| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 105 === Fault activities play a key role in crustal evolution. Fluid appears to be one of the most important factors controlling the behaviour of fault zone. High pore-water pressure reduces the effective normal stress of a fault, providing a lubricating agent to trigger fault slip. In addition, the fluid-rock interactions within a fault zone would redistribute the elemental composition between fluid and wall rock, leading to the change the rock composition. The subsequent precipitation in fracture from fluid reduces the permeability of a damage zone for fluid transport and enhance the fault zone strength. However, the exact role of the fluid chemical compositions related to faulting mechanisms remains largely unknown. This study aims to constrain the compositions and temperatures of fluids using the isotopic compositions and elemental abundances of calcite veins sampled from the Taiwan Chelungpu-fault Drilling Project (TCDP). The analyses yielded that Fe, Mg, Li, Be, Pb, Rb, Th and Cs concentrations were higher in calcite veins around the Chelungpu fault zone than those from the adjacent formations. For comparison, Sr concentrations exhibited a pattern contrast from the elements described above. The overall pattern is in contrast to that for the host rocks described in a previous study. As calcite veins represent the archive of fluids percolating through the fracture network, such elemental variations could be best explained with the redistribution of elements in the fault zone, which may result from melting of clay minerals and pyrite and high-temperature fluid-rock reaction during the shearing. The carbonate clumped isotope analysis yielded that calcite veins distributed around the Chelungpu fault zone were precipitated at temperatures generally less than 100 °C. Calculation of isotopic equilibrium also indicated that the related fluids were depleted in 18O. In contrast, the fluids of calcite veins outside the fault zone were at temperatures higher than 100 °C and enriched in 18O. These results indicate that fluid source of calcite veins outside the fault zone is the formation water, whereas fault-related fluid sources are the mixture of formation water and meteoric water. Such isotopic and elemental variations across the fault domain suggest that the fault activities facilitate to mobilize elements and provide a fluid conduit that enables the circulation and infiltration of shallow-ranging meteoric water into great depths.
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