Investigating the interior of terrestrial planets and moons using electrical laboratory measurements

1648. [10] McKinnon W.B. et al. (2001) Geology, 29, 103-106. [11] Pommier A. (2014) Surveys in Geophysics, 35, 41-84. [12] Pommier A. et al. (2015) EPSL, 425, 242-255. [13] deKleer, K. et al. (2019), https://kiss.caltech.edu/ workshops/tidal_heating/tidal_heating.html. [14] Pommier A. (2020), American Mineralogist, 105, 1069-1077. [15] Breuer D. et al. (2015) PEPS, 2:39. [16] Margot J.L. et al. (2018) Mercury the View after MESSENGER. CUP. [17] Buske M. (2006) PhD thesis. [18] Knibbe J.S. and van Westrenen W. (2018) EPSL, 482, 147-159. POMMIER: ELECTRICAL RESISTIVITY OF FE-NI(-S) ALLOYS UNDER PRESSURE 1074 American Mineralogist, vol. 105, 2020 Fe-S and Fe-Ni-S samples and is negligible between Fe-Ni and Fe-Ni-S samples. This suggests that the effect of pressure on resistivity depends on the alloy chemistry, and in the Fe-10Ni5S sample, the dependence of electrical resistivity to pressure is controlled by Ni impurity, not by S impurity. Different factors might explain these two observations; in particular, differences in compressibility and the phase assemblage could contribute to the contrasting pressure effect on resistivity. First, the Fe-S alloy is less dense than Fe and Ni-bearing iron alloys (e.g., Sanloup et al. 2000; Lin et al. 2004; Kawaguchi et al. 2017), and Fe-Ni alloys present a slightly higher density than pure Fe (e.g., Martorell et al. 2015; Watanabe et al. 2016). For instance, Fe-10S at 5 GPa and 1770 K has a density of about 5.65 g/cm3 (Sanloup et al. 2000) whereas the density of Fe-7.6Ni and Fe-7.6Ni-10S at a similar temperature and extrapolated to the same pressure is about 7.8 and 6.8 g/cm3, respectively (Kawaguchi et al. 2017). The Fe-S alloy is thus more compressible than pure Fe and Fe-Ni alloys, which can explain at least partly the higher pressure effect on the resistivity of Fe-5S than on one of the other investigated alloys. Second, in the Fe-Ni-S sample, the small pressure-dependence of resistivity suggests a control of the electrical properties by Ni rather than S, and this might be explained by the multi-phase assemblage of the starting materials. The Fe-Ni-S sample is likely a mixture of Fe-Ni(-S) alloy with a small volume fraction of Fe1–xS, as the solubility of sulfur is low in solid fcc iron (e.g., Li et al. 2001; Hayashi et al. 2009). The low S solubility in fcc Fe could result in a minor role of sulfur in controlling he bulk resistivity, compared to the effect of nickel that substitutes for Fe. The presence of two phases in the solid Fe-Ni-S sample may account for why Fe-Ni-S and Fe-Ni resistivity present a similar P dependence. Further work is required to demonstrate whether or not these observations about the relative effect of nickel and sulfur on electrical resistivity also apply to pressures higher than 8 GPa. Thermal conductivity estimates of Fe-Ni alloys Watanabe et al. (2019) demonstrated that experimentally measured thermal conductivities of Fe-Ni melts at atmospheric pressure and high temperature (1700–2000 K) are larger than those calculated using the Wiedemann-Franz law, due to the contribution of the thermal vibration of atoms to the thermal conductivity of Fe-Ni alloys. The empirical Wiedemann-Franz law relates thermal conductivity and electrical resistivity as follows