Influence of Foliation Orientation on Melt Interconnectivity and Rock Strength in a Natural Fine-Grained Gneiss

Abstract:

In nature, granitic melts are commonly observed in layers parallel to foliations of source rocks but are also observed cross-cutting through the foliations. These observations indicate that foliation can control melt migration at the source, which can also influence the strength of these source rocks. In order to determine how foliation and lineation orientation affects the development of melt interconnectivity and rock strength evolution, we performed a series of experiments on cores of the Gneiss Minuti, a natural fine-grained gneiss consisting of 43% quartz, 40% plagioclase, 16% biotite, and 1% other. Cores parallel, 45°, and perpendicular to the foliation were deformed at a constant temperature of 900°C, pressure of 1.5 GPa, and strain rate of 10^-6/s using the D-DIA apparatus at Beamline 6-BMB at the Advanced Photon Source at Argonne National Laboratory.

The viscosity of the cores with the foliation perpendicular to the compression direction are about 2x that of the parallel. Surprisingly, the viscosity of the core with the foliation orientation at 45° to the compression direction is slightly greater than the foliation parallel core. Melt interconnectivity is greatest in the foliation parallel orientation sample where interconnecting melt channels parallel to the compression direction (l = 80-100 microns) are observed along grain boundaries and cross-cutting through grains. Melt interconnectivity parallel to the compression direction in the perpendicular and 45° orientation cores is limited to single grain scale lengths (l = 10-50 microns) cross-cutting single grains or along grain boundaries. The melt concentrations ranged from about 2.5 vol% in the foliation perpendicular core to 4 vol% in the foliation parallel core. These results indicate that certain foliation orientations more readily allow for the development of interconnected melt channels and, therefore, influence the viscous anisotropy and overall rock strength.