Fluid migration within the Earth's crust significantly influences the development of shear zones. The Bergen Arcs have been a focal point for investigating the localization of rheologically weaker zones that facilitate shear zone formation, with various hypotheses proposed (Jamtveit et al. 2019; Incel et al. 2022). These zones may originate from seismic events, inducing brittle fracturing and creating pathways for mineral re-equilibration and ductile deformation.

This research concentrates on the island of Krossøy, located in the northernmost part of the Bergen Arcs, Western Norway, offering a unique perspective on deformation, textural evolution, and metamorphism compared to the extensively studied southern regions of Holsnøy and Radøy (Austrheim 1987; Mukai et al. 2014; Moore et al. 2020). Krossøy exposes anorthosites from the old granulitic basement, intruded by a series of subparallel mafic granulitic dykes forming a distinctive "dyke swarm," not documented elsewhere in the Bergen Arcs. We present our results on microstructural analysis through Electron Backscattered Diffraction (EBSD) and, mineral and chemical evolution using Electron Microprobe analysis (EMPA).

Given the plagioclase-rich nature of anorthosites, our results delineate the chemical and textural evolution of feldspars, tracing their journey from early-stage granulitic anorthosite formation approximately 930 Ma ago to the development of Caledonian mylonite (440-420 Ma) during shear zone activity under amphibolite facies conditions. By examining the fluid pathways, we seek to determine their relationship with the formation of rheologically weaker areas in the crust and how the dynamic interplay between fluid infiltration and deformation mechanisms may play an important role by changing the metamorphic conditions, textures and mineral assemblages in the rocks.

Austrheim, H. (1987). Eclogitization of lower crustal granulites by fluid migration through shear zones. Earth and Planetary Science Letters, 81(2–3), 221-232.

Incel, S., Labrousse, L., Hilairet, N. et al. (2022). Reaction-induced embrittlement of the lower continental crust. Geology, 47(3).

Jamtveit, B., Petley‐Ragan, A. et al. (2019). The Effects of Earthquakes and Fluids on the Metamorphism of the Lower Continental Crust. Journal of Geophysical Research: Solid Earth, 124(8), 7725-7755.

Moore J., Beinlich A., Piazolo S., Austrheim H. and Putnis A. Metamorphic differentiation via enhanced dissolution along high permeability zones. Journal of Petrology 61, 10, egaa096 (2020)

Mukai, H., Austrheim, H., Putnis, C. V., and Putnis, A. (2014). Textural Evolution of Plagioclase Feldspar across a Shear Zone: Implications for Deformation Mechanism and Rock Strength Journal of Petrology, 55(8), 1457-1477.

How to cite: Hernández Filiberto, L., Austrheim, H., and Putnis, A.: Fluid pathways and metamorphic evolution in the northern part of the Bergen Arcs: the island of Krossøy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20323, https://doi.org/10.5194/egusphere-egu24-20323, 2024.

Sediment-hosted Cu deposits are a significant global source of copper. This study employs a mineral system approach, focusing on basin-scale groundwater flow as a key mechanism for Cu transport from source to trap. Numerical experiments using the open-source IC-FERST code investigate the controls on Cu transport in the Katangan Basin, Central African Copperbelt. The models developed for early and late stages of basin evolution incorporate fluid flow, heat and solute transport, and dynamic mesh optimization to enhance computational efficiency.

The early-stage model, corresponding to the salt deposition period, attributes the formation of saline brine to a dense residual phase resulting from evaporite formation. In the late-stage model, corresponding to Cu mobilization and mineralization, Cu dissolution and mineralization are simulated using a partition coefficient informed by experimental data.

Results reveal that density gradients induced by salinity and temperature variations play a crucial role in initiating convective groundwater flow. Highly saline, dense brines generated during salt deposition or dissolution form complex, downward-propagating plumes influenced by flow instabilities and geologic heterogeneity. Permeable faults and fractures in basement rocks enable groundwater to percolate, potentially mobilizing Cu from intra- or extra-basinal source rocks. Salinity and temperature gradients drive upwelling plumes, transporting Cu from deeper source rocks to shallower, organic-rich sedimentary rocks where mineralization occurs.

