Displays

Breccia structure is a ubiquitous feature that is characterized by angular fragment in a matrix composed of smaller grain size, often associated to brittle tectonics or to specific sedimentary environment such as karst collapse. Many different studies across the world describe breccia related to dolomite geobodies, themselves associated to ore deposits occurring during major extensional events (Hungary, Spain, France, Canada, Poland, Canada). The mineralogical textures of these structures, i.e. angular fragments of dark dolomicrite bound by elongated blocky, white, dolomite crystals in veins, are interpreted as univocal markers of fluid overpressure and hydrofracturing, hydrothermal dolomite breccia (HDB) being a precious tool to help to reconstruct pressure history.

This contribution presents a case study that challenge this hydrofracturing origin of HDB, questioning the role of fluid-mediated replacement in the observed crystallographic textures. The Mano Formation located in the Mail Arrouy, an anticline related to Mesozoic hyper-extension of the crust located in the Chaînons Béarnais (Northwest Pyrenees, France), presents classical HDB, i.e. characterised by black dolomite fragment surrounded by a white dolomite-matrix supposedly related to hydrofracturing. Yet, in some places, it is possible to observe this angular black fragment in contact with a brown dolomite matrix. As attested by the presence of dolomitized fossils, the brown dolomite and black fragments constitute an initial sedimentary breccia structure, that is described regionally.

Textural and chemical analyses of the HDB and of the initial sedimentary breccia have been carried out by scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and electron probe microanalyzer (EPMA) across different dolomite-dolomite interfaces. Quantitative data has been obtained by image processing, showing that oxide particles that are randomly distributed in the brown matrix appears pushed at the tips of the white crystals of dolomites, suggesting a cleaning process during growth. Also, the initial breccia comprises small-size around 1830 µm² (surface area) clasts that are absent from the HDB. Moreover, the contact between black, white and brown dolomite show a roughness similar to what is observed in fluid-mediated dissolution/replacement processes. Finally, EBSD results show that white dolomite crystal grew under local stress generated by a competition between grain growth, typical of slow, fluid-limited, grain growth.

This array of results leads us to propose that the HDB results from texturally controlled, fluid initiated hydrothermal recrystallization of initial sedimentary dolomicrite. This model is further tested by 2D numerical simulations of phase separation process using the modelling environment “ELLE” that reproduce the patterns observed in natural samples. Hence, a critical reappraisal of the origin and process behind HDB must be conducted, as we show that, in the case of the Mano Formation in the Mail Arrouy, no fluid overpressure were required to create HDB.

How to cite: Centrella, S., Beaudoin, N., Motte, G., Hoareau, G., Koehn, D., and Callot, J.-P.: Hydrothermal dolomite breccia: when pre-existing rock heterogeneities control fluid-mediated replacement patterns and mimic tectonic features., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18931, https://doi.org/10.5194/egusphere-egu2020-18931, 2020.

Subsurface activities, such as energy production or geo-storage, affect the natural equilibrium of the reservoir and surrounding geological system. Fluid production from porous sandstones, for example, is often associated with reservoir compaction and induced seismicity, such as seen in the Groningen Gas Field. Production-induced stress changes lead to compaction by elastic and inelastic mechanisms. Partitioning between elastic and inelastic processes control the energy budget available for driving seismogenic events. To predict the amount of inelastic strain, it is key to identify the microscopic mechanisms controlling it. One of the current hypotheses is that micro-strains are accommodated by localized compaction of inter-granular clay films. In contrast to sandstones, claystones offer potential both as source rocks for shale gas and containment for the storage of radioactive waste and CO₂. It is known that fluid flow in intact and fractured claystones is slow due to pore throats below 10 nm. However, it is unclear whether fractured shales contain a hierarchy of multi-scale highways and byways for fluid transport that is either poorly connected or more easily cross-linked and stable under in-situ conditions. Depending on how fractures change due to in-situ conditions, the shales may have a high potential as barriers in geo-storage systems, or they are of interest in relation to energy production.

This leads to two widely different research questions:

• How do sandstones compact due to changing stress conditions?
• How do fractures influence fluid flow in shales under in-situ stress conditions?

Despite the distance between these research questions, both can be addressed using in-situ imaging. We have developed a compaction cell and a fluid flow cell to perform experiments at the D50/NeXT beamline of the Institut Laue-Langevin in Grenoble, France. Here, combined X-ray and neutron imaging is possible.

With the compaction cell, sandstone samples from the Groningen gas field were uniaxially compacted to axial stresses of 45 MPa. At different intervals, 3D neutron and X-ray computed tomography scans were taken. As such, 4D representations (3D volumetric + time) of the in-situ changes were obtained using both neutron and X-ray tomography. The X-ray imaging allows a thorough inspection of the grain-scale deformation of the sample, while the neutron imaging highlights the changes in porosity and gives an indication of the role of clay films.

With the fluid flow cell, fractured samples of the Whitby mudstone were subjected to fluid flow under different hydrostatic pressures. The flow path evolution within the sample was visualized using neutron radiography, giving an indication of the differences between fracture and matrix permeability.

In this contribution, we will show preliminary results of four experiments performed at the D50/NeXT beamline in October 2019. We will discuss the applicability of using neutron imaging to study grain-scale processes occurring in compacting sandstone, as well as for understanding the fluid pathways in clay-rich shales, with direct implications for energy production and geo-storage.

