台灣車籠埔斷層鑽井A井岩心之碳酸鹽類礦物碳氧同位素研究

Oxygen and Carbon Isotopic Studies of Carbonate Minerals from TCDP Hole-A Drill Cores in Taiwan

巫佳靜(Jia-Jing Wu)

指導教授 ： 王珮玲

國立臺灣大學/理學院/海洋研究所/碩士(2006年)

https://doi.org/10.6342/NTU.2006.02697

摘要

斷層活動和剪動帶形成的裂隙、裂面，被視為地殼中流體流動的主要通道，當環境條件適當時，原本充填流體的裂隙，開始沉澱結晶出適合生長的礦物而形成礦脈。礦脈帶有流體的同位素訊號，能反應生長條件，因此分析礦脈的同位素組成能瞭解流體的來源與破裂帶的特性。台灣地處活動造山帶，構造活動和礦脈的生成關係密不可分，本研究分析西部麓山帶地層中礦脈之碳氧同位素組成與地層變化和斷層活動間的關係，推測可能的碳源與流體來源，探討構造活動、流體作用與礦脈生長間的關係。
車籠埔斷層鑽井之A井岩心中出現的方解石脈、斷層擦痕充填物、生物碎屑及孔隙膠結物的碳氧同位素成分分析結果如下：方解石脈的 δ13CVPDB 值為 -14‰ 至 -2‰，δ18OVSMOW 值為 10‰ 至 24‰，斷層擦痕充填物的 δ13CVPDB 值為-10‰ 至 -2‰，δ18OVSMOW 值為14‰ 至 20‰，生物碎屑的 δ13CVPDB 值為 -10.0‰ 至 2‰，δ18OVSMOW值為12‰ 至 30‰，孔隙膠結物的 δ13CVPDB 值為 -11‰ 至 -3‰，δ18OVSMOW 值為16‰ 至 20‰。
比對礦脈之碳同位素組成隨深度的變化及岩心構造資料，推測影響碳酸鹽礦脈碳同位素組成的因素，可能與斷層構造活動及沉積岩層的改變有關。在岩心深度 400 至 1290 公尺出現的礦脈，碳同位素組成隨深度增加由 -10‰ 變重至 -2‰ ，顯示地下水中溶解的無機碳和海水碳酸鹽類為主要碳源的混合端成分。深度介於 1290 至 1710 公尺之礦脈的碳同位素組成，隨深度增加而由 -2‰ 變輕至 -12‰ ，由於此深度的岩心位於車籠埔斷層之下，所在地層由錦水頁岩轉變為桂竹林層，因此推論碳源的轉變應與斷層活動或岩層變化有關，而碳源的變化均受控於微生物作用。由礦脈氧同位素組成估算的流體氧同位素成分，其值隨著深度加深而變大，由 -14‰ 變重至 -4‰ ，推測流體的來源應為天水與海水或地層水之混合。

關鍵字

車籠埔斷層鑽井 ； 碳酸鹽類礦物 ； 碳氧同位素

並列摘要

Faults and shear zones are generally thought to be major fluid conduits in crustal environment. Fluid circulation and migration may deposit or recrystallize carbonate minerals in fractures within the fault zone. Isotopic signatures of such crack-fill materials will serve as a good indicator of both sources of fluid and processes of fluid-rock interaction.
This study analyzed carbon and oxygen isotopic compositions of calcite veins, fillings of slickensides, cementations and fossils retrieved by Taiwan Chelungpu Drilling Program (TCDP) Hole-A Drill cores. The core penetrated the active Chelungpu fault zone at around 1100 m depth and was composed of the Cholan, Chinshui and Keichulin formation. The calcite veins mainly appeared in sandstone and siltstone with several mm in width.
Calcite veins revealed variable δ18OVSMOW values ranging from 10 to 20‰, and δ13CVPDB values of -13 to -2‰. Filling of slickensides shown δ18OVSMOW values from 14 to 20‰ and δ13CVPDB values from -10 to -2‰. The δ18O VSMOW and δ13CVPDB values of cementation were between 16 to 20‰ and -11 to -3‰, respectively, and the valves for fossils varied from 12 to 30‰ in δ18O VSMOW values and -10 to 2‰ in δ13CVPDB values.
The δ13CVPDB values of calcite veins and cementations generally increased with depth between 400 to 1290 m, then decreased with depth below 1290 m. A mixture of dissolved inorganic carbon in groundwater and marine carbonate was suggested to be the carbon sources for these carbonate precipitation from 400-1290 m interval. The carbon isotopic compositions of calcite veins from 1290-1710 m interval might be controlled by the fault events or the hosted formations. The composition reflected the microbial processes involved during fault activity or strata deposition. Calculated δ18O values of fluids in chemical equilibrium with calcite veins were ranging from -14 to -4‰, which were in between that of meteoric water and seawater. These results inferred that the calcite veins were formed from fluids originating from meteoric water mixed with seawater or formation water and carrying bicarbonate dissolved from fossil or diagenetic carbonate.

並列關鍵字

TCDP ； carbonate minerals ； carbon and oxygen isotope

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