Torsion experiments on synthetic magmatic suspensions
Résumé
The microstructures and the rheological behaviour of synthetic magmatic suspensions in the range of 0% to 80% solid fraction have been investigated in torsion experiments. The liquid phase is a haplogranitic melt (HPG) with 2.5 Wt % H20. The solid phase is composed of a sieved fraction (45Μm<Φ<90Μm) of crushed alumina grains that avoid any chemical interaction with the melt such as dissolution/crystallisation during synthesis of the suspension and during the experiments. Four analogous suspensions with respectively 0, 20, 60 and 80% solid fraction were synthesised at 100 MPa and 950°C during one month ensuring a homogeneous repartition of water in the melt. Torsion experiments were performed in a high-pressure, high temperature Paterson apparatus at 300 MPa confining pressure, temperatures ranging from 475°C to 1000°C and shear strain rates from 2.1x10-3 to 2.0x10-5 s-1. Pure melt was deformed up to Γ=8.8. Steping tests at various shear strain rates yielded a stress exponent of ~1.1 with an apparent viscosity ~1010.9 Pa.s at T=475°C and a stress exponent of ~1.0 with an apparent viscosity ~1010.3 Pa.s at T=500°C. These results confirm the Newtonian character of pure haplogranitic melts at high pressure. The 20% solid fraction suspension departs from Newtonian behaviour with a stress exponent n=1.4 and an apparent log viscosity varying from 11.5 (T=500°C) to 10.6 (T=600°C) both at a strain rate ~2.10-4 s-1. Shape Preferred Orientations (SPO) of alumina grains and shapes and orientations of initially spherical (22 Μm in diameter) residual air bubbles were analysed using the inertia tensor method on polished sections normal to the shear direction. For Γ=2.6 the solid fraction developed a strong SPO with a mean direction Α=2° anticlockwise from the shear direction and bubbles passively deformed with a mean long axis orientation 24° anticlockwise and a mean aspect ratio R=2.07. For Γ=17.5, the SPO of the alumina grains became significantly weaker but still homogeneous in orientation with Α=6° anticlockwise while long axis of bubbles were oriented 17° anticlockwise with R=2.13. For the 60% solid fraction suspension, shear softening behaviour is evidenced by a stress exponent n=2.2 for T=550°C and n=2.0 for T=600°C. Measured apparent viscosities from 1010.1 Pa.s to 1011.6 Pa.s are four orders lower that predicted by theoretical models. From Γ=8.8 to 21.3, SPO of the alumina phase determined by the intercept method is composed of two mean heterogeneously developed directions respectively ~10° anticlockwise and ~25° clockwise from the shear direction. These two associated directions that match S/C structures are locally disturbed by strain localisation developing synthetic R and P Riedel shear bands that suggest granular flow-like behaviour of the solid suspension. This strain localisation with possibly melt enhanced weakening in strain localised domains is a possible cause of low apparent viscosities recorded in these experiments. For 80% solid fraction, the samples were too strong to be deformed in the stress range of the apparatus. This behaviour implies a rheological threshold between 60% and 80% solid fraction that matches with the Particle Locking Threshold suggested by theoretical models.