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Tracing the terrestrial input to back-arc magmatism in the middle Okinawa Trough by lithium geochemistry
Li isotopes,back-arc basin,subduction zone,the middle Okinawa Trough,
|Publication Year :||2019|
|Abstract:||穩定鋰同位素 (7Li) 被認為具有示蹤殼函物質循環的潛力，其因為 (1) 鋰元素在矽酸鹽類岩石中屬於親石微量元素，具有相當高的水溶性; (2) 其兩個穩定同位素6Li與7Li在近地表的水岩交互作用中會發生顯著分餾。然而洋內弧火成岩具有與MORB相仿的鋰同位素值，部分研究將此結果歸因於隱沒陸源物質的高鋰同位素值特徵並沒有完整地傳輸到岩漿源區，近期研究則指出隱沒沉積物也可能是控制火山弧鋰同位素值特徵的重要因子，因此鋰在隱沒後的演化仍有待探討。而弧後盆地玄武岩質岩漿是由地函橄欖岩減壓熔融所形成，其地球化學特徵可以作為另一個瞭解地殼物質再循環的研究窗口，以便更好釐清鋰在隱沒後的演化。
沖繩海槽為琉球弧溝系統中的弧後盆地，本研究分析了弧後張裂過程較典型的海槽中段玄武岩鋰濃度 與鋰同位素值 (7Li)。中沖繩海槽玄武岩的地球化學特徵已由Shinjo et al. (1999) 進行詳細的描述，該研究主要根據輕稀土元素富集程度數據將基性岩分為兩群：高富集型與低富集型，並指出高型玄武岩的岩漿源區相較低型玄武岩有更明顯的隱沒組分貢獻。本研究分析結果表明，高型玄武岩的鋰濃度和Li/Yb值 ([Li] = 4.6 ~ 6.0 ppm, Li/Yb = 1.8 ~ 2.2) 相較低型玄武岩 ([Li] = 2.3 ~ 3.5 ppm, Li/Yb = 1.2 ~ 1.8) 都有較高的數值。根據釹同位素值與沉積物熔體指標等地球化學特徵 (Th/Yb and La/Sm) 推斷，樣本鋰濃度的提高主要源於岩漿源中隱沒沉積物的參與。然而高型玄武岩 (7Li = +2.3 ~ +2.9) 與低型玄武岩 (7Li = +2.1 ~ +2.8) 之間的鋰同位素值差異相當有限，這說明在中沖繩海槽岩漿源區的隱沒沉積物與該區地函有相似的鋰同位素組成。此外一筆具有顯著低7Li (+0.7) 的玄武岩樣本在不同於高型與低型樣本採撈點的地塹中取得，顯示地函在小於100公里的區域範圍內仍可能保留了相當大的鋰同位素值變化，儘管該變化的成因很難透過其他地球化學數據如放射性同位素和微量元素解釋，但這也表明鋰同位素值在追蹤隱沒物質上可能提供了更敏感的變化。
Stable lithium isotopic ratio, commonly expressed as 7Li, is considered to be a potential tracer for the recycling of crustal materials because (1) lithium is a lithophile trace element in silicate rocks with high solubility in aqueous fluid, and (2) its stable isotopes, 6Li and 7Li, significantly fractionate during the near-surface fluid-rock interaction. However, results in early researches have revealed basic volcanic rocks in intra-oceanic arcs show MORB-like values of 7Li, and thus imply that subduction materials, expected to have high 7Li values, might not be completely delivered to the mantle wedge. Moreover, recent studies suggest that the subducted sediments can play a key role in the Li isotopic compositions of volcanic arc magmas as well as of their mantle sources. The recycling evolution of lithium remains obscure. Basaltic magmas in back-arc basins are generated by decompression melting of mantle peridotites, and thus their geochemical characteristics can be a window as MORB to better understand the evolution of lithium after subduction.
New data of lithium contents and isotopic ratios in basalts from the middle Okinawa Trough (MOT), a back-arc basin of the Ryukyu arc, were determined in this study. The geochemistry of these MOT basalts has been well characterized in Shinjo et al. (1999) and two endmembers were identified mainly by REE patterns, i.e. the upper type and the lower type. The mantle source of the upper-type basalts is indicated to have significant contribution of subduction components in contrast to that of the lower-type basalts. Our analytical results show that the upper-type basalts have relatively higher values of both Li contents and Li/Yb ratios ([Li] = 4.6 ~ 6.0 ppm; Li/Yb = 1.8 ~ 2.2) than the lower-type basalts ([Li] = 2.3 ~ 3.5 ppm; Li/Yb = 1.2 ~ 1.8). The elevated Li concentration of the upper-type basalts is inferred to mainly originate from the subducted sediments in the magma source based on their relative lower Nd isotopic ratios and corresponding features of trace elements (e.g., higher Th/Yb & La/Sm). The difference of 7Li between upper-type basalts (7Li = +2.3 ~ +2.9) and lower-type basalts (7Li = +2.1 ~ +2.8), however, is indistinguishable. This proposes that subducted sediments contain similar lithium isotopic value to the pre-existing mantle beneath the middle Okinawa Trough. In addition, there is one basalt sample from the other graben in MOT showing significantly low 7Li (+0.7), suggesting that mantle can retain considerable variation of 7Li in a small domain (within a distance of <100 km). This heterogeneity is not identified from other geochemical data such as those of radiogenic isotopes and trace elements, and thus Li isotopes reveal the potential in tracking the involvement of other subducted materials.
The geochemical data of basalts from the other back-arc basins are integrated in this study as well. The overall dataset shows wider variation in both Li content and 7Li than that of MOT basalts, suggesting that the lithium geochemistry derived from multiple subduction components can be well retained in mantle. The basalts with lower Nd generally have higher contents of lithium and a relatively limited range of 7Li, which can be attributed to the incorporation of subducted sediments, rather than other subduction components. The subducted sediment is an important contributor of lithium abundances to basalts in back-arc basins and thus to the crust recycling.
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