Fluid release from subducting slabs during recycling of oceanic lithosphere plays an important role in concentrating select elements near the Earth’s surface. Yet the extent to which these fluids are oxidising or reducing (fO₂), one of the key characteristics that controls element mobility, remains poorly understood. Additionally, it is unclear whether the fO₂ signature of arc fluids is generated deep within the slab during subduction or if it reflects later, shallower subarc crustal processes. Previous experimental work has shown that the extent to which highly incompatible W partitions into aqueous fluids depends on the temperature and fO₂ conditions of the fluids. In the subducting slab, W partitions into rutile with a different coordination number to that which it exhibits in fluids, a process which may generate stable W isotopic fractionation. Therefore, if the competition between partitioning into aqueous fluids versus rutile is controlled by the conditions within the subducting slab, the elemental and stable W isotopic systematics of arc lavas may provide a novel tool with which to investigate subduction zones. We present isotopic compositions of a well characterised suite of arc lavas from the central island province of the Mariana arc (W. Pacific), and rutile separates from exhumed fragments of subducted, depleted, mafic oceanic crust from the Raspas Complex, Ecuador. Our data show that lavas from Guguan that exhibit the strongest geochemical signature of subduction zone fluids (e.g., high Ba/Th, Mo/Ce, Pb/Ce), contain the highest abundances of W compared to similarly magmatically incompatible Th, in agreement with anticipated fluid mobile behaviour. These Guguan lavas have distinct, heavy stable W isotopic compositions (δ^186/184W = +0.134 to +0.156 ‰) compared to MORB (δ^186/184W = +0.078 to +0.099 ‰) and those from sediments-dominated islands like Uracas and Agrigan (δ^186/184W = +0.080 to +0.111 ‰). The W isotopic compositions of sediments off-board the Mariana arc are more variable, with the volcaniclastic sediments having δ^186/184W values closest to those of the sediment dominated arc lavas. We show that rutile from representative subducting mafic crust incorporates isotopically light W, consistent with a coordination change from tetrahedral in fluids and melts to octahedral in the structure of rutile. We suggest that isotopically heavy fluids, complementary to the residual rutile in the mafic crust, account for the high δ^186/184W of the fluid-dominated Guguan lavas, a process previously invoked to control Mo systematics in the same samples. Combining our new elemental and isotopic W data with these existing Mo data, we model the influence of oxygen fugacity on fluid compositions in equilibrium with the mafic crust, and the fraction of such fluids required to be added to the mantle wedge to reproduce the Mo-W systematics of erupted arc lavas. Our models show that more fluid is required (F = 6 %) than can be generated internally in the mafic oceanic crust, and that this fluid must be oxidised, perhaps as high as FMQ +5. We suggest that these requirements document the interaction of subducted mafic crust with an external source of oxidising fluids sourced from dehydration of underlying serpentinites.

在海洋岩石圈再循环过程中，俯冲板片的流体释放在将精选元素集中在地球表面附近发挥着重要作用。然而，这些流体的氧化或还原程度（f O ₂）是控制元素迁移率的关键特征之一，目前仍知之甚少。此外，尚不清楚弧流体的f O ₂特征是在俯冲过程中在板片深处产生还是反映了较晚的、较浅的弧下地壳过程。先前的实验工作表明，高度不相容的 W 分配到水性流体中的程度取决于温度和f O ₂流体的条件。在俯冲板片中，W 分裂成金红石，其配位数与其在流体中表现出的配位数不同，这一过程可能会产生稳定的 W 同位素分馏。因此，如果划分成含水流体与金红石之间的竞争是由俯冲板片内的条件控制的，那么弧熔岩的元素和稳定的 W 同位素系统可能会提供一种新的工具来研究俯冲带。我们展示了来自马里亚纳弧中部岛屿省（西太平洋）的一组特征良好的弧熔岩的同位素组成，金红石与厄瓜多尔拉斯帕斯复合体的俯冲、枯竭、镁铁质海洋地壳的挖掘碎片分离。我们的数据表明，古关熔岩表现出最强的俯冲带流体地球化学特征（例如，高 Ba/Th、Mo/Ce、Pb/Ce），与类似岩浆不相容的 Th 相比，W 的丰度最高，与预期的流动移动行为。这些古关熔岩具有独特的、重的稳定的 W 同位素组成（δ ^186/184 W = +0.134 至 +0.156 ‰) 与 MORB ( δ ^186/184 W = +0.078 至 +0.099 ‰) 以及乌拉卡斯和阿格里甘等以沉积物为主的岛屿 ( δ ^186/184 W = +0.080) 相比至 +0.111 ‰)。马里亚纳弧外沉积物的 W 同位素组成变化较大，火山碎屑沉积物的δ ^186/184W 值最接近以沉积物为主的弧形熔岩。我们表明，来自代表性俯冲镁铁质地壳的金红石包含同位素轻 W，这与金红石结构中从流体和熔体中的四面体到八面体的配位变化一致。我们认为，与镁铁质地壳中残留的金红石互补的同位素重流体是高δ ^{186/184的原因}W 以流体为主的古关熔岩，这是以前在相同样品中用于控制 Mo 系统学的过程。将我们新的元素和同位素 W 数据与这些现有的 Mo 数据相结合，我们模拟了氧逸度对与镁铁质地壳平衡的流体成分的影响，以及需要添加到地幔楔中以再现 Mo-的此类流体的比例。 W 爆发的弧形熔岩系统学。我们的模型表明，需要的流体（F = 6 %）比镁铁质洋壳内部产生的流体要多，而且这种流体必须被氧化，可能高达 FMQ +5。我们建议，这些要求记录了俯冲镁铁质地壳与来自下方蛇纹岩脱水的外部氧化流体源的相互作用。