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Pommier, A, Laurenz V, Davies CJ, Frost DJ.  2018.  Melting phase relations in the Fe-S and Fe-S-O systems at core conditions in small terrestrial bodies. Icarus. 306:150-162.   10.1016/j.icarus.2018.01.021   AbstractWebsite

We report an experimental investigation of phase equilibria in the Fe-S and Fe-S-O systems. Experiments were performed at high temperatures (1400-1850 degrees C) and high pressures (14 and 20 GPa) using a multi anvil apparatus. The results of this study are used to understand the effect of sulfur and oxygen on core dynamics in small terrestrial bodies. We observe that the formation of solid FeO grains occurs at the FeS liquid - Fe solid interface at high temperature ( > 1400 degrees C at 20 GPa). Oxygen fugacities calculated for each O-bearing sample show that redox conditions vary from Delta 1W= 0.65 to 0. Considering the relative density of each phase and existing evolutionary models of terrestrial cores, we apply our experimental results to the cores of Mars and Ganymede. We suggest that the presence of FeO in small terrestrial bodies tends to contribute to outer-core compositional stratification. Depending on the redox and thermal history of the planet, FeO may also help form a transitional redox zone at the core-mantle boundary. (c) 2018 Elsevier Inc. All rights reserved.

Pommier, A, Leinenweber K, Tran T.  2019.  Mercury's thermal evolution controlled by an insulating liquid outermost core? Earth and Planetary Science Letters. 517:125-134.   10.1016/j.epsl.2019.04.022   AbstractWebsite

The weak intrinsic magnetic field of Mercury is intimately tied to the structure and cooling history of its metallic core. Recent constraints about the planet's internal structure suggest the presence of a FeS layer overlying a silicon-bearing core. We performed 4-electrode resistivity experiments on core analogues up to 10 GPa and over wide temperature ranges in order to investigate the insulating properties of core materials. Our results show that the FeS layer is liquid and insulating, and that the electrical resistivity of a miscible Fe-Si(-S) core is comparable to the one of an immiscible Fe-S, Fe-Si core. The difference in electrical resistivity between the FeS-rich layer and the underlying Fe-Si(-S) core is at least 1 log unit at pressure and temperature conditions relevant to Mercury's interior. Estimates of the lower bound of thermal conductivity for FeS and Fe-Si(-S) materials are calculated using the Wiedemann-Franz law. A thick (>40 km) FeS-rich shell is expected to maintain high temperatures across the core, and if temperature in this layer departs from an adiabat, then this might affect the core cooling rate. The presence of a liquid and insulating shell is not inconsistent with a thermally stratified core in Mercury and is likely to impact the generation and sustainability of a magnetic field. (C) 2019 Elsevier B.V. All rights reserved.

Pommier, A, Gaillard F, Malki M, Pichavant M.  2010.  Methodological re-evaluation of the electrical conductivity of silicate melts. American Mineralogist. 95:284-291.   10.2138/Am.2010.3314   AbstractWebsite

Electrical impedance measurements in the laboratory oil silicate melts are used to interpret magnetotelluric anomalies. On the basis of 2- and 4-electrode measurements, we show that the influence of the electrodes of the 2-electrode system Oil the measured resistivity call be of significant importance for low-resistivity melts and increases with temperature. At 1400 degrees C, the resistivity of very conductive melts measured with two electrodes call reach six times the resistivity value measured with four electrodes. A short-circuit experiment is needed to correct the 2-electrode data. Electrodes contribution is also estimated for samples from other studies, for which the resistance of the electrical cell call be as high as the resistance of the sample. A correction of the resistivity data from the literature is proposed and Values of the corresponding Arrhenian parameters are recommended.