Study of banded skarns and instabilities of dissolution/precipitation front
Etude des alternances récurrentes dans les skarns et des instabilités du front de dissolution/précipitation
Résumé
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.
Cette thèse se propose de discuter la formation des alternances récurrentes dans les skarns et la précipitation oscillante par métasomatose en contexte métamorphique. On commence par discuter deux types d'alternances de Rio Marina (Ile d'Elbe, Italie). Les directions des bandes sont parallèles à la zonation des skarns ou au contact entre le skarn et le calcaire. Les études géochimiques montrent que les skarns à alternances et les skarns ordinaires sont formés sur des calcaires massifs et homogènes et que la formation des skarns nécessite des apports importants de silice et de fer et de lessivages de calcium et de gaz carbonique. La comparaison des alternances de Rio Marina avec d'autres exemples d'alternances a permis d'énoncer les règles suivantes: (1). Les alternances apparaissent généralement sous 2 formes: des strates alternantes ou des structures orbiculaires. (2) Les épaisseurs des bandes varient de 0,2 mm a 6 mm ; les épaisseurs des bandes claires et des bandes sombres sont les mêmes dans la plupart des cas. (3) Dans les alternances, il y a au moins un minéral qui est constitué non seulement d'un élément provenant de la dissolution des carbonates comme Ca et Mg mais aussi d'un élément provenant du fluide comme Si et Fe. le deuxième minéral dans les alternances est compose essentiellement d'éléments provenant du fluide comme Si et Fe. La formation des bandes requiert le couplage de la précipitation des minéraux en alternances et de la dissolution de la roche carbonatée. Des modèles qualitatifs sont établis pour discuter le mécanisme possible d'apparition de ce type de structure spatiale.