How to cite: Bahlali, M., Woitischek, J., Jacquemyn, C., Purkiss, M., and Jackson, M.: Integrated Modeling of Cu-Rich Fluid Migration and Mineralization in the Katangan Basin, Central African Copperbelt: Insights from Numerical Experiments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11453, https://doi.org/10.5194/egusphere-egu24-11453, 2024.

The alteration of ferroan brucite, a common by-product of serpentinization, has been proposed as a H₂ source at low temperature. Here, synthetic ferroan brucite with Fe/(Fe+Mg) = 0.2 was reacted with pure water at temperatures ranging from 348 to 573 K in 29 experiments either conducted in gold capsules or Ti-based reactors. H₂ production monitoring with time and characterization of the reaction products revealed the occurrence of the following reaction: 3 Fe(OH)₂^brucite = Fe₃O₄ + H₂ + 2 H₂O. This reaction proceeds completely in ~ 2 months at 378 K and is thermally activated. The small grain size of the synthetic brucite (40-100 nm) is similar to observations in natural samples, and is probably responsible for the high reaction rate measured. H₂ production reached a plateau and Fe-bearing brucite also precipitated as a reaction product, suggesting the achievement of equilibrium. The thermodynamic properties of Fe(OH)₂ were refined based on the experimental dataset and differ by less than 5 % from previous estimates. However, ferroan brucite is predicted to be stable at an hydrogen activity one order of magnitude lower than previously calculated. As a result, significant H₂ production during ferroan brucite alteration at low temperature requires efficient fluid renewal. Such a mechanism strongly differs from olivine serpentinization which can occur even at high activity in H₂ and thus with limited water renewal.

How to cite: Malvoisin, B., Carlin, W., Brunet, F., Lanson, B., Findling, N., Lanson, M., Jeannin, L., Fargetton, T., and Lhote, O.: New thermodynamic and kinetic constraints on H2 production during ferroan brucite reaction at low temperature, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20271, https://doi.org/10.5194/egusphere-egu24-20271, 2024.

Coupled dissolution-precipitation reactions have been studied extensively recently for their ability to retain elements of interest into a stable solid form that can sequester potentially toxic elements. This is achieved through the initial dissolution of a substrate mineral in a fluid containing the target (often toxic) element. The dissolution leads to the supersaturation of a boundary layer at the mineral surface with respect to another solid phase containing the element of interest¹. When the relative solubilities of the different minerals (in the aqueous fluid at the reaction interface) and their molar volume difference allow it, a coupled dissolution-precipitation can lead to the pseudomorphic replacement of the original substrate². We tested this reaction for CaCO₃ and cadmium (Cd) containing solutions as calcite (CaCO₃) and otavite (CdCO₃) form an almost perfect solid solution. We compared the reaction in similar solutions with different types of CaCO₃: calcite single crystal, Carrara marble (polycrystalline calcite) and aragonite single crystals. For single calcite crystals, the reaction in a Cd-solution passivates the crystal’s surface due to the epitaxial growth of a (Ca,Cd)CO₃ solid solution layer of low solubility. However, the random orientation of the grains in the Carrara marble samples and the change of crystal structure for the aragonite crystals modified the mechanism and allow the replacement of CaCO₃ by (Ca,Cd)CO₃ to take place³. Hydrothermal experiments and in situ fluid-cell atomic force microscopy (AFM) were used to observe the reaction both at room temperature and high pressure and temperature (200°C). In addition to SEM, BSE and EDX observations, synchrotron X-ray microtomography images were acquired on Carrara marble and aragonite samples at different stages of the reaction in order to gather more information about the replacement mechanism and the extent of Cd-uptake achievable through this process. The extent of the reaction was shown to be similar for the different solution concentrations used and limited in the case of Carrara marble. The porosity closes fast after the start of the reaction blocking the fluid pathways necessary for the reaction to proceed. The reaction in the aragonite samples seems to progress mainly through a reaction-induced fracture network probably created by the stress caused by the crystallographic structural differences between parent and product phases. Overall, these results demonstrate the capacity of CaCO₃ to trap and store cadmium into a solid phase by a mechanism of coupled dissolution-precipitation.