How to cite: Van Stappen, J. F., Houben, M. E., Wolterbeek, T. K. T., Tengattini, A., Shinohara, T., Teuling, F. S. R., Zhang, M., Spiers, C. J., and Hangx, S. J. T.: Visualization of localized deformation and fluid flow in sedimentary rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5293, https://doi.org/10.5194/egusphere-egu2020-5293, 2020.

Long-distance transport along weak basal detachments in thin-skinned tectonics is often accomplished by rheologically weak evaporites. This weakness can be attributed to  the behavior of gypsum and/or halite. While the former dehydrates and the released fluid reduces the effective stress in the system, the latter is known to be extremely weak at the corresponding conditions. Separately, both minerals and their behavior under tectonic loading have been studied in great detail. However, these studies on single minerals are limited in that natural detachments are often not monomineralic and are clearly affected by interdependencies between different mineral species. In evaporitic sequences, two key couplings that can be expected are: 1) the sensitivity of the dehydration reaction to the pore fluid pressure versus the notoriously low permeability of rock salt (a potentially negative feedback), and 2) the exposure of halite to undersaturated water released from the gypsum dehydration reaction, versus the response of the dehydration reaction to lower water activity due to dissolved salt species (a potentially positive feedback). 

Here we present insights from experiments that used time-resolved (4D) synchrotron tomographic microscopy and our x-ray transparent triaxial deformation rig Mjølnir to document the evolution of layered gypsum-halite samples that were simultaneously deformed and dehydrated. Our data, which were acquired at the TOMCAT beamline at the Swiss Light Source, allow us to visualise chemical-hydraulic-mechanical feedbacks on the grain scale, and quantify the microscale evolution of transport properties. In this contribution, we show that gypsum dehydration affects the capacity of the halite layers to retain the liberated fluids. The reaction itself generates the pore fluid pressure to create permeability in the salt layers through hydraulic fracturing. Dissolved salt significantly accelerates the reaction, and the evolving interconnected porosity facilitates the transport and precipitation of solutes, which contributes to the rheological complexity. These insights have, potentially significant, repercussions on the long-standing assumption about the significance of the gypsum dehydration on thrust fault formation within evaporitic sequences.

How to cite: Marti, S., Fusseis, F., Butler, I. B., Schlepütz, C., Marone Welford, F., Gilgannon, J., Kilian, R., and Yang, Y.: Chemical-mechanical-hydraulic coupling in deforming, dehydrating halite-gypsum rocks - implications for basal detachments in thin-skinned tectonics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9560, https://doi.org/10.5194/egusphere-egu2020-9560, 2020.

Strain localisation in the upper crust is strongly influenced by the presence of phyllosilicates (e.g. white mica, biotite, chlorite), systematically observed in shear zones in granites. Identifying reactions involving phyllosilicates at low-grade metamorphic conditions is crucial to understand crust mechanics and fluid-granite interactions during deformation. In the 305 Ma old basement of the Bielsa massif (Axial Zone, Pyrenees), extensive pre-orogenic (i.e. pre-Alpine) alteration related to feldspar sericitization and chloritization of biotite and amphibole occurred at temperatures of 270–350°C at 230–300 Ma. This event was followed by mylonitization and fracturing at 40–70 Ma, and fluid–rock interaction at 200–280°C marked by replacement and new crystallization of chlorite and white mica. In undeformed parts of the granite, compositional maps reveal in situ reaction, high local heterogeneities and low element mobility (migration over few µm) for most elements. Transmission electron microscopy (TEM) shows disconnected reaction-induced nanoporosity in chloritized amphiboles and ripplocations in chloritized biotite. Chloritization reaction varies over tens of nanometres, indicating high variability of element availability. Equilibrium is reached locally due to isolation of fluid in pockets. In samples with fractures, both elemental maps and TEM images show two chlorite groups: alpine chlorites in fractures have homogeneous composition while pre-alpine chlorites in the matrix show patchy compositions. Channelization of fluids in fractures and sealing by chlorite prevented replacement of the matrix chlorite. High element mobility was therefore limited to fractures. In mylonites, compositional maps show secondary chlorites up to 1 mm around cracks and only partial replacement of chlorite within the matrix. This suggests fluids could percolate from cracks to the matrix along chlorite grain boundaries. TEM images show nanocracks at the boundary of chlorite crystallites where replacement is localised. Crystallites were individually replaced by dissolution-reprecipitation reactions and not by intra-crystallite mineral replacement, explaining the patchy compositional variations. While fracturing did not allow chlorite sheets to be progressively re-oriented, a continuous, brittle-ductile deformation in mylonites did, making preferential fluid pathways progressively change.  Despite high strain, chlorite replacement was not complete even in mylonites. Replacement appears to be controlled by matrix-fracture porosity contrasts and the location and connection of nanoporosity between crystallites, criteria that may be only transiently met in space during deformation. These mechanisms need to be taken into account when attempting to reconstruct the metamorphic history of shear zones as well as the evolution of their mechanical behaviour since they affect the scale of the thermodynamic equilibrium and the preservation of hydrothermal metamorphism in granites.

How to cite: Airaghi, L., Dubacq, B., Alexandre, G., Anne, V., and Nicolas, B.: Chloritization of granites in shear zones: an open window on fluid pathways, equilibrium length-scales and porosity formation down to nanoscale, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5519, https://doi.org/10.5194/egusphere-egu2020-5519, 2020.

