Fluid permeability and speciation in subduction zones

俯冲带的流体渗透率和形态

• 批准号: 539187065
• 负责人: Professor Dr. Daniel J. Frost
• 金额: --
• 依托单位: Bayerisches Forschungsinstitut für Experimentelle Geochemie und Geophysik (BGI)
• 依托单位国家: 德国
• 项目类别: Research Grants
• 资助国家: 德国
• 项目状态: 未结题
• 来源: https://gepris.dfg.de/gepris/projekt/539187065?language=en
• 关键词: Fluid; permeability; speciation; subduction; zones

Subduction-related H2O-rich fluids raise the degree of melting and cause metasomatism and possibly oxidation of the mantle source of island arc magmas. Serpentinites are important carriers of H2O and ferric iron into subduction zones. Although serpentine dehydration has been extensively studied, it is not known whether, and on what timescale the released fluids can migrate into the mantle wedge or whether oxidised species exist in these fluids that could explain arc mantle source oxidation. The mechanism of mantle fluid migration, and whether focused or pervasive flow occurs, is important as it may control the position of island arc volcanos and the extent to which H2O-soluble elements are removed from the slab. If the timescale for fluid transport is slow compared to the rate of subduction, fluids may be subducted into the deep mantle. In preliminary work we have developed a new experimental method for determining the flux of H2O through a mineral assemblage at the conditions of slab dehydration. A fluid source, e.g. serpentinite, is dehydrated to produce a pore fluid overpressure. The fluid migrates through an overlying core of hot-pressed olivine or pyroxene and is captured in an MgO sink on the other side of the core, as it forms brucite. From the volume of brucite formed the fluid flux can be determined. We also place FeS in the serpentinite fluid source and iron metal in the MgO sink to trace the passage of an oxidised fluid phase. The results show that H2O migrates predominantly by diffusion through an olivine assemblage at serpentinite dehydration conditions, rendering olivine rocks effectively impermeable. In this proposal we will examine the conditional limits of this behaviour and determine lattice and grain boundary diffusion coefficients for H2O in mantle assemblages with the new method. The experiments also show that porous flow can occur in orthopyroxene. We will explore the conditions and timescale through which orthopyroxene rich rocks can transport H2O and the effect of grain size and olivine content on this transition. Finally, we will further investigate our preliminary findings that a mobile sulphur bearing fluid is produced by serpentinite dehydration and determine its mobility and redox potential. Thus, we will build on very promising preliminary results to determine the time scale required for H2O-rich fluids to pass through initially dry overlying peridotitic and eclogitic rocks and evaluate whether and to what extent a mobile sulphur-bearing fluid can oxidise the mantle wedge.

与俯冲有关的富含H2O的流体提高了熔融程度，导致岛弧岩浆的地幔来源发生交代和可能的氧化。蛇纹岩是水和铁进入俯冲带的重要载体。尽管蛇纹石脱水作用已经得到了广泛的研究，但尚不清楚释放的流体是否以及在什么时间尺度上可以迁移到地幔楔体中，也不知道这些流体中是否存在可以解释弧地幔来源氧化的氧化物种。地幔流体迁移的机制，无论是集中流动还是普遍流动的发生，都是重要的，因为它可以控制岛弧火山的位置和H2O可溶元素从板块中清除的程度。如果流体运移的时间尺度与俯冲速度相比较慢，则流体可能会被俯冲到地幔深处。在前期工作中，我们发展了一种在板坯脱水条件下通过矿物组合测定H2O通量的新实验方法。流体来源，例如蛇纹岩，脱水以产生孔隙流体超压。流体通过上覆的热压橄榄石或辉石岩心迁移，并在形成水镁石时被岩心另一侧的氧化镁水槽捕获。从形成的水镁石的体积可以确定流体的流量。我们还将FeS放置在蛇纹岩流体源中，并将铁金属放置在氧化镁水槽中，以追踪氧化的流体相的通道。结果表明，在蛇纹岩脱水条件下，H2O主要通过橄榄石组合扩散迁移，使橄榄石岩石有效地不透水。在这个建议中，我们将考察这种行为的条件极限，并用新方法确定地幔组合中H2O的晶格和晶界扩散系数。实验还表明，斜方辉石中可以发生多孔流动。我们将探索斜方辉石富集岩输送H2O的条件和时间尺度，以及粒度和橄榄石含量对这一转变的影响。最后，我们将进一步研究我们的初步发现，即蛇纹岩脱水产生了可移动的含硫流体，并确定了其迁移率和氧化还原电位。因此，我们将以非常有希望的初步结果为基础，确定富含H2O的流体通过最初干燥的上覆橄榄岩和榴辉岩所需的时间，并评估流动的含硫流体是否以及在多大程度上可以氧化地幔楔体。