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 ∼52vol% plagioclase, ∼40vol% pyroxene, ∼3vol% quartz, ∼5vol% magnetite and ilmenite. Water content was ∼0.17±0.05wt% in the deformed samples. 40 creep tests were performed on 13 samples at 300MPa 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 12–764MPa 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 nMT of 5.7±0.1, activation energies QMT of 525±34kJ/mol, logAMT 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 nHT of 4.8±0.1, QHT of 1392±63kJ/mol, and logAHT 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 ∼2vol%. 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.