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
Final Report Abstract
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. Samples from ODP leg 169 at Middle Valley, Juan de Fuca Ridge replenished with dredged basalts from the East Pacific Rise were used to capture a wide range of rock types from strongly altered sediments to nearly unchanged basement rocks. In order to correlate results of diffusion transport with material properties of the rocks, various methods were applied for the characterization of textures, porosity, mineral abundance and composition. Pore volumes and pore size distributions were analyzed by mercury intrusion porosimetry for different alteration stages of basalts. Lowest pore volume of 0.5 vol.% was found for a dredged basalt increasing up to 17.1 vol.% in strongly altered basalts. Pore volumes are confirmed by analyzing the water release of pre-saturated samples using Karl Fischer titration. In pore size distribution broad maxima are found between 15 and 150 nm. Only a few pore radii are larger than 1 µm. Pore structure visualization by Wood`s metal intrusion reveals that in the dredged basalt samples the pores propagated from cracks. Wood´s metal intrusion offers the possibility for a detailed microscopic characterization of pore morphology and is useful for the critical evaluation of data obtained from Hg porosimetry e.g. by detecting pores related to fractures and issues for uneven distribution of pore size. Leaching experiments show that the degree of alteration has strong influence on the mobilization of Na, K, Mg and Ca. Due to dissolution processes, the connected porosity and the diameter of pores might increase and promote further dissolution reactions. Secondary minerals can impair further weathering by the extension of the diffusion paths and the covering of surfaces. Diffusion processes were studied in-situ at ambient pressure using a novel experimental cell attached to a FTIR-microscope. The effective diffusion coefficients of proton in porous medium of samples from oceanic crust were determined to be in the range of 10^-10 - 10^-11 m2/s that is more than two orders of magnitude slower than the diffusion of H+ and H2Omol in free liquid water. If pore characteristics (geometry, distribution, connectivity, constructivity etc.) are described in terms of diffusional tortuosity factor (X), samples without (B10) or with small amount of secondary minerals (B4) have X values varying in a narrow range from 34, while strongly altered sample ODP5 has X value of about 70. The activation energy of the diffusion process increases from 12.29 (0.71) kJ/mol for fresh and moderately altered basalts to 14.37 (1.33) kJ/mol for completely altered basalts. Our results show that porosity is not the limiting parameter, controlling diffusivity processes in porous medium. The sample with the largest amount of secondary minerals has the lowest activation energy for diffusion whereas the highest activation energy is found for the sample with well-developed pore network and relatively small amount of secondary phase. The difference in activation energies for samples with similar porosity suggests that additional factors have to be considered. The relation between Deff and porosity was found to be weak for both rocks, different additional factors such as tortuosity, uniformity of pore space characteristics, mineralogical composition of secondary minerals, formation of bound H2O layers, and total and connected porosity have to be considered for ameliorating the prediction of Deff from porosity. A new spectroscopic cell was developed to extend the experimental range towards higher pressure (up to 200 bar) and higher temperature (up to 150 °C) 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. The detailed knowledge of weathering processes from the fresh to the highly altered rocks including changes in mineralogical composition, porosity and Deff can ameliorate the prognosis of the present function in element turnover of rocks as well as from future states. Here different processes together with their dependencies can be explained. In further investigations the relationship between transport in pore systems and dissolution kinetics has to be examined experimentally.
Publications
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(2006): Diffusion as a control of element transfer in rocks of the oceanic crust: a study with in situ FTIR spectroscopy. Annual meeting of the Deutsche Mineralogische Gesellschaft in Hannover. Beih. Z. Eur. J. Mineral. Vol. 18, 129
Simonyan, A.V., S. Dultz, H. Behrens, U. Schwarz-Schampera
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(2006): Transport in microporous minerals and rocks studied by in situ FTIR microscopy, EMPG XI in Bristol, September 11-13, Abstract volume, 69
Simonyan, A.V., H. Behrens, S. Dultz, U. Schwarz-Schampera
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(2006): Transport in microporous rocks studied by in situ FTIR microscopy – implications for hydrothermal alteration of the oceanic crust. IODP-ICDP-Kolloquium Greifswald 27.-29.3.2006. Proceedings page 104
Simonyan, A.V., S. Dultz, H. Behrens, U. Schwarz-Schampera
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(2007): Porosity and element mobility in different alteration types of the oceanic crust. Jahrestagung der Geologischen Vereinigung, Bremen, 1.-5. Oktober 2007, University of Bremen
Simonyan, A.V., S. Dultz, H. Behrens, U. Schwarz-Schampera
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(2007): Porosity and permeability in different alteration types of the oceanic crust as a control of element mobilization. ICDP/IODP colloquium from 19.-21. March 2007 in Golm, University of Potsdam
Simonyan, A.V., H. Behrens, S. Dultz, U. Schwarz-Schampera
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(2007): Porosität und Permeabilität in Mineralen und Gesteinen als steuernder Faktor innerer Verwitterungsreaktionen. Mitteilungen der Deutschen Bodenkundlichen Gesellschaft 110, 641-642
Dultz, S., Simonyan, A.V., Behrens, H., Kübler, S.
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(2008): 3-D-Bildverarbeitung von röntgenmikrotomografischen Bodenanalysen im Mikrometerbereich. Bornimer Agrartechnische Berichte, Heft 62, 24-30. Leibniz-Institut für Agrartechnik Potsdam-Bornim. ISSN 0947-7314
Rath, T., Pastrana, J., Dultz, S.
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(2008): Porosity in different alteration types of the oceanic crust as a control of element mobilization – determination of diffusion transport by in-situ FTIR-spectroscopy. ICDP/IODP colloquium, 12.-14. March 2008, Leibniz University Hannover
Simonyan, A.V., S. Dultz, H. Behrens, J. Pastrana, U. Schwarz-Schampera
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(2009): Determination of porosity and diffusion transport in differently altered basalts of the oceanic crust. ICDP/IODP colloquium, 16.- 18. March 2009, University of Potsdam, GFZ Potsdam
Simonyan, A.V., S. Dultz, H. Behrens, J. Pastrana, U. Schwarz-Schampera
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(2009): Experimental determination of water diffusion in porous basaltic rocks of the oceanic crust. Conference Information: 19th Annual VM Goldschmidt Conference, June 21, 2009 Davos, Switzerland. Geochimica et Cosmochimica Acta 73, A1227-A1227
Simonyan, A.V., Behrens, H., Dultz, S.
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(2009): Transport in porous feldspars: in-situ experiments using FTIR spectroscopy. Geochimica et Cosmochimica Acta 73, 7019-7033
Simonyan, A.V., H. Behrens, S. Dultz
