Tungsten geochemistry and implications for understanding the Earth's interior

The concentration of tungsten (W) in basaltic melts provides a window into the behavior of this element during core-mantle separation, crust formation, silicate differentiation, and potentially core-mantle interaction. We have analyzed an extensive suite of modern basalts (n =86) for their trace element chemistry via laser ablation ICP-MS, with barium (Ba), thorium (Th), uranium (U), and W concentrations typically determined to <= 5% (2 sigma) uncertainty. We find that the partitioning behavior of U mirrors that of W during basalt genesis, whereas Ba and Th both behave more incompatibly. The W/U ratio of our complete sample suite (0.65 +/- 0.45, 2(sigma) is representative of the mean modern mantle, and is indistinguishable from that of mid-ocean ridge basalts (W/U-MORB=0.65 +/- 0.41, n=52), ocean island basalts (W/U-OIB=0.63 +/- 0.07, n=10), and back-arc basin basalts (W/U-BABB=0.62 +/- 0.09, n=12). This ratio is also consistent with the W/U ratio of the continental crust, and thus represents the W/U ratio of the entire silicate portion of the Earth. Assuming a concentration of 20 +/- 8 (2 sigma) ng/g U in the bulk silicate Earth, the abundance of W in the silicate Earth is 13 +/- 10 ng/g. Following mass balance, this implies a mean modern mantle and core composition of 8.3 +/- 7.1 ng/g W and 500 +/- 120 ng/g W. respectively. Additionally, the MORB Source is modeled to contain approximately 3.0 +/- 2.3 ng/g W, indicating a four-fold depletion of the highly incompatible elements in the MORB source relative to the silicate Earth. Although both the isotopic composition of W and the constancy of the silicate Earth W/U ratio allow for potential insight into core-mantle exchange, both of these proxies are extremely dependent on the chemical Composition Of the Source. A case Study of three Hawaiian picrites with enrichments in Os-186-Os-187 but terrestrial;MVV can be explained by: i) a lack of a core component in the Hawaiian plume, ii) crustal contamination, or iii) a source composition enriched in incompatible trace elements relative to the bulk silicate Earth. Published by Elsevier B.V.