THE DEPLETION OF TUNGSTEN IN THE BULK SILICATE EARTH - CONSTRAINTS ONCORE FORMATION

The depletion of the siderophile element tungsten (W) in the bulk sili cate Earth (BSE), due to core formation, provides clues to the formati on and early evolution of the Earth. This study significantly improves our knowledge of the abundance of W in the continental crust, an impo rtant reservoir for W in the BSE. Tungsten is a highly incompatible el ement, whose absolute concentrations are variable due to igneous proce sses. Therefore, the abundance of W is normalized to the highly incomp atible lithophile element Th, to correct for igneous fractionation sin ce the end of core formation. Similar W/Th ratios are observed in seve ral terrestrial reservoirs, including the depleted mantle (nodules and Mid-Ocean Ridge Basalts), the old continental crust (upper continenta l 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 du e to loss of Th (and U) compared to other incompatible elements includ ing Ba and W. The few komatiite samples investigated have high ratios of W to the normalizing elements, possibly due to loss of the normaliz ing elements during alteration. The depletion of W in the BSE is deter mined, by using a mass balance calculation based on the W/Th ratios of mantle and crustal reservoirs, and including the uncertainty in the i nitial abundance of W in the Earth, based on W abundances in chondriti c 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 depleti on range for W overlaps the depletions of the compatible siderophile e lements Co and Ni. This observation is consistent with the heterogeneo us accretion theory for the Earth (Newsom, 1990; O'Neill, 1991). In co ntrast, the observed depletions for W, Co, and Ni are not consistent w ith their experimental high temperature equilibrium metal-silicate par tition coefficients (Hillgren and Drake, 1994). The depletion of W als o provides important constraints on the timing of core formation, base d on the decay of the now extinct isotope Hf-182 to the isotope W-182 in the early solar system (Harper and Jacobsen, 1994).