Fluid–rock interactions link mass and energy transfer with large-scale tectonic deformation, drive the formation of mineral deposits, carbon sequestration, and rheological changes of the lithosphere. While spatial evidence indicates that fluid–rock interactions operate on length scales ranging from the grain boundary to tectonic plates, the timescales of regional fluid–rock interactions remain essentially unconstrained, despite being critically important for quantifying the duration of fundamental geodynamic processes. Here we show that reaction-induced transiently high permeability significantly facilitates fast fluid flow through low-permeability rock of the mid-crust. Using observations from an exceptionally well-exposed fossil hydrothermal system to inform a multi-element advective–diffusive–reactive transport model, we show that fluid-driven reaction fronts propagate with ~10 cm year^-1_, equivalent to the fastest tectonic plate motion and mid-ocean ridge spreading rates. Consequently, in the presence of reactive fluids, large-scale fluid-mediated rock transformations in continental collision and subduction zones occur on timescales of tens of years, implying that natural carbon sequestration, ore deposit formation, and transient and long-term petrophysical changes of the crust proceed, from a geological perspective, instantaneously.

How to cite: Beinlich, A., John, T., Vrijmoed, J. C., Tominaga, M., Magna, T., and Podladchikov, Y. Y.: Instantaneous rock transformations in the deep crust driven by reactive fluid flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1266, https://doi.org/10.5194/egusphere-egu2020-1266, 2020.

High-grade anorthositic granulites from Holsnøy in the Bergen Arcs (western Norway) were subducted and underwent high pressure (HP) eclogite-facies metamorphism during Caledonian orogeny. They indicate that local eclogitization is linked to an interplay between deformation, fluid infiltration and subsequent fluid-rock interaction. The final result is an interconnected network of hydrous eclogite-facies shear zones surrounded by pristine unreacted granulites. This local transient eclogitization process temporarily weakens the subducting plate and therefore, might have had a strong impact on its deformation.  

In a first quantitative study we combined detailed field-mapping with numerical modelling to investigate the evolution of hydrous eclogite-facies shear zones with respect to the regional far-field stress and we discuss the strain partitioning. Although it is supposed that strain localises within the shear zones, we were able to show that widening overcomes the effect of stretching because of the fluid-rock interaction during deformation. The availability of a free fluid phase, which is continuously infiltrating the system, has a strong effect on shear zone widening. The most appropriate effective diffusion coefficient to emulate nature-like structures and hydration front widths by simple, hydro-mechanical numerical models was 10^-12 m².s^-1. Our first conclusions suggest that a continuous fluid infiltration seems to be required to reproduce the observed structures. However, a complex model is necessary to understand how the fluid infiltrates and consequently, transforms the granulite adjacent to the shear zone widening.

Mass balance considerations reveal that the eclogitization of the granulite did not result in significant compositional changes, hence the fluid composition was quickly rock buffered. In order to better understand the link between enhancing deformation and fluid-infiltration fronts, we aim to determine the H₂O content stored in minerals (including nominally anhydrous minerals, NAMs) perpendicular to the deformation structure from the core of the eclogite-facies shear zone to the macroscopically unaffected granulite. Hydrogen in garnet, pyroxene, plagioclase can significantly weaken the mineral structure, especially when substituting for silica. Additionally, it is crucial to constrain the amount of H₂O needed for the transition from nominally anhydrous to hydrous assemblages. The H₂O content was measured using transmission Fourier transform infrared spectroscopy using single points and maps to investigate potential zoning. An entire 20 cm wide transect was investigated, between unaltered granulite and the core of the eclogite-facies shear zone. This study will provide new constraints on the dynamic weakening processes affecting metastable dry and rigid crustal rocks.

How to cite: Kaatz, L., Reynes, J., John, T., Schmalholz, S., Hermann, J., and Moulas, E.: The distribution of the H2O content in nominally anhydrous minerals and its effect on shear zone widening (Holsnøy, West-Norway), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8198, https://doi.org/10.5194/egusphere-egu2020-8198, 2020.

Molecular dynamics simulations of diffusive properties of stressed water films in quartz and clay grain contacts

Floris S.R. Teuling¹, Marthe G. Guren², François Renard², Martyn R. Drury¹, Suzanne J.T. Hangx¹, Helen E. King¹, Oliver Plümper¹, Henrik A. Sveinsson²

Hydrocarbon extraction can increase effective normal stresses in geological reservoirs, potentially inducing deformation and seismicity¹. The kinetics of time-dependent creep processes that could persist long after production has ended, such as pressure solution and stress corrosion, are poorly quantified. These processes can be limited by diffusion efficiency at stressed grain contacts, which depends strongly on fluid film thickness as well as interfacial and surface energies. The diffusive properties of stressed fluid films between various crystallographic surfaces of the rock forming minerals clay and quartz are critical to predict long term deformation of reservoir. Due to the small length scales of grain contacts, experimental data on these quantities are difficult to acquire. Therefore, we use molecular dynamic simulations to elucidate the physico-chemical behaviour of fluid films at different mineral interfaces.

We apply large-scale classical molecular dynamics in LAMMPS to numerically resolve fluid film behaviour in grain contacts. The silicate-water system is modelled using a modified ClayFF force field². A β-quartz block was placed within a water-filled nanopore with either hydroxylated  β-quartz or basal illite clay surfaces as walls.  This geometry was built using the software packages Atomic Simulation Environment, Ovito and Packmol. The system was first equilibrated using an NVT thermostat and an NPT barostat for tens of picoseconds under conditions of 8 MPa fluid pressure and a temperature of 100°C. Then, a force was applied on the quartz block, corresponding to 10-200 MPa normal contact stress, such that a thin water film is squeezed at the interface between two grains. Self-diffusion constants were calculated by mean square displacements and velocity autocorrelation in films at steady state thicknesses.

