The Fe-FeS melt is thought to be the major candidate of the outer core material. Its viscosity is one of the most important physical properties to study the dynamics of the convection in the outer core. We performed the in situ viscosity measurement of the Fe-FeS melt under high pressure using X-ray radiography falling sphere method with a novel sample assembly. Viscosity was measured in the temperature, pressure, and compositional conditions of 1233-1923 K, 1.5-6.9 GPa, and Fe-Fe72S28 (wt%), respectively. The viscosity coefficients obtained by 17 measurements change systematically in the range of 0.008-0.036 Pa s. An activation energy of the viscous flow, Q=30.0±8.6 kJ/mol, and the activation volume, △V=1.5±0.7 X 10-6 m3/mol, are determined as the temperature and pressure dependence, and the viscosity of the Fe72S28 melt is found to be smaller than that of the Fe melt by 15 ± 10%. These tendencies can be well correlated with the structural variation of the Fe-FeS melt.
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