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It is widely accepted that a large part of the Earth was molten during its early history, thus forming a `magma ocean'. Silicate melt - (liquid) metal interaction during the separation of core material, as well as mineral-melt interaction during the cooling of the magma ocean, are therefore among the possible reactions that took place in the accreting Earth. The extent to which these different reactions have contributed to establishing current mantle abundances of siderophile elements is the object of ongoing investigations and dispute. The importance of silicate melt composition ( X_melt) for the distribution of siderophile elements between liquid metal and silicate melt has recently been explored, thus introducing an additional parameter to P, T and fO₂ that has to be considered when interpreting the results of partitioning studies. Large inconsistencies between existing studies (e.g. McFarlane et al, GCA 58, 23, 5161 (1994); Agee, Nature, 346, 834 (1990)) on magnesiowüstite-silicate melt distribution coefficients (D^Mw/Sil) may possibly be resolved by considering differences in melt composition. Moreover such experimental data may provide new insights into the question of whether mineral fractionation during the early history of the Earth contributed to differentiation processes in the mantle.

We performed element partitioning experiments between magnesiowüstite and silicate melt using a multi-anvil apparatus at 9 to 25 GPa and 1800° to 2500°C for 2 to 20 min. Samples were analysed using a CAMECA SX50 electron microprobe. The two element partitioning coefficients K_D^Mw/Sil between magnesiowüstite and silicate melt are defined as K_D(M/Mg) ^Mw/Sil = (c_M^Mw/c_M^Sil ) / (c_Mg^Mw /c_Mg^Sil ). Instead of the usually employed structural parameter NBO/T, we use other chemical parameters such as the Si/Mg ratio in the quenched silicate melts, which serve equally well for investigating the partitioning behaviour as a function of melt composition.

The K_D^Mw/Sil obtained for Fe, Ni, Co, Mn, Cr and V exhibit dependencies on both temperature (at constant X_melt) and melt composition (at constant T) at 9 GPa. As an example, the partitioning of some elements between magnesiowüstite and silicate melt as a function of melt composition and temperature at constant pressure are shown in Fig. 3.2-1a and 3.2-1b. Additional experiments at pressures above 20 GPa exhibit distinct pressure effects for the partitioning behaviour of some elements (e.g. Co, Cr, V) while almost identical K_D^Mw/Sil values were determined for other elements (e.g. Mn, Fe) at high and low pressures (e.g. Fig. 3.2-1b). In the present experiments Mn preferentially partitions into silicate melt while Ni and Co are more compatible in magnesiowüstite irrespective of the P, T, or melt composition of the experiments. Cr and V prefer silicate melt at high pressures and magnesiowüstite at low pressures. Our results generally agree with those of McFarlane et al. (1994) and the pressure, temperature and composition effects on magnesiowüstite-silicate melt partitioning determined here cannot solve the inconsistencies (i.e. significantly different K_D^Mw/Sil at comparable P, T conditions) between McFarlane et al. (1994) and Agee (1990). In contrast to the other experiments reported here S-rich metal liquid was present in the experiment of Agee (1990). However, additional S-bearing experiments in our study clearly show that sulphur has a negligible influence on oxide-silicate partitioning (Fig. 3.2-1b). Because all experiments have been performed at oxygen fugacities at or below the iron-wüstite buffer, the presence of ferric iron in magnesiowüstite and/or silicate liquid is unlikely to be responsible for the differences between the various studies. Our study indicates that other variables, for instance compositional effects such as the magnesiowüstite composition (e.g. its FeO content) may be responsible for the observed differences between literature data. Further experiments are underway to investigate this possibility.
 

Fig. 3.2-1: (a) Magnesiowüstite-silicate melt partitioning coefficients for Fe, Mn and Ni as functions of silicate melt composition (Si/Mg ratio) at constant temperature and pressure. Ni preferably partitions into magnesiowüstite while Mn prefers the silicate melt. All partition coefficients decrease with increasing polymerization of the silicate melt.  (b) Magnesiowüstite-silicate melt partitioning coefficients for Mn as a function of temperature at 9 GPa and 20 to 25 GPa in comparison to literature data. The regression line is given for the 9 GPa results. The triangles denote S-bearing experiments. Grey symbols (experiments below 2000°C) show partitioning between solid silicate and magnesiowüstite.

The present experimental results contribute important information to the discussion about the extent to which mineral fractionation processes were responsible for the observed siderophile element anomaly in the Earth´s mantle. In particular the abundances of Mn, Cr and V are difficult to explain by simple metal-silicate equilibrium processes. Because none of the determined K_D values for these elements significantly exceeds unity, the present results indicate, in accordance with McFarlane et al. (1994), that it is unlikely that magnesiowüstite fractionation has contributed significantly to the abundance patterns of the investigated elements in the mantle.