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% (2σ) 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, 2σ) 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 (2σ) 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 ¹⁸⁶Os-¹⁸⁷Os but terrestrial ε_182W 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.