台灣車籠埔斷層之流體成分及溫度

Abstract

斷層活動對於地殼演化扮演著重要的角色，其中流體為影響斷層活動的重要因子之一。高孔隙水壓會降低岩石有效正向力，進而使斷層易於滑動。流體進入斷層帶會與岩石進行岩水交換反應，促使新礦物形成與元素重新分布，流體後期結晶充填於破裂帶中，則會降低岩石滲透率，使流體不易再進入滑移帶進而提升斷層強度。然而流體化學成份與斷層活動機制之間的關聯性研究還相對稀少，且兩者之間的關係未被完全釐清。 本研究分析來自於車籠埔鑽井計畫中岩心A井中方解石脈穩定同位素成份及元素含量，探討流體成分及溫度。分析結果顯示方解石脈的鐵、鎂、鋰、鈹、鉛、銣、釷及銫濃度在車籠埔斷層帶周圍均有相對高值，相反地斷層帶周圍方解石脈的鍶濃度有相對低值。上述的濃度變化趨勢與前人車籠埔斷層滑移帶中岩石濃度變化趨勢相反，可能由於斷層錯動造成摩擦熱熔融了綠泥石和黃鐵礦而釋出部分元素，以及高溫環境下的岩水交換反應，使得流體中元素重新分布。碳酸鹽叢同位素溫度計分析結果顯示車籠埔斷層帶附近的流體溫度大多小於攝氏100度，且流體的氧同位素值較小。相反的斷層帶以外的流體溫度皆高於攝氏100度，且流體的氧同位素值較大。流體溫度及同位素資料顯示遠離斷層帶的流體以地層水為主，而斷層帶附近的流體則是以地層水及天水混合。藉由研究斷層帶與其周遭方解石脈化學成份及溫度紀錄，顯示斷層活動促使了岩石及流體元素重新分布，並可能提供了流體通道使天水從淺層遷移至深處斷層帶。

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.