Simulations reach a steady state after several nanoseconds run time. Under reservoir conditions, fluid film thicknesses are reduced to less than one nanometre. Two to three layers of adsorbed water remain in the grain contact, a result consistent with reported fluid film properties for grain contacts in upper crustal systems. Our results quantify how various juxtaposed quartz surfaces and quartz-clay interfaces influence fluid film thickness, self-diffusion and the dynamics of the water layer, which allows for constraining the kinetics of pressure solution creep in sandstone reservoirs.

Acknowledgements

This project received funding from the DeepNL programme

How to cite: Teuling, F., Guren, M. G., Renard, F., Drury, M. R., Hangx, S. J. T., King, H. E., Plümper, O., and Sveinsson, H. A.: Molecular dynamics simulations of diffusive properties of stressed water films in quartz and clay grain contacts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4250, https://doi.org/10.5194/egusphere-egu2020-4250, 2020.

A paleohydrothermal giant quartz reef (at least 75 m wide, 40 km long) and abundant hot springs at the Heyuan fault, South China, provide an excellent opportunity to investigate hydrothermal flows from the Mesozoic through to present-day.

The giant quartz reef has formed in the extensional regime initiated in the Mesozoic, while a change to  compressional stress on the Heyuan in the Cenozoic led to the development of cross-cutting strike-slip faults and associated vertical fracture network. Here, we present multiscale observations and analyses from the earlier long-term extensional phase.

Detailed microstructural analyses identified a 'quartz-reef window' of formation occurring between ~200-350˚C, linking in both quasi-static criteria (accommodation space; massive fluid sources; and a cap rock/seal)  and dynamic mechanisms (episodic-dynamic permeability; brittle-ductile cycles; and fluid injection though brittle-ductile equivalent of Sibson's 'fault-valve' behaviour.

This oscillatory brittle-ductile fault-valve is recorded in the field through its apparent contradiction between idiomorphic 5 cm long quartz crystal growth in mode-I fractures, embedded at large-scale inside far from equilibrium fault zones with mylonitic and cataclastic microstructures. Another characteristic feature is the increasing quartz vein frequency towards the core shown by enrichment of SiO₂, with depletion of K₂O and  Na₂O in tectonites during alteration from the host granite; a reaction partly sourcing the SiO₂ for the quartz reef.

We present a first theoretical model compatible with the observation of oscillatory macroscale far from equilibrium conditions, followed by long periods of micro-scale local equilibrium. The model can in particular describe mechanisms of abundant SiO₂ dominated fluid release reaching episodically above hydrostatic pressures followed by long periods of SiO₂ precipitation, allowing growth of up to 5 cm long idiomorphic quartz  crystals in subparallel open channels, which presumably were held open by high fluid pressures. In this interpretation, the observations instabilities are seen to stem from the multiscale and multiphysics of the mineral reactions at the brittle-ductile transition, promoted by a slow extensional geodynamic driver at the Heyuan fault.

The new approach allows interpretation of rock physics properties in terms of recently discovered Thermo-Hydro-Mechanical-Chemical (THMC) multiscale wave-like instabilities. In the model short wavelength chemical dissolution-precipitation reaction waves are bouncing between the phyllonitic cap rock and the mylonitic shear zone below. A resonance phenomenon of constructive interference in a finite width around the future quartz-reef triggers the long-time scale steady-state attractor allowing quartz reef growth over geodynamic time scales. We show that this solitary wave limit forms a standing wave matching the characteristic periodic pattern of mode-I quartz veining around the reef and also explaining the fluid overpressures leading to local hydro-fracturing.

How to cite: Tannock, L., Herwegh, M., Berger, A., and Regenauer-Lieb, K.: Building a giant quartz reef at the Heyuan fault, South China: observations and multiphysics model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17237, https://doi.org/10.5194/egusphere-egu2020-17237, 2020.

Hangjinqi area is located in the north of Ordos basin, China. The capping strata are mainly comprised of argillaceous rocks of the upper Shihezi and Shiqifeng formation. In this paper, the thickness and displacement pressure of the cap layers are analyzed by logging data, the sealing ability of the cap layers is comprehensively estimated based on the geochemical characteristics of the formation water.

 Based on logging data and well logging interpretation, The cumulative thickness of mudstone cover in the upper Shihezi formation is 32~112m, the highest thickness is 112m in Jin51, and 108m in Jin29. The thickness has the characteristics of being thicker in the east, which is bounded by the junction of Boerjianghaizi and Wulanjilinmiao fault. According to the statistical results of maximum thickness of monolayer, in upper Shihezi formation is generally 4~40m, the maximum in Jin91 is 40m. The spatial distribution shows that the maximum thickness is r in the central Jin72-Jin71-Jin 99 and in the western Jin29. The cumulative thickness of Shiqianfeng formation caprock is relatively thick, 80~201.5m. The maximum thickness of monolayer generally is 2~44m, the maximum is 44m in Jin83.

Based on the logging data of 69 Wells, the displacement pressure data of the upper Shihezi and the Shiqianfeng formation were calculated according to the calculation formula between the measured displacement pressure and the acoustic time difference, the statistics of maximum displacement pressures were conducted. The maximum displacement pressure of the upper Shihezi and Shiqianfeng formation changes from 23MPa in the west to 15MPa in the east, which shows the characteristics of higher displacement pressure in the middle west and lower in the east and northeast.

