The Śnieżnik Massif forms the eastern part of the Orlica-Śnieżnik Dome (OSD), located in the north-eastern part of the European Variscan Belt. The OSD, which exposes the root zone of the Variscan Orogen, comprises mostly orthogneisses containing small bodies of ultra-high pressure (UHP) eclogites. Previous studies on the metamorphic conditions recorded by these eclogites yielded inconsistent results. Some authors suggest that they were metamorphosed in conditions of ~1.9-2.2 GPa and ~700-750 °C ¹. Others, however, argue that the eclogites experienced nearly-UHP peak metamorphic conditions of ~2.6-3.0 GPa and 800-930 °C.²

This study provides the first evidence of UHP metamorphic episode recorded in eclogites from the OSD, as coesite inclusions were discovered in garnet and omphacite grains. This finding is consistent with our results obtained using Grt-Cpx-Ky-Ph-Coe/Qtz geothermobarometry and phase equilibria modelling, which both indicated conditions of peak metamorphism of ~2.8 – 3.2 GPa and ~830-870 °C, partially overlapping the coesite stability field.

We also applied quartz-in-garnet elastic barometry to provide additional constraints on the pressure conditions of metamorphism. About 60 inclusions of quartz were identified using Raman spectroscopy. The residual pressure calculated from the spectral shifts of 464 cm^-1 characteristic quartz Raman band reaches a maximum of ~0.73 GPa. This corresponds to the entrapment pressure of ~2.1 GPa, calculated based on the elastic solution for an isotropic spherical inclusion. This estimation contradicts the results coming from methods based on equilibrium thermodynamics. Moreover, such low peak pressure would not explain the presence of the observed coesite inclusions. We hypothesize that the discrepancy might be related to viscous relaxation of garnet host grains under such high peak metamorphic temperatures.

References

[1]     Štípská, P. et al. The juxtaposition of eclogite and mid-crustal rocks in the Orlica-Śnieżnik Dome, Bohemian Massif. J. Metamorph. Geol. 30, 213–234 (2012).

[2]     Majka, J. et al. Integrating X-ray mapping and microtomography of garnet with thermobarometry to define the P-T evolution of the (near) UHP Międzygórze eclogite, Sudetes, SW Poland. J. Metamorph. Geol. 37, 97–112 (2019).

How to cite: Nowak, M., Szczepanski, J., and Dabrowski, M.: Discrepancy between equilibrium thermodynamics-based P-T calculations and quartz-in-garnet elastic barometry in coesite-bearing eclogite (Śnieżnik Massif, NE Bohemian Massif), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-785, https://doi.org/10.5194/egusphere-egu23-785, 2023.

Elastic geo-thermobarometry has become an important technique in determining the pressure-temperature (P-T) conditions of entrapment during metamorphism. A prerequisite is that the inclusion’s over- or under-pressure is not reset during exhumation. This would be the case if the host-inclusion pair interacts elastically only, which is an oversimplification. It is thus not yet been fully understood how fast the inclusion pressure may become reset. In this study, we performed heating experiment on an almandine-rich (from an eclogite) and a spessartine-rich garnet (gem-stone) under 1) graphite (dry), 2) forming gas (5% H₂ and 95% N₂) and 3) water vapour (wet) buffered conditions at high T and room P. Raman spectroscopy is used to measure the same quartz and zircon inclusions at room T before and after different heating times. In wet and forming gas conditions, the Raman band wavenumber changes are dependent on time, decreasing for quartz and increasing for zircon inclusions. Under dry condition, the Raman band wavenumber exhibits a small amount of shift and becomes stable shortly. Raman mappings reveal that the stress heterogeneity of the garnet host develops stronger at the early stage of the wet heating experiments and fade away afterward, potentially indicating a diffusion-like behaviour of the dislocation density. A visco-elastic model is performed to fit the measured data. The calculated flow law parameters of garnet around quartz inclusions is comparable to the flow law extracted from deformation experiments, while zircon shows substantially faster relaxation rate. This study highlights that fluid can be an important trigger for fast viscous relaxation together with temperature, time and inclusion mineralogy. The study may have implications for elastic thermobarometry, garnet rheology, and the preservation of coesite inclusions.

How to cite: Zhong, X., Wallis, D., Kingsbery, P., and John, T.: On the sensitivity of inclusion pressures after entrapment: the drastic effect of aqueous fluid on garnet viscous relaxation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9047, https://doi.org/10.5194/egusphere-egu23-9047, 2023.

