Fluid-rock interaction during retrograde greenschist-facies conditions may result in a wide variety of quartz textures as observed by optical and SEM cathodoluminescence (CL) (Behr, 1989). The textures are formed by water-quartz diffusional processes and related with aqueous fluid inclusions like brines (not with CO2 inclusions). The majority of secondary CL textures are formed at temperatures between about 200C and 350-400C. By the CL study of high-grade metamorphic rocks the texture development can be used as a measure for the role of metasomatic alteration by late and post-metamorphic fluids. Although the CL textures are mainly related with relic fluids the observations by CL may support also the recognition of aqueous fluid activity as an important factor for the charnockitization process (Perchuk, 1993; Touret, 1995). Secondary CL textures in quartz from different terrains have been compared with mineral and fluid inclusion data.
Diffusional and quartz healing textures include grain boundary and channelway alteration, concentrations
of micropores, healed microfractures, and haloes around fluid inclusions. The latter have a star-shaped or amoeboid geometry and are related to decrepitated fluid inclusions. Decrepitation centres are typically connected by thin
(<1 mm) healed microcracks. The complex halo textures most frequently occur in rocks which underwent an episode of fast uplift (e.g. magmatic charnockites of the Bamble sector - Norway); fluid inclusions in these rocks are extensively modified during the retrograde PT-path. Furthermore, the textures are well developed around fluid inclusions containing H2O-CH4-CO2-graphite formed by chemical re-equilibration at lower temperatures (Rogaland - Norway; Victoria Land - Antarctica).
CL intensity and wavelength are basically related to aqua-metallic defects in quartz induced or restored during rock evolution. The textures are assumed to form by one
or a combination of different fluid-controlled processes:
(a) water diffusion; (b) radioactivity of circulating fluids;
(c) quartz dissolution-precipitation; (d) quartz-quartz replacement; (e) hydrofracturing. The concentration of defect structures around fluid inclusions in quartz under stress conditions is assumed one of the most effective
mechanisms. On the other hand some textures indicate quartz healing by precipitation (pore closure) or replacement crystallization. In some cases (Bamble) idiomorphic quartz crystals formed in situ from growth nuclei.
Diffusional alteration textures like micropore clusters and alteration along grain boundaries and along microcracks show better contrasts by partial CL extinction after longer irradiation (1-10 minutes) of the electron beam. This change is explained by increasing amorphization and diffusion of released crystal water into micropores. Darkly contrasting textures (in SEM-CL) observed by this "secondary" CL are interpreted as zones of pervasive fluid flow related with metasomatic alteration e.g. in the incipient charnockites of the Port Edward area, South Africa (Van de Kerkhof and Grantham, 1995).
CL microscopy is the only method by which the effects of fluid-quartz interaction can be directly visualized and therefore is a powerful tool for the interpretation of the fluid role during metamorphism.
Behr, H. J., Nds. Akad. Geowiss. Veröfftl. 1, 7-41 (1989).
Perchuk, L. L., Chemical Geology 108, 175-186 (1993).
Touret, J. L. R., Boletin de la Sociedad Espanola de Mineralogia 18-1, 250-251 (1995).
Van den Kerkhof, A. M. & Grantham, G. H., Extended Abstracts of the Centennial Geocongress Johannesburg (1995).