Large strain experiments on crystal- and bubble-bearing silicic magmas

We present novel experimental results on the rheological behavior of magmas composed of liquid and both, gas-pressurized bubbles and crystals, deformed at magmatic pressures and temperatures. This study aims to constrain the dependence of rheological and physical properties of magmas on the viscosity of the silicate melt, on the relative contents of crystals and bubbles and on the interactions between these three phases. The starting material constitutes of a haplogranitic melt containing variable amounts of H2O (2.68 wt% and 5.25 wt%) and CO2 (2.06 wt% and 4.97 wt%) and different proportions of quartz crystals (between 24 and 65 vol%; 63-125 μm in diameter) and bubbles (between 7 and 26 vol%; 5-150 μm in diameter). The experiments were performed in simple shear using a HT-HP internally-heated Paterson-type rock deformation apparatus (Paterson and Olgaard, 2000) at strain rates ranging between 5*10-5 s-1 and 1*10-3 s-1, at a pressure of 200 MPa and temperatures between 773 and 923 K. At these temperature and strain rate conditions the silicate melt behaves as a Newtonian liquid (Webb and Dingwell, 1990). Consequently, non-Newtonian effects can entirely be related to the presence of bubbles and crystals. In all experimental runs a marked weakening behavior (decrease of stress with increasing strain) was observed. Back-scattered electron images were acquired on external portions of the samples, where the simple shear configuration is best appreciated. These images clearly highlight the presence of melt-enriched portions of the samples were bubbles are strongly deformed. In contrast, in regions surrounding these melt-enriched bands, bubbles are almost spherical testifying that these portions of the material suffered a significantly lower amount of deformation. Locally, deformed bubbles between crystals resulting from local stress concentration generated by the solid particles can be observed. The measured weakening is most probably related to the generation of melt-enriched shear bands. The localization of deformation in these lower viscosity regions results in a decrease of viscosity with increasing strain (shear thinning effects). BIBLIOGRAPHY Paterson M.S., Olgaard D.L. (2000). Rock deformation tests to large shear strains in torsion. Journal of Structural Geology 22, 1341-1358. Webb S.L., Dingwell D.B. (1990). Non-Newtonian rheology of igneous melts at high stresses and strain rates: experimental results for rhyolite, andesite, basalt and nephelinite. Journal of Geophysical Research 95 (B10), 15695-15701.