The strength of the lithosphere strongly influences the plate tectonics and mantle convection. The flow behavior of the lithospheric mantle is largely controlled by low-temperature plasticity of olivine, the dominant mineral in the upper mantle. Many experimental studies have explored the low-temperature rheological behaviors of olivine but result in strengths that are highly variable when extrapolated to geological conditions. Kumamoto et al. (2017) performed nanoindentation experiments using Berkovich and spherical indenters on olivine at room temperature and proposed that the strength of olivine depends on the length scale of deformation, with experiments on smaller volumes of material exhibiting larger yield stress, that is, the indentation size effect (ISE). However, their nanoindentation tests were done at room temperature, while traditional creep tests were often done at elevated temperatures of ⩾400°C, the temperature dependence in the ISE must be considered in synthesizing experimental results from different studies. Here, we conducted nanoindentation experiments on a single crystal of Fe-free olivine, eliminating the influence from grain size, using a diamond Berkovich indenter at temperatures of 28, 100, 200, 400 and 600°C. In all tests, the hardness decreases with increasing contact depth that is characteristic of the ISE. Taking our data into the classic hardness-depth relationship of H = H₀(1+h^*/h_c)^1/2, where H is hardness, H₀ is the so-called “infinite hardness”, corresponding to the hardness at the infinite indentation depth, h_c is contact depth, and h^∗ is the material length scale parameter. We found h^∗decreases with increasing temperature, which can be attributed to an increase of the storage volume of geometrically necessary dislocations during nanoindentation test. The decrease of h^∗means that the ISE weakens with increasing temperature, suggesting that at lithospheric temperatures the size effect is not strong enough to explain the disagreements between different experiments and between experiments and geophysical observations. Other aspects, such as grain size effect (Hall-Petch effect) and strain-weakening mechanisms may contribute significantly and need to be revisited.

How to cite: Qi, C. and Wang, Q.: Temperature dependence of indentation size effect in olivine and its implications to low-temperature plasticity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17012, https://doi.org/10.5194/egusphere-egu23-17012, 2023.

Thanks to plate tectonics, the Earth lithosphere is composed of very different lithologies, most of which consisting of peridotites, usually covered by either oceanic or continental crust. Depending on several parameters including composition, pressure, temperature, and strain rate, lithospheric materials can deform smoothly and silently or generate seismic ruptures. Collision belts and subduction systems, including subducted materials being heated and sheared in the mantle transition zone, are characterized by intense seismicity; in contrast, the bottom of lithospheric plates, known as lithosphere-asthenosphere boundary (LAB), is not associated with any seismicity, giving the impression that oceanic plates have the intrinsic ability to maintain their basal stress at relatively low values. Comparing results from experimental geophysics, field geology, geodynamics modelling and seismology, I discuss the representativity of experimental findings and potential consequences on our understanding of the rheology of the lithosphere.

The idea that lithospheric materials at intermediate depths or deeper cannot support high deviatoric stresses is still supported by many studies in geosciences or physics. Plenty of authors start by recalling that brittle failure cannot occur at high pressure, and thus conclude that deep earthquakes and their shallow counterparts should consist of totally different events relying on totally different physical processes. Yet, deep seismicity is characterized by double-couple mechanisms and thus is an actual proof of seismic ruptures at great depths. Here I recall achievements from experiments under synchrotron radiation, suggesting that differential stresses can reach several gigapascals within subducting slabs at intermediate depths (30-300 km). In either peridotites or lawsonite blueschists, high-energy X-rays reveal differential stresses above 2 GPa for confining pressures of 1-1.5 GPa, and reaching ≈ 3 GPa for confining pressures of 2.5-3.5 GPa. This is further supported by both field geology studies and numerical modelling.

While mean stresses in seismogenic zones exhibit severe deviations from lithostatic pressure, the base of lithospheric plates deforms in a way that never triggers seismicity. The coupling between lithospheric plates and the underlying asthenosphere is still a matter of debate. According to global dynamics modelling, a basal shear stress as low as only 10-100 MPa would suffice to allow decoupling at the LAB. While partial melting has recently been favoured as an explanation for plate motion, experimental results on an analogue (germanium peridotite) suggest a solid-state lubrication process, involving grain-boundary disordering, and would confirm that mechanical stresses do not exceed 200 MPa at the LAB (60-120 km).

How to cite: Ferrand, T. P.: How high can mechanical stresses be within lithospheric materials?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17411, https://doi.org/10.5194/egusphere-egu23-17411, 2023.

