FLUID CHANNELIZATION AT THE SUBMICROSCOPIC SCALE IN EXSOLVED ORTHOAMPHIBOLES

Transmission electron microscope study of incipient alteration of exso lved orthoamphiboles has shown that the planar exsolution microstructu re exerts a strong influence on the fluid pathways during the initial stages of hydration. The exsolution microstructure consists of (010) o r (120) lamellar mixtures of anthophyllite and gedrite. As the metamor phic fluids infiltrate the amphibole grains, the gedrite lamellae are preferentially attacked by the reactive fluid, being transformed to a mixture of chlorite plus quartz. The anthophyllite lamellae remain rel atively unaffected by the fluid and thereby act as walls between the n arrow channels of advancing fluid. The ability of the exsolution micro structure to channelize fluids depends on the scale of the lamellae, w ith the coarsest microstructures being the most effective. The majorit y of chlorite domains are coherently intergrown with the gedrite with a(amph) parallel to c(chl)*. Analytical electron microscope data allo ws the formulation of the following hydration reaction: 1.0 gedrite 0.24 Fe2+ + 1.56 Mg2+ + 6.46 H2O --> 1.51 chlorite + 1.64 quartz + 0.5 3 Na+ + 0.01 Mn2+ + 2.84 H+.