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蚀变的速率取决于岩石的渗透性和可接近的比表面。孔隙中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，其中大多数孔隙位于未破裂的岩石碎片中。我们的项目的范围是系统地研究孔隙和岩石渗透性对海洋热液系统中元素周转的作用。在Juan de Fuca海岭的Middle Valley，ODP 169航次的样品补充了来自东太平洋海隆的疏浚玄武岩，用于捕获从强烈蚀变沉积物到几乎不变的基底岩石的各种岩石类型。在项目初期，采用了各种分析和光谱技术对样品进行表征，特别是确定孔径分布和孔结构。首先，在岩石样品和流体之间的氢同位素交换的原位测量表明，在多孔岩石内的扩散是两个数量级慢于在自由流体，强调孔隙的孔隙度和弯曲度的作用岩石内运输。开发了一种新的光谱池，将实验范围扩展到更高的压力（高达300 bar）和更高的温度（高达200°C）。淋滤实验表明，蚀变程度对Na、K、Mg、Ca的活化有很大影响。我们工作的下一步将是系统地研究高压下多孔岩石中的扩散，因为在这种条件下没有实验数据。此后，我们计划原位研究岩石与含有溶解盐、酸和/或有机物质的反应性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渗透率和岩石/流体在高温高压下的相互作用。结合文献资料，可以加深对洋壳金属来源和热液演化的认识。

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.