The results of the comparative analysis of the plane distribution characteristics of the geochemical parameters of formation water and the gas production of natural gas show that, in Jin30 and Jin63 near the Wulanjilinmiao and the Sanyanjing fault, the salinity and metamorphic coefficients of the formation water are relatively high, while the sodium-chloride coefficient, desulfurization coefficient and carbonate rock equilibrium coefficient are relatively low, which indicate that the stratigraphic sealing conditions are good and beneficial to the preservation of oil and gas reservoirs. To the east, the sodium chloride coefficient of the formation water increased obviously, while the salinity and metamorphism coefficient decreased, which indicated that the formation sealing condition became worse. Further to the east, due to the influence of the Boerjianghaizi fault, the fault zone is characterized by low values of salinity and metamorphic coefficient, high values of sodium-chlorine coefficient, desulfurization coefficient and carbonate rock balance coefficient, indicating that the formation has poor sealing conditions, which is not conducive to the preservation of oil and gas reservoirs. But in Jin53-JIn72 and Yishen1-Jinping1-Jin33 located in the south and north sides of fault zone, the salinity and metamorphic coefficient is relatively high, and sodium chloride, desulfurization and carbonate balance coefficient are relatively low, indicating stratigraphic sealing and preservation condition in the south and north areas near the fault zone are in favor of the preservation of the gas reservoirs.

How to cite: zhao, G.: Evaluation on sealing ability of caprocks for gas reservoirs in Hangjinqi area, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20569, https://doi.org/10.5194/egusphere-egu2020-20569, 2020.

Revealing the alteration mechanism of reservoir-cap rock system during CO₂-rich fluid charging is meaningful to the study of CO₂ geological storage, as well as when CO₂ enhance oil recovery. The study is taking the Permian Longtan reservoir formation and Dalong cap layer of Huangqiao and Jurong region in Lower Yangtze area in China as comparative study objects, in order to understand the differences between presence and absence of CO₂ in the similar geological background. The samples of reservoirs and cap rock in both regions are analysized through petrological and geochemistry method. The authigenic minerals in the reservoirs of Huangqiao region are mainly overgrowth quartz and kaolinite. A small amount of dawsonite is developed in Huangqiao, while undeveloped in Jurong region due to the absent of CO₂. The content of secondary quartz is lower in Jurong than in Huangqiao. The reservoir’s average porosity in Huangqiao is obviously higher than in Jurong, because of the feldspar’s dissolution during CO₂ charging. The cap rocks in the two areas are both black block mudstones. There were micro-cracks developed in the cap rocks of Huangqiao region, in which have been refilled with calcite veins. Carbon isotope data shows that calcite was formed from CO₂-water-rock interaction. The result indicates that CO₂ charging could cause a major dissolution of feldspar in reservoir, and precipitate a typical authigenic mineral assemblage of dawsonite, secondary quartz and kaolinite. The continuous activity of the CO₂-rich fluid leads to re-precipitation of carbonate minerals in cap rock, which is improving its sealing ability.

How to cite: Zhou, B. and Lun, Z.: The Alteration of Reservoir-Cap’s System During CO2 Charging in Huangqiao Region, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2314, https://doi.org/10.5194/egusphere-egu2020-2314, 2020.

The outcrop areas of metamorphic rocks in continental setting cover significant regions and their extension zones are even more significant if we also consider the shallow and medium subsurface areas under the sedimentary cover. However, the metamorphic rocks are usually disregarded as potential geothermal reservoirs since they are considered as tight rocks with no or very limited porosity and permeability. Even if this statement is correct, the metamorphic units are frequently associated with a long and complex tectonic evolution and in particular with pre-, syn- and post-metamorphism fractures, which represent potential target zones for the development of geothermal reservoirs. Another limitation to assess the geothermal potentiality of the metamorphic units is the very limited number of deep exploration wells, especially in comparison to other well-investigated reservoirs, such as those located in sedimentary formations.

The anchizone and epizone metasedimentary rocks in Southern Belgium (Wallonia) cover more than 30% of the territory and probably more than 40% if we also consider the metamorphic rocks under the non-metamorphic formations. These statistics are based on the depth interval between 0 km (outcrop) and 6 km, which are reasonable depths for the development of geothermal projects. The encountered lithologies consist of a few km-thick quartzite and slate formations of Lower Palaeozoic and Lower Devonian ages. These formations are associated with different events with the most significant ones regarding this study being the fracturing events related to the formation of the Rhenohercynian basin during the Devonian and the Dinantian times followed  by the Variscan orogeny during Upper Carboniferous.

The Havelange borehole was drilled by the Geological Survey of Belgium between 1980-1985 as a gas exploration reaching a maximum depth of 5648 m (MD). The aim of this borehole was to investigate the presence/absence of a Carboniferous gas reservoir located under the main décollement level of the Ardenne Allochthon. Even if the borehole never reached any Carboniferous rocks, it allowed a better characterization of the transition between shallow Lower Famennian shale units and Lower Devonian meta-sedimentary formations, along with Middle Devonian rocks at intermediate depth. The study conducted in the framework of the H2020 MEET project (www.meet-h2020.com) for the Havelange study-site includes the re-investigation of cores, cuttings and log data acquired during the drilling. The laboratory study entails the mineralogical characterisation of the host rocks and fracture zones as well as the petrophysical and rock mechanical characterisation and this borehole material is completed with outcrop analyses and comparable measurements in analogue zones. The lab and field results are cross-matched with the drilling archives and in particular the drilling report indicating the depths of mud losses, representing intervals of great interest for the potential reconversion of this borehole into a geothermal well.

