186/184W values (+0.101 to +0.195‰, 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 mantle-derived rocks (+0.085 to +0.144‰ in δ186/184W, n = 5). Lavas from the Sunda arc show regional variability with a tendency towards lower δ186/184W values in the Central/East Java region (as low as −0.009‰). However, sediments from this region show uniformly high δ186/184W values of up to +0.301‰ 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 δ186/184W 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 δ186/184W values (+0.109 ± 0.027‰, 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.