台灣溫泉水中溶解氣成分研究

Abstract

台灣位處菲律賓海板塊與歐亞大陸板塊交界處，劇烈的構造活動及高地溫梯度提供加熱源，加熱天水、地層水或岩漿水，是以溫泉分布很廣。前人研究中，溫泉分類依據可有三種：外觀、化學組成、地質環境等。本研究希望利用溫泉水中溶解氣成分來分類溫泉，並討論溶解氣與地質環境的關係，以及溶解氣體的可能來源。水中溶解氣體主要可分成一般溶解氣與稀有氣體溶解氣，主要一般溶解氣為氧氣、氮氣、甲烷、二氧化碳等，稀有氣體溶解氣為氦、氖、氬、氪、氙、氡等六種。 本研究共取台灣五十六處溫泉水樣，封存在塑膠樣品瓶中，以進行水化學分析，進而繪製piper圖；利用灌頂空間（Head space）方法進行一般溶解氣分析；另取水樣封存在高真空的玻璃瓶中，進行溶氦同位素比值、氦氖比值研究。由溫泉氣泡與溶解氣的成分比較可以發現，在大部分地區，兩者成分有相同的富集趨勢。依照採樣點位所在岩區的不同，水中主要溶解陰陽離子成分有三大類特徵，與前人研究大致符合；而一般溶解氣亦有三大類特徵，分別富集氮氣、甲烷及二氧化碳，在變質岩區點位沒有觀察到甲烷，而沉積岩區有大量甲烷加入，顯示溶解氣體富集分布與岩區有一定程度相關。配合氦同位素比值發現，台灣地底具有富集氣體儲藏庫，可能源自前期岩漿活動；而溶解氣的氦同位素比值亦有助於吾人對於溫泉水中天水與深部來源水體混合關係的瞭解。

Taiwan, a geologically active island, is located on the boundary of the Philippine Sea Plate and the Eurasian Plate. High heat flow and geothermal gradient generated by the complex collision and orogeny, and heat up the meteoric water and/or the ground water. The heated water becomes geothermal fluids. In previous studies, some researchers tried to categorize hot springs based on their appearance, chemical compositions and lithological areas. Because of the chemical inertness, the concentrations and isotopic composition of dissolved noble gases are considered to be good indicators of the mantle degassing, climate changes, geothermal conditions, and so on. In this study, it is first time to classify the hot springs in Taiwan in terms of dissolved gases. In order to discuss different origins of hot springs, 55 hot springs had been sampled in this study. Dissolved gases are distinguished as major and minor gases. Major gases include O2, N2, CH4 and CO2, while minor gases are He, Ne, Ar, Kr, Xe and Rn. Hot spring water was sampled and stored in pre-evacuated glass bottles for analyzing chemical compositions of minor gases. The abundance of five noble gases is determined by a quadrupole mass spectrometer (QMA200, Pfeiffer Vacuum Co.) based on the isotope dilution technique. Furthermore, helium isotopic ratios and helium-neon ratios are measured on a conventional noble gas mass spectrometer (MM5400, MicroMass Co.). For hydrochemistry analysis, water samples are analyzed by IC, ICP-MS and titration. Samples with glass vials are introduced to RAD 7 and GC for dissolved Rn and major dissolved gases analyses. We can classify the hot springs into three major groups based on anion concentration data, and then subdivide them into nine minor groups by cation concentration data. Moreover, according to major dissolved gases compositions, three major gas components: CH4, N2 and CO2, are identified. The result indicates that there are three gas reservoirs in study area. Dissolved noble gases can provide more detailed clues about the sources of hot springs in Taiwan, such as the degree of mixing between meteoric water and deep-source water. The circulation model also can be identified.