| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 99 === 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.
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