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 these textures originate by a magmatic flow mechanism. We are currently testing another hypothesis, namely: the possibility that deformation affects crystallisation and thus, fabric patterns. The main difficulty in such experiments comes from an antagonism between the low viscosity of magmas (i.e. fast deformation) at relevant geological temperatures and the slow kinetic of crystallisation. To overcome this problem, we start our deformation experiments with an already partially crystallised magma (crystallinity = 30%), which increases the viscosity by about 1 order of magnitude. The experimental procedure is divided in two steps: synthesis in IHPV at 850°C, 300 MPa, during 10 days, of partially crystallised glass cylinders (starting material: a rhyodacitic glass, 69% SiO2, with 3 wt% bulk H2O), followed by the deformation of the cylinder at the same pressure but at a temperature below that of synthesis (in the range 700-800°C), in order to induce a second crystallisation stage. The effect of deformation is characterised by comparing deformed and undeformed charges produced at the same temperature from the same starting materials. To this end, we have designed a deformation apparatus (Champallier et al., 2001). This first prototype works at high strain rate and does not allow to accurately determine the onset of deformation at HP-HT. The resulting deformed cylinders all displayed complex crossed fabric patterns difficult to interpret. A new deformation apparatus, overcoming the problems of both onset and strain rate, is currently being tested. It will be presented with the results on fabrics and nucleation obtained from deformation/crystallisation experiments. Champallier R, Scaillet B, Pichavant M, Pons J, Peilleron L, Bellenoue D & Lefèvre A, Journal of abstracts of the 26th Assembly of the European Geophysical Society, (2001).
