大屯火山與菲律賓火山氣體成分之時空變化及其成因探討

Abstract

火山氣體的成分和變化直接反應了其底下岩漿庫的特性和其活動性。我們在台灣大屯火山地區以及菲律賓的Taal火山和Negros島上利用吉氏瓶採集火山氣體樣品，這三個地區的噴氣口溫度都不高，僅接近沸點。氣體組成都以水氣為主，除水之後為CO2為主，其他還有H2S、N2等，顯示是蒸氣在淺處與沸水分離的結果，為典型的低溫火山噴氣口的氣體組成。這三個地區的氦同位素比值都相當高，表示大部分的氦氣來自於地函，亦指示底下可能仍有岩漿庫存在。Negros島的氦同位素比值有由北向南增加的趨勢，和北部較接近碰撞帶相關。大屯火山地區和Taal火山的碳同位素指示CO2為岩漿來源；而Negros島則除了有岩漿來源外，其底下的石灰岩層亦可能提供其他的CO2至逸氣系統中。此外，我們還分析大屯火山中的有機氣體，除了大油坑地區之外，其他採樣點大致和熱液地區的有機種類分佈相同。然而相較世界其他火山而言本地區存在過多甲烷，根據甲烷的碳氫同位素分析結果顯示，其來源為深部有機物經火山或熱液作用高度成熟而產生。 而大屯火山的長期觀察結果來看，從1999年到2003年之間，氣體成分並沒有重大改變。從2004年8月之後，大油坑的氣體樣品在HCl含量以及SO2/H2S比值開始有明顯的變化，這些異常現象和溫度上升有一致性的改變，而大屯火山其他地區則維持穩定。大油坑的異常現象可能和岩漿活動或是底下裂縫增加/變大有關。2010年中在大部分採樣點發生一次短暫的HCl含量同步增加的現象，目前又回復穩定。由於變化期間在大屯火山地區並沒有大規模地震發生，是否與微震活動有關還有待釐清。

Volcanic gases can provide us much useful information for understanding the characteristics and activity of magmas. Samples were collected by using ‘Giggenbach bottles’ from Tatun Volcano Group (TVG), Taiwan, and Taal Volcano and Negros Island, Philippine. The highest temperatures of these geothermal areas are around boiling point. These samples all show a similar composition as those of low-temperature fumaroles in other parts of the world. H2O is the major species of these gas samples, and CO2 is the dominant component after de-watering. Both of the gas compositions and temperatures indicate that vapor separated from boiling water near the surface. The high 3He/4He ratios of all samples indicate a mantle-derived degassing source in origin, i.e., magma chambers could still exist beneath these volcanoes. Helium isotopes ratios show a decreasing trend from north to south in Negros Island, such distribution could be due to more crustal contamination caused by the collision event which happened in the northern part of the island. The carbon isotopic values of CO2 of Taal Volcano and the TVG area exhibit a magmatic source. The carbon isotopic values of CO2 of Negros Island samples are far less negative than the values from a magma source. There are other carbon sources of CO2, most likely a thick sequence of limestone formation in Negros Island. We also analyzed the volatile organic compounds (VOCs) in the TVG area. The VOCs in most sampling sites show a similar distribution as hydrothermal areas, except Da-you-keng area. However, compared to other volcanoes in the world, there is an excess of methane in the TVG area. According to carbon and hydrogen isotopic values of methane, this excess methane might be produced by the organic matter which in the deep stratum. In addition, the results of long-term observation in the TVG area show that there is no significant change in gas composition from 1999 to 2003. However, since August 2004, progressive increases of HCl concentrations and SO2/H2S ratio in fumaroles from Da-you-keng have been observed. These variations were accompanied by rising temperature of fumaroles. We propose two possible processes, 1) new magma supply and 2) recent opening of fractures in local area, to explain these observations. In 2010, brief increases of HCl concentrations were observed in most sampling sites simultaneously. There is no large-scale earthquake take place in this period. We need to check if this phenomenon is associated with microseismic activity.