Flow of synthetic, wet, partially molten “granite” under undrained conditions: An experimental study

Abstract

Major problems in experimental studies of the flow of partially molten granitic rocks are (1) maintaining control of the grain size of the matrix of solid grains, (2) controlling the melt fraction, and (3) controlling the melt viscosity (closely linked to water content). To overcome these problems, we used a synthetic “granitoid” comprising a solid matrix of quartz grains (50 μm grain size) mixed with an albite‐quartz melt prepared from oxides, with added water (2.5 wt %) dissolved in the melt phase (viscosity 9 × 10 4 Pa s). Experiments were performed undrained, with melt fractions of 0.1, 0.2, 0.25, and 0.3. Constant strain rate, creep, and stress relaxation experiments were carried out in a gas medium apparatus at 1273 and 1173 K, mostly at 300 MPa confining pressure, up to ∼15% shortening strain. Strain rates ranged between 10 −4 to less than 10 −7 s −1 , and fully ductile mechanical behavior was observed. Melt fraction has a more profound effect on strength than water content, temperature, or total confining pressure. Low strain rate (<10 −6 s −1 ) data were fitted to an empirical flow law of the form dɛ/d t = A exp( B ϕ m ) exp(− H / RT ) σ n with the parameters log A = −1.39, m = 3, n = 1.8, B = 192, H = 220 kJmol −1 , where dɛ/d t is strain rate (s −1 ), σ is flow stress (MPa), ϕ is melt fraction, and T is temperature (K). Strength is expected to fall rapidly above about 0.4 melt fraction to levels characteristic of flow of the melt containing suspended crystallites. Extrapolation to geological strain rates using this flow law shows that in nature, migmatites bearing granitic melt will be extremely weak, much weaker than silicate rocks deforming by intracrystalline plasticity. Microstructural study shows grains remain equant at all strains, with no discernable formation of a grain flattening fabric nor a large strain contribution from microfracturing; thus bulk intracrystalline plasticity is of minor importance, and intergranular sliding is implied. The nonlinear flow observed is tentatively attributed to a combination of sintering of grain contacts and sliding failure of such contacts, with a substantial contribution from diffusive transfer processes.