(1)        Putnis, C. V.; Putnis, A. A Mechanism of Ion Exchange by Interface-Coupled Dissolution-Precipitation in the Presence of an Aqueous Fluid. Journal of Crystal Growth 2022, 600, 126840. https://doi.org/10.1016/j.jcrysgro.2022.126840.

(2)        Pollok, K.; Putnis, C. V.; Putnis, A. Mineral Replacement Reactions in Solid Solution-Aqueous Solution Systems: Volume Changes, Reactions Paths and End-Points Using the Example of Model Salt Systems. American Journal of Science 2011, 311 (3), 211–236. https://doi.org/10.2475/03.2011.02.

(3)        Julia, M.; Putnis, C. V.; King, H. E.; Renard, F. Coupled Dissolution-Precipitation and Growth Processes on Calcite, Aragonite, and Carrara Marble Exposed to Cadmium-Rich Aqueous Solutions. Chemical Geology 2023, 621, 121364. https://doi.org/10.1016/j.chemgeo.2023.121364.

How to cite: Julia, M., Putnis, C. V., Renard, F., and Plümper, O.: Potential sequestration of toxic elements: the specific example of cadmium and carbonates, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5717, https://doi.org/10.5194/egusphere-egu24-5717, 2024.

The circulation of toxic elements through soils, sediments and aquatic environments remains a significant environmental problem, which implies several, often unrecognized health risks. Dissolution and precipitation reactions that result in formation of sparingly soluble crystalline compounds containing toxic elements e.g. As, appear to be a promising strategy for reducing their chemical mobility and bioavailability (Magalhães, 2002; Wang et. al., 2013).

In this study, the processes occurring at the interface of anglesite and solutions containing AsO₄^3- ions were investigated. The results provide insight into the mechanism of As immobilization on the mineral surface through transformation of labile form into less reactive and more thermodynamically stable phases. Synthetic anglesite powder and fragments of natural anglesite crystals (Tsumeb, Namibia) were reacted for up to 3 months with solutions containing AsO₄^3- (50 mg As/l) in the presence of Cl^- ions at pH range 2 - 8. The experiments were conducted at room temperature (20 ^oC) or in the autoclave (120 ^oC).

Rapid sequestration of As from the solution was associated with precipitation of mimetite Pb₅(AsO₄)₃Cl: over 66% of As was removed within 24 hours. After 7 days of the reaction, the concentration of As decreased from 50 to less than 11 mg As/L. The concentration of Pb²⁺ ranged from 0.6 mg Pb/L at pH 8 to 20 mg Pb/L at pH 2. Both, homogeneous and heterogeneous precipitation of mimetite was observed, and some anglesite crystals were covered by incrustations. At pH=2, mimetite was associated with schultenite PbHAsO₄.

Scanning Electron Microscopy observations of the natural crystals surface, indicated that the reaction of As-containing solutions with anglesite at pH 4-8, involves dissolution of PbSO₄, resulting in formation of etch pits, followed by precipitation of randomly intergrown mimetite as a loosely bound crust. The crust was formed in the vicinity but is separated from anglesite surface. In contrary, at very acidic conditions (pH=2), anglesite is replaced by a secondary phase as a result of a coupled dissolution and precipitation, leading to formation of lead arsenate pseudomorph. Surprisingly, temperature had no significant effect on the reaction, while the pH control is of great importance and determines the mechanism.

References:

Magalhães, M. C. F. (2002). Arsenic. An environmental problem limited by solubility. Pure and Applied Chemistry, 74(10), 1843–1850.

Wang, L., Putnis, C. V., Ruiz-Agudo, E., King, H. E., & Putnis, A. (2013). Coupled dissolution and precipitation at the cerussite-phosphate solution interface: Implications for immobilization of lead in soils. Environmental science & technology, 47(23), 13502-13510.

This research was funded by AGH University of Kraków project No 16.16.140.315.  

How to cite: Stępień, E. and Manecki, M.: Arsenate aqueous solution – anglesite PbSO4 interface: dissolution, precipitation, and arsenic sequestration, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11970, https://doi.org/10.5194/egusphere-egu24-11970, 2024.

