Porosity and permeability in different alteration types of the oceanic crust as a control of element mobilization - a case study on ODP Leg 169, Middle Valley Juan de Fuca Ridge

海水/岩石相互作用和洋壳变化的速度取决于岩石渗透率和可接近的特定表面。孔隙内的扩散和反应过程对热液流体中元素的活化和固定有很大的影响，其中大部分位于未破裂的岩石碎片中。我们的项目范围是系统地研究大洋热液系统中孔隙和岩石渗透率对元素周转的作用。来自中山谷胡安德富卡山脊的ODP169段的样品被用来捕捉从强烈蚀变的沉积物到几乎不变的基底岩石等各种岩石类型。在项目的初始阶段，采用了各种分析和光谱技术来表征样品，特别是确定了孔的大小分布和孔结构。首次现场测量了岩石样品和流体之间的氢同位素交换，结果表明，多孔岩石中的扩散比自由流体中的扩散慢两个数量级，强调了孔隙度和孔隙的曲折性对岩石内传输的作用。开发了一种新的光谱池，以将实验范围扩展到更高的压力(高达300bar)和更高的温度(高达200°C)。淋溶实验表明，蚀变程度对Na、K、Mg、Ca的活化有很大影响。我们下一步的工作将是系统地研究高压下多孔岩石中的扩散，因为没有关于这种条件的实验数据。此后，我们计划在现场研究岩石和含有溶解的盐、酸和/或有机物质的反应流体之间的相互作用。这项工作的一部分将是开发一种新的工具来测量高温和压力下的渗透率和岩石/流体相互作用。利用所获得的数据，结合文献数据，可以更好地了解洋壳中金属的来源和热液流体的演化。

The rate of seawater/rock interaction and alteration of the oceanic crust depends on the rock permeability and on the accessible specific surfaces. Diffusion and reaction processes within pores, most of them located inside unfractured rock fragments, have strong influence on mobilization and immobilization of elements in hydrothermal fluids. The scope of our project is to investigate systematically the role of pores and rock permeability on element turnover in oceanic hydrothermal systems. Samples from ODP leg 169 at Middle Valley, Juan de Fuca Ridge replenished with dredged basalts from the East Pacific Rise are used to capture a wide range of rock types from strongly altered sediments to nearly unchanged basement rocks. In the initial period of the project various analytical and spectroscopic techniques were employed for sample characterization, in particular for the determination of pore size distribution and pore structure. First in situ measurements of hydrogen isotope exchange between rock samples and fluid show that diffusion within the porous rocks is two orders of magnitude slower than in the free fluid, emphasizing the role of porosity and tortuosity of pores on intra-rock transport. A new spectroscopic cell was developed to extend the experimental range towards higher pressure (up to 300 bar) and higher temperature (up to 200°C). Leaching experiments show that the degree of alteration has strong influence on the mobilization of Na, K, Mg and Ca. The next steps of our work will be to investigate systematically diffusion within porous rocks at elevated pressure because no experimental data are available for such conditions. Thereafter, we plan to study in situ the interaction between rocks and reactive fluids containing dissolved salts, acids and/or organic materials. Part of this work will be to develop a new tool to measure permeabilities and rock/fluid interaction at elevated temperatures and pressures. Using the obtained data in combination with literature data, the understanding of the sources of metals and the evolution of hydrothermal fluids in the oceanic crust can be improved.