Lithophile elements planetary differentiation

Energy released from impacts, together with heat from the decay of radioactive isotopes, led to differentiation in planetary embryos, once these objects became partially molten (Tonks and Melosh, 1992). Iron and siderophile elements (e.g., platinum, palladium, and gold) preferentially sank to the center to form a core, while the lighter silicates and lithophile elements formed a mantle. Differentiation was probably a continuous process rather than a single event, so that large planets like Earth accreted from embryos that were already partially or wholly differentiated. 

As refractory lithophile elements, the REE play an important role in constraining the overall composition and history of the silicate fraction of planets, which for the terrestrial planets is also termed their primitive mantle (equivalent to the present-day crust plus mantle). Since there is no evidence for significant planetary-scale fractionation of refractory elements during the assembly and differentiation of planetary bodies, it is widely accepted that the primitive mantles of terrestrial planets and moon possess chondritic proportions of the REE. As such, the absolute concentrations of REE (and other refractory elements) in primitive mantles provide an important constraint on the proportions of volatile elements to refractory elements and on the oxidation state (i.e., metal/silicate ratio) of the body. To date, the only major planetary bodies for which REE data are directly available are the Earth, Moon, and Mars, and Taylor and McLennan" recently reviewed these data. 

The decay of Hf to is well suited to date core formation in planetary objects mainly for three reasons. First, owing to the Hf half-life of 9 Myr, detectable W isotope variations can only be produced in the first -60 Myr of the solar system. This timescale is appropriate for the formation of the Earth and Moon in particular and to planetary accretion and differentiation in general. Second, both Hf and W are refractory elements such that there is only limited fractionation of Hf and W in the solar nebula or among different planetary bodies (see above). The HfrW ratio of the bulk Earth therefore can be assumed to be chondritic and hence can be measured today. Third, Hf is a lithophile and W is a siderophile element such that the chondritic HfrW ratio of the Earth is fractionated internally by core formation. If core formation took place during the effective lifetime of Hf, the metal core (HfrW-O) will develop a deficit in the abundance of whereas the silicate mantle, owing to its enhanced Hf/W, will develop an excess of (7-P). 

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