Reactive transport of solutes in porous media involving dissolution/precipitation reactions may alter the physical properties of intact rocks, such as porosity and permeability. Such processes are difficult to predict and characterize due to heterogeneity at the pore scale, caused by the intimate coupling of chemical and transport processes. So far, experimental studies have mainly focused either on chemical aspects, using short time batch or microfluidic experiments, or on transport, using a porous medium in classical counter diffusion cells. The former approach offers a large flexibility in selecting and modifying parameters like pH, ionic strength, and element concentrations, and thereby allows the systematic derivation of chemical data such as mineral precipitation and dissolution at ex-situ conditions. The latter approach allows studying changes of (bulk) rock transport properties under realistic (in-situ) conditions over longer times, with limited knowledge of pore scale physical and chemical parameters, which may vary in space and time. Therefore, each of these methods limits a full characterization of the evolution of chemistry and transport properties in a realistic porous medium, particularly near the clogging zones that may form due to precipitation.

In this study, we present a methodology that allows the full chemical and physical characterization of reactive transport processes within a porous medium at the microscopic scale (1-10 um). We present experimental datasets that record, the chemical and physical evolution in 300 mm tapered glass capillaries filled with silica gel, in which precipitation of CaCO₃ polymorphs has been induced via counter diffusion of Na₂CO₃ and CaCl₂ reservoir solutions. Two counter diffusion systems, one containing 1 mM ZnCl₂ in the CaCl₂ titrant, the other without Zn, were prepared and evolved for three years. Optical microscopy and synchrotron-based techniques (micro XRF/XRT/XRD, micro XANES and SAXS) were used to characterize the system at selected times (1, 6, 12, 24, 36 months). In both experiments the precipitation of several calcite and/or vaterite crystal aggregates collectively reduced the gel porosity and formed clogging zones in the central part of the capillary. The data obtained for the Zn-free system show that vaterite crystals formed in the calcium rich part of the capillary (i.e., on the side of the CaCl₂ titrant) remain stable for the entire duration of the experiment without converting to the more stable calcite. In the Zn-doped system, the vaterite crystals remain stable for 1 year and start to dissolve afterwards. The carbonate ions released via vaterite dissolution did not lead to the formation of new CaCO₃ precipitates in the Ca-rich part of the capillary. Instead, a zinc hydroxy carbonate phase is formed.

Through this study, we show for the first time the evolution of a clogging zone in a porous medium at the microscopic scale over an extended period and under truly undisturbed in-situ conditions. Our results show that small concentrations of Zn²⁺ ions in the pore water have a strong effect on the precipitation/dissolution kinetics of CaCO₃ polymorphs, as well as on their crystal morphology.

How to cite: Rajyaguru, A., Curti, E., Ferreira Sanchez, D., Appel, C., and Grolimund, D.: Zinc(II) ions under diffusive regime controls the stability of vaterite: Derivation of pore scale chemical dynamics near CaCO3 clogging interface via coupled micro-XRF/XRD/XANES and SAXS imaging., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17340, https://doi.org/10.5194/egusphere-egu24-17340, 2024.

Water, a principal component of Earth’s fluids, interacts with rocks in ways that profoundly influence lithospheric phenomena. These interactions are fundamental to both geochemical and geodynamic processes, extending their impact to areas of societal importance such as subsurface energy extraction and storage, and the formation of vital metal deposits for green energy technology. Additionally, the interplay between fluids and rocks significantly affects the Earth's carbon cycle, influencing atmospheric CO₂ levels, climate, and overall planetary habitability. The traditional perspective suggests that fluids move through the lithosphere without being affected by the unique properties that emerge when matter is confined to the nanoscale. Contradicting this view, our research reveals that rocks involved in a variety of lithospheric processes consistently show nanoporosity, primarily with pores smaller than 100 nanometers. Within these small spaces, water behaves differently than it does in larger environments. Through molecular dynamics simulations, we have quantified water's relative permittivity—a critical factor in its geochemical behavior— when confined in natural nanopores. Our findings demonstrate that, under a wide range of lithospheric conditions, from ambient to extreme temperatures up to 700 °C and pressures up to 5 GPa, water's permittivity within these nanopores significantly deviates from that in its bulk state. Our thermodynamic equilibrium models indicate that this difference markedly reduces mineral solubility and alters ion speciation in natural settings. These pore-size-dependent properties may exert a primary influence on rock reactivity and the evolution of aqueous geochemistry during fluid-rock interactions.

