海底熱液循環系統為影響海水化學成分的重要機制。尤其在低溫蝕變過程中次生礦物的形成將顯著改變熱液流體化學組成。在低溫蝕變過程中鎂同位素會產生顯著同位素分化，其同位素比值有潛力作為指示低溫熱液系統水岩反應程度的指標。為理解不同型態熱液循環系統的水岩反應程度，本研究樣品取自希臘愛琴海域米羅斯島的熱液湧泉系統。米羅斯島的地質以中性火成岩為主，其海底熱液系統包含兩種不同的流體循環模式。其一為深度約0.5公里的淺部循環，主要因海水下滲循環後湧出形成；另一為深度約1-2公里的深部循環，其流體經過次臨界相分離作用，分化為蒸氣相與鹵水相後，沿著岩層裂隙往上流動而湧出。本研究探討米羅斯島不同熱液循環系統的化學組成與鎂同位素比值變化，以探討可能水岩反應機制及過程。我們使用感應耦合電漿光譜儀(ICP-OES)與感應耦合電漿質譜儀(ICP-MS)測定樣本的主要元素濃度和微量元素濃度；使用Wombacher et al. (2009)純化方法，最後利用多接收器感應耦合電漿質譜儀(MC-ICP-MS)精確測量鎂同位素比值。
結果顯示，除深部循環的蒸氣相顯示出與海鹽水流體混合趨勢有較大偏差，鎂濃度和主要元素呈負相關，藉由元素濃度變化的特性，探討端成分的來源。整體熱液系統δ26Mg變化約為0.66‰。相分離作用分離出的蒸氣相δ26Mg介於-0.52~-0.63‰；鹵水相δ26Mg介於-0.65~-1.00‰，提供鎂同位素在相分離作用下可能的分化機制以及混和趨勢。淺部循環似海水δ26Mg介於-0.64~-1.18‰，大部分樣品符合雷利分化α=1.00020，顯示在開放系統重同位素有利於進入矽酸鹽固相。藉由比較δ26Mg與主要元素變化，搭配文獻鋰、硼同位素數據，歸納出米羅斯島淺水熱液系統模擬圖。

The input of submarine hydrothermal vent fluids is an important factor to influence the compositions of seawater, particularly during secondary minerals formation at low-temperatures. During water/rock alteration, exchange of Mg isotopes take place and to cause isotopic fractionation in vent fluids. The study of δ26Mg in vent fluids, therefore, has became a sensitive tracer for tracing water/rock interactions in hydrothermal system. To assess the degree of water/rock interactions under different types of hydrothermal circulations, this study analyzes the δ26Mg in vent fluids, which collected from a shallow submerged hydrothermal system in Milos Island.
The aims of this study are (1) to investigate the chemical compositions and Mg isotopes in different hydrothermal vent fluids in Milos; and (2) to explore their possible water/rock interaction mechanisms/processes involved. Major and trace elements were analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES) and/or inductively coupled plasma mass spectrometry (ICP-MS). Sample solutions were treated with the chemical separation processes for Mg isotopic measurement, which were analyzed using multi-collector inductively coupled plasma mass-spectrometer (MC-ICP-MS).
The δ26Mg in Milos hydrothermal fluid is varied of 0.66‰, the seawater-like ranges from -0.64 to -1.18‰, and most data for seawater-like fall on a trend line of Rayleigh fractionation, with an α about 1.00020 ± 0.00011, which may represent chlorite mineral formation. The δ26Mg of the Cave fluids range from -0.52 to -0.63‰; the δ26Mg of Submarine-brine range from -0.65 to -1.00‰, which provide possible mechanisms for explaining Mg isotopic fractionation under phase separation. By comparing the available major/trace elemental concentrations and several isotopic systems (Li, B, and Mg) in literature, this study explores possible mechanisms that affecting submarine hydrothermal circulation in Milos.

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