Metal-silicate partitioning

Chondrite-normalized abundances of siderophile elements in the Earth s mantle. The measured concentrations do not match those expected from low-pressure metal-silicate partition coefficients determined by experiments. Modified from Tolstikhin and Kramers (2008). 

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. 

Righter K. and Drake M. J. (1999) Effect of water on metal-silicate partitioning of siderophile elements a higji pressure and temperature magma ocean and core formation. Earth Planet Sci. Lett. 171, 383—399. 

Walter M. J., Newsom H. E., Ertel W., and Holzheid A. (2000) Siderophile elements in the Earth and Moon Metal/silicate partitioning and implications for core formation. In Origin of the Earth and Moon (eds. R. M. Canup and K. Righter). University of Arizona Press, Tucson, pp. 265-289. 

RSEs comprise two groups of metals the HSEs—osmium, rhenium, ruthenium, iridium, platinum, and rhodium with metal/silicate partition coefficients >10" —and the two moderately siderophile elements—molybdenum and tungsten (Table 2). As the major fractions of these elements are in the core of the Earth, it is not possible to establish independently whether the iDulk Earth has chondritic ratios of RLE to RSE, i.e., whether ratios such as Ir/Sc or W/Hf are chondritic in the bulk Earth. Support for the similar behavior of RLE and RSE in chondritic meteorites is provided by Figure 9. The ratio of the RSE, Ir, to the nonrefractory siderophile element, Au, is plotted against the ratio of the RLE, Al, to the nonrefractory lithophile element, Si. Figure 9 demonstrates that RLEs and RSEs are correlated... 

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). 

Holzheid A., Sylvester P., O Neill H., St C., Ruble D. C., and Palme H. (1998) Late chondritic veneer as source of the highly siderophile elements in the Earth s mantle insights from high pressure-high temperature metal-silicate partition behavior of Pd. Nature 406, 396-399. 

A strong effect of temperature on metal/silicate partition coefficients is expected on thermodynamic grounds for those metal/silicate equilibria possessing a large entropy change (e.g., Capobianco et aL, 1993). Although Murthy... 

Any effect of pressure on the magnitude of the metal/silicate partition coefficient can be related to the volumetric properties of the equilibrium... 

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 7 The effect of sulfur content of metallic liquid on the magnitude of the solid metal/liquid metal (SM/LM) partition coefficient. Note that copper and silver have an affinity for S-bearing liquid, whereas nickel, gallium, tungsten, osmium, and rhenium all prefer the solid metal. The connection to core formation is that the latter group of elements will have a lower metal/silicate partition coefficient if the metal is liquid and contains sulfur. Similar effects have been documented for carbon (Willis and Goldstein, 1982) (sources Chabot et al., 2003 Malvin et al., 1986 Jones and Drake, 1983 Liu and Reel, 2001 Fleet et al., 1999).

Until the advent of multi-anvil experiments, only low-pressure metal/silicate partition coefficients were available. It was known that D(Ni) (metal/silicate) > D(Co) (metal/silicate) at low pressures, so it was concluded that the chondritic Ni/Co ratio indicated a lack of... 

If a small amount of core-forming metal was left behind in the mantle and then later oxidized, the mantle could potentially acquire a chon-dritic Ni/Co and even near chondritic HSE concentrations. This idea, called inefficient core formation, was advocated by Jones and Drake (1986) and provides an explanation by combining low-pressure and low-temperature metal/silicate partition coefficients and metal mobility constraints. This model awaits integration with more realistic mobility data acquired in a dynamic... 

Capobianco C. J. and Amelin A. (1994) Metal/silicate partitioning of nickel and cobalt the influence of temperature and oxygen fugacity. Geochim. Cosmochim. Acta 58, 125-140. 

Ito E., Katsura T., and Suzuki T. (1998) Metal/silicate partitioning of Mn, Co, and Ni at high pressures and high temperatures and imphcations for core formation in a deep magma ocean. In Properties of Earth and Planetary Materials at High Pressure and Temperature, Geophysical... 

Righter K. (2003) Metal/silicate partitioning of siderophile elements and core formation in the early Earth. Ann. Rev. Earth Planet. Sci. 31, 135—174. 

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... 

Kilburn M. R. and Wood B. J. (1997) Metal-silicate partitioning and the incompatibility of S and Si during core formation. Earth Planet. Sci. Lett. 152, 139—148. 

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