How to cite: Plümper, O., Chogani, A., King, H. E., and Tutolo, B.: Nanopore Influence on the Geochemical Properties of Water in Earth's Lithosphere, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12279, https://doi.org/10.5194/egusphere-egu24-12279, 2024.

The interpretation of mineral equilibria and reactions in rocks assumes uniform hydrostatic pressure across all phases. However, such condition is typically not satisfied in geological systems that are composed of multiple phases that are in contact with one another.  Stress gradients and non-hydrostatic stresses are to be expected in rocks in the lithosphere, even in the presence of fluids. This complexity challenges the reliability of existing hydrostatic thermodynamic models, and, currently, there is still no accepted theory for evaluating the thermodynamic effect of non-hydrostatic stress on reactions [e.g. 1, 2, 3, 4].

Molecular dynamics (MD) allows us to investigate first-order phase transitions in solid-liquid systems under stress, without the a-priori assumption of a specific thermodynamic potential [5]. With MD simulations the energy of the system, the pressure of the fluid, the stress of the solid, as well as the overall melting and crystallization process can be monitored until the stressed system attains the equilibrium conditions. Our findings indicate that under differential stress, the mean stress of the solid deviates progressively from the pressure of the fluid with which it is in equilibrium. At low differential stresses, deviations from the reference hydrostatic equilibrium are small, allowing accurate phase equilibria predictions by considering the fluid pressure as a proxy for equilibration pressure, as suggested by previous experimental investigations [6].

In the presence of substantial non-hydrostatic stresses, equilibrium between solid and fluid occurs at a fluid pressure significantly higher than hydrostatic thermodynamics predicts. However, the stressed system becomes unstable and a rim of a quasi-hydrostatically stressed solid eventually crystallizes around the initial highly stressed solid core. During the crystallization, the total stress balance is preserved until the newly formed solid-solid-fluid system reaches again a stable equilibrium. At the final equilibrium conditions only the low-stressed solid is exposed to the fluid, bringing back the equilibrium fluid pressure close to the value expected for the equilibrium at homogeneous hydrostatic conditions. Therefore, our results suggest that models describing equilibria and reactions in minerals and rocks under stress can assume that phase equilibria are accurately predicted by using the fluid pressure as a proxy of the equilibration pressure.

References

[1] Frolov, T., & Mishin, Y. 2010: Effect of nonhydrostatic stresses on solid-fluid equilibrium. I. Bulk thermodynamics. Physical Review B - Condensed Matter and Materials Physics, 82(17), 1–14.

[2] Hobbs, B. E., & Ord, A. 2015: Dramatic effects of stress on metamorphic reactions. Geology, 43(11), e372.

[3] Tajčmanová, L., Vrijmoed, J., & Moulas, E. 2015: Grain-scale pressure variations in metamorphic rocks: Implications for the interpretation of petrographic observations. Lithos, 216–217, 338–351.

[4] Wheeler, J. 2014: Dramatic effects of stress on metamorphic reactions. Geology, 42(8), 647–650.

[5] Mazzucchelli M., Moulas E., Kaus, B., Speck T. submitted: Fluid-mineral equilibrium under stress: insight from molecular dynamics. American Journal of Science.

[6] Llana-Fúnez, S., Wheeler, J., & Faulkner, D. R. (2012). Metamorphic reaction rate controlled by fluid pressure not confining pressure: Implications of dehydration experiments with gypsum. Contributions to Mineralogy and Petrology, 164(1), 69–79.

How to cite: Mazzucchelli, M. L., Moulas, E., Schmalholz, S. M., Kaus, B., and Speck, T.: Precipitation in fluid-mineral systems under differential stress investigated through molecular dynamics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13089, https://doi.org/10.5194/egusphere-egu24-13089, 2024.