Oxygen solubility in Fe-Ni-S alloy at high pressure and implications for the formation and compositions of planetary cores
Zusammenfassung der Projektergebnisse
In order to constrain the concentration of oxygen in the Earth’s core, the partitioning of oxygen between liquid iron alloy and magnesiowüstite has been studied experimentally at pressure-temperature conditions of 3-70 GPa and 2273-3500 K. Experiments were performed using both multianvil presses and laser-heated diamond anvil cells. Sample analysis involved electron microprobe and TEM techniques (including EDX and EELS spectroscopies), with the focused ion beam (FIB) technique being used for TEM sample preparation. Independently of the resulting partitioning data, a thermodynamic model of oxygen partitioning has been developed based on additional multianvil experiments that were performed to study the miscibility gap between Fe-rich and FeO-rich liquids in the Fe-FeO system as a function of pressure. The results of the model, which are in excellent agreement with the partitioning data, show that oxygen partitioning is strongly dependent on temperature (which favours oxygen in the metal) but only weakly dependent on pressure. An important new finding is that partitioning becomes strongly dependent on the magnesiowüstite (or silicate) composition at pressures above 25-30 GPa due to changes in the non-ideality of mixing. Further multianvil experiments have been performed to study the effects of nickel and sulfur in the Fe alloy on oxygen partitioning. These two elements have opposite effects. For example, with 10 wt% Ni in the metal, the oxygen metal-silicate partitioning coefficient is reduced by about 0.5 natural log units whereas it is increased when sulfur is dissolved in the metal. A thermodynamic model has been developed that enables the combined effects of Ni and S to be calculated as a function of pressure and temperature. These oxygen partitioning studies show that the oxygen content of the Earth’s outer core must be undersaturated with respect to equilibrium with the bulk of the Earth’s mantle. Based on this conclusion, there are two possible end-member consequences: (a) The very base of the mantle is strongly depleted in the FeO component. (b) The outer core is compositionally stratified (as suggested by some seismological studies) with a outer buoyant layer being enriched in oxygen relative to the bulk of the outer core. A new model of multistage core formation has been developed on the basis of the oxygen partitioning results together with recent experimental data on the metal-silicate partitioning of Fe, Si, Ni, Co, W, Nb, V, Ta and Cr. This is based on a simplified version of accretion models whereby the Earth accretes through impacts with smaller bodies, with each collision creating a magma ocean and an episode of core-mantle segregation. The model is based on the bulk composition of accreting bodies and compositions of equilibrated metal and silicate are determined by mass balance. The results are constrained by a least squares fit to mantle composition. The results show that: (a) Accretion was heterogeneous, with the initial 60-70 % of the Earth’s mass accreting from highly-reduced volatile-free material and the final 30-40 % from more oxidized material that contained the Earth’s budget of at least the moderately volatile elements. (b) The cores of the larger impacting bodies only partially equilibrated in the magma ocean – i.e. there was significant disequilibrium. (c) Magma oceans may have extended to the core-mantle boundary, thus making core formation an extremely efficient process. (d) The Earth’s core is predicted to contain ~8 wt% Si and <1 wt% oxygen, in addition to ~5 wt% Ni and <2 wt% S.
Projektbezogene Publikationen (Auswahl)
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(2007) Formation of Earth’s core. In: Treatise on Geophysics Vol. 9: Evolution of the Earth, D.J. Stevenson (ed.). Elsevier, Amsterdam, pp. 51-90
Rubie, D.C., Nimmo, F., Melosh, H.J.
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(2007) Partitioning of FeO between magnesiowüstite and liquid iron at high pressures and temperatures: Implications for the composition of the Earth’s outer core. Earth and Planetary Science Letters 257, 435-449
Asahara, Y., Frost, D.J., Rubie, D.C.
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(2008) The redox state of the mantle during and just after core formation. Phil. Trans. R. Soc. A, 366, 4315-4337
Frost, D.J., Mann, U., Asahara, Y., Rubie, D.C.
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(2010) Combining FIB milling and conventional Argon ion milling techniques to prepare high quality site-specific TEM samples for quantitative EELS analysis of oxygen in molten iron. Journal of Microscopy 238, 200-209
Miyajima, N., Holzapfel, C., Asahara, Y., Dubrovinsky, L., Frost, D.J., Rubie, D.C., Drechsler, M., Niwa, K., Ichihara, M., Yagi, T.
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(2010) The partitioning of oxygen between the Earth’s mantle and core. Journal of Geophysical Research 115, B02202
Frost, D.J., Asahara, Y., Rubie, D.C., Miyajima, N., Dubrovinsky, L.S., Holzapfel, C., Ohtani, E., Miyahara, M., Sakai, T.
