Growth mechanisms of metasomatic reaction veins in dolomite marbles from the Bergell Alps

Tremolite and olivine reaction veins in dolomite marble inclusions in the B ergell granite formed by a crack-reaction-seal mechanism during the cooling history of the area. Brittle failure of stressed marble opened extension c racks that served as conduits for infiltrating silica-rich aqueous fluids. Reaction of the fluids at 450-550 degrees C with dolomite along the fractur e walls resulted in partial replacement of dolomite by reaction products wh ose mineralogy was controlled mostly by temperature. Aqueous silica dissolved in the fluid in the central fracture was transport ed by diffusion from the fracture wall to the reaction front in the dolomit e marble. The velocity of the replacement reaction front in the marble itse lf was controlled by the slower of the two processes: the surface-reaction kinetics of the replacement reaction and the diffusion rate of silica to th e reaction site. Reaction veins with very different reaction front morphologies occur. Tremo lite veins always have straight reaction fronts parallel to the central fis sure and formed at about 450 degrees C. Olivine veins are typically bounded by highly irregular, wavy, undulating reaction fronts. Ol veins formed at about 550 degrees C. It is concluded that tremolite-vein growth was control led by surface-reaction kinetics, whereas diffusion kinetics controlled the growth of olivine veins. The difference in the morphology of the reaction front surface is probably a consequence of small scale texture variations i n the deposited vein rock and associated porosity differences. Both types o f veins formed in about 1000 years.