Fluid channelization at the submicroscopic scale in exsolved orthoamphiboles

TEM study of incipient alteration of two exsolved orthoamphiboles from metamorphic terrains in New England, U.S.A., has shown that the planar exsolution microstructure exerts a strong influence on the fluid pathways during the initial stages of hydration. The exsolution microstructure consists of (010) or (120) lamellar mixtures of anthophyllite and gedrite; as the metamorphic fluids infiltrate the amphibole grains, the gedrite lamellae are preferentially attacked by the reactive fluid and transformed to a mixture of chlorite and quartz. The anthophyllite lamellae remain relatively unaffected by the fluid and thereby act as walls between the narrow channels of advancing fluid. The ability of the exsolution microstructure to channelize fluids depends on the scale of the lamellae, with the coarsest microstructures being the most effective. Most of the chlorite domains are coherently intergrown with the gedrite with a* (sub amph) c (super *) (sub chl) . AEM data allows the hydration reaction to be formulated: 1.0 gedrite + 0.24 Fe (super 2+) + 1.56 Mg (super 2+) + 6.46 H (sub 2) O ->> 1.51 chlorite + 1.64 quartz + 0.53 Na (super +) + 0.01 Mn (super 2+) + 2.84 H (super +) .