A systematically investigation of metal-oxide partitioning behaviour of siderophile elements at high pressures and temperatures as a function of oxygen fugacity allows
the calculation of the valence state of these elements in magnesiowüstite. Although their valences are predictable from low P,T studies and thermodynamic considerations, only rare experimental evidence has so far been provided (e.g. Thibault and Walter, 1995; Walter and Thibault, 1995). A careful observation of our data base shows some restrictions which might be of importance to similar studies. Moreover the results indicate nonideal mixing of Fe and Ni in liquid metal at high pressures and temperatures.
Fe-Ni powders were doped with various siderophile and light elements (Co, Cr, Mn, V and Si, Ti, O) and packed into MgO capsules. A range of oxygen fugacities (IW to 3.5 log bar units below IW) was imposed by variation of Si and O in the starting material. Experiments were performed in a multi-anvil apparatus at 9 and 18 GPa and 2200 °C. At run conditions the metal was always a liquid. Iron as well as the other elements diffuse into the capsule material, forming magnesiowüstite solid solutions. The quenched liquid metal and magnesiowüstite compositions were analysed by
electron microprobe. Oxygen fugacities were calculated
relative to the iron wüstite buffer from the Fe content in liquid metal and FeO content in magnesiowüstite respectively, assuming ideal Fe-Ni and Fe-Mg mixing behaviour in the metal alloy and in the oxide solid solutions.
The metal/magnesiowüstite partition coefficients of the siderophile elements are - as expected - significantly dependent on oxygen fugacity. The slope of the regression lines allows the calculation of the valence state of the element of interest.
The maximum solubility for the elements Mn, V , Cr and Si in magnesiowüstite at the observed pressures and temperatures and high oxygen fugacities will lead to erroneous valences and therefore restrict the evaluation of correct valence states.
The resulting valence states indicate that Ni, Fe, Co, Cr, V and Mn exist as divalent cations in magnesiowüstite solid solutions at 9 to 18 GPa and 2200 °C while Si has a valence of 4+. Slight deviations were obtained for Cr (calculated valence 2.3) and Ni (calculated valence 1.6). The low valence state for Ni might be attributed to nonideal mixing effects in the Fe-Ni alloy, which is indicated by the compositional variation of the metal as a function of oxygen fugacity rather than to Ni0 as suggested by Colson (1992).
Colson, R.O., Nature 357, 65-68 (1992).
Thibault, Y. & Walter, M.J., Geochim. Cosmochim. Acta 59, 991-1002 (1995).
Walter, M.J. & Thibault, Y., (1995, submitted).