Alterationsmechanismen von basaltischen und rhyolitischen Gläsern unter Berücksichtigung derLösungschemie und passivierender Eigenschaften von Palagonit - eine Fallstudie an den ICDP Bohrungen in Hawaii und Snake River Plain
Mineralogie, Petrologie und Geochemie
Zusammenfassung der Projektergebnisse
Volcanic glasses are thermodynamically instable and easily react with hydrous fluids resulting in large elemental release to the environment. Palagonite layers on basaltic and rhyolitic glasses may act as a diffusion barrier that slows down glass alteration. Different altered basaltic and rhyolitic glass samples available from ICDP drilling cores Hawaii and Snake River Plain were used to determine the dependency of glass alteration from palagonite properties, in particular effects of porosity of layers and solution chemistry on glass weathering rate. From investigations on drilling cores evidence for strong glass alteration at both localities was obtained. Palagonite formation is common at HSDP but in contrary palagonite was absent at SRSDP indicating that high shares of glass compounds released are removed by leaching. Indication for a congruent dissolution of the glass and precipitation at an inward moving reaction front of palagonite, potentially mediated by microorganisms was obtained for glassy rims of pillow basalts from HSDP. Open spaces in zones with microtunnels between the primary glass and the palagonite and the sharp interface between the pristine glass and altered layers point to a dissolutionreprecipitation mechanism of glass alteration, contrary to the commonly accepted leaching mechanism. Variations of chemical composition along transects from the glass into the palagonite support this hypothesis, i.e. lower concentrations of Fe near the glass/palagonite transition are unlikely in presence of a leaching mechanism prevalent at glass alteration. Indication for microbial colonization not only on glass surfaces but also in palagonite was obtained by tomography. In palagonite formed on the glassy rim of a pillow at a depth of 8808 ftbsl, longish branching structures with cross sectional dimensions at 10-20 µm reminding to microbial morphologies were separated by their marked contents of Fe-oxides. Organic matter in this sample could be identified by 2D mapping on thick sections with an IR-microscope. The pore volume at 15% in the palagonite sample and the presence of voids in fractures allows fluid exchange, which is a prerequisite for microbial life. The intensification of glass dissolution by microorganisms could clearly be traced by micromorphological observations with pits and longish channels formed on the glass surface. By active removal of weakly soluble compounds from the glass surface, microorganisms can have a significant contribution to glass alteration affecting dissolution and reprecipitation, thereby shaping the secondary phase. Measurement of ionic effects on glass dissolution traced by element release and determination of by zeta potential provided new insights on changes of surface chemistry and the effect of solution composition hereby. The presence of monovalent cations Na+ and K+ suppressed Si release from the glass by favoring polymerization, whereas it is accelerated by bivalent cations Mg2+ and Ca2+. Upon addition of Al3+ the formation of Si-O-Al-O-Si bonds suppresses Si release. The presence of strongly adsorbing anions enhanced dissolution by increasing coordination of network forming cations hereby weakening bonds. The affinity of different anions and cations to glass surfaces was observed by establishing sorption isotherms and was also clearly traceable in zeta potential measurements by the shift in surface charge at the solid-liquid interface. F-, HPO42- and C2O42- were identified as strongly adsorbing anions, accelerating glass dissolution rate hereby. Over time a striking shift of negative surface charge of the glass to positive charges was obtained, most marked in presence of divalent cations but also clearly observable in deionized H2O for basaltic and rhyolitic glasses. The generation of positive surface charge is a major factor for the sorption of negatively charged compounds such as dissolved organic matter and cell walls of bacteria on the glass surface and might be hereby an important factor for the bioalteration of glass. Basaltic glasses have a widespread occurrence and together with their relatively high reactivity, they have strong significance for local and global element cycles. Increased knowledge of decisive factors for glass alteration rate improves predictions of element release rates. Fundamental differences in the modeled course of glass corrosion can be expected for a diffusion-controlled and an interface-controlled glass alteration process.
Projektbezogene Publikationen (Auswahl)
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(2012) Biogenic weathering of basaltic glass and authigenic mineral formation. 6th Mid-European Clay Conference, Průhonice/Prague, Czech Republic, 4.-9. September 2012
Dultz, S., Boy, J., Dupont, C., Behrens, H.
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(2013) Determination of microbial driven weathering of volcanic glasses with morphological and chemical methods. Session `Soil Mineralogy´ within the annual meeting of the German Soil Science Society (DBG). Universität Rostock, 7.-12. September 2013
Dultz, S., Dupont, C., Behrens, H., Tramm, F., Plötze, M.
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(2013) Passivating properties of the palagonite layer on subsurface basaltic and rhyolithic glass alteration from ICDP sites Hawaii and Snake River Plain. IODP/ICDP Colloquium 2013, TU Bergakademie Freiberg, 25.-27. March 2013
Dultz, S., Behrens, H., Plötze, M.
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(2014) Alteration of a submarine basaltic glass under environmental conditions conducive for microorganisms: Growth patterns of the microbial community and mechanism of palagonite formation. Geomicrobiology Journal 31, 813-834
Dultz, S., Boy, J., Dupont, C., Halisch, M., Behrens, H., Welsch, A.-M., Erdmann, M., Cramm, S., Helsch, G., Deubener, J.
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(2014) Controlling factors of alteration rate of basaltic glass and palagonite formation in pillow lavas from Hawaii Scientific Drilling Project. 7th Mid-European Clay Conference, Dresden, 16.-19. September 2014
Dultz, S., Behrens, H., Dupont, C., Tramm, F., Ohrt, C., Plötze, M.
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(2015) Mechanisms of alteration of basaltic and rhyolitic glasses considering solution chemistry and passivating properties of palagonite – a case study on ICDP drilling sites Hawaii and Snake River Plain. IODP/ICDP Colloquium 2015, Universität Bonn, 2.-4. March 2015
Dultz, S., Behrens, H., Tramm, F., Plötze, M.
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(2016) Electrolyte effects on surface chemistry of basaltic glass in the initial stages of dissolution. Chemical Geology 426, 71-84
Dultz, S., Behrens, H., Helsch, G., Deubener, J.