How to cite: Vanbrabant, Y., Stenmans, V., Burlet, C., Petitclerc, E., Meyvis, B., Stasi, G., Arbarim, R., Bär, K., and Goovaerts, T.: Havelange deep borehole (Belgium): a study case for the evaluation of metasedimentary formations as potential geothermal reservoir – H2020 MEET project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10943, https://doi.org/10.5194/egusphere-egu2020-10943, 2020.

The widespread occurrence of the quartz–carbonate alteration assemblage (listvenite) in ophiolites indicates that ultramafic rock represents an effective sink for dissolved CO₂. However, the understanding of the carbonation mechanisms is almost exclusively based on surface samples, which adds significant uncertainty to the interpretation of fossil hydrothermal systems. Here we present novel insight into the reaction textures and mechanisms of ultramafic rock carbonation obtained from the 300 m deep BT1B drill hole, ICDP Oman Drilling Project. Hole BT1B recovered continuous drill core intersecting surface alluvium, 200 meters of altered ultramafic rock comprising serpentinite and listvenite, and 100meters of the underlying metamorphic sole. The ultramafic part of BT1B is dominated by listvenite with only two thin intercalated serpentinite bands at 90 m and 180 m depth. Microstructural analyses indicate an evolution beginning with non-equilibrium growth of spheroidal carbonate composed of interlayered magnesite and dolomite in the completely serpentinized harzburgite, and magnesite and Ca-magnesite in the listvenite. Carbonate spheroids are characterized by sectorial zoning resulting from radially oriented low-angle boundaries. In the listvenite spheroidal carbonate is overgrown by euhedral magnesite indicative of near-equilibrium growth. Carbonate clumped isotope thermometry indicates carbonate crystallization predominantly between 100°C and 200°C. The strong macroscopic brecciation and veining of listvenite indicate that carbonation was facilitated by significant tectonic deformation allowing for infiltration of reactive fluids over an extended duration.

How to cite: Plümper, O., Beinlich, A., Boter, E., Müller, I. A., Kourim, F., Ziegler, M., Harigane, Y., Lafay, R., Kelemen, P. B., and Project Science Team, T. O. D.: Ophiolite carbonation: Constraints from listvenite core BT1B, Oman Drilling Project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1287, https://doi.org/10.5194/egusphere-egu2020-1287, 2020.

Long-term oscillations of the Earth’s atmospheric carbon dioxide concentration and climate are intrinsically linked to tectonic plate motion controlling CO₂ uptake in rocks, their transport into the Earth’s mantle and recycling back into the atmosphere by volcanic activity. In this long-term deep carbon cycle, the efficiency of mantle ingassing is controlled by the stability of carbon carrier phases at subduction zone pressure-temperature conditions, during deformation and their interaction with subduction zone dehydration fluids. However, the current understanding of carbonate stability under these conditions is controversial. This is reflected by studies predicting carbonate transport deep into the asthenospheric mantle [1, 2] in contrast to more recently postulated shallow-depth carbon release from subducting slabs [e.g. 3]. Some of this controversy is related to the lack of available field sites that allow for the quantification of subduction-related decarbonation and its driving force. Here we present novel observations on the release of carbon during subduction of previously carbonated, ultramafic, oceanic lithosphere. Our observations are based on a recently discovered, exceptionally well-exposed, outcrop in northern Norway [4] containing frozen-in decarbonation reaction textures at the km scale. Our observations and textural analyses indicate breakdown of magnesium carbonate and serpentine to secondary olivine at depths shallower than 20 km. Secondary olivine is present as up to fist-sized nodules pseudomorphically replacing magnesite and as veins delineating escape pathways for the carbon-bearing aqueous fluid. We present first field observations and reaction textures and will discuss implications for the efficiency of carbon transport into the Earth’s mantle by subduction of carbonate-bearing oceanic lithosphere.

[1] Kerrick, D.M. & Connolly, J.A.D. (1998). Geology 26, 375-378.

[2] Dasgupta, R. & Hirschmann, M.M. (2010). EPSL 298, 1-13.

[3] Kelemen, P.B. & Manning, C.E. (2015). PNAS 112, E3997-E4006.

[4] Beinlich, A., Plümper, O., Hövelmann, J., Austrheim, H. & Jamtveit, B. (2012). Terra Nova 24, 446-455.

How to cite: Strobl, L., Beinlich, A., Ohl, M., and Plümper, O.: Shallow-depth slab decarbonation prevents recharge of the deep carbon cycle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1374, https://doi.org/10.5194/egusphere-egu2020-1374, 2020.

The apparently low abundance of pseudotachylytes in field outcrops of fault zones may be due to their alteration and hence destruction of characteristic microstructures. The potential for alteration of rocks is largely controlled by the availability of water that in turn depends on the rocks’ permeability. The permeability of pseudotachylytes, which generally exhibit a fine-grained matrix, is expected to be low relative to their host rock, such that infiltration by fluids should be minimal. We are therefore conducting flow-through experiments at elevated temperatures on pseudotachylyte samples from the Gole Larghe fault zone, Italian Southern Alps. We are monitoring the permeability and its evolution with time due to hydrothermal alteration processes using the pore-pressure oscillation technique. Microstructural analyses of naturally and experimentally altered pseudotachylytes will help to constrain the alteration processes and associated kinetics. Our results contribute to answer the question how pseudotachylytes are lost from the rock record.

