Models metal-silicate equilibria

In polymeric models for silicate melts, it is postulated that, at each composition, for given values of P and T, the melt is characterized by an equilibrium distribution of several ionic species of oxygen, metal cations, and ionic polymers of monomeric units SiOt. ... 

Although HSE concentrations are low in the Earth s mantle, they are not as low as one would expect from equilibrium partitioning between core forming metal and residual mantle silicate, as emphasized by new data on metal/silicate partition coefficients for these elements (Borisov and Palme, 1997 Borisov et al., 1994). Murthy (1991) suggested that partition coefficients are dependent on temperature and pressure in such a way that at the high P-T conditions where core formation may have occurred, the observed mantle concentrations of HSEs would be obtained by metal/silicate equilibration. This hypothesis has been rejected on various grounds (O Neill, 1992), and high P-T experiments have not provided support for the drastic decrease of metal/silicate partition coefficients of HSE required by the Murthy model (Holzheid et al., 1998). 

It is clear, however, that metal mobility through liquid silicate is rapid (Stevenson, 1990 Arculus et al., 1990) and equilibration mechanisms involving liquid metal and liquid silicate have been evaluated most extensively. Since the 1990s, many new experiments on metal-silicate systems have clearly increased our knowledge and understanding of chemical equilibrium in differentiated planets. Models for a deep magma ocean have... 

The geochemical models of Wanke and Dreibus (1988), Lodders and Fegley (1997), and Sanloup et al. (1999) suggest that the core comprises 20.6-23.0% of the mass of Mars. All these model cores are sulfur rich, but differ significantly in core mass and sulfur abundance (Table 5). Measured siderophile element abundances in martian meteorites are consistent with equilibrium between sulfur-bearing metal and silicate at high temperature and pressure (Righter and Drake, 1996). 

The simplest models for the composition of the planets presume that the differences between them can be explained in terms of an equilibrium condensation. At the highest temperatures a sequence of mixed oxides of calcium, titanium, and aluminum would be found (>1,400 K). This would be followed, at lower temperatures, by metal and silicate fractions. At temperatures somewhat greater than 600 K alkali metals enter the silicate phase along with sulfur, which combines with iron at 650 K to form triolite... 

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