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.

与俯冲作用有关的富H2O流体提高了岛弧岩浆的熔融程度，并引起地幔源的交代作用和可能的氧化作用。蛇纹岩是水和三价铁进入俯冲带的重要载体。虽然蛇纹岩脱水已被广泛研究，它是不知道是否，以及在什么时间尺度上释放的流体可以迁移到地幔楔或是否氧化的物种存在于这些流体中，可以解释弧地幔源氧化。地幔流体迁移的机制，无论是集中或普遍的流动发生，是很重要的，因为它可以控制岛弧火山的位置和H2O可溶的元素从板去除的程度。如果流体输运的时间尺度与俯冲速率相比很慢，则流体可能会俯冲到地幔深处。在初步工作中，我们已经开发了一种新的实验方法，用于确定通过矿物组合在板坯脱水条件下的H2O通量。流体源（例如蛇纹岩）脱水以产生孔隙流体超压。流体通过热压橄榄石或辉石的覆盖核心迁移，并被捕获在核心的另一侧的MgO汇，因为它形成水镁石。从水镁石形成的体积可以确定流体通量。我们还将硫化亚铁的蛇纹岩流体源和铁金属的氧化镁汇跟踪的氧化流体相的通道。结果表明，在蛇纹岩脱水条件下，H2O主要通过扩散通过橄榄石组合迁移，使橄榄石岩石有效地不可渗透。在这个建议中，我们将研究这种行为的条件限制，并确定晶格和晶界扩散系数的H2O在地幔组合的新方法。实验还表明斜方辉石中可以发生渗流。我们将探讨条件和时间尺度，通过斜方辉石丰富的岩石可以运输水和粒度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，一个移动的含硫流体是由蛇纹岩脱水和确定其流动性和氧化还原电位。因此，我们将建立在非常有希望的初步结果，以确定所需的时间尺度为富H2O流体通过最初干燥的覆盖橄榄岩和榴辉岩，并评估是否以及在何种程度上移动的含硫流体可以氧化地幔楔。