How to cite: Rempe, M. and Renner, J.: Permeability evolution of pseudotachylytes during hydrothermal alteration experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8269, https://doi.org/10.5194/egusphere-egu2020-8269, 2020.

Fluids play a key role in weakening rocks, controlling crustal deformation from early fracture development to mature strain localization, fault nucleation and propagation through cumulative slip. In particular, at the brittle-ductile transition zone crustal deformation and fluid flow are mutually interconnected by repeating cycles of transient frictional and viscous deformation. Uncertainties remain, however, on the details of the micromechanical and chemical influence of fluids in facilitating strain localization processes.

N-S to NW-SE sub-vertical brittle-ductile faults cut across the Paleoproterozoic migmatitic basement of southwestern Finland on the island of Olkiluoto, where the Finnish authorities plan the construction of a deep repository for high-grade nuclear waste. The faults are characterized by a brittle–ductile to fully brittle deformation style resulting from transient fluid pressurization. We investigated a representative fault by combining field and microstructural observations with fluid inclusion and mineral chemistry analysis on synkinematic and authigenic minerals in order to reconstruct the temporal variations of pressure, temperature, composition and salinity of the synkinematic fluids that controlled strain localization. Combined laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-TOFMS) and electron back-scattered diffraction analysis (EBSD) were also applied on authigenic sulphides to gain insights into their role upon strain accommodation and deformation-induced elemental transport and distribution at the microscopic scale.

Initial embrittlement of the Olkiluoto basement occurred under a first event of fluid overpressure conditions (> 210 MPa) with formation of a diffuse network of joints and/or hybrid–shear fractures in a volume that corresponds to the fault damage zone. Subsequent deformation was caused by repeated hydrofracturing induced by fluid pressure up to 210 MPa. Brittle ruptures affected a system that was otherwise under overall ductile conditions, as demonstrated by mutually overprinting veining, cataclasis and plastic deformation.

Later exhumation and cooling of the fault system to fully brittle conditions was aided by reactivation triggered by a distinct fluid ingress at lower pressure (140-180 MPa) and temperature (≤ 300° C). Deformation was accommodated at that stage by the interplay of brittle fracturing and low-temperature crystal-plastic in sulphides. Strain and fluid flow created high diffusivity pathways within the pyrite crystal lattices contributing to- and enhancing the net transport of a significant range of heavy elements (e.g. Co, Ni, Cu, Sn, Ag, As, Sb, Pb). These data indicate that the studied fault zone acted as a chemically open system and fault valve.

How to cite: Marchesini, B., Viola, G., Menegon, L., Mattila, J., Schwarz, G., Hattendorf, B., and Günther, D.: The role of fluids on strain localization at seismogenic depth: a case study from brittle-ductile faults from Olkiluoto Island, SW Finland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1134, https://doi.org/10.5194/egusphere-egu2020-1134, 2020.

How to cite: Sanchez, C., Saldi, G., Mitchell, T., lacoviello, F., Meredith, P., Jones, A., Oelkers, E., and Striolo, A.: The role of fluid chemistry on permeability and fault strength evolution in granite, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21850, https://doi.org/10.5194/egusphere-egu2020-21850, 2020.

The widespread presence of epidote-bearing veins and hydrous minerals such as micas in meta-granitoid rocks attests to the large extent of hydration of the exhuming continental crust. The ability of epidote (Ca₂Al₃Si₃O₁₂(OH) – Ca₂Al₂Fe³⁺Si₃O₁₂(OH)) to incorporate a wide variety of trace elements renders this mineral a promising geochemical tracer of circulating fluid(s).

We report trace element and microstructural data on epidote-bearing veins from the Aar Massif (Central Alps) and from the Albula Pass (Eastern Alps). We characterized and classified the epidote-bearing veins based on their extent of deformation, shape and size of the epidote grains, coexisting minerals, and degree of dynamic recrystallization of associated quartz. Laser ablation ICP-MS data of individual epidote crystals reveal prominent zoning, confirmed by electron probe maps for Sr and Mn. Overall, low to very low Th/U ratios (down to 0.0005 in the Aar Massif veins and 0.001 in the Albula ones) with Th often below limits of detection (< 0.1 µg/g at 16 µm beam size) go along with variably LREE-depleted patterns (and CI Chondrite-normalized La_N/Yb_N ~0.35 in the Aar Massif veins and ~0.60 in the Albula Pass veins). Strontium contents are variable (hundreds to thousands of µg/g) and mostly high (up to 10100 µg/g in the Aar Massif samples and 12800 µg/g in the Albula Pass samples). The in-situ geochemical data are linked to the microstructures in order to assess whether microstructures can be related to variations in trace elements, also considering the role of coexisting phases. Moreover, trace element data of samples from the Aar Massif are compared to metamorphic host-rock epidotes and cleft epidotes from the same massif.

We find that REE patterns of Aar Massif vein epidotes are clearly different than those of metamorphic host-rock epidotes and of cleft epidotes. In addition, REE patterns vary based on the microstructural characteristics of veins. Overall REE patterns of the Albula Pass vein epidotes resemble those from the Aar Massif. Different veins and microstructures define clusters in Sr vs. Y, Eu anomaly vs. Th/U ratios, and Eu anomaly vs. U values. Geochemical heterogeneities are observed among sampling localities within the Aar Massif.

