Abstract Fluids play a crucial role in element mobility and mass transfer at the slab-mantle interface in subduction zones. However, tracing the source and chemical composition of subduction zone fluids still remains challenging. High-pressure (HP) metamorphic leucophyllites, mainly composed of quartz, muscovite/phengite and Mg-chlorite, occur in several localities in the Eastern Alps and experienced significant Mg-metasomatism at forearc depths during the Eoalpine orogeny. They thus provide a good opportunity to explore the origin of Mg-rich fluids in the continental subduction zone. The leucophyllites are rich in Mg and poor in Fe, Na, and Ca and occur as lenses and layers in the country rocks of metagranites. Both leucophyllites and metagranites show similar whole-rock REE distribution patterns, and magmatic zircons from them exhibit similar U-Pb ages of ∼271 Ma, suggesting that both types of rock have the same protolith of early Permian granite. One metagranite that preserves magmatic composition shows a low δ26Mg value of −0.64‰, suggesting a contribution of carbonate-rich sediment when granitic melts were produced. A profile of samples from metagranite through transitional gneiss to leucophyllite in Hungary shows two trends in δ26Mg values. In the Vashegy sections, δ26Mg values firstly decrease from −0.64‰ to −1.30‰ during gneiss formation, and then increase to 0.29‰ during leucophyllite formation. In a second locality, the TV tower section, the δ26Mg values firstly decrease from −0.29‰ to −0.89‰ during gneiss formation, and then remain constant during leucophyllite formation. Two leucophyllites in Austria exhibit higher δ26Mg values of 0.05 to 0.09‰ compared to their country rocks of about −0.20‰. Based on the Mg isotope systematics and their relationships to whole-rock geochemical compositions, we propose two types of fluid in the subduction zone: (1) a low δ26Mg ( 0.3‰) fluid derived from dehydration of talc-rich serpentinite. This is for the first time to find subduction zone fluids with distinct δ26Mg values at forearc depths. Both fluids possibly originated from dehydration of the sediment-serpentinite melange at the slab-mantle interface. The fluids were heterogeneous in δ26Mg values and they metasomatized slices of continental crust along shear zones. The presence of fluids with δ26Mg values as low as −1.3‰ suggests that the dissolution of Mg-rich carbonates can be significant at forearc depths. Our results provide an excellent example of tracing different sources of subduction zone fluids by coupling Mg isotopes with other petrological and geochemical variables. They may also have great implications for the generation of magmas with variable Mg isotope compositions at convergent plate boundaries.

中文翻译：

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追踪具有不同镁同位素组成的俯冲带流体：来自东阿尔卑斯山的高压交代岩（白蜡石）的见解

摘要 流体在俯冲带板片-地幔界面的元素迁移和质量传递中起着至关重要的作用。然而，追踪俯冲带流体的来源和化学成分仍然具有挑战性。主要由石英、白云母/菱镁矿和镁绿泥石组成的高压（HP）变质白蜡石出现在东阿尔卑斯山的几个地方，并且在始高山造山运动期间在弧前深度经历了显着的镁交代作用。因此，它们为探索大陆俯冲带富镁流体的起源提供了一个很好的机会。白蜡石富含镁，缺乏铁、钠和钙，在变花岗岩的围岩中以透镜状和层状出现。白蜡石和变花岗岩均显示出相似的全岩稀土元素分布模式，和来自它们的岩浆锆石的 U-Pb 年龄相似，约为 271 Ma，表明这两种岩石具有相同的早二叠世花岗岩原岩。一块保留岩浆成分的变花岗岩显示出-0.64‰的低δ26Mg值，表明在产生花岗岩熔体时富含碳酸盐的沉积物的贡献。匈牙利变花岗岩到过渡片麻岩到白蜡石的样品剖面图显示了 δ26Mg 值的两种趋势。Vashegy断面δ26Mg值在片麻岩形成过程中先从-0.64‰下降到-1.30‰，然后在白蜡石形成过程中上升到0.29‰。在第二个地点，电视塔部分，δ26Mg 值在片麻岩形成过程中首先从-0.29‰ 下降到-0.89‰，然后在白蜡石形成过程中保持不变。奥地利的两个白蜡石显示出较高的 δ26Mg 值，为 0.05 到 0.09‰，而它们的围岩约为 -0.20‰。基于镁同位素系统及其与全岩地球化学成分的关系，我们提出俯冲带中的两种流体类型：（1）来自富含滑石的蛇纹岩脱水的低δ26Mg（0.3‰）流体。这是首次在弧前深度发现具有明显 δ26Mg 值的俯冲带流体。这两种流体可能起源于板片-地幔界面处沉积物-蛇纹石混杂物的脱水作用。这些流体的 δ26Mg 值是不均匀的，它们沿着剪切带交代了大陆地壳切片。δ26Mg 值低至 -1.3‰ 的流体的存在表明富镁碳酸盐的溶解在弧前深度可能很显着。我们的结果提供了一个很好的例子，通过将镁同位素与其他岩石学和地球化学变量耦合来追踪俯冲带流体的不同来源。它们也可能对会聚板块边界处具有可变 Mg 同位素组成的岩浆的产生具有重要意义。