Fluid permeability and speciation in subduction zones

俯冲相关的富氢流体提高了熔融程度，并引起岛弧岩浆地幔源的交代作用和可能的氧化作用。蛇纹岩是水和铁进入俯冲带的重要载体。尽管对蛇纹岩脱水进行了广泛的研究，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中，也不知道这些流体中是否存在可以解释弧源地幔氧化的氧化物质。地幔流体运移的机制，以及是否发生集中流动或普遍流动，是重要的，因为它可以控制岛弧火山的位置和从板块中去除水溶性元素的程度。如果流体输运的时间尺度比俯冲速度慢，流体可能被俯冲到深部地幔中。在初步工作中，我们开发了一种新的实验方法来测定在板坯脱水条件下水通过矿物组合的通量。流体源（如蛇纹岩）脱水后产生孔隙流体超压。流体通过热压橄榄石或辉石的上覆岩心迁移，并在岩心另一侧的MgO汇中捕获，形成水镁石。由水镁石形成的体积可以确定流体通量。我们还在蛇纹岩流体源中放置了FeS，在MgO水槽中放置了铁金属，以追踪氧化流体相的通过。结果表明，在蛇纹岩脱水条件下，水主要通过橄榄石组合扩散运移，使橄榄石有效不渗透。在本提案中，我们将研究这种行为的条件极限，并使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，正辉石中也会发生多孔流动。我们将探讨富正辉石岩石输运水的条件和时间尺度，以及晶粒尺寸和橄榄石含量对这种转变的影响。最后，我们将进一步研究我们的初步发现，即蛇纹岩脱水产生可移动的含硫流体，并确定其流动性和氧化还原电位。因此，我们将以非常有希望的初步结果为基础，确定富氢流体通过最初干燥的上覆橄榄岩和榴辉岩所需的时间尺度，并评估一种可移动的含硫流体是否以及在多大程度上氧化地幔楔。

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.