AN SEM STUDY OF POROSITY AND GRAIN-BOUNDARY MICROSTRUCTURE IN QUARTZ MYLONITES, SIMPLON FAULT ZONE, CENTRAL ALPS

A backscattered and secondary electron SEM study of the grain boundary microstructure in quartz mylonites sampled along the length of the re trograde Simplon Fault Zone established three characteristic component s. (1) Fine isolated pores (less than or equal to 1 mu m diameter) are scattered across two-grain interfaces, preferentially concentrated on surfaces in extension. Pores are uncommon on three-grain junctions an d there is no evidence for fluid interconnectivity along three-and fou r-grain junctions. The fine porosity may develop by accumulation of or iginal, mainly intragranular fluid inclusions to the grain boundary du ring deformation and recrystallization and by cavitation of grain boun daries during grain boundary sliding. Dynamic cavitation implies that the ''ductile'' mylonitic deformation is at least locally dilatant and therefore pressure sensitive. (2) Large ''vug''-like pores (up to mm- scale) extend along multi-grain boundaries. Observed in all samples, t hey are most common in the higher initial temperature, coarse-grained samples with a microstructure dominated by grain boundary migration re crystallization. Grains bordering this connected porosity develop perf ect crystal faces, undecorated by fine pores or pits. The irregular '' lobate'' optical microstructure of many migrating grain boundaries act ually consists of a series of straight crystal faces. The coarse poros ity is probably due to accumulation during dynamic recrystallization o f (CO2-rich ?) fluid with a high wetting angle against quartz. (3) In one sample, interconnected sinuous ridges, less than or equal to 0.2 m u m high, are observed to follow three-and four-grain junctions and di sjoint into more isolated worms and spheroidal globules. On two-grain interfaces, these are transitional to more branching vein-like or conv oluted brain-like forms. The brain-like and globular forms have been o bserved, with varying frequency, through the range of samples, with th e globules attaining sizes of up to 60 mu m. Vein structures have also been observed on intragranular fractures. These topologies do not mat ch across adjoining surfaces and must have developed into free space. The ridge-vein-brain-spheroid structure is distinctly different to tha t previously observed on experimentally healed microcracks and its ori gin is not unequivocally established. They could represent unstable me niscus necking of a thin grain-boundary phase of low viscosity, develo ped due to quasi-adiabatic shear and/or local stress-induced dilatancy during microcracking.