琉球隱沒系統中熱液之氣體地球化學特徵及其隱示

Abstract

地球內部的熱液可藉由陸上火山與海底熱液活動釋放至大氣圈與水圈，而透過研究熱液來源和通量可提供氣體在大氣圈和水圈的演化訊息。本研究藉由分析琉球隱沒系統海域、九州南部和台灣北部火山群熱液中氣體組成(特別著重在甲烷和與氦氣)，藉由氦、碳同位素成分以探討琉球隱沒系統的氣體地球化學特性和地體構造。調查地區由東北向西南包含九州南部的鹿兒島灣火山群、琉球隱沒系統北段的吐噶喇列島以及南段的伊良部海丘、鳩間海丘、第四与那國海丘和龜山島和台灣北部大屯火山群。 先透過聲納探測器產生水柱反射影像圖協助判定海底熱液噴出位置，隨即進行海底熱水樣本採集，同時量測海水溫度、濁度與酸鹼值等物理化學特性，由低酸鹼度和高濁度等異常特性，可視為海底熱液存在的佐證。鹿兒島灣若尊火山口(Wakamiko Crater)主要是受微生物氧化、消耗較輕的甲烷後的非生物來源甲烷；吐噶喇列島(Tokara Islands)地區的甲烷主要由熱作用形成。氦同位素表示鹿兒島灣若尊火山口是以隱沒型的地函氦氣為主，而在吐噶喇列島顯示受到較多的地殼訊號。氣體地球化學資料顯示吐噶喇列島海底火山在琉球隱沒系統中位於轉換帶，由火山前緣轉換成弧後張裂盆地的關鍵證據。 琉球隱沒系統南段的鳩間海丘(Hatoma Knoll)和第四与那國(Yonaguni Knoll Ⅳ)熱液中的氦氣屬於中洋脊(MOR)類型，主要由上部地函所貢獻。而伊良部海丘(Irabu Knoll)的熱液含有較多的地殼訊號，推測是因為上部地函熱液上湧與淺處地層中的沉積物質交互作用所致。位於最南端的龜山島，亦具有中洋脊類型的高氦同位比值。因此，南段在沖繩海槽在張裂環境下，主要是中洋脊類型熱液攜來的氦，會與淺處地殼類型熱液的氦出現不同程度的二次反應，使得熱液的氦同位素比值在各地有所不同，反映各熱液環境的特性。 台灣北部大屯火山群火山氣體組成和氦同位素屬板塊隱沒型來源，受到上部地函和地殼成分混合而成，地體構造上屬聚合性板塊交界的島弧類型。主要熱液活動區的火山氣體與土壤氣體之氦同位素以及氣體組成表示大屯火山群的逸氣系統可能有數個火山流體通道。大油坑和八煙熱液區的火山氣體是來自相同的通道，但在淺處產生分道，並於近地表地區發生次級反應造成碳同位素變化。四磺坪和焿子坪熱液區有另一個不同於大油坑地熱區的氣體通道，而出露地表的區域受控於地表的弱帶範圍。對應至九州南部的鹿兒島灣火山群中的櫻島火山和若尊火山口也有相似的逸氣型態，火山流體源自於同一個岩漿庫但有各自的逸氣通道彼此不相互干擾。 整合琉球隱沒系統在不同的地體構造單元上的氦同位素值，在弧前地區氦同位素值低，具有較多的氦-4氣體；而在島弧以及弧後盆地地區的氦同位素較高具隱沒型地函訊號。氦同位素比值於琉球隱沒系統出現自南北兩端向中段遞減：九州陸上與海底火山往南向吐噶喇列島海底火山以及南段的南沖繩海槽往中部沖繩海槽出現氦同位素比值降低，中部沖繩海槽卻有較多的地殼氦氣的訊號。琉球隱沒系統由東北端向西南端的火山熱液特性，可探討地體構造上的隱示：日本九州鹿兒島火山群與海底火山以及吐噶喇列島海底火山是屬火山前緣的琉球島弧，向南延伸到龜山島則轉為弧後張裂盆地的沖繩海槽，台灣北部大屯火山群又轉為島弧火山。

Hydrothermal fluids emitted from seafloor vents and/or volcanoes generally form plumes which preserve the information about these fluids. Therefore, submarine hydrothermal plumes represent a means to observe and understand regional tectonic settings and fluid circulation. In this study, we report the volatile (helium and methane) geochemistry of hydrothermal plumes/fluids and volcanic gases related to the Ryukyu subduction system in the region stretching from southern Kyushu in southern Japan to northern Taiwan, in order to understand the geochemical characteristics of hydrothermal fluids in the Ryukyu subduction system. Hydrothermal plumes were sampled by using a CTD-RMS system after analyzing water column images collected by multi-beam echo sounder surveys. The hydrothermal activity can be identified by low pH value and high turbidity in the water column near seafloor. Methane at the Wakamiko Crater in Kagoshima Bay is of abiotic origin but affected by isotopic fractionation through rapid microbial oxidation. The methane in the region of the Tokara Islands is a mix between abiotic methane similar to that found in the East Pacific Rise and thermogenic one. The helium isotopes suggest the presence of subduction-type mantle helium at the Wakamiko Crater, while a larger crustal component is found close to the Tokara Islands. The hydrothermal fluid of Hatoma Knoll, Yonaguni Knoll Ⅳ, and Kueishantao in the southern Ryukyu subduction system are a typical MOR signature characterized by high 3He/4He. The Irabu hydrothermal fluid shows lower 3He/4He values which might be due to fluid-sediment interaction. Based on the gas composition and helium isotopes results, it is evidenced that the gas source in the Tatun Volcano Group is composed of upper mantle and crustal composition. The geographical distribution of 3He/4He ratios and carbon isotopes help in delineating the degassing model with several pathways of magmatic fluid transportation in different hydrothermal areas. It can be observed that there is a similar degassing system in the Aira caldera. The degassing activity is independent between the Sakurajima volcano and Wakamiko Crater, but both have the same magmatic source. Compilation of the available data in the subduction system in Japan helps in characterizing crustal helium and microbial methane at the frontal arc region, and mantle helium and thermogenic methane at the volcanic arc region. The spatial variation of helium isotopic ratios in the Ryukyu subduction system exhibits two decreasing trends from northern and southern system to the central section, probably related to the depth of Ryukyu Trench due to the mount of terrestrial helium. The helium isotopes suggest the presence of subduction-type mantle helium at the Southern Kyushu volcano and Southern Ryukyu subduction system, while a larger crustal component is found close to the Northern and Central Ryukyu subduction system. This suggests that the Tokara Islands submarine volcanoes, and also the Jade and Hakurei hydrothermal systems are a key feature of the transition zone between the volcanic front and the spreading back-arc basin.