Ophiolites and high-pressure/low-temperature (HP–LT) terranes are important sites for the study of geochemical cycling in ancient oceanic lithosphere. We have analysed Li abundances and isotope composition in a series of ultrabasic and basic rocks from the Variscan Limousin ophiolite, as well as in basic and pelitic rocks from the Ile de Groix HP–LT terrane. Both bulk and in situ analyses are employed to evaluate Li mobility and isotope fractionation in the oceanic lithosphere during fluid–rock interactions related to seafloor and sub-seafloor hydrothermal alteration, subduction and exhumation processes. In the Limousin ophiolite, early stages of high-temperature (high-T) hydrothermal alteration of oceanic ultrabasic rocks produced serpentine with low Li abundances (0·9–4·6 ppm) and low δ⁷Li (–8·9‰). The δ⁷Li increase from –2·2 to +4·2‰ in the following generations of serpentine during late-stage hydrothermal alteration results from changes in the fluid composition and temperature conditions. Therefore, even if dehydrating subducted serpentinites generate high amounts of fluids during subduction, abyssal serpentinites do not constitute an important source of Li for Li-rich metabasic rocks. In the associated amphibolites, hornblende displays typical Li contents (3·1–8·2 ppm) and isotopic compositions (+3·5 to +12·5‰) similar to hydrothermally altered sheeted dykes and gabbros. In contrast, the low Li abundances and extremely high δ⁷Li values recorded by omphacite and pargasitic amphibole in the ultrahigh-pressure (UHP) zoisite-eclogite from the Limousin probably reflect interaction with a heavy-Li sediment-derived fluid. The HP–LT metabasites of the Ile de Groix record different Li behaviour, with high Li abundances and low δ⁷Li. They contain Li abundances significantly higher than fresh mid-ocean ridge basalts (MORB) (16–124 ppm), indicating a metasomatic overprint by fluids derived from the neighbouring Li-rich mica-schist (15–52 ppm) in addition to seawater during the early stages of subduction. Lithium is mainly hosted by (1) glaucophane and omphacite in blueschists and eclogites, (2) chlorite and albite in retrograde greenschists, and (3) phengite and chlorite in mica-schists. The metabasites have δ⁷Li values of –4·8 to +3·2‰ that are generally lower than those of fresh and altered MORB. The intercalated mica-schists display δ⁷Li values ranging from –1·7 to +0·2‰ that are typical of subducted sediments. The δ⁷Li decrease from blueschists to eclogites from +1·8 to –4·8‰, as well as the rimward δ⁷Li decrease in glaucophane from MORB-like δ⁷Li values to negative values in blueschists (core: –2·4 to +8·8‰; rims: –7·1 to +2·2‰), reveals that significant fluid-induced Li isotope fractionation occurred at the transition from the lawsonite-blueschist facies to the epidote-blueschist facies, and may be triggered by prograde lawsonite breakdown. In eclogites, the low δ⁷Li measured in whole-rocks (–4·8 to –2·5‰), omphacite (–22·4 to +3·3‰) and glaucophane (–6·9 to +1·4‰) indicates that Li isotope kinetic fractionation had stronger effects under eclogite-facies conditions. The δ⁷Li increase toward positive values in the most retrogressed greenschist samples suggests Li mineral–fluid isotopic exchange during rehydration reactions and interaction with a Li-heavy fluid that is probably derived from the dehydrating metabasites. Thus, lithium isotope fractionation in the HP–LT rocks of the Ile de Groix highlights migration of heavy-Li fluids along the oceanic crust–mantle interface in the subduction zone.

中文翻译：

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Variscan大洋缝合带流体-岩石相互作用过程中的锂行为和同位素分馏：利木赞蛇绿岩和Ile de Groix高压地层（法国）

蛇绿岩和高压/低温（HP-LT）地形是古代海洋岩石圈地球化学循环研究的重要场所。我们分析了Variscan Limousin蛇绿岩的一系列超基性和碱性岩石以及Ile de Groix HP-LT地层的碱性和胶质岩石中的锂丰度和同位素组成。在与海底和海底热液蚀变，俯冲和掘出过程有关的流体-岩石相互作用过程中，大量分析和原位分析都用于评估大洋岩石圈中的锂迁移率和同位素分馏。在利木赞蛇绿岩中，高温的早期阶段（高T）热液海洋超基性岩的改变产生蛇形具有低丰度李（0·9-4·6 PPM）和低δ ⁷李（-8·9‰）。的δ ⁷李增加从-2·2〜+ 4·2‰蛇形中的以下世代期间从所述流体组合物和温度条件的变化晚期热液蚀变结果。因此，即使俯冲过程中脱水的俯冲蛇纹岩产生大量流体，深海蛇纹岩也不会构成富含锂的变质岩的重要锂来源。在相关的闪石中，角闪石显示出典型的Li含量（3·1-8·2 ppm）和同位素组成（+ 3·5至+ 12·5‰），类似于水热蚀变的片状堤坝和辉长岩。与此相反，低丰度丽和极高的δ ⁷Limousin的超高压（Z）辉绿岩-榴辉岩中的绿辉石和次生闪石所记录的Li值可能反映了与重Li沉积物衍生的流体的相互作用。法兰西岛格鲁瓦记录不同李的行为的HP-LT metabasites，高丰李，低δ ⁷李。它们所含的锂含量显着高于新鲜的中洋脊玄武岩（16–124 ppm），表明在此期间除了海水外，还由邻近的富含锂的云母片岩（15–52 ppm）衍生的流体发生了交代交叠叠印。俯冲的早期阶段。锂主要由（1）蓝片岩和榴辉岩中的葡聚糖和绿辉石，（2）逆向绿片岩中的绿泥石和钠长石和（3）云母片岩中的绿泥石和绿泥石所占据。该metabasites有δ ⁷Li值为–4·8到+ 3·2‰，通常低于新鲜和改良的MORB的Li值。插层云母片岩显示δ ^7个李值从-1·7〜+ 0·2‰是典型俯冲沉积物测距。的δ ⁷从蓝片岩到榴辉栗减少从+ 1·8至-4·8‰，以及所述rimwardδ ⁷在闪石李减少从MORB状δ ⁷在蓝片岩李值负值（芯：-2 ·4到+ 8·8‰；边缘：–7·1到+ 2·2‰），表明在从钙钠铁石-蓝胶岩相到附生-蓝胶岩相的过渡过程中，发生了明显的流体诱导的Li同位素分馏。可能是由腐烂的钙钠矿分解引起的。在榴辉，低Δ ⁷在整个岩石（–4·8至–2·5‰），绿辉石（–22·4至+ 3·3‰）和青石岩（–6·9至+ 1·4‰）中测量的Li表示Li同位素在榴辉岩相条件下，动力学分级具有更强的作用。的δ ⁷朝向最倒退绿样品中的正值李增加预示期间补液反应和与被可能从脱水metabasites衍生的锂重流体相互作用锂矿物流体同位素交换。因此，Ile de Groix的HP–LT岩石中的锂同位素分馏突出显示了重Li流体沿俯冲带大洋地壳—地幔界面的迁移。