Etude des alternances récurrentes dans les skarns et des instabilités du front de dissolution/précipitation

This thesis discusses the development under metamorphic conditions of banded skarns and oscillatory precipitation by metasomatism. Two types of banded skarns are described at Rio Marina (Elba, Italy), the first one is composed of light pyroxene (hedenbergite) bands and dark ilvaite bands, and another one is composed of light quartz bands and dark ilvaite bands. The directions of the bands are parallel to the zoning of the skarns or to the contact between the skarn and the limestone. The geochemical studies indicate that the banded skarns and the ordinary skarns are formed upon the same massive and homogeneous limestones and that the formation of the skarns require important gains of Si and Fe and losses of Ca and CO₂. The same fluid is responsible for the formation of the ordinary and the banded skarns. The comparison of the banded skarns of Rio Marina with other examples of banded skarns allow us to state the following rules: (1) The banded skarns can occur as alternating strata or as orbicular structures. (2) The width of the bands varies from 0. 2 to 6 mm; the widths of the light bands and the dark bands are the same in most cases. (3) In banded skarns there is at least one mineral constituted not only of an element, such as Ca or Mg, derived from the dissolution of the carbonates, but also of an element, such as Si or Fe, derived from the fluid. The formation of banded skarns requires the coupling between the precipitation of the minerals in the bands and the dissolution of the carbonates. The second mineral is composed essentially of the elements derived from the fluid, such as Si and Fe. Based on the previous observations and on the analysis of the conditions for the coupling between the different processes operating in the formation of banded skarns, the qualitative models have been set forth to discuss the possible mechanism for the appearance of banding. The main mechanism is the dissolution-precipitation loop where coupling operates thanks to the aqueous species liberated or consumed by the dissolution and precipitation reactions. This is a positive feedback loop and an indirect auto-catalytic reaction; it creates a delay between the reaction and the transport, that gives rise to the pause of the activity of the loop, then its starting again when the transport fills the delay. The banded skarns are the result of the repetitive appearance of the loop due to the coupling between the reaction and transport. The stability analysis on a quantified model indicates that one necessary condition for the formation of the banded skarns is a high concentration of Si or Fe and a low concentration of Ca in the original fluid.