MICROSCALE ISOTOPIC STUDY OF FLUID MIGRATION DURING METAMORPHISM.

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Isotopic characterization of graphite and calcite in various associations from the granulite facies supracrustal terrance of southern Kerala provide valuable information on fluid processes in the continental lower crust. From carbon isotope signature, I clearly distinguish two categories of graphites : one derived from biogenic sources through the conversion of organic matter trapped within sediments and the other precipitatioed from magmatically derived CO_2 rich fluids. Graphites associated with incipient charnockites and those filling mesoscopic sheares and faults represent crystallization from isotopically homogenous CO_2 rich fluids which were derived from sublithospheric magmas and which propagated by hydraulic fracturing, forming an interconnected structurally controlled fluid network. Extermal fluid ingress is also borne out by the carbon and oxygen isotopic compositions of calcite from calc-silicates. On ther other hand, the marked isotopic zonations on a micrometer scale displayd by single crystals of graphite from pegmatities indicate slow crystallization from isolated fluid-filled pockets.In the lower crust, carbon dioxide and water will move through fractures and the grain boundaries. I proposed the mechanism which clealy showed the different behavior of carbon dioxide and water in the chemical fronts of a contact metamorphic terrance. Carbon normally migrates as CO_2, or CO_3 form through lattice diffusion, but oxygen can migrate through grain boundary as water molecules for a long distances. This showed the most important factors of such different behavior is caused the ionic radii or the size of the chemical forms.