THE FLOW BEHAVIOR OF PARIALLY MOLTEN CRUSTAL ROCKS AND THE EXTRACTION OF CRUSTAL MELTS FROM THEIR PROTOLITHS

In the source region for crustal melts, melt producing reactions yield a rock permeated with what may be a very viscous fluid. Here we are concerned the problem of collecting the fluid into substantial bodies that can ascend to higher crustal levels. Local circumstances may prevent the melt from escaping at all, in which case we are concerned with the mechanical properties of a partially molten interval of crustal rocks, which may influence the behavior of the surrounding rock mass, particularly during tectonism. It is widely believed that momentum imparted to the melt phase by non-hydrostatic stress gradients can significantly enhance the effects of gravity in causing segregation of melt, with concomitant compaction of the solid matrix.

The relatively few experimental studies made on partially molten granitic rocks have shown that deformation can be accomplished by (a) cataclastic deformation and frictional sliding of the matrix of solid grains, facilitated by high melt pressure, or (b) intracrystalline plastic deformation of the solid grains. There is only equivocal evidence for flow of the solid matrix controlled by diffusional mass transfer processes, presumably accelerated by the presence of the high diffusivity melt phase. Theoretically derived flow laws for diffusion creep tend to suggest that it may be experimentally accessible, but only at rather low strain rates. We have performed preliminary experiments at 300 MPa confining pressure using a 'synthetic' granite, comprising quartz grains (60 to 100 micron grain size) and a melt prepared from oxides and near the albite-quartz eutectic, with melt fractions between 15 and 30%. Non-linear, power-law creep with stress exponents between 2 (wet) and 3.5 (dry) was observed, but strain rates were too high to access diffusion creep according to flow law models. It is suggested that granular flow occurred, but rate limited by neck growth at grain contacts followed by time-dependent fracturing of those contacts to permit grain rolling/sliding events. Natural rock textures suggest that granular flow is important or dominant in migmatites, but the nature of the rate controlling process is rarely evident.