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首页> 外文期刊>Proceedings of the National Academy of Sciences of the United States of America >Separation of supercritical slab-fluids to form aqueous fluid and melt components in subduction zone magmatism
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Separation of supercritical slab-fluids to form aqueous fluid and melt components in subduction zone magmatism

机译：在俯冲带岩浆作用中分离超临界板状流体以形成含水流体和熔融组分

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Subduction-zone magmatism is triggered by the addition of H_2O- rich slab-derived components: aqueous fluid, hydrous partial melts, or supercritical fluids from the subducting slab. Geochemical analyses of island arc basalts suggest two slab-derived signatures of a melt and a fluid. These two liquids unite to a supercritical fluid under pressure and temperature conditions beyond a critical end-point. We ascertain critical endpoints between aqueous fluids and sediment or high-Mg andesite (HMA) melts located, respectively, at 83-km and 92-km depths by using an in situ observation technique. These depths are within the mantle wedge underlying volcanic fronts, which are formed 90 to 200 km above subducting slabs. These data suggest that sediment-derived supercritical fluids, which are fed to the mantle wedge from the subducting slab, react with mantle peridotite to form HMA supercritical fluids. Such HMA supercritical fluids separate into aqueous fluids and HMA melts at 92 km depth during ascent. The aqueous fluids are fluxed into the asthenospheric mantle to form arc basalts, which are locally associated with HMAs in hot subduction zones. The separated HMA melts retain their composition in limited equilibrium with the surrounding mantle. Alternatively, they equilibrate with the surrounding mantle and change the major element chemistry to basaltic composition. However, trace element signatures of sediment-derived supercritical fluids remain more in the melt-derived magma than in the fluid-induced magma, which inherits only fluid-mobile elements from the sediment-derived supercritical fluids. Separation of slab-derived supercritical fluids into melts and aqueous fluids can elucidate the two slab-derived components observed in subduction zone magma chemistry.
机译：俯冲带的岩浆作用是由富含H_2O的板坯衍生成分引起的：俯冲板块中的含水流体，含水部分熔体或超临界流体。岛弧玄武岩的地球化学分析表明，有两个平板状的熔体和流体特征。在压力和温度条件下超过临界点时，这两种液体会合并为超临界流体。通过现场观察技术，我们确定了分别位于83公里和92公里深度的含水流体与沉积物或高镁安山岩（HMA）熔体之间的关键终点。这些深度在俯冲板块上方90至200 km处形成的火山前沿下的地幔楔内。这些数据表明，从俯冲板块进入地幔楔的沉积物超临界流体与地幔橄榄岩反应形成HMA超临界流体。此类HMA超临界流体会分离为水性流体，并且在上升过程中HMA会在92 km的深度处融化。含水流体被流入软流圈地幔中以形成弧形玄武岩，这些玄武岩在热俯冲带中与HMAs局部相关。分离的HMA熔体与周围地幔保持有限的组成平衡。或者，它们与周围的地幔保持平衡，并将主要元素化学成分改变为玄武质成分。然而，源自熔岩的超临界流体中的痕量元素特征在熔体衍生的岩浆中要比在流体诱导的岩浆中更多，后者仅从源自沉积物的超临界流体中继承了可流动的元素。将板坯衍生的超临界流体分离成熔体和含水流体可以阐明在俯冲带岩浆化学中观察到的两种板坯衍生的成分。

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