Radial microcracks surrounding retrogressed SiO₂ inclusions in UHP garnets are key microstructural observations allowing to constrain the mechanisms of exhumation of ultra-high-pressure (UHP) rocks. The major challenge lies in identifying whether the microstructures formed during their ascent from mantle depths, or as a consequence of transient variations in the tectonic regime. By combining petrographic observations, petrochronological data and numerical thermo-mechanical modelling, we show that radial cracks around SiO₂ inclusions in ultrahigh-pressure garnets from Dora Maira are caused by ultrafast decompression during the early stage of exhumation (< 0.5 Ma). Decompression rates higher than 10^-14 s^-1 are, for the first time, inferred from natural microstructures independently of current petrochronological estimates¹. We demonstrate that the SiO₂ phase transition generates shear stresses sufficiently large to trigger plastic yielding, resulting in the generation and propagation of radial and bent shear bands, mimicking the fractures observed in UHP garnet. Our results question the traditional interpretation of the exhumation from great depth of ultrahigh-pressure tectonic. Instead, we propose that such ultrafast decompression rates are related to transient changes in the stress state of the buried continental lithosphere, favoring an exhumation mechanism involving nappe stacking.

1 Rubatto, D. & Hermann, J. Exhumation as fast as subduction? Geology 29, 3–6 (2001).

How to cite: Duretz, T., Luisier, C., Tajčmanová, L., and Yamato, P.: Garnet microstructures suggest ultra-fast decompression of ultrahigh-pressure rocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17302, https://doi.org/10.5194/egusphere-egu23-17302, 2023.

Fluid-rock interaction is one of the most important factors regarding the evolution of the Earth’s crust, as it is strongly affecting its petrophysical properties and enabling chemical transport. Therefore, its impact on the Earth’s crustal chemical reservoirs and geodynamic processes can be significant. Fluid-mediated mineral reactions are dependent on the availability of fluids and their capability to percolate through the rock and interact with the minerals, often through pre-existing fluid pathways.

The Kråkenes Gabbro is a mafic enclave, embedded in the felsic gneisses of the Western Gneiss Region in Norway. Although the whole region reached (ultra-)high pressure metamorphic conditions, the gabbro remained in a metastable state and preserved its igneous textures and magmatic minerals. The dry and low permeability gabbro is cut by a N-S-trending fracture network of mode-I cracks, which opened during exhumation. These fractures served as fluid pathways for an aqueous fluid to infiltrate the rock and trigger mineral reactions. Along these fractures the dry gabbro is “hydrated” under amphibolite-facies conditions. The resulting amphibolite reaction front is sharp on outcrop scale and propagates on dm-scale into the gabbro. A complete profile of rock spanning 32 cm in length was taken perpendicular to the vein, including sample material from the vein, the alteration zone, and the mostly pristine gabbroic wall rock.

The gabbro-amphibolite-transition is displayed by the development of a hydrous mineral assemblage, accompanied with a densification and therefore porosity formation. The main cause of this is a drop in the abundance of plagioclase during the amphibolitization. Thermodynamic analysis using Thermolab were done to predict the amphibolite mineral assemblage from the original bulk rock composition of the gabbro. The calculations reveal that mainly H₂O is added to the system and minor further element transport is needed. Furthermore, we observe that even the most reacted amphibolite still contains unaffected gabbroic mineral relicts and the main chemical reactions during amphibolitization are limited to a few minerals. The incoming fluid is consumed as soon as the hydrous phases of the amphibolite are formed. As amphibolitization favors porosity formation, a free fluid phase remains in the pore space as soon as the gabbro at the reactive surface of the affected minerals is completely transformed. The fluid progresses through the newly formed pore space and advances as a sharp the amphibolitization front.

In order to test our hypothesis, we formulate a reactive flow model based on local equilibrium thermodynamics, mass balance and Darcy flow, that simulates the hydration of the dry gabbro to amphibolite including the porosity and fluid pressure evolution. Results confirm the formation of a sharp reaction front and the decrease in porosity during the hydration as a potential physical explanation for the observations without the further need for kinetically delayed reactions. We conclude that the metastability of gabbro is mostly controlled by the availability of fluid to the rock.

How to cite: Bläsing, S., John, T., and Vrijmoed, J. C.: How to hydrate almost non-permeable, dry and mafic crust – A mechanistic view on the Kråkenes Gabbro (Western Gneiss Region, Norway), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12498, https://doi.org/10.5194/egusphere-egu23-12498, 2023.

