Rates and mechanisms of element transfer during mineral reactions in the presence of a fluid phase
Final Report Abstract
Fluid-rock interaction is fundamental to chemical segregation in the Earth crust, including the formation of ore deposits, the creation of porosity essential for petroleum reservoirs, and chemical exchange between the hydrosphere and geosphere. This often leads to the formation of geochemical fronts (both in terms on mineralogy and isotopes) between reacted and unreacted rocks. Most of such fronts are directly related to fluid-present reactions. Key features of such systems are a) reaction induced creation of porosity increasing permeability and further focussing of fluid into the zone of reaction; hence this results in a positive feedback between reaction, fluid ingress and further reaction, and b) recrystallization of mineral phases releasing potentially economic elements into the fluid. In this project, the mechanisms controlling hydrothermal replacement of CaCO3 by Mg-bearing carbonates were studied using a combined approach of hydrothermal experiments and modeling with special focus on the role of fluid mediated element fluxes. The experimental results revealed that the replacement of CaCO3 sample material by a porous reaction rim of magnesite and Ca-rich dolomite takes place by a dissolution-precipitation mechanism. The diffusive transport of the aqueous species across the reaction rim is the rate-limiting process controlling both the rate of the replacement process and the chemical composition of the replacement product on a microscopic scale. The effective element flux is, in turn, controlled by three specific parameters: (I) The concentration gradient of the major elements across the reaction rim, (II) the degree of primary permeability and reactive surface area in the original solid precursor, and (III) the degree of secondary permeability in the product replacement phase. In summary, our results reveal the evolution and transport-controlled nature of the replacement reaction. More specifically, the time series experiments provide convincing evidence that the reaction proceeds via a dissolution-precipitation process, but the reaction progress, i.e. the rate of reaction is controlled by element transport through the evolving pore network. Our findings therefore contradict previous assumptions of interface controlled dissolution-precipitation reaction rate, and highlight the critical importance to understanding rates of carbonate replacement.
Publications
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(2014): Hydrothermal replacement of aragonite by Mg-carbonates: Geological Society of America Annual Meeting, Vancouver, Canada
Jonas, L., Müller, T., Dohmen, R., and Immenhauser, A.
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(2015): Mg: Invited plenary keynote, IAS 31st Meeting of Sedimentology, Krakow, Poland
Immenhauser, A., Riechelmann, S., Wassenburg, J., Jonas, L., Müller, T., Dohmen, R., and Niedermayr, A.
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(2015): Numerical modeling of hydrothermal carbonate replacement reactions: Goldschmidt conference, Prague, Czech Republic
Jonas, L., Müller, T., and Dohmen, R.
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(2015): Transportcontrolled hydrothermal replacement of calcite by Mg-carbonates. Geology, 43(9), 779-782
Jonas, L., Müller, T., Dohmen, R., Baumgartner, L., & Putlitz, B.
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Rates and mechanisms of element transfer in the presence of a fluid phase: A case study on carbonate replacement reactions. Dissertation, Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, October 2015
Jonas, L.
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(2017): Hydrothermal replacement of biogenic and abiogenic aragonite by Mg-carbonates–Relation between textural control on effective element fluxes and resulting carbonate phase. Geochimica et Cosmochimica Acta, 196, 289-306
Jonas, L., Müller, T., Dohmen, R., Immenhauser, A., & Putlitz, B.
