MICROSCALE ISOTOPIC STUDY OF FLUID MIGRATION DURING METAMORPHISM.

喀拉拉邦南部麻粒岩相上地壳阶地不同组合中石墨和方解石的同位素特征为研究大陆下地壳流体过程提供了有价值的信息。从碳同位素特征上可以清楚地区分出两类石墨：一类是由沉积物中有机质转化而来的生物源石墨，另一类是由岩浆来源的富CO_2流体沉积而来的石墨。与早期炭屑岩伴生的石墨以及充填介观剪切和断裂的石墨代表了源自岩石圈下岩浆的同位素均质富CO_2流体的结晶，这些流体由水力压裂传播，形成了一个相互联系的构造控制的流体网络。钙硅酸盐中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，在微米尺度上石墨单晶所显示的明显的同位素分带表明，在孤立的充满液体的口袋中缓慢结晶。在下地壳中，二氧化碳和水将通过裂缝和晶界移动。我提出的机制清楚地显示了二氧化碳和水在接触变质梯田的化学前沿的不同行为。碳通常以CO_2或CO_3的形式通过晶格扩散迁移，而氧可以以水分子的形式通过晶界进行长距离迁移。由此可见，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.