Novel fluid medium pressure cells were used to deform antigorite under
constant stress creep conditions at low temperature, low strain rate
(10^-9 - 10^-4 1/s), and high pressure (1 GPa) in a Griggs-type
apparatus. Antigorite cores were deformed at constant temperatures
between 75°C and 550°C, applying 8-12 stress-strain steps per
temperature. The microstructures of deformed samples highlight the
importance of basal shear and kinks to antigorite plasticity.
Rheological data were fit with a low temperature plasticity law,
consistent with a deformation mechanism involving large lattice
resistance. When applied at geologic stresses and strain rates, the
extrapolated viscosity agrees well with predictions based on subduction
zone thermal models.