Measurement of activation volume for creep of dry olivine at upper mantle pressure

Olivine is the most abundant and weakest phase in the upper mantle, and thus its rheological properties have a critical role in controlling convective flow in this region. A resilient obstacle to understanding the behavior of olivine in the mantle has been the difficulty of determining activation volume (V*), the influence of hydrostatic pressure on flow strength. The bulk of previous studies examining V* were conducted at low pressure (<300 MPa) and small pressure ranges in gas-medium deformation apparatuses, limiting precision and raising questions about application to relevant geological conditions. For this study, we conducted deformation experiments on dry polycrystalline olivine in the D-DIA apparatus. The development of a new hybrid soft-fired pyrophyllite/mullite sample assembly allowed for a broadened pressure range (2-9 GPa), while stress and strain were measured in-situ with synchrotron x rays. Refinement in diffraction technique has allowed stress resolution of ±0.01 GPa. For the pressure range in this study, we have measured an average activation volume of about 17 cm^3/mol for dry polycrystalline San Carlos olivine. This is a substantial pressure effect, representing a pressure-induced viscosity increase of nearly 7 orders of magnitude from the base of the lithosphere to the bottom of the upper mantle.