Partitioning silicate melt composition

Figure 6 Effect of silicate melt composition on metal/silicate partition coefficients for cobalt ( ), gallium (+), tungsten (o), and phosphorus ( ) (Jaeger and Drake, 2000 Pak and Fruehan, 1986). NBO/t is calculated according to Mysen (1991) and corresponds to basalt values of 1, komatiite —1.7, and peridotite —2.8. In general, high-valence elements such as tungsten and phosphorus are affected more strongly than lower valence elements such as cobalt (or nickel).

Figure 24. Lattice strain model applied to zircon-melt partition coefficients from Hinton et al. (written comm.) for a zircon phenocryst in peralkaline rhyolite SMN59 from Kenya. Ionic radii are for Vlll-fold coordination (Shannon 1976). The curves are fits to Equation (1) at an estimated eraption temperature of 700°C (Scaillet and Macdonald 2001). Note the excellent fit of the trivalent lanAanides, with the exception of Ce, whose elevated partition coefficient is due to the presence of both Ce and Ce" in the melt, with the latter having a much higher partition coefficient into zircon. The 4+ parabola cradely fits the data from Dj, and Dy, through Dzi to Dih, but does not reproduce the observed DuIDjh ratio. We speculate that this is due to melt compositional effects on Dzt and (Linnen and Keppler 2002), and possibly other 4+ cations, in very silicic melts. Because of its Vlll-fold ionic radius of 0.91 A (vertical line), Dpa is likely to be at least as high as Dwh, and probably considerably higher.

Kinzler, R. J., Grove, T. L. Recca, S. I. (1990). An experimental study of the effect of temperature and melt composition on the partitioning of nickel between olivine and silicate melt. Geochim. Cosmochim. Acta, 54, 1255-65. 

Righter, K., Drake, M. J. and Yaxley, G. (1999) Prediction of siderophile element metal-silicate partition coefficients to 20 GPa and 2800 °C the effects of pressure, temperature, oxygen fugacity, and silicate and metallic melt compositions. Physics of the Earth and Planetary Interiors, 100, 115—134. 

Although there is no general model for activity-composition relationships in silicate melts, several studies have shown that, under certain circumstances, a semi-empirical approach works reasonably well. Blundy et al. (1995) investigated the partitioning behavior of sodium between clinopyroxene and silicate melts over wide ranges of pressure and temperature. They showed that the crystal-liquid partition coefficient Dfja bears a very simple relationship to the equilibrium constant Xj,ja for the melting reaction ... 

REE and Y. Based on the approach of Wood and Blundy (1997), the partitioning of the rare earths between clinopyroxene and silicate melt depends on chemical compositions of the melt and pyroxene, pressure, and temperature ... 

The behavior of some siderophile elements is controlled by the composition of the metal, and, in particular, the Fe/Ni ratio. Because the activity coefficient of a siderophile element, M, may be different in a nickel-rich metal than an iron-rich metal, an understanding of this effect is necessary before partition coefficients can be successfully applied to a natural system. A good example is tin, which has a low activity coefficient in iron-rich metal, and a high activity coefficient in nickel-rich metal (Figure 8 Capobianco et al., 1999). Metal composition can change as a function of/o, and this can then change the solubility in the silicate melt. In addition, the Fe/Ni ratio in metal is sometimes used to impose a specific/o on a system. These effects are linked and must be unraveled first in order to understand tin in planetary mantles (Righter and Drake, 2000). 

Righter K., Drake M. J., and Yaxley G. (1997) Prediction of siderophile element metal—silicate partition coefficients to 20 GPa and 2,800 °C the effect of pressure, temperature,/o and silicate and metalhc melt composition. Phys. Earth... 

Chabot N. E. and Drake M. J. (1999) Potassium solubility in metal the effects of composition at 15 kbar and 1,900 degrees C on partitioning between iron alloys and silicate melts. Earth Planet. Sci. Lett. 172, 323—335. 

The partition coefhcents for selected REE between sphene and a silicic melt of intermediate composition, plotted as a function of pressure. There is an increase in partition coefficient with increasing pressure in the range 7.5 to 30 kb (after Green and Pearson, 1983). 

Colson er al. (1988) in a detailed study of trace element partitioning between olivine and silicic melt, and orthopyroxene and silicic melt, have shown that many partition coefficients are strongly dependent upon temperature and melt composition. They show that these partition coefficients vary according to ionic size and they have modelled temperature and composition dependence as a function of these variables. On the basis of their equations it is possible to predict partidon coefficients between olivine and melt, and orthopyroxene and melt, for a wide range of tri- arid di-valent cations under a7yiriety of magmatic conditions. 

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