Program > Browse abstracts by speaker > Pichavant Michel

Natural magmas react to thermal or compositional perturbations by developing specific textures. Igneous textures are diverse and often very complex. They result from crystal-melt reactions such as crystallization and crystal dissolution. Natural textures are potentially rich in information but their interpretation in terms of reaction mechanisms and timescales requires experiments. Crystal dissolution can be either simple or more complex texturally. It is controlled by three main processes, convection, diffusion and interface kinetics. In the absence of convection, crystal dissolution is mostly diffusive, as marked by chemical zonation in the melt and dissolution rates that are proportional to t0.5. However, diffusion in the melt is coupled with atom detachment at the crystal-melt interface. One basic assumption is that interface kinetics are very fast and so dissolution is generally controlled by diffusion in the melt. Although experimentally checked, exceptions to this general “rule” exist. Another assumption is that chemical equilibrium is established at the crystal-melt interface. However, because of multicomponent diffusion in the melt, interface melts depleted and enriched in fast- and slow-diffusing components respectively can be produced. The dissolution of solid solutions has been investigated numerically but is less well documented from experiments. The dissolution of Fo-poor Ol in a primitive basalt produces highly complex textures. Chemical diffusion is observed both in the melt and in the crystal. A new more Fo-rich Ol crystallizes at the interface. Melt inclusions with compositions deviating from the far-field melt form in the reacting crystal. Plagioclase (Plag) dissolution experiments demonstrate the pile up of Al and Ca at the interface which promotes crystallization of a new An-rich Plag and makes the dissolution rate to decrease with time. For this type of complex crystal-melt reactions, rate laws derived from experiments, although empirical, are useful and can directly inform on magma dynamics.