Abstract Subduction zones are the focal points of mass transfer between the surface and deep Earth. Despite their significance, there remains substantial debate regarding the specific mechanisms of material transport from the slab to the overlying magmatic arc. Broadly, models accounting for slab material transport focus on the relative sequence of events promoting arc volcanism and, in particular, whether mobilization of the down-going slab leads or lags mixing with the mantle wedge. To address these uncertainties, we outline the utility of barium (Ba) isotope mass balance in subduction zones as a means to test different slab material transport models. Barium is a highly fluid-mobile element that is significantly enriched in arc magmas and is thus thought to be a sensitive tracer of slab material transport in arcs. We also present qualitative Ba isotopic mass balances for two well-characterized subduction zones—the Aleutian and Ryukyu magmatic arcs—by analyzing the Ba isotope systematics of their respective subduction inputs and outputs. Despite the narrow (and similar) Ba-isotope range of slab inputs to both systems, we find that erupted magmas exhibit systematic variations indicative of a small negative isotope fractionation during Ba mobilization (≈20–40 ppm AMU−1). We suggest that AOC (altered oceanic crust) is not the principal source of these negative isotope values using other geochemical parameters (e.g., Rb/Ba, Pb isotopes), and infer that the Ba isotope composition of AOC—though contributing a minor amount of Ba in these systems—is isotopically heavier than the overlying sediment package and the depleted mantle. Altogether, these findings are significant as they indicate that the magnitude of isotope fractionation associated with Ba mobilization is small relative to the likely isotopic contrast between subduction inputs in other subduction zones, such as beneath areas of strong ocean upwelling (e.g., South Sandwich, Kamchatka). Thus, we propose that the Ba isotope composition of erupted arc magmas holds great promise for constraining the importance of different slab components, which could help address uncertainties regarding the mechanism of slab material transport in subduction zones.

中文翻译：

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俯冲带钡同位素系统学

摘要 俯冲带是地表与地球深处传质的焦点。尽管它们很重要，但关于物质从板片到上覆岩浆弧的具体传输机制仍存在大量争论。从广义上讲，解释板片物质运输的模型侧重于促进弧形火山活动的事件的相对顺序，特别是向下移动的板片是否超前或滞后与地幔楔混合。为了解决这些不确定性，我们概述了俯冲带中钡 (Ba) 同位素质量平衡的效用，作为测试不同板坯材料传输模型的一种手段。钡是一种高度流体流动的元素，在弧形岩浆中显着富集，因此被认为是弧形中板片材料传输的敏感示踪剂。我们还通过分析各自俯冲输入和输出的 Ba 同位素系统学，提出了两个特征明确的俯冲带——阿留申和琉球岩浆弧的定性 Ba 同位素质量平衡。尽管两个系统的板坯输入的 Ba 同位素范围很窄（且相似），但我们发现喷发的岩浆表现出系统变化，表明在 Ba 动员期间存在小的负同位素分馏（≈20-40 ppm AMU-1）。我们建议使用其他地球化学参数（例如 Rb/Ba、Pb 同位素），AOC（改变的洋壳）不是这些负同位素值的主要来源，并推断 AOC 的 Ba 同位素组成——尽管贡献了少量这些系统中的 Ba 在同位素上比上覆的沉积物包和枯竭的地幔重。共，这些发现是重要的，因为它们表明与 Ba 动员相关的同位素分馏的幅度相对于其他俯冲带中俯冲输入之间可能的同位素差异很小，例如在强烈海洋上升流区域（例如，南桑威奇，堪察加半岛）的下方。因此，我们提出喷发的弧形岩浆的 Ba 同位素组成对于限制不同板块成分的重要性具有很大的希望，这有助于解决俯冲带中板块物质输运机制的不确定性。