Abstract Molybdenum isotope variations in mafic arc lavas have mainly been attributed to the influence of slab-derived components, such as subducted sediment melts and aqueous fluids. The latter have been hypothesised to fractionate Mo isotopes through interaction with the oceanic crust and carry an isotopically heavy signal that is transferred to the source of arc magmas. Thus, understanding Mo isotope systematics in subduction zones requires characterising the Mo isotope composition of slab-derived fluids and their influence on the Mo isotope budget of arc magmas. However, Mo isotope data reported to date show a considerable influence from subducted sediments that complicate accurate constraints being placed on the fluid contribution. We present Mo isotope data for mafic lavas from the Izu arc, a highly depleted oceanic island arc whose magma compositions show a dominant control from slab-derived fluids. The lavas from the Izu volcanic front are isotopically heavier than MORB and the depleted mantle. Their δ98/95Mo (the relative difference in measured 98Mo/95Mo to NIST 3134) systematically varies with indicators for fluid-mobile element enrichment, suggesting that slab-derived fluids in the Izu arc have heavy Mo isotope compositions. Additionally, co-variations with radiogenic 143Nd/144Nd and 176Hf/177Hf point to a relationship between the addition of aqueous fluids and compositional heterogeneity of the sub-arc mantle. We present mass balance models that show that the influence of subduction zone fluids on the trace element pattern of arc magmas is more dominant when these are added to a more depleted and refractory sub-arc mantle, which preferentially melts due to a relatively higher fluid flux. The mass balance of Mo in the Izu arc predicts a light Mo isotope composition for the residual oceanic crust as a result of the preferential removal of isotopically heavy Mo during slab dehydration, consistent with previous suggestions for the Mariana arc and isotopically light Mo previously reported for eclogites.

中文翻译：

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伊豆弧玄武岩钼同位素比：俯冲带流体对弧火山系统成分变化的控制

摘要 镁铁质弧熔岩中钼同位素的变化主要归因于板片衍生成分的影响，如俯冲沉积物熔体和含水流体。后者被假设通过与大洋地壳的相互作用来分离 Mo 同位素并携带同位素重信号，该信号被转移到弧形岩浆源。因此，了解俯冲带中的钼同位素系统需要表征板片衍生流体的钼同位素组成及其对弧形岩浆钼同位素收支的影响。然而，迄今为止报告的 Mo 同位素数据显示，俯冲沉积物的影响相当大，这使得对流体贡献的准确限制变得复杂。我们提供了来自伊豆弧的镁铁质熔岩的 Mo 同位素数据，一个高度枯竭的大洋岛弧，其岩浆成分显示出主要来自板块衍生流体的控制。来自伊豆火山前缘的熔岩的同位素比 MORB 和枯竭的地幔重。它们的 δ98/95Mo（测量的 98Mo/95Mo 与 NIST 3134 的相对差异）随着流体-移动元素富集指标的变化而系统地变化，这表明伊豆弧中的板片衍生流体具有重的 Mo 同位素组成。此外，与放射成因 143Nd/144Nd 和 176Hf/177Hf 的共变表明含水流体的添加与亚弧地幔的成分异质性之间存在关系。我们提出的质量平衡模型表明，当将俯冲带流体添加到更耗竭和更难熔的亚弧地幔时，俯冲带流体对弧形岩浆微量元素模式的影响更为显着，由于相对较高的流体通量，它优先熔化。伊豆弧中钼的质量平衡预测，由于板片脱水过程中优先去除同位素重的钼，残余洋壳的轻钼同位素组成与先前对马里亚纳弧和同位素轻钼的建议一致榴辉岩。