We discuss a model for temperature-induced rock dehydration that features fluid liberation through mineral reactions, diffusion of chemically released species, and flow through porous media. This model can be derived either by considering standard conservation laws and flux definitions (Pl¨umper et al.[2017], Beinlich et al. [2020]) or, alternatively, using the variational framework of GENERIC (General Equations for Non-Equilibrium Reversible Irreversible Coupling)(Zafferi et al. [2021]) introduced by M. Grmela and H.C. ¨ Ottinger. The latter approach is based on the abstract definition of thermodynamical driving potential and operators characterizing the reversible and dissipative contributions of the processes. By doing so we can show that local equilibrium assumptions are recovered as fast limit of irreversible processes. Ultimately, we rigorously prove that the PDE model so derived admits solutions using a discretization strategy that imitates the numerical implementations.

References

Andreas Beinlich, Timm John, Johannes C Vrijmoed, Masako Tominaga, Tomas Magna, and Yuri Y Podladchikov. Instantaneous rock transformations in the deep crust driven by reactive fluid flow. Nature Geoscience, 13(4):307–311, 2020.

Oliver Plümper, Timm John, Yuri Y Podladchikov, Johannes C Vrijmoed, and Marco Scambelluri. Fluid escape from subduction zones controlled by channel-forming reactive porosity. Nature Geoscience, 10(2):150–156, 2017.

Andrea Zafferi, Dirk Peschka, and Marita Thomas. Generic framework for reactive fluid flows. ZAMM-Journal of Applied Mathematics and Mechanics/Zeitschrift für Angewandte Mathematik und Mechanik, page e202100254, 2021.

How to cite: Zafferi, A., Huber, K., Peschka, D., Vrijmoed, J., John, T., and Thomas, M.: A porous-media model for reactive fluid-rock interaction in a dehydrating rock, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17245, https://doi.org/10.5194/egusphere-egu23-17245, 2023.

Hydration and dehydration reactions as well as the associated fluid flow are important features of geodynamic processes. For example, hydration of rocks can significantly decrease rock strength and generate shear localization or fluids liberated by dehydration reactions in subducting rocks can flow into the mantle wedge and cause melting and magmatism. However, several aspects of (de)hydration related fluid flow and the propagation of (de)hydration reaction fronts remain unclear.

Here, we study hydration and dehydration reactions with hydro-chemical numerical models based on continuum mechanics and local equilibrium thermodynamics. For simplicity, we mainly consider 1D isothermal models. We focus on the propagation velocity and direction of the (de)hydration reaction front. We define hydration as an increase of chemically, or lattice, bound water in the solid phase. Therefore, hydration requires fluid flow towards the hydration reaction front. Contrary, dehydration is a decrease of chemically bound water in the solid phase. Hence, dehydration requires fluid escape from the dehydration reaction front.

Our models show that hydration requires a negative sign of the solid volume change with pressure increase across the reaction boundary, whereas dehydration requires a positive sign of solid volume change with pressure increase across the reaction boundary. The reason for this difference in sign is due to the fluid flow associated with the (de)hydration reaction which is driven by the fluid pressure gradient following Darcy’s law. Thus, for hydration to happen it must occur on the lower fluid pressure side of the reaction front compared to the side with more porous fluid. Porosity is directly related to the solid density change, so it is larger on the high solid density side of the reaction front. Therefore, the hydration reaction requires that the rock that should be hydrated is on the lower fluid pressure side of the front. Opposite can be reasoned for the dehydration front. We also include in our models the case of zero porosity, and hence zero permeability, on one side of the (de)hydration reaction front. This zero-permeability limit involves a singularity at the reaction front due to the multiplication of zero permeability with an infinite pressure gradient. We resolve this singularity in our numerical algorithm by applying a fully conservative form of the governing equations. Resolving this zero-permeability limit is in agreement with the well-established theory of non-linear degenerate parabolic equations. We apply our model to two natural settings: First, eclogite shear zones in the Bergen Arcs, Norway, where hydration of dry granulite formed eclogite. Second, olivine veins in the Erro-Tobbio unit, Ligurian Alps of Italy, where dehydration of serpentinite during subduction formed olivine.

How to cite: Schmalholz, S. M. and Podladchikov, Y.: Hydration versus dehydration reactions: increase versus decrease of solid density with pressure rise, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14012, https://doi.org/10.5194/egusphere-egu23-14012, 2023.

