The silicate Earth ways of reconstruction

There are several approaches that allow us to trace the way back to the composition of the silicate Earth about 4.5 Gyr ago. Each of these approaches addresses a specific aspect but as they are combined the results converge.

The assumption that the starting material in the terrestrial feeding zone was primitive and chondrite-like directly constrains the terrestrial abundances of the major and trace involatile elements, although this material must have originated under enhanced temperatures compared with the asteroid belt (Fig. 14.1). This is a forward model. In a complementary, inverse, approach the presently observed differentiated silicate reservoirs, the mantle and continental crust, which are respectively depleted and enriched in incompatible elements, can be recombined into a hypothetical bulk silicate Earth (BSE). The ratios of elements with similar chemical behaviour, e.g. K/U as discussed earlier (see Fig. 14.4 and the related text), allow the abundances of some volatile elements (e.g. K, Rb) to be quantified using involatile ones (U).

Isotopic systematics give time-integrated elemental (parent/daughter) ratios even for reservoirs inaccessible to direct sampling, and they are therefore especially important for such an inverse approach. Below, the direct and inverse approaches are discussed mainly via examples illustrating the "restoration techniques" for the case of selected elements. The terrestrial inventories of all elements can be found in e.g. Rudnick and Gao (2003), Veizer and Mackenzie (2003) and Palme and O'Neill (2003) and in scientific contributions by Ringwood (1975), Morgan and Anders (1980), Hart and Zindler (1986), Hofmann (1988), McDonough and Sun (1995), Allegre etal. (1995b, 2001).

Observational and experimental petrology give the pressure and temperature conditions under which melting would occur and geochemistry sheds light on the partitioning of elements between melts and residues. Together, these disciplines help us to understand the numerous and, in part, huge deviations from chondritic compositions that are seen in subsystems of the silicate Earth and to interpret them in terms of fractionation processes.

Seismology and other geophysical observations related to the physical properties of terrestrial materials under different conditions, first of all temperature and pressure, allow the composition and state of matter, even at great depths, to be evaluated.

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