In subduction systems, the forearc (FA) serpentinized mantle plays a key role as a reservoir for fluid-mobile elements, especially those prone to early mobilisation from the slab. FA serpentinites therefore provide an indication of the fluid characteristics that have been steamed out of the slab, and allow better quantification of elemental recycling and fluid flow. FA serpentinites outcropping along the Indus Suture Zone are formed by hydration of mantle peridotites by fluids derived from the Indian slab, providing a geochemical record of fluid-rock interactions during the India-Eurasia convergence. Systematic trace and multi-isotope (Sr and Pb) analyses of serpentinites from three major sites, namely Shergol-Tingdo, Kargil and Tso Morari, along the Indus Suture Zone in Ladakh (NW Himalaya) show that these rocks record a major change in the origin of metasomatic agents, from oceanic to continental subduction. The Shergol-Tingdo and Kargil FA serpentinites, formed under low temperature and pressure conditions, are characterised by high B enrichment and As and Sb depletion, high alkali/U ratios and relatively unradiogenic Sr and Pb isotope ratios. These geochemical characteristics are identical to those reported for modern oceanic subduction zones (e.g., Mariana forearc) and are consistent with input by fluids derived from subducted oceanic crust. In contrast, FA serpentinites from the Tso Morari ultra-high pressure unit show enrichment in fluid mobile elements such as As, Sb and U, low alkali/U ratios and radiogenic Sr and Pb isotopic compositions. These features suggest a change in the metasomatizing agent with a strong influence from subducted continental material. Consistent with this scenario, the Sr-Pb isotopic composition of the Kargil FA serpentinites can be reproduced by mixing between an Indian MORB-depleted mantle source and fluids derived from dewatering of blueschist facies oceanic metasediments, whereas in the case of Tso Morari a fluid end-member derived from various eclogitic continental metasediments is required. We propose that the observed geochemical signature of the Indus Suture Zone FA serpentinites can be used to reconstruct the geochemical exchanges between the slab and the overlying mantle from intra-oceanic to continental subduction. This successful systematic approach could be applied to other collisional contexts.

中文翻译：

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从海洋到大陆俯冲的前弧蛇纹岩的地球化学特征


在俯冲系统中，前弧 （FA） 蛇纹岩化地幔作为流体移动元件的储存库起着关键作用，尤其是那些容易从板坯中早期动员的元件。因此，FA 蛇纹岩可以指示从板中蒸出的流体特性，并允许更好地量化元素循环和流体流动。沿印度河缝合带露头的 FA 蛇纹岩是由来自印度板块的流体对地幔橄榄岩进行水化形成的，提供了印度-欧亚大陆辐合期间流体-岩石相互作用的地球化学记录。对拉达克（喜马拉雅西北）印度河缝合带沿线三个主要地点（即 Shergol-Tingdo、Kargil 和 Tso Morari）的蛇纹岩进行系统痕量和多同位素（Sr 和 Pb）分析表明，这些岩石记录了交代剂起源的重大变化，从海洋俯冲到大陆俯冲。Shergol-Tingdo 和 Kargil FA 蛇纹岩在低温和压力条件下形成，其特征是高 B 富集和 As 和 Sb 消耗、高碱/U 比值以及相对不放射性的 Sr 和 Pb 同位素比值。这些地球化学特征与现代海洋俯冲带（例如马里亚纳前弧）报告的地球化学特征相同，并且与俯冲洋壳衍生的流体的输入一致。相比之下，来自 Tso Morari 超高压装置的 FA 蛇纹岩显示出流体移动元素（如 As、Sb 和 U）的富集、低碱/U 比率以及放射性 Sr 和 Pb 同位素组成。这些特征表明交代剂发生了变化，受到俯冲大陆物质的强烈影响。 与这种情况一致，Kargil FA 蛇纹岩的 Sr-Pb 同位素组成可以通过混合印度耗尽的 MORB 地幔源和蓝片岩相海洋变质沉积物脱水产生的流体来复制，而在 Tso Morari 的情况下，需要来自各种榴辉大陆变质沉积物的流体端元。我们提出，观察到的印度河缝合带 FA 蛇纹岩的地球化学特征可用于重建板块与上覆地幔之间从大洋内俯冲到大陆俯冲的地球化学交换。这种成功的系统方法可以应用于其他碰撞环境。