Eclogitization constitutes one of the most emblematic transformations in continental subduction zones, where conversion of initially dry lower crustal rocks into eclogite facies rocks correlates with the occurrence of seismogenic events. This reaction is generally considered to occur at high pressure conditions during hydration of dry granulite. Several models using « ad hoc » diffusion equation exist to model this hydration process and the consequences of reaction-induced changes in terms of rheology and density. However, to our knowledge, there is no quantitative model allowing to physically explain how fluids propagate inside a dry rock (i.e. with no porosity at all) and how reaction-induced alteration front widens over time. In this study, we therefore propose a new fully coupled hydro-chemical model wherein a two-phase flow model is coupled with the eclogitization reaction. We use a mass conservative approach, solving total mass and solid mass equations, in a closed isothermal system. Fluid and solid densities are calculated with lookup tables from equilibrium thermodynamics. Our model shows that a fluid pressure pulse generates a pressure gradient that can be associated with the densification reaction when the pressure required for the eclogitization is reached. This reaction generates a large increase in porosity (0 to ~16%) and subsequent porous fluid flow inside the initially dry granulite. This process is then sustained as long as the fluid pulse is maintained, and ends shortly after the fluid pressure pulse stops. However, high pressure within the reacted area can persist for a long period of time. A parametric study allowing to constrain both the duration and the widening of the reaction area is proposed as well as an application to the emblematic case study of the eclogitized granulites of Holsnoy (Bergen Arcs, Norway).

How to cite: Bras, E., Yamato, P., Duretz, T., Schmalholz, S., and Podladchikov, Y.: Fluid-pressure induced eclogitisation of a dry granulite: Insights from Hydro-Chemical model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17232, https://doi.org/10.5194/egusphere-egu23-17232, 2023.

The hydration of initially dry, lower crustal metamorphic rocks during orogenesis is a commonly observed phenomenon and hydration/reaction interfaces are also often preserved, providing a unique insight into the evolution of the lithosphere. Often the interfaces between unreacted and reacted rock are very sharp, even on a thin section scale, and various explanations have been proposed to account for the abrupt changes in mineral assemblage on such a small spatial scale. Common to a wide range of specific examples is the role of an infiltrating aqueous fluid that is generally assumed to be required for the reaction to take place, although other features of such reaction fronts can differ widely in terms of density changes and the apparent difference in metamorphic grade across a sharp interface. 

The examples discussed here all involve the hydration of basement granulite rocks formed during the Caledonian Orogeny and now exposed in the Bergen Arcs in Norway. All stages of hydration can be observed from totally unreacted dry granulites with a wide range of composition to either eclogite facies or amphibolite facies overprints. In these cases the density changes across the interface can either be positive (in the case of eclogite formation from anorthosite granulites), can be negative (in the amphibolitisation of basic rocks) or virtually zero (during the amphibolitisation of garnet bearing anorthosites). The preservation of volume across such interfaces has led to investigations of the coupling between the consequent stress generation and mass transfer, which in turn focusses on the evolution of porosity/permeability in the parent dry rock. The extent of hydration in the Bergen Arcs as a whole ( 90% hydration of ~10⁵ km³ of granulite) suggests a plentiful supply of aqueous solution introduced seismically by fracturing and the consequent generation of shear zones from which hydration fronts spread. The hydration to either eclogite or amphibolite, often observed at the same structural level (i.e. depth in the crust) continues to be an enigma.

Although the details of the reactions and density changes are different, a common feature is the need for an infiltrating aqueous solution and hence the question of what drives the fluid and the hydration reaction and finally why the reaction stops at the sharp interfaces observed in the field. Terminated reactions can be studied by the extent of alteration around fracture planes by modelling the likely fluid pressure gradients that drive Darcy flow from the fluid source towards the reaction interface. Fracture planes represent zones of localised high permeability that facilitate the infiltration of fluid. The difference in fluid saturation between the fracture plane and the alteration halo is thought to be responsible for both the degree of reaction and the difference in assemblage. Additionally, the width of the initial fracture plane is thought to be proportional to the extent of the alteration halo. Examples will be given of hydration fronts associated with amphibolite facies shear zones as well as the observation of eclogite fingers within a partly hydrated granulite host. Combined reaction-fluid flow models attempt to explain these phenomena.

How to cite: Putnis, A., Moore, J., and Podladchikov, Y.: What controls the preservation of hydration interfaces in high grade metamorphic rocks?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17238, https://doi.org/10.5194/egusphere-egu23-17238, 2023.