隱沒帶的鋰同位素特徵：以琉球弧為例

Abstract

鋰有兩個穩定同位素，6Li與7Li，在過去的研究中，鋰同位素被視為具有探討殼幔循環潛力的同位素系統，這是基於以下兩個特性：(一)、鋰同位素在地表的低溫環境下，容易發生同位素分化；(二)、7Li相較6Li更快速由岩石進入流體中，造成流體中的d7Li會明顯上升。然而，在大部分洋內弧的火山岩中，卻發現：(一)、d7Li和流體指標(e.g. Li/Y, Pb/Ce, U/Th, and Sr/Y)不存線性關係；(二)、d7Li與普通中洋脊玄武岩的範圍接近，且大部分樣本落在普通中洋脊玄武岩的範圍內，這些特徵使得鋰同位素的示蹤能力受到質疑。因此，本研究採用第四紀、SiO2< 57 wt.%且有完善主量元素、微量元素、鍶、釹與鉛同位素資料的琉球弧火山岩(九州南部、琉球北部與琉球中部)，藉由報導其鋰同位素來釐清鋰同位素在隱沒帶的特徵與演化。 實驗方法上，前處理皆於國立臺灣大學海洋研究所建置的無塵室完成，每個樣本秤重約25 mg的岩石粉末，並確保粉末在溶樣後完全溶解於溶劑中；樣本的純化使用兩次的陽離子交換層析法，搭配樹酯AG50W-X8 resin (200-400 mesh; hydrogen form)與1M HNO3+80v/v%的提洗液，第一次去除樣本中大部分的元素，第二次則將會干擾分析的Ti、Na進一步排除。洗出液的檢測於國立臺灣大學地質科學系的孫賢鉥紀念實驗室完成，鋰濃度使用四極桿感應耦合電漿質譜儀Agilent 7700x分析，確認各樣本濃度後，再將各樣本稀釋至相同濃度並利用多接收感應耦合電漿質譜儀(Nu plasma II)分析其d7Li。 琉球弧火山岩的鋰同位素系統(Li: 4.61 ~ 13.3 ppm, d7Li: -2.44 ~ +9.40)與大部分全球洋內弧有相似的特徵，值得注意的是，在琉球弧火山岩各區段的樣本中，九州南部(d7Li: -2.44 ~ +2.86)比琉球北部與中部(d7Li: +3.49 ~ +9.40)的d7Li更低，推測是受到隱沒物質脫水釋出的流體與隱沒板塊表層溫度的影響。隱沒入琉球隱沒系統的沉積物有兩個，四萬十頁岩(d7Li: -2.7 and -1.5)與菲律賓海盆沉積物(d7Li: +5.9)，其中四萬十頁岩分佈在九州南部以及琉球北部，這顯示九州南部的低鋰同位素特徵與沉積物的分佈位置無關。本研究認為這個現象與隱沒板塊表層的溫度有關，在九州南部地區，隱沒板塊表層的溫度是整個琉球最低的區段，低溫的狀態使得隱沒板塊在表層的脫水程度較低，隱沒物質—四萬十頁岩的低d7Li訊號越有機會被保存在隱沒板塊中，接著在隱沒板塊隱沒達90公里深左右的位置脫水，將低d7Li的訊號帶入島弧岩漿源，塑造出九州南部較低的d7Li特徵。 基於琉球弧火山岩的研究可以更加釐清鋰同位素在隱沒帶的地球化學特徵與演化：在島弧火山岩中，鋰同位素主要取決於，岩漿源獲取的流體訊號，而這個流體訊號受到隱沒沉積物與隱沒板塊表層溫度的影響，也就是說鋰同位素不僅可以探討殼幔循環的議題，還可以幫助鑑別隱沒沉積物與隱沒板塊表層的溫度。

Li stable isotopes, 6Li and 7Li, have been proposed to be a potential tracer for crust-mantle recycling, based on two important geochemical characteristics: Firstly, the fractionation of Li isotopes occurs in the near-surface rock-fluid interactions; secondly, d7Li value in the liquid phase increases as a result. However, d7Li values in volcanic rocks of most intra-oceanic arcs do not vary with indicators of fluid addition in magma sources (e.g. Li/Y, Pb/Ce, and U/Th), and have similar range to those of N-MORB. To better understand the evolution of Li in the subduction zone, lithium isotopic compositions of Quaternary mafic rocks from the volcanic front of the Ryukyu arc, including the south Kyushu-, the north Ryukyu- and the central Ryukyu arcs, were determined and compiled with their abundances of major elements, trace elements, and Sr-Nd-Pb radiogenic isotopes in this study. Li in sample solution digested from ~25 mg rock powder, was extracted and purified by cation exchange chromatography using AG50W-X8 resin (200-400 mesh; hydrogen form) and an eluent of 1M HNO3+80v/v% methanol in the clean room of Institute of Oceanography, NTU. Dual column separation was conducted in our method in order to sufficiently remove Ti and Na from the Li eluates. For the following instrumental analyses, Li concentration was analyzed by ICP-QMS (Agilent 7700x), and d7Li was determined by MC-ICP-MS (Nu plasma II) in the Shen-Su Sun Memorial Lab, Department of Geosciences, NTU. The variation and correlation between Li contents (4.61 to 13.3 ppm) and d7Li (-2.44 to +9.40) in the Ryukyu arc volcanic rocks also have the common geochemical characteristics in those of the other intra-oceanic arcs that have been mentioned above. Nevertheless, mafic volcanic rocks in the south Kyushu show relative lighter Li isotopic compositions (d7Li: -2.44 to +2.86) than those in the north and central Ryukyu arcs (d7Li: +3.49 to +9.40). The fluid from dehydration of sediments and altered oceanic crust as well as the thermal structure in the subducting slab are the main causes of the spatial difference in d7Li. Shimanto shale (d7Li: -2.7 and -1.5) and pelagic sediments on the Philippine Sea Plate (d7Li: +5.9) representatives of subducting sediments in the Ryukyu arc, resulted in the large variation of d7Li in the Ryukyu arc magmatism. As a matter of fact, the Shimanto shale of low d7Li covers on both of the south Kyushu and north Ryukyu arcs, and then surface temperature of the subducting slab plays the key role that relative lower temperature of the subducting plate in the south Kyushu area cause low degree of dehydration to shallow depth, and addition of fluid with significant slab Li isotopic features in the arc magma source. This study of Ryukyu arc volcanic rocks gives an insight into Li isotopic characteristics and evolution in subduction system: Li isotopic variation of mafic rocks in an intra-oceanic arc is mainly affected by the fluid that is dehydrated from subducting slabs and adding into the corresponding magma source. Li isotopes are a powerful tracer for crust-mantle recycling as a proxy for sediments and thermal structure in the subducted slab.