The depletion of tungsten in the bulk silicate Earth: Constraints on core formation

The depletion of the siderophile element tungsten (W) in the bulk silicate Earth (BSE), due to core formation, provides clues to the formation and early evolution of the Earth. This study significantly improves our knowledge of the abundance of W in the continental crust, an important reservoir for W in the BSE. Tungsten is a highly incompatible element, whose absolute concentrations are variable due to igneous processes. Therefore, the abundance of W is normalized to the highly incompatible lithophile element Th, to correct for igneous fractionation since the end of core formation. Similar W/Th ratios are observed in several terrestrial reservoirs, including the depleted mantle (nodules and Mid-Ocean Ridge Basalts), the old continental crust (upper continental sediments and Archean granulites), and the young continental crust (continental and oceanic arcs). The use of Th as a normalizing element is inappropriate. however, in the case of granulite xenoliths from the lowermost continental crust. These samples have higher W/Th ratios due to loss of Th (and U) compared to other incompatible elements including Ba and W. The few komatiite samples investigated have high ratios of W to the normalizing elements, possibly due to loss of the normalizing elements during alteration. The depletion of W in the BSE is determined, by using a mass balance calculation based on the W/Th ratios of mantle and crustal reservoirs, and including the uncertainty in the initial abundance of W in the Earth, based on W abundances in chondritic meteorites. The resulting depletion of W relative to the refractory lithophile element Th is 0.06, with a range from 0.03-0.1. The depletion range for W overlaps the depletions of the compatible siderophile elements Co and Ni. This observation is consistent with the heterogeneous accretion theory for the Earth (Newsom, 1990; O'Neill, 1991). In contrast, the observed depletions for W, Co, and Ni are not consistent with their experimental high temperature equilibrium metal-silicate partition coefficients (Hillgren and Drake, 1994). The depletion of W also provides important constraints on the timing of core formation, based on the decay of the now extinct isotope Hf-182 to the isotope W-182 in the early solar system (Harper and Jacobsen, 1994).