Direct measurement of olivine grain boundary viscosity

The deformation of olivine grain boundaries plays an important role for a variety of geophysical phenomena in Earth's upper mantle, but no direct measurement of olivine grain-boundary viscosity has ever been made. Current estimates vary by orders of magnitude and come indirectly from measurements of relaxation times inferred from laboratory studies of attenuation. Furthermore, while microphysical models have been developed to describe the deformation of crystal interiors, descriptions of grain-boundary deformation remain phenomenological. Thus, it is difficult to confidently extrapolate the already variable laboratory results to natural conditions.

To directly measure olivine grain-boundary viscosity and to constrain the microphysical mechanism of deformation, we have performed a series of experiments on forsterite bicrystals using two different uniaxial deformation apparatuses. We deformed samples with a single ~20° grain boundary oriented for direct shear at atmospheric pressure and 1573 K. Grain-boundary strain rates (displacement rate divided by grain-boundary width, taken to be 1 nm) measured at shear stresses of 5-25 MPa reveal a stress exponent n close to 1, in strong contrast to olivine single crystals deformed at the same conditions (n = 3.5). Our experiments suggest that the grain-boundary viscosity is 3.3-5.2 x 10⁵ Pa s at these conditions, more than 8 orders of magnitude less viscous than forsterite crystal interiors. The measured stress exponent and grain boundary viscosity are in quantitative agreement with theoretical predictions by Ashby (1972), which suggests that the deformation is controlled by grain-boundary diffusion of silicon. These results provide the first direct measurement of grain-boundary viscosity in a geologic material and provide constraints for grain-boundary related deformation and attenuation phenomena in the mantle.