南沖繩海槽石林隆堆場址沉積物間隙水的鋰豐度

海底熱液循環是水圈、岩石圈間物質交換和形成各類海底硫化礦床的重要途徑，而海底熱液作為熱液循環的產物，其化學性質隨水岩交互作用而發生顯著變化，所以可以作為研究海底熱液循環演化的工具，特別是海底熱液的鋰豐度（鋰含量與鋰同位素值）已被證實具有示蹤二次改性作用與分辨固體端元的潛力，儘管僅能區分沉積物或火成岩端元，無法分辨岩性。本研究地點為聚焦在位於南沖繩海槽最南端、新發現的石林隆堆熱液場址，藉由測量OR1-1164航次石林隆堆熱液場址重力岩心（128 cm）和複管岩心（31 cm）間隙水樣品的鋰豐度，並結合以往調查所得的其他地球化學數據，為石林隆堆熱液的成因和演化提供制約。為了建立適用於熱液流體的鋰同位素測定方法，在樣本前處理開始時便加入氫氟酸，移除樣本中的矽，接著以1.6 ml AG 50W-X8氫型陽離子交換樹脂與1 M硝酸 + 80% (v/v)甲醇提洗液進行層析，以提純出樣品中的鋰；根據提洗曲線，收集第6到第17毫升洗出液，即便是熱液中的鋰含量異常高（達20000 ng鋰），鋰亦能被完全收集。藉由本方法分析NASS-6海水國際參考樣品，其鋰同位素組成（d7Li）為+30.5 ± 0.4（1 S.D., n=3），與文獻值一致。石林隆堆間隙水樣品的鋰濃度在複管與重力岩心分別為23.5 ~ 50.3和24.5 ~ 757 μM，鋰同位素組成則分別是+22.0 ~ +30.6和+13.5 ~ +28.8。兩岩心的深度剖面中，最表層樣本的鋰豐度接近海水值，而隨著深度的增加，鋰濃度逐漸上升，而d7Li則降低，且兩剖面在鋰同位素變化上相似，但在鋰含量上，複管岩心隨深度上升的幅度不如重力岩心。雖然傳統海水混染示蹤劑「鎂」並未在複管岩心有顯著變化，但其鋰含量與其同位素間的線性相關明確指示了海水與熱液的二元混合，間隙水的鋰豐度顯然在追蹤海水混合時更具優勢。重力岩心間隙水的鋰豐度和鎂含量具有線性相關，這意味著重力岩心同樣發生了海水混合，不過其鋰含量與鋰同位素值呈現非線性的證據，進一步表明熱液在上湧過程受到高溫（350℃）的沉積物二次改性作用影響，以鋰豐度推估的反應溫度與過去調查的成礦溫度（370 ~ 450℃）接近。在剔除了海水混染的影響，並用沖繩海槽熱液場址在前人文獻中報導的最高鋰含量作為上限，本研究估計出石林隆堆熱液端元的鋰含量約為2071 ~ 5600 μM，鋰同位素值為+5.6 ~ +10.0。與中洋脊沉積物匱乏場址及沉積物富集場址相比，石林隆堆的熱液端元有更高的鋰含量，但有相似的鋰同位素組成；相反地，石林隆堆熱液端元與同樣富含沉積物的沖繩海槽熱液場址的鋰濃度變化相近，但鋰同位素值更高。基於溶解－沉澱模型，石林隆堆熱液端元的非典型鋰豐度，來自於有流紋岩參與了石林隆堆熱液的形成，顯然前人研究低估岩性影響熱液循環系統鋰豐度的重要性。

關鍵字

熱液作用；鋰豐度；間隙水；南沖繩海槽；沉積物富集熱液系統

並列摘要

Hydrothermal circulation is a key process associated with mass transport between hydrosphere and lithosphere, and submarine massive sulfide deposits. To better understand a hydrothermal system, the chemistry of hydrothermal fluids, as a product of the circulation, can vary significantly with and thus constrain the water-rock interactions. Specifically, the lithium abundances, concentrations and isotopic ratios (d7Li), have been demonstrated to trace the secondary process and solid end-member, sediments or igneous rocks regardless of lithology. In the southernmost Okinawa Trough, Geolin Mounds (GLM) hydrothermal site was newly discovered. Li abundances of the interstitial water sampled from a gravity core (128 cm) and a multi-core (31 cm) in cruise OR1-1164 at the GLM hydrothermal site, were measured and combined with other geochemical data from past investigations to constrain the origin and evolution of GLM hydrothermal fluids. For the method of lithium isotopes, pretreatment procedures were modified specifically for hydrothermal fluids in this study by the addition of HF into samples in the first step to remove silicon. Following routine column chromatography (using 1.6 ml AG 50W-X8 cation exchange resin with 1 M HNO3 + 80% (v/v) methanol eluent), the Li fraction, even of high-Li (up to 20000 ng) hydrothermal fluids, was able to be completely collected from the 6th to 17th ml eluates. The d7Li value of international reference material NASS-6 (seawater) analyzed as unknown is +30.5 ± 0.4‰ (1 S.D., n=3) that is consistent with reference values. Lithium concentrations of pore water in the multi-core and gravity core are 23.5 ~ 50.3 and 24.5 ~ 757 μM, and the isotopic compositions of them are +22.0 ~ +30.6 and +13.5 ~ +28.8‰, respectively. In the depth profile of both cores, the shallowest samples have lithium abundances identical to the seawater, and with depth, lithium concentrations increase gradually accompanied with decreasing lithium isotopic composition. The two profiles are consistent in lithium isotopic variations, but lithium contents in the multi-core rise not as rapidly as the gravity core with depth. Although there is no significant variation in Mg contents, a conventional tracer of the addition of sea water, the linear correlation between Li contents and isotopic ratios in the multi-core indicates simple mixing between hydrothermal fluid and seawater. The lithium abundance in interstitial water thus is more powerful in tracing seawater mixing. Li abundances in gravity core show a linear correlation with Mg concentrations suggesting the mixing with seawater. However, the non-linear relationship between Li concentrations and isotopic ratios further demonstrates secondary modification in sediments occurred under high-temperature (350℃) which is similar to GLM mineralization temperature (370 ~ 450℃) reported. Based on the Mg contents to remove contamination by seawater and constraints of the highest Li concentration reported in Okinawa Trough vent fluids, the lithium concentrations and isotopic ratios of the GLM hydrothermal end-member fluid are about 2071 ~ 5600 μM, and +5.6 ~ +10.0, respectively. This composition shows similar lithium isotopic values but higher Li concentration as compared with those from both of the sediment-starved and sediment-rich hydrothermal sites in mid-ocean ridges. On the contrary, it has similar Li concentrations to and higher lithium isotopic values than those of hydrothermal sites in Okinawa Trough that all are sediment-rich. Based on the dissolution-precipitation model, such untypical Li abundances of GLM hydrothermal fluid end-member are proposed to result from the involvement of rhyolite in the reaction zone of the GLM hydrothermal system. The case of the GLM hydrothermal system tells us that the lithology indeed can play an important role in controlling the Li abundances of hydrothermal circulation.