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

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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.