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The viscosity of the outer core is a critical parameter in geodynamo modelling. It has been assumed from theoretical arguments that the liquid Fe-alloy in the outer core is inviscid. This has never been verified experimentally and the many estimates of outer core viscosity, from geomagnetism, seismology, geodesy, and theory, span 14 orders of magnitude. Viscosity experiments on Fe-S liquids are being carried out in a multianvil apparatus at pressures and temperatures that are low compared with core conditions but with the aim of determining pressure and temperature dependences (activation volume and energy, respectively) of viscosity in this important geophysical system. The typically used high pressure Stokes viscometry 'quench and probe' technique is not suitable for this system because of the low viscosities and the lack of requisite control of the rise/fall time of a sphere. Instead, we employ the 'electro-detection method' which was developed for in-situ sphere speedometry measurement. The method is based on the detection of sphere position from an electrical resistance anomaly produced by an insulating sphere passing through a pair of electrodes located in the conducting liquid sample (Fig. 3.6-14). Spheres of ruby or sapphire,
 

Fig. 3.6-14: Example of electro-detection signal at 7 GPa and 1165°C using a 0.5 mm sapphire sphere.

0.5-0.7 mm diameter, are used in boron nitride contained samples of 2.5 mm diameter and 2.5-3.0 mm height. Ruby is inert in Fe-S liquids and provides the requisite electrical conductivity contrast (~10⁷) with the sample. Using Fe electrodes, the measured sphere velocity incorporates the electrical size of the sphere, determined from previous potential field modelling experiments, which is ~2.6 times the physical size. The experiments are carried out with heating rates as high as 2750^oC/min using LaCrO₃ furnaces and W/Re thermocouples. Viscosity results for Fe-20.7wt% at 7 GPa show an Arrhenian T-dependence (Fig. 3.6-15). At pressures of 6 and 8 GPa and for temperatures in the range 1100-1350^oC, the viscosities of Fe-27wt%S are in the range 1-6 Pa-s, or ~3 orders of magnitude higher than pure liquid Fe at 1 atm. These values are in agreement with extrapolations of previous viscosity measurements on the same composition up to 5 GPa and 1300^oC in a cubic anvil press. Using the same configuration and sample volume, we plan to extend the pressure range of viscosity measurement to 15 GPa in a 5000 ton press.
 

Fig. 3.6-15: Arrhenian T-dependence of viscosity of liquid Fe-20.7 wt%S at 7 GPa.