MICROSCALE ISOTOPIC STUDY OF FLUID MIGRATION DURING METAMORPHISM.

南喀拉拉邦麻粒岩相表壳地体中各种组合中的石墨和方解石的同位素特征为大陆下地壳的流体作用提供了有价值的信息。根据碳同位素特征，我清楚地区分了两种类型的石墨岩：一种来自生物源，来自沉积物中捕获的有机质的转化；另一种来自岩浆来源的富含CO_2流体的沉淀。与早期紫苏花岗岩伴生的石墨岩和充填在中观岩片和断层中的石墨岩代表了来自亚层岩浆的同位素均质富CO_2流体的结晶，这些流体通过水力压裂传播，形成了一个相互关联的构造控制的流体网络。来自方解硅酸盐的方解石的碳、氧同位素组成也证实了外部流体的侵入。另一方面，来自伟晶岩的石墨单晶在微米尺度上显示的明显的同位素分带表明，孤立的充满流体的口袋结晶缓慢。在下地壳，二氧化碳和水将通过裂缝和晶界移动。提出了接触变质地体化学锋面上二氧化碳和水的不同行为机制。碳通常以CO_2或CO_3的形式通过晶格扩散迁移，但氧可以作为水分子长距离地通过晶界迁移。这表明造成这种不同行为的最重要因素是离子半径或化学形态的大小。

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.