台灣北部地熱區低度變質岩之碳氮硫地球化學特徵

Abstract

台灣北部宜蘭地區擁有豐富的地熱資源，隨著近年來地熱發電技術的提升和能源缺乏的議題逐漸浮現，宜蘭地區的地熱發電潛能再次受到重視。本研究分析取自宜蘭地區三口地熱鑽探井樣品之碳、氮、硫等元素和同位素特徵，以期提供宜蘭地區板岩帶的基本岩石化學特性、沉積環境、沉積物來源等訊息，並推測地熱來源的可能。由於宜蘭地區的岩石屬於輕度變質的板岩帶，為了比對這些元素在板岩的地化特徵，也分析了台灣中部橫貫公路沿線，從西側的沉積岩逐漸變質到東部的多次變質綠色片岩相的樣品，比較變質程度對這些地球化學特徵的影響。 分析結果顯示取自宜蘭地區的三口地熱井之地下岩樣，其碳、氮、硫等元素可能大多受到變質作用大幅影響，可依照其有機碳與總氮之比值判斷其有機物質的來源應為海源，而根據有機碳與黃鐵礦含量之關係，認為其原始沉積環境可能屬於有機物質、硫酸鹽含量與可反應含鐵物質皆充足，且硫酸還原反應快速的缺氧海洋環境。此外，根據黃鐵礦之產狀以及穩定硫同位素特徵，可知區域內之黃鐵礦有兩種成因，一種為典型的沉積型黃鐵礦，其黃鐵礦顆粒結晶較小、δ34S 值變化很大，而另外一種大顆粒黃鐵礦之 δ34S 值的變化較小，可能是沉積型黃鐵礦受到變質作用或與熱液反應再結晶而成，使其δ34S 數值較為集中，熱液活動徵兆與前人研究認為研究地域內可能有地底岩漿庫造成較高地溫梯度有關。至於中部橫貫公路沿線樣品之碳、氮、硫等元素的變化，隨變質度增加到多次變質綠色片岩相而稍有碳、氮流失和硫同位素值一致化的現象，但因樣品數量和種類的涵蓋性不足而無法定論。

Due to the shortage of non-regenerated energy resources and the development of advanced technology in geothermal energy exploration, the potential geothermal energy in Yilan is under revaluation. Three drilled cores from the Chingshui geothermal field and Yilan plain were examined for basic geochemical characteristics (C, N, S and S isotopes) of low-grade metamorphic rocks in slate belt and also for possible signature of geothermal resource. In order to nvestigate possible change of geochemical characteristics during progressive metamorphism, this study also examined a set of samples from sedimentary to greenschist-facies rocks in a transection of the central Taiwan mountain belt. The results indicated that the C, N, and S contents of slate in Yilan area might still inherit their sedimentary origin and did not be modified during low-grade metamorphism. The low C/N ratios demonstrated that their organic matter is predominant marine origin. The relationship between organic carbon and pyrite contents suggested that the original sedimentary environment may be euxinic, where sulfate, organic matter and reactive iron were available and the sulfate reduction rate was high. The pyrite can be classified into two categories, according to their textures and δ34S signatures. One is fine-grained with highly variable δ34S values, representing a general characteristic of sedimentary pyrite; the other is coarse-grained with a narrower range of δ34S values, inferring a homogenization during metamorphism or recrystallization of a new growth from a specific geothermal fluid. Our findings seems consistent with previous study that the higher geothermal gradient in Yilan may caused by a magma chamber. The samples from the transection of the central Taiwan mountain belt showed that their C, N, and S contents was gradually depleted and sulfur isotopic compositions seemed homogenized in rocks of polymetamorphosed greenschist facies. Since the numbers and types of samples were limited currently, the result is still inconclusive.