Serpentinite-derived components play an important role in chemical cycling in subduction zones. However, it is difficult to identify recycled serpentinite in subarc mantle due to the overprinting of other slab components, and how serpentinite contributes to arc magmatism is poorly understood. Here we report Mo and Mg isotopic compositions of Early Paleozoic Qushiang arc mafic rocks in the East Kunlun orogen. The Qushiang mafic rocks have variable δMo values of −0.50‰ to 0.78% and δMg values of −0.31‰ to −0.05‰. These features, combining with arc-type trace element distribution patterns, enriched SrNd isotopes, and variable zircon HfO isotopes, suggest the incorporation of different subducting components in their mantle source. Some mafic rocks show high δMo values together with low Mo/Ce ratios and enriched SrNd isotopes, suggesting the contribution of subducted sediment (SSD)-derived melts. Additionally, light Mo isotopes are also identified in mafic rocks, which are closely related to depleted SrNd isotopes. These features are mainly inherited from the subducted dehydrated altered oceanic crust (SAOC)-derived melts. More importantly, the coexisting and significantly high δMg and δMo values of these rocks could not be explained by the SSD or SAOC components, but require serpentinite components in the subarc mantle source. Our study reveals that combined Mo and Mg isotopic studies are potential tools for tracing serpentinite recycling, and provides new insight into the mechanisms by which different subducting components contribute to arc magmatism.

中文翻译：

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钼镁同位素追踪蛇纹岩在产生弧岩浆作用中的作用

蛇纹岩衍生的成分在俯冲带的化学循环中发挥着重要作用。然而，由于其他板块成分的叠印，很难识别弧下地幔中的再生蛇纹岩，而且蛇纹岩如何促进弧岩浆作用也知之甚少。本文报道了东昆仑造山带早古生代曲象岛弧镁铁质岩石的钼、镁同位素组成。曲乡镁铁质岩的δMo值变化范围为-0.50‰至0.78%，δMg值变化范围为-0.31‰至-0.05‰。这些特征与弧型微量元素分布模式、富集的SrNd同位素和可变的锆石HfO同位素相结合，表明其地幔来源中掺入了不同的俯冲成分。一些基性岩显示出高 δMo 值以及低 Mo/Ce 比和富集的 SrNd 同位素，表明俯冲沉积物 (SSD) 衍生熔体的贡献。此外，在镁铁质岩石中还发现了轻Mo同位素，它们与贫化SrNd同位素密切相关。这些特征主要继承自俯冲脱水蚀变洋壳（SAOC）衍生的熔体。更重要的是，这些岩石共存且显着高的δMg和δMo值不能用SSD或SAOC成分来解释，而是需要弧下地幔源中存在蛇纹岩成分。我们的研究表明，钼和镁同位素的联合研究是追踪蛇纹岩再循环的潜在工具，并为不同俯冲成分促成弧岩浆作用的机制提供了新的见解。