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3.2 a. Are Si and O the light elements in the core? A study of Si and O solubilities in liquid metal as a function of P, T and fO2 (C.K. Gessmann and D.C. Rubie)

According to geophysical measurements, the density of the Earth's metallic core is lower than expected based on experimental determinations of the density of iron at high pressure and temperature. This density deficit is usually explained by the presence of one or more light elements dissolved in the Fe-Ni metal. Understanding the cause of the density deficit and the identity of the light elements is one of the important problems related to the early history of the Earth and core formation. While a number of possible candidates for light components, including C, Si, O and S, have been proposed and discussed, experimental data on the solubility of these elements in liquid metal alloys are scarce.

In order to determine if a combination of Si and O can account for the density deficit, the solubilities of these elements in liquid Fe-Ni metal have been measured in multianvil experiments over the P-T range 5-23 GPa and 1800-2400°C. A Fe-Ni powder doped with Co, Ti, Si, O and other trace elements was used as a starting material and was contained in MgO capsules. At run conditions the metal was always a liquid. During the experiments, the liquid metal reacted with the MgO capsule to produce (Mg,Fe)O. A 3.5 log unit range of oxygen fugacity was systematically imposed on the samples by varying the Si and O contents of the starting material. The compositions of the quenched liquid metal and (Mg,Fe)O were analyzed by electron microprobe. Oxygen fugacities were calculated relative to the iron-wüstite buffer from the Fe contents of liquid metal and the FeO content of (Mg,Fe)O, assuming ideal Fe-Ni and Fe-Mg mixing behaviour in the metal alloy and in the oxide solid solutions, respectively.

The solubilities of both Si and O in Fe-Ni metal liquid at experimental conditions are greater than expected, even at pressures as low as 5-9 GPa. The highest concentrations of Si in liquid metal are present in the most reducing experiments (~4 log bar units below the iron-wüstite (IW) buffer), while those of oxygen show the opposite trend. O is most soluble in liquid metal under relatively oxidising conditions (close to the IW buffer curve) (Fig. 3.2-1).

Fig. 3.2-1: Si and O contents of the quenched Fe-Ni liquid metal as a function of oxygen fugacity. The data cover pressures and temperatures in the range 5-23 GPa and 1800-2400°C. The solubilities of Si and O in the metal show opposite trends as a function of oxygen fugacity. Up to 6 wt% of Si have been determined in quenched Fe-Ni alloys. The maximum O-content is 1 wt%.

The solubility of silicon in liquid metal increases considerably with decreasing oxygen fugacity (at constant P,T) reaching values around 4-6 wt% at IW�3 (Fig. 3.2-2). In addition, its solubility also increases with increasing pressure (at constant fO2, T) and with increasing temperature (at constant P, fO2). The observed solubility behaviour suggests that considerable amounts of Si can dissolve in liquid metal (i) at extremely reducing conditions and (ii) at fO2 values close to the IW buffer curve at higher pressures and temperatures than those of the present study. However, the solubility of O as a function of fO2 and pressure varies inversely with that of Si. The highest oxygen contents in liquid metal (around 1 wt%) have been observed at oxygen fugacities slightly below the IW-buffer curve. Moreover, in contrast to Si, the O solubility decreases with increasing pressure. Only an increase in temperature appears to increase the solubility of O in liquid metal.

Fig. 3.2-2: Si contents in quenched Fe-Ni liquid metal as a function of oxygen fugacity. Regression lines are shown for sets of experiments performed at constant pressure and temperature. For a given fO2 value, an increase in P (at constant T) and also increasing T (at constant P) both increase Si solubility.

Extrapolating the experimental results to higher pressures and temperatures suggests that significant quantities of Si can dissolve in liquid metal even at moderate oxygen fugacities, i.e. fO2 values that are relevant to core formation. However, because of the opposite solubility trends of Si and O as a function of both oxygen fugacity and pressure, it is unlikely that the presence of both Si and O simultaneously contribute significantly to the density deficit of the Earth's core.

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