The fact that the geochemical characteristics of retrograde hydrothermal vein epidotes are clearly different than those of high-grade metamorphic and metamorphic host-rock epidotes, and the relationship between geochemical characteristics and microstructures support the hypothesis that the deformation did not alter the original geochemical record through neomineralization. Our data argue for the potential of epidote as a powerful fluid tracer in the granitoid continental crust.

How to cite: Peverelli, V., Berger, A., Pettke, T., Stunitz, H., Lanari, P., and Herwegh, M.: The retrograde hydration of the continental granitoid crust as seen from epidote-bearing veins: Trace elements and microstructures, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6785, https://doi.org/10.5194/egusphere-egu2020-6785, 2020.

Back-arc basins are known to be controlled by deep subduction dynamics. In the Aegean domain, the slab retreat led to the formation of crustal-scale Low Angle Normal Faults (LANFs) that were involved in the exhumation of Metamorphic Core-Complexes (MCC) in this region. The North Cycladic Detachment System (NCDS) is an example of these LANFs. These large-scale structures are associated with heat exchange and fluid circulations representing a major interest in the understanding of metallogenic provinces and hydrothermal systems. The Menderes massif of Western Anatolia is the location of an active exploitation of high-temperature geothermal resources related to extension and the activity of the main detachments. However, there the rock-fluid interactions in the deep part of the geothermal reservoir are not accessible to observation. The Miocene MCC of Mykonos (Cyclades) represents instead a perfect example to study those systems because it combines detachment faults, a magmatic event, a sedimentary basin and baryte-iron-hydroxides veins exploited until the 80’s. The NE-SW post-orogenic extension is accommodated in the island by the Livada and Mykonos detachments that belong to the NCDS. These detachments are coeval with the emplacement of granitoids and associated to the formation of a supra-detachment sedimentary basin during the Late Miocene. These detachments are strongly related to a dense network of barite, Fe-oxy/hydroxide or Fe-sulfur veins that emplaced during the synkinematic cooling of Mykonos granitic laccolith. The observed fluids in the granite below the detachments show two distinct sources, seawater and a magmatic fluid. However, in the sedimentary basin, the emplacement and the nature of fluids and their interaction with deformation remain poorly investigated. Based on field observations and geochemical analyses, this study aims to propose a scenario of fluid circulations in the Mykonos sedimentary basin by characterizing and tracking them. Raman spectroscopy on fluid inclusions and bulk-rock geochemical analyses were performed to respectively understand fluid sources and hydrothermal circulations. Our observations led us to suggest a mineralization emplacement model during the synkinematic cooling of the laccolith intrusion. First, Mykonos detachment isolates two different domains in term of fluid circulations: strongly reduced magmatic fluids below the detachment and oxidized fluids above it. Further extension and formation of normal faults promoted the progressive connection of these domains. In barite from the detachment, the coexistence of low-salinity fluids and brines in coeval fluid inclusions suggest a boiling phase that could be related to the opening of the system by pulse. Moreover, iron-leached infiltration zones in the overlying sediments witness the percolation of magmatic reduced fluids, able to mobilize Fe²⁺ and to transport it towards oxy-hydroxide-rich veins. These reduced fluids also allowed baryum leaching from magmatic feldspar, while mixing with seawater (rich in SO₄^2-) in the detachment could be responsible for barite mineralization during and after the deposition of the sedimentary pile in Mykonos.

How to cite: Faure, A., Jolivet, L., Verlaguet, A., and Do Couto, D.: Fluid circulations in detachment faults : insights from Mykonos Metamorphic Core Complex, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17187, https://doi.org/10.5194/egusphere-egu2020-17187, 2020.

Caledonian eclogite- and amphibolite-facies metamorphism of initially dry Proterozoic granulites in the Lindås Nappe of the Bergen Arcs, Western Norway, is driven by fluid infiltration along faults and shear zones. The granulites are also cut by numerous dykes and pegmatites that are spatially associated with metamorphosed host rocks. U-Pb geochronology was performed to constrain the age of fluid infiltration and metamorphism. The ages obtained demonstrate that eclogite- and amphibolite-facies metamorphism were synchronous within the uncertainties of our results and occurred within a maximum time interval of 5 Myr, with a mean age of ca. 426 Ma.  Caledonian dykes and pegmatites are granitic rocks characterised by a high Na/K-ration, low REE-abundance and positive anomalies of Eu, Ba, Pb, and Sr. The most REE-poor compositions show HREE-enrichment. Melt compositions are consistent with wet melting of plagioclase- and garnet-bearing source rocks. The most likely fluid source is dehydration of Paleozoic metapelites, located immediately below the Lindås part of the Jotun-Lindås microcontinent, during eastward thrusting over the extended margin of Baltica. Melt compositions and thermal modelling suggest that short-lived fluid-driven metamorphism of the Lindås Nappe granulites was related to shear heating at lithostatic pressures in the range 1.0-1.5 GPa. High-P (≈2 GPa) metamorphism within the Nappe was related to weakening-induced pressure perturbations, not to deep burial. Our results emphasize that both prograde and retrograde metamorphism may proceed rapidly during regional metamorphism and that their time-scales may be coupled through local production and consumption of fluids.

How to cite: Jamtveit, B., Dunkel, K. G., Petley-Ragan, A., Corfu, F., and Schmid, D. W.: Rapid fluid-driven transformation of lower continental crust associated with thrust-induced shear heating, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2154, https://doi.org/10.5194/egusphere-egu2020-2154, 2020.