Influence of water fugacity and activation volume on the flow properties of fine‐grained anorthite aggregates

<jats:p>To specify quantitatively the effect of pressure and water weakening on the flow strength of feldspar we performed triaxial creep experiments in a gas deformation apparatus at temperatures of 1000–1150°C, confining pressures of 100–450 MPa, and axial stresses of 10–400 MPa, resulting in strain rates of ∼6 × 10<jats:sup>−7</jats:sup> to 3 × 10<jats:sup>−3</jats:sup> s<jats:sup>−1</jats:sup>. Dense samples with a grain size of ∼3 μm were prepared by hot‐isostatic pressing of anorthite glass powder. Hydrous samples contain about 0.33 ± 0.14 wt % H<jats:sub>2</jats:sub>O and dry specimens 0.0005–0.02 wt % H<jats:sub>2</jats:sub>O. The estimated residual glass content of wet samples is <2 vol %. Samples deformed by grain boundary diffusion‐controlled creep at low stresses and dislocation creep at stresses ≳150 MPa. We estimate an activation volume of <jats:italic>V</jats:italic> ≈ 24 cm<jats:sup>3</jats:sup>mol<jats:sup>−1</jats:sup> for anhydrous samples deforming in diffusion creep. For wet samples, deformed in hydrous conditions with varying buffers fixing oxygen fugacity, the activation volume is about 38 cm<jats:sup>3</jats:sup> mol<jats:sup>−1</jats:sup>. Creep rate of hydrous anorthite aggregates depends on water fugacity raised to a power of <jats:italic>r</jats:italic> = 1.0 ± 0.3, suggesting hydrolysis of oxygen bonds. Considering the effect of activation volume and water fugacity on extrapolation of constitutive laws to conditions prevailing in the continental lower crust, viscosities of hydrous feldspar aggregates increase by a factor of <3.</jats:p>