Experimental deformation of partially molten granite and implications for strain localization

To improve our understanding of partially molten systems we conducted a set of hydrostatic, general shear and axial compression experiments on sintered aggregates composed of equal amounts by weight of quartz, albite and microcline (grain size of 37-53μm). All experiments were conducted using a Griggs solid medium apparatus at T=900°C, P=1.5GPa and strain rates from 10^-4/s to 10^-6/s. Previous hydrostatic and axial compression experiments conducted on partial molten granitic rocks have shown that the initial grain size, amount of melt and strain rate are important parameters for the development of distinct microstructures, LPO, and melt distribution. In addition, some of these studies demonstrated that the strength of granite and aplite decrease significantly for melt contents up to 15%, when compared to similar melt-free rocks. The rock's strength deep within the Earth decreases owing to partial melting which brings up some questions: would strain localization take place when partial melt affects rheology? Would brittle and/or ductile shear zones act as potential regions for concentration of partial melt? Is there a critical fraction of melt responsible for strain localization? How is melt distribution influenced by deformation? How does the kinematics of deformation (i.e., axial compression versus general shear) affect melt distribution? The purpose of our experiments is to investigate the role of melting on the rheological properties of crustal rocks. In addition, we seek to provide new constraints on the grain scale processes that control the properties of partially molten rocks and the importance of these processes in understanding shear localization in the lithosphere. Samples were made from crushed Amelia albite (Ab₉₇Or₂An₁), Hugo Microcline (Or₉₀) and Black Hills quartzite, which have all been used in previous experimental deformation studies. The albite is essentially pure; the microcline contains ~ 1% of muscovite. The Black Hills quartzite contains < 1% feldspar, iron oxide and clays, and trace amounts of apatite, zircon and rutile. Our experiments were performed on "as-is" synthetic aggregates or adding ~ 1 wt. % of de-ionized water to samples, to produce a small melt percentage (~ 3-5%, according to previous data). At axial strain rate of 1.5 x 10^-6/s, which translates to a shear strain rate of 1.6 to 2.5 x 10^-5/s, the maximum strength of nominally melt-free synthetic aggregate is 180 MPa, while samples with 3-7% melt show strength values of ~ 110 to 145 MPa. Our results indicate that the strength of partially molten granitic aggregates is similar to that of quartzite with 0.17 wt.% water added and are significantly weaker than feldspar aggregates with 0.1 wt.% adsorbed water or < 0.22 wt.% water added. Under hydrostatic conditions melt is found in isolated pools and/or confined to triple grain junctions. Deformed samples show more homogeneously distributed melt; melt is observed both in triple junctions and wetting grain boundaries. Wetted boundaries are more common in more highly strained regions. These observations suggest that changes in melt distribution promote shear localization.