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

Abstract

[1] 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 mm grain size) mixed with an albite-quartz melt prepared from oxides, with added water (2.5 wt %) dissolved in the melt phase (viscosity 9 Â 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 À4 to less than 10 À7 s À1 , 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 À6 s À1 ) data were fitted to an empirical flow law of the form de/dt = A exp(B f m ) exp(ÀH/RT) s n with the parameters logA = À1.39, m = 3, n = 1.8, B = 192, H = 220 kJmol À1 , where de/dt is strain rate (s À1 ), s is flow stress (MPa), f 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.

The Viscous to Brittle Transition in Crystal- and Bubble-Bearing Magmas

“…This inversion of the relationship between the τ c and magma crystallinity can be reconciled with microscopic processes (Figures 2C,D). Once φ reaches about 0.4-0.5, shear-thinning behavior is observed because local dilation and compaction are required for magmas to flow at such high crystallinity (Rutter et al, 2006;Caricchi et al, 2007;Pistone et al, 2012). When the φ increases at constant β, the β/µ increases leading to a relative bubble enrichment in the melt phase.…”

Experimental studies on seismic anisotropy enhanced by alignment of melt: Implication for the origin of abnormal anisotropy in Tibetan crust

“…On the other hand, experimental studies demonstrated that partial melting led to a general reduction in rock strength and concentrated deformation (Rutter et al, 2006;Mecklenburgh and Rutter, 2003;Kohlstedt and Zimmerman, 1996;Rutter and Neumann, 1995;Dell'Angelo et al, 1987). As shown by our experiment, the anisotropy enhanced by alignment of melt is likely comparable to or larger than that induced by LPO of rock-forming minerals.…”

Insights on high-grade deformation in quartzo-feldspathic gneisses during the early Variscan exhumation of the Cabo Ortegal nappe, NW Iberia

“…Melting occurring during a pervasive deformation episode will impart extreme weakening and ductile behaviour to rocks (e.g. Rosenberg and Handy 2005;Rutter et al, 2006), ultimately to the whole crustal section. This environment should promote widespread thinning in the subducting slab in which the Cabo Ortegal nappe was involved during plate convergence prior to the Variscan collision.…”

“…In the case of the Himalayan-Tibet system, the great crustal thickness beneath the Tibetan Plateau contributes significantly to the lithostatic pressure gradient required to force the lateral and frontal flow of a ductile lower crust (e.g. Beaumont et al, 2004;Rutter et al, 2011). Highly sheared and migmatized rocks of the greater Himalayan sequence between the Main Central Thrust and the South Tibetan Detachment are effectively extruded towards the foreland.…”