Preliminary results on experimental deformation of partially crystallised magmas.
Résumé
Magmatic fabrics are common in volcanic and plutonic rocks. In the latter, these textures are often used as markers of regional deformation patterns. However, the origin of such fabrics is still not well understood. The most commonly accepted hypothesis is that such textures originate by a magmatic flow mechanism. In order to characterise this mechanism and test the possibility of generation of strong magmatic fabrics, in-situ experiments on the orientation of crystals in an already partially crystallised magma are being conducted. The experimental procedure is divided in two steps: partial crystallisation of glass cylinders in Internally Heated Pressure Vessels (IHPV), followed by the deformation of the cylinders at 0.1MPa. Experiments are conducted on the Eburru rhyolite (Kenya), a massive, bubble- and crystal-free, peralkaline obsidian with 0.2 wt % H2O, from which glass cylinders have been cored. In order to explore a wide range of crystallisation, partially crystallised cylinders are produced at different temperatures between 700°C and 900°C and 150 MPa. Kinetics of crystallisation of peralkaline magmas ensure a fast attainment of chemical equilibrium (100 to 300 hours depending on temperature). Deformation experiments are conduced in a new apparatus designed to deform cylindrical samples (L=10-15mm – Ø=5mm) of crystal-bearing magma. This new apparatus works on the principle of the parallel-plate viscosimeter (coaxial strain). It allows deformation to be performed at different temperatures, pressures, strain rates, shortening ratios, and viscosity to be measured as well. Calibration measurements of the apparatus on viscosity of the standard melt DGG1 and the Eburru rhyolite at 0.1 MPa have demonstrated its precision to be within ±0.2 log units. Deformation experiments are conduced at 0.1 MPa between 600°C and 850°C. Due to the short duration of experiments, neither new crystallisation nor bubble nucleation occurs. The results obtained from deformation at 0.1 MPa will be presented and discussed.