An experimental study on creep of partially molten granulite under high temperature and wet conditions

Abstract Samples of natural granulite were deformed in a gas medium apparatus to evaluate the flow strength of the lower crust. The sample consists of ∼52 vol% plagioclase, ∼40 vol% pyroxene, ∼3 vol% quartz, ∼5 vol% magnetite and ilmenite. Water content was ∼0.17 ± 0.05 wt% in the deformed samples. 40 creep tests were performed on 13 samples at 300 MPa confining pressure, temperatures of 900–1200 °C, and strain rates between 3.13 × 10 −6 and 5 × 10 −5 /s, resulting in axial stresses of 12–764 MPa and the total strain up to 7.8–20.5%. At low temperatures of 900–1000 °C, the microstructural observations show that the granulite samples were deformed in semi-brittle deformation regime, mainly by dislocation glide and intragranular microcracking. At medium temperatures (MT) of 1050–1100 °C, deformation was observed to be dominated by grain boundary migration recrystallization, corresponding to stress exponent n MT of 5.7 ± 0.1, activation energies Q MT of 525 ± 34 kJ/mol, log  A MT of 1.3. At high temperatures (HT) of 1125–1150 °C, the samples was deformed mainly by grain boundary migration recrystallization accommodated by partial melting and metamorphic reactions characterized by neo-crystallization of fine-grained olivine, with n HT of 4.8 ± 0.1, Q HT of 1392 ± 63 kJ/mol, and log  A HT of 37.5. Partial melting at high temperatures of 1125–1200 °C, which induces grain boundaries slip and enhances diffusion, has a significant weakening effect on the rheology of granulite, with an estimated strain rate enhancement by 5 times at melt fraction of ∼2 vol%. Reaction from pyroxene to olivine may affect the flow law parameters and deformation mechanism. Based on our data, a wet and cool continental lower crust may still be in brittle deformation regime, whereas a hot lower crust may likely have a weak layer with plastic deformation.