Effect of water on rheological properties of garnet at high temperatures and pressures

- Water solubility in garnet first increases then decreases with increasing pressure.
- Flow law was determined for garnet under water-saturated conditions.
- Water significantly influences the creep strength of garnet.
- At shallow mantle depths, garnet viscosity is strongly affected by water.
- Deeper in the mantle, garnet viscosity is governed by the dry flow law.

The effect of water on the rheological properties of garnet was investigated with triaxial compressive creep experiments at high temperatures (1223⩽T⩽1423 K) and high pressures (1.6⩽P⩽5.6 GPa) using a deformation-DIA (D-DIA) apparatus. Samples fabricated from a natural polycrystalline garnet (Pyr23Alm20Gro57) together with samples prepared from San Carlos olivine were deformed under water-saturated conditions with water supplied by the dehydration of talc during the experiments. Experiments were carried out on a synchrotron X-ray beamline. In each experiment, differential stress and sample displacement were determined using synchrotron X-ray diffraction and time-resolved radiography techniques, respectively. Water fugacity was calculated based on equation of state for water at experimental P-T conditions. A fit of our experimental results on the creep of garnet samples to a power law demonstrates strong dependencies of strain rate on water fugacity with a water fugacity exponent of r=1 and on pressure with an activation volume of V⁎=28×10−6 m3/mol. Importantly, under water-saturated conditions, the water content of deformed garnet samples measured by infrared spectroscopy increases with increasing pressure (i.e., increasing water fugacity) up to an average value of ∼21 000 H/106Si at ∼3.0 GPa and then decreases with increasing pressure down to ∼3000 H/106Si at 5.6 GPa. The flow law for garnet quantified in this study provides an important constraint on the strength of the subducted oceanic crust. For a water-enriched environment, the viscosity of a garnet-rich layer will be low above the shallow upper mantle because the water content in garnet increases with increasing pressure at relatively low pressures. However, at greater depth, the viscosity will be high because the direct effect of pressure on the creep through thermally activated processes outweighs the indirect effect on the creep through water solubility.