台灣北部及沖繩海槽氣體地球化學研究：氣體通量、地體構造與地球逸氣之隱示

Abstract

流體地球化學是探討地球生成起源、地球物質之來源與遷移、全球環境變遷等重要議題的主要工具之一。隨著科技進展，我們得以不同分析技術與各種出發點探討流體地化在地球科學研究上的應用與意義。本論文結合三項研究工作，希望藉由流體地球化學在不同地區之應用與探討，展示流體地球化學研究的重要性及其研究價值。 火山氣體中的二氧化碳是岩漿上升的早期指標。藉由觀測土壤氣二氧化碳通量的變化，讓觀察者可以在安全的範圍內監測火山活動。氣體通量的測量也有助於準確地計算出火山地區土壤氣體的逸散總量，進而與世界上的火山比較，並探討火山的活動性質。結合通量、氣體成分與同位素資料，有助於了解通量與火山活動之間的關聯。本研究結果也顯示：藉由二氧化碳濃度與氦同位素良好的正相關，分析成本低且易測量的二氧化碳可以為監測火山活動的替代指標。 氦同位素是追蹤流體來源相當重要的工具，地表偵測到的氦氣濃度與同位素值代表著氣體由深部來源向地表遷移的最終結果。藉由測量土壤氣中的氦同位素比值及濃度，可以個別探討成因不同的氦三與氦四各自的地質意義，在結合兩者的數值與空間上的分佈情形，其結果不僅指示了來自深部與淺部之流體遷移的機制，更讓我們確定：來自於地函的流體已經進入了因為地體構造拉張而造成地殼逐漸減薄的宜蘭平原。 氦同位素在海洋研究中也扮演了相當重要的角色，然而受限於採樣技術，氦同位素在海洋環境的應用仍有相當大的進步空間。本研究使用新設計的採樣裝置「孔隙水稀有氣體採樣器」，成功於沖繩海槽海底沈積物中，測量到溶解於孔隙水中之氦同位素濃度與隨深度變化之梯度，並估算出研究區域的氦三通量。將氦三通量與其他地體構造單元比較，我們發現隱沒帶的氦三通量是中洋脊的四分之一；隱沒帶的氦四通量在過去的研究中則可能被低估。

Fluid geochemistry plays an essential role in Earth sciences. It has been particularly influential in aquiring information on migration of geo-fliuds, global warming, and chemical evolution of the Earth. Over the last few decades, the continuing improvements in analytical technology have led to many new and fascinating applications in this field, and we could further look into Earth sciences from different aspects. This dissertation focuses on different applications of fluid geochemistry in three areas: the Liu-Huang-Ku hydrothermal area, the Ilan Plain and the Mid Okinawa Trough backarc basin. Carbon dioxide flux is an important index of early magma ascending. By observing the variation of soil CO2 flux, scientists could monitor volcanic activities in a safe distance. Quantifying soil CO2 flux also enhances the precision of gas budget, by which we could do a worldwide comparison of soil flux and better improve the knowledge of current volcanic activity in our research area. Combined gas flux, gas compositions and isotope data of soil gas from Liu-Huang-Ku were carried out in order to unravel the relationship between soil CO2 flux and volcanic activities. The results show a clear correlation between soil CO2 concentration and Helium isotope ratios, which also suggest that soil CO2 could be a good parameter for future monitoring in this area. Helium is a key tracer of fluid flow. Helium concentration and isotope ratios of samples from the Earth surface are the ultimate results of mixing and integrating processes during migration from mantle to the atmosphere. By coupling Helium isotope ratios and concentration, we can identify their sources and geological significance. The findings of this study indicate the migration mechanism of Helium-3 and Helium-4 respectively; and most importantly, mantle fluids have migrated into the crust-thinning Ilan Plain. Helium isotope studies are valuable in ocean research; however, one of the thorniest problems researchers face is air contamination. In this study, we successfully used a newly developed device ‘noble gas porewater sampler’ to sample marine sediments from the Mid Okinawa Trough backarc basin. We measured the helium isotope ratios in the porewater and derived helium-3 flux from a backarc basin. The results of this study suggest that helium-3 flux from subduction zone is a quarter of that from mid ocean ridge magmatism, and helium-4 flux from subduction zone might have been underestimated.