南沖繩海槽海底熱液噴泉之海水溶解氣體氦同位素分析

Abstract

沖繩海槽南端（Southernmost Part of Okinawa Trough，簡稱SPOT）在地體構造上為菲律賓海板塊隱沒入歐亞大陸板塊的弧陸碰撞系統所形成的弧後擴張盆地。此處海底熱泉活動十分活躍，噴出型的熱泉噴口多達六十幾座，海底地熱梯度極高。本研究於宜蘭外海，分別於 14 個可能的熱泉噴口處，多次利用海研二號航次及潛水取得不同深度水層共 134 個海水樣品，進行海水樣品溶解氣體及氦碳同位素之分析研究。 結果顯示，溶解無機碳含量大於北大西洋海水正常值外，其濃度隨海水深度增加，並於多數採樣點中最深層水樣亦有明顯增加的現象；而溶解氧含量則有完全相反的趨勢。於龜山島旁潛水至水深20公尺處之熱泉噴口採集到的海水樣本，其3He/4He比值最高可達5.5 RA，顯示很高比例的地函源氣體持續噴氣中，且此處兩種噴泉口（黃泉、白泉）之樣品具有不同之氦同位素分析比值，暗示龜山島附近之噴口流體成分有不同地函來源混染比例（59.7 %與80.2 %）。本研究亦計算過剩3He值，繪製出精細的氦同位素深度濃度剖面圖，探討南沖繩海槽底部海水以及上部水層200公尺大於北太平洋海水的背景值之訊號來源，並將此結果與溶解無機碳同位素相互討論，推論溶解無機碳碳同位素成份較不適合單獨用於追溯海底熱液來源。

Many active submarine volcanoes have been found in southern Okinawa Trough. Water column samples from the hydrothermal plumes above venting volcanoes were collected during the OR2-1897, -1984, and -2003 cruises. Meanwhile, diving at shallower depths were conducted several times to collect the water samples near the venting sites. In total, 134 water samples from various depths in the offshore area of NE Taiwan were collected for dissolved gases and helium isotopes measurement. The dissolved gases of water column samples show that the CO2 concentration and the alkalinity increase with depth and become higher at the bottom, while the result of O2 concentration shows a reverse pattern. The 3He/4He ratios near the vicinity of active Kueishantao volcano show highest value, 1.92 to 5.57 RA, where RA is the atmospheric ratios of 1.39 x 10-6. The plot of 3He/4He and 4He/20Ne ratios suggests that the influence of 59.7 % and 80.2 % by mantle component at KST-Y and KST-W, and the excess-3He profiles in this study suggesting that the helium-3 isotope is a nice tracer for the magmatic-component input from venting volcanoes. Furthermore, the correlation between helium and carbon isotopic ratios is low in this study. Therefore, the carbon isotopic ratio along itself may not be a sensitive proxy of geothermal.