- 1The University of Manchester, Earth and Environmental Science, Manchester, United Kingdom of Great Britain – England, Scotland, Wales (julian.mecklenburgh@manchester.ac.uk)
- 2Federal Institute for Materials Research and Testing (BAM), Berlin, Germany
- 3Department of Earth, Ocean and Ecological Sciences, University of Liverpool, Liverpool, L69 3BX, UK
- 4Department of Materials, University of Oxford, Oxford OX1 3PH, UK
- 5Department of Earth and Environmental Sciences, University of Minnesota-Twin Cities, Minneapolis, Minnesota, 55455, USA
- 6Rhenium Alloys, Ohio, USA
Olivine is the most abundant mineral in Earth’s mantle, and its rheological behaviour is likely to control upper-mantle deformation. While the rheological behaviour of olivine is widely studied, relatively little is known about the behaviour of individual olivine grain-boundaries. There is a pressing need to advance our understanding of their physical and chemical properties. Forsterite bicrystals, synthesized by direct bonding of highly polished single crystals at high temperature, were tested in a creep apparatus to investigate sliding along a single planar grain-boundary at high temperature (1300°C and 1400°C). Prior to deformation, the lateral surfaces of the bicrystals parallel to the shear direction were polished, and fiducial markers were scribed perpendicular to the grain-boundary trace to track grain-boundary sliding. Bicrystals were deformed in shear between two polycrystalline alumina pistons or two single crystal forsterite pistons, at 1 atm, with applied resolved shear stresses ranging from 1 to 30 MPa. Post-deformation microstructural analysis using a scanning electron microscope (SEM) shows discrete offsets of fiducial markers, which is the first direct evidence of grain-boundary sliding in olivine bicrystals. These results establish that the studied grain-boundaries are significantly weaker than crystal interiors, and that, crucially, grain-boundary sliding is controlled by the crystallography of crystal interiors and is favoured in a direction nearly parallel to the weakest slip direction in both crystals of the bicrystal. The measured effective grain-boundary viscosities fit well theoretical models of a dislocation grain-boundary sliding mechanism and are higher than measurements inferred from attenuation. This evidence may highlight the important role of boundary dislocations in accommodating grain-boundary sliding in large grain sizes. These new results indicate that grain-boundary sliding in olivine could play a crucial role in the development of crystallographic preferred orientation and the resulting seismic anisotropy in the upper mantle and should therefore be accounted for in geodynamic models of Earth’s interior.
How to cite: Mecklenburgh, J., Singh, S., Mariani, E., Thom, C., Marquardt, K., Wheeler, J., and Hansen, L.: Direct Measurement of Grain-Boundary Sliding in Forsterite Bicrystals, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19836, https://doi.org/10.5194/egusphere-egu25-19836, 2025.
