Shallow Depth, Substantial Change: Fluid-Metasomatism Causes Major Compositional Modifications of Subducted Volcanics (Mariana Forearc)
Shallow Depth, Substantial Change: Fluid-Metasomatism Causes Major Compositional Modifications of Subducted Volcanics (Mariana Forearc)
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DOI：
10.3389/feart.2022.826312
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发表时间：
2021-12
期刊：
影响因子：
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通讯作者：
E. Albers;J. Shervais;Christian Hansen;Y. Ichiyama;P. Fryer
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文献类型：
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作者：
E. Albers;J. Shervais;Christian Hansen;Y. Ichiyama;P. Fryer
浅俯冲层次的传质及其对更深层次过程的影响仍然不完全受到限制。来自马里亚纳弧前的洋岛玄武岩碎屑提供了新的见解，这些碎屑经历了俯冲到∼25-30公里深和蓝片岩相变质作用；此后，碎屑通过蛇纹岩泥火山作用循环到弧前海底。此外，我们还证明这些岩石具有强烈的交代作用：它们的K2O含量(中位数=4.6wt%)和点火损失(中位数=5.3wt%)远高于太平洋板块上的OIB，这意味着它们是在俯冲过程中添加的。这一解释与样品中富含多硅白云母相一致。质量平衡计算进一步显示，所有样品的二氧化硅都有不同程度的增加，其中一个增加了氧化镁和氧化钠，但另一个研究地点损失了氧化镁和三氧化二铁。Cs和Rb浓度的升高表明碎屑的吸收，而Ba和Sr的低含量表明所有碎屑中的微量元素都被清除了。交代作用可能是由俯冲通道中的OIBs与富K流体相互作用引起的。我们的热力学模型表明，这种流体是从俯冲沉积物和蚀变火成岩地壳中释放出来的，温度为5kbar，甚至低于200℃。平衡组合图解表明，多硅白云母的稳定场随着交代作用的增加而显著增加，与未交代的OIB相比，多硅白云母在交代岩石中形成的多硅白云母可高达4倍。多硅白云母又被认为是K2O、H2O和流动元素进入弧下深度的重要载体。这些发现表明，俯冲岩石圈的质量传递始于低P/T条件。富含溶质的流体的释放可以在与这些流体相互作用的岩石中引起深远的成分和矿物学变化。因此，浅层(30公里)的过程有助于控制哪些组分以及处于哪种状态(即，结合在哪些矿物中)，这些组分最终到达更深的地方，在那里它们可能有助于或可能不有助于弧岩浆活动。为了从整体上理解深部地球化学循环，需要从浅层开始认识交代作用和岩石改造。
Mass transfer at shallow subduction levels and its ramifications for deeper processes remain incompletely constrained. New insights are provided by ocean island basalt (OIB) clasts from the Mariana forearc that experienced subduction to up to ∼25–30 km depth and up to blueschist-facies metamorphism; thereafter, the clasts were recycled to the forearc seafloor via serpentinite mud volcanism. We demonstrate that the rocks were, in addition, strongly metasomatized: they exhibit K2O contents (median = 4.6 wt%) and loss on ignition (median = 5.3 wt%, as a proxy for H2O) much higher than OIB situated on the Pacific Plate, implying that these were added during subduction. This interpretation is consistent with abundant phengite in the samples. Mass balance calculations further reveal variable gains in SiO2 for all samples, and increased MgO and Na2O at one but losses of MgO and Fe2O3* at the other study site. Elevated Cs and Rb concentrations suggest an uptake whereas low Ba and Sr contents indicate the removal of trace elements throughout all clasts. The metasomatism was likely induced by the OIBs’ interaction with K-rich fluids in the subduction channel. Our thermodynamic models imply that such fluids are released from subducted sediments and altered igneous crust at 5 kbar and even below 200°C. Equilibrium assemblage diagrams show that the stability field of phengite significantly increases with the metasomatism and that, relative to not-metasomatized OIB, up to four times as much phengite may form in the metasomatized rocks. Phengite in turn is considered as an important carrier for K2O, H2O, and fluid-mobile elements to sub-arc depths. These findings demonstrate that mass transfer from the subducting lithosphere starts at low P/T conditions. The liberation of solute-rich fluids can evoke far-reaching compositional and mineralogical changes in rocks that interact with these fluids. Processes at shallow depths (<30 km) thereby contribute to controlling which components as well as in which state (i.e., bound in which minerals) these components ultimately reach greater depths where they may or may not contribute to arc magmatism. For a holistic understanding of deep geochemical cycling, metasomatism and rock transformation need to be acknowledged from shallow depths on.