The Earth’s tungsten budget during mantle melting and crust formation

During silicate melting on Earth, W is one of the most incompatible trace elements, similar to Th, Ba or U. As W is also moderately siderophile during metal segregation, ratios of W and the lithophile Th and U in silicate rocks have therefore been used to constrain the W abundance of the Earth’s mantle and the Hf–W age of core formation. This study presents high-precision W concentration data obtained by isotope dilution for samples covering important silicate reservoirs on Earth. The data reveal significant fractionations of W from other highly incompatible lithophile elements such as Th, U, and Ta. Many arc lavas exhibit a selective enrichment of W relative to Th, U, and Nb–Ta, reflecting W enrichment in the sub-arc mantle via fluid-like components derived from subducting plates. In contrast, during enrichment by melt-like subduction components, W is generally slightly depleted relative to Th and U, but is still enriched relative to Ta. Hence, all arc rocks and the continental crust exhibit uniformly low Ta/W (ca. 1), whereas W/Th and W/U may show opposite fractionation trends, depending on the role of fluid- and melt-like subduction components. Further high-precision W data for OIBs and MORBs reveal a systematic depletion of W in both rock types relative to other HFSE, resulting in high Ta/W that are complementary to the low Ta/W observed in arc rocks and the continental crust. Similar to previous interpretations based on Nb/U and Ce/Pb systematics, our Ta/W data confirm a depletion of the depleted upper mantle (DM) in fluid mobile elements relative to the primitive mantle (PRIMA). The abundance of W in the depleted upper mantle relative to other immobile and highly incompatible elements such as Nb and Ta is therefore not representative of the bulk silicate Earth. Based on mass balance calculations using Ta–W systematics in the major silicate reservoirs, the W abundance of the Earth’s primitive mantle can be constrained to 12ppb, resulting in revised ratios of W–U and W–Th of 0.53 and 0.14, respectively. The newly constrained Hf–W ratio of the silicate Earth is 25.8, significantly higher than previously estimated (18.7) and overlaps within error the Hf–W ratio proposed for the Moon (ca. 24.9). The 182Hf–182W model age for the formation of the Earth’s core that is inferred from the 182W abundance and the Hf/W of the silicate Earth is therefore younger than previously calculated, by up to 5Myrs after solar system formation depending on the accretion models used. The similar Hf/W ratios and 182W compositions of the Earth and the silicate Moon suggest a strong link between the Moon forming giant impact and final metal–silicate equilibration on the Earth.