台灣北部地區大屯火山群火山噴氣之硫同位素分析

Abstract

大屯火山地區，目前已有對噴氣口及溫泉水的採樣分析方法，可以定期監測火山的活動，並了解其火山噴氣及溫泉水之化學成份變化。但是前人研究中，一直缺少於大屯火山地區有系統的硫同位素分析，本研究將以大屯火山地區的噴氣硫（包含SO2和H2S）、溫泉水中的硫酸根以及噴氣口旁之結晶硫為材料，分析硫同位素成份之變化，討論硫的來源與分異之現象，並希望綜合前人研究之其他同位素資料，探討火山噴氣來源。 本研究的氣體樣本，在大屯火山地區的噴氣口共有九處，硫同位素值為-4.4‰ ~ 10.3‰。另外，本研究也同時分析大屯火山群之溫泉水及結晶硫的硫同位素成份，分別落在-2.6‰~29.0‰與-3.0‰~4.5‰之間。 大油坑噴氣有最高的氦同位素比值，接近上部地函成分。本研究分析結果顯示大油坑噴氣之硫同位素比值比較高，前人認為可能是本地區的硫化作用強烈，造成硫同位素組成偏重。但是，我們利用硫化物之間的分異係數，計算其平衡溫度，結果顯示大油坑的二氧化硫和元素硫之噴氣平衡溫度約為347℃，而元素硫和硫化氫之間的平衡溫度約為92℃，二氧化硫較重的硫同位素不能以元素硫的形成來解釋，說明，大油坑噴氣中有較高硫同位素比值的二氧化硫成份帶有明顯的島弧岩漿訊號。 在大屯火山地區的硫（包括火山噴氣硫、溫泉水的硫及元素硫）有兩個明顯的來源，為岩漿來源及地層滷水（或海水）來源，而這些不同來源的硫於地底上升至地表的途中有可能因為圍岩的作用而改變其硫同位素比值。 綜合前人研究的噴氣口氣體成分分析、溫泉水之化學成分分析及其氦、氫、氧、碳同位素分析研究，我們將大油坑當作最接近岩漿來源的地區，其他地區的噴氣硫及溫泉水中的硫同位素，由於受到圍岩作用，表現出隨著愈遠離大油坑而其硫同位素比值愈小的趨勢；另外，硫磺谷、大埔及地熱谷由於地層滷水的影響，具有非常高的硫同位素比值，並且大埔及地熱谷的低氦同位素比值也證實其有較高的地殼來源；特別的是焿子坪及四磺坪在大屯火山地區氦同位素比值相當高，卻有相對小的硫同位素比值，我們推測其為底下熱液蒸氣之硫化氫氣體上升於淺層水體並再次酸化所造成的結果。

Previous studies revealed that compositions of volcanic gas and hot spring water are closely related to the volcanic activity. Sulfur species are principle constituents in volcanic gas. Usually they are very rare in atmosphere so that we do not have to consider air contamination when we collect samples for sulfur analysis. In this study, it is first time to systematically study the sulfur isotopes in volcanic gas （including both H2S and SO2）, hot spring water （SO4-2） and sulfur elements （S8） at eleven sites of the TVG （Tatun Volcano Group） geothermal area to better understand the source of sulfur species and the fractionation of sulfur. The results of the δ34S ratios of total sulfur gas from fumaroles fall in the range of -4.4 to 10.3 ‰. In addition, we have also analyzed the spring water （from -2.6 to 29.0 ‰） and native sulfur isotopic compositions （from -3.0 to 4.5 ‰） of sulfur in the studied area. It indicates that there are multiple sources for the sulfur species in studied area. The highest sulfur isotopic ratio of the SO2 gas found in Da-yiou-keng （DYK） area was suggested by previous studies that the isotopic fractionation may be induced by the active process of sulfurous production from fumarolic gas. However, the estimated equilibrium temperature between H2S gas and native sulfur, and SO2 gas and native sulfur is ca. 92 and 347℃, respectively for DYK sulfurous samples. It implies that the native sulfur is unlikely to deposit directly from SO2 gas in this area. Therefore, the heavier sulfur isotopic composition of SO2 gas cannot explain by isotopic fractionation due to native sulfur deposition from venting gas. Furthermore, we can conclude that DYK fumarolic gas exhibits geochemical signature of island-arc related magma which shows higher sulfur isotopic ratios than typical MORB samples. Magmatic sulfur and formation brine （or seawater） sulfur are two distinct sources for the TVG fumarolic samples. The sulfur isotopic value of these sulfurous samples would be modified by reacting with host rock while the magmatic gas/fluid moving upward to surface. Available geochemical data show that fumarolic samples from DYK hydrothermal area exhibit significant magma related characteristics, therefore, many researchers suggest that there may exist an active magma chamber in this area. In this study, the sulfur isotopic data also support the conclusion that DYK sulfurous samples exhibit the highest sulfur isotopic values among all TVG samples. The other samples show lighter sulfur isotopic values as the sampling site is away from DYK and then the fumarolic gas/fluids may have more chance to react with host rock to modify its compositions. It is worthy to note that samples from LHK, DP and TRK exhibit very high δ34SSO4 values and show strong signals of formation brine water （or seawater）. The lower helium isotopic ratios of DP and TRK also support that more crustal contribution input for the degassing system at these two sites. Samples from SHP and GTP show relative high helium isotopic values, however, with relative negative sulfur isotopic values. We suggest that they may result from secondary acidification of hydrogen sulfide gas at shallow level while the magmatic fluids migrating upward to surface.