Barium isotope systematics of subduction zones
Barium isotope systematics of subduction zones
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DOI：
10.1016/j.gca.2020.02.006
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发表时间：
2020-04
影响因子：
5
通讯作者：
S. Nielsen;Yunchao Shu;M. Auro;G. Yogodzinski;R. Shinjo;T. Plank;S. Kay;T. Horner
中科院分区：
文献类型：
--
作者：
S. Nielsen;Yunchao Shu;M. Auro;G. Yogodzinski;R. Shinjo;T. Plank;S. Kay;T. Horner
俯冲带是地球表面和深处之间质量传递的焦点。尽管它们具有重要意义，但关于从板块到上覆岩浆弧的物质运输的具体机制仍存在大量争论。总的来说，考虑板块物质搬运的模型关注的是促进弧火山作用的事件的相对顺序，特别是下沉板块的动员是先于还是滞后于地幔楔的混合。为了解决这些不确定性，我们概述了在俯冲带中钡（Ba）同位素质量平衡的效用，作为测试不同板块物质运输模型的一种手段。钡是一种高度流动的元素，在弧岩浆中显著富集，因此被认为是弧中板状物质搬运的敏感示踪剂。通过对阿留申和琉球岩浆弧两个俯冲带各自俯冲输入和输出的Ba同位素系统分析，得出了两个特征较好的俯冲带的Ba同位素质量平衡。尽管这两个系统的板岩输入的Ba同位素范围很窄（且相似），但我们发现喷发的岩浆在Ba活化过程中表现出了小的负同位素分异（≈20-40 ppm AMU−1）。利用其他地球化学参数（例如Rb/Ba、Pb同位素），我们认为蚀变洋壳不是这些负同位素值的主要来源，并推断蚀变洋壳的Ba同位素组成虽然在这些体系中贡献了少量的Ba，但在同位素上比上覆沉积物包体和贫化地幔重。总的来说，这些发现意义重大，因为它们表明，与其他俯冲带(如在强烈的海洋上升流区域（如堪察加半岛的南桑威奇）下的俯冲输入之间可能的同位素对比相比，与钡动员相关的同位素分异幅度很小。因此，我们认为喷发弧岩浆的Ba同位素组成在限制不同板块成分的重要性方面具有很大的前景，这有助于解决关于俯冲带板块物质运输机制的不确定性。
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.