Effects of Melt Interconnectivity on Strength of a Fine-Grained Gneiss

Abstract:

Before magma can be segregated from the source and ascend to Earth’s surface, it must first become interconnected at the grain scale in the source. In order to determine the mechanisms that cause melt interconnectivity in a common granite melt source rock, we deformed stacked cylinders of the Gneiss Minuti with different foliation orientations at three different temperatures (850, 900, and 950 °C), but constant pressure (1.5 GPa), strain rate and total strain (30%) using the D-DIA deformation apparatus at beamline 6-BMB at the Advanced Photon Source at Argonne National Laboratory. The Gneiss Minuti is a fine-grained, foliated rock composed of 52% plagioclase, 34% quartz, and 13% biotite from the Strona-Ceneri Zone, Italy. We deformed stacked cylinders of Gneiss Minuti with two different foliation orientations, 45° and perpendicular to the compression direction.

Cores with foliation oriented at 45° are weaker than the cores with the foliation perpendicular to the compression direction at 850 and 900°C, but both orientations have the same strength at 950°C. Plagioclase, quartz, and biotite deformed by crystal plastic mechanisms, but are sometimes cross cut by melt-filled cracks parallel or sub-parallel to the compression direction. Melt content increases with increasing temperature (1% to 5%). Melt is found as vertical veins along grain boundaries and crosscutting individual grains in all samples but is only interconnected around mica grain boundaries at lower melt fractions. These veins of melt vary in length from 10μm to 20μm, though the melt pockets become shorter and wider as the deformation temperature increases. These results indicate that the effect of small amounts of interconnected melt (2 vol%) has a larger effect on the strength of rock than foliation orientation.