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Tectonic (or non-volcanic) tremors have been extensively documented at subduction zones and are considered as the signature of transport processes of dehydration-related fluids in subduction zones, often recorded in close association with geodetically observed shear induced slow-slip events. However, to the best of our knowledge, they have not yet been reproduced in the laboratory at subduction zones P–T conditions in such way that their first-order controlling mechanisms remain enigmatic. This work investigates the mechanism of these seismic events by performing dehydration-deformation experiments combined with detailed investigations of mineral reactions and acoustic emissions. Experiments were carried out on chlorite-peridotite powders (Balmuccia peridotite with synthetically added chlorite, a mineral that is typically found in subduction zone lithologies), following a subduction zone geothermal gradient using a high-pressure apparatus (Griggs-type). The experiments were conducted from ambient conditions to maximum pressures of 1.5-3.0 GPa and temperatures of 750-800 °C. Experiments were executed under hydrostatic conditions and an additional one with deformation. An ultrasonic transducer (0.5-10MHz dynamic range) was employed to monitor and detect the micro-seismic events. High-resolution electron beam techniques (EMPA, SEM and TEM) have been applied for analyzing the sample material.Dehydration of ~15 vol.% of the initial chlorite suffices to trigger acoustic emissions, which display waveforms reminiscent of those of tectonic tremors. The moment distribution statistics of these laboratory tremor-like signals follows the Gutenberg-Richter relationship and a scaling between moment vs. event duration. Finally, we observe a match between the ratios of size and typical frequency of natural over laboratory tremors. Microstructural observations document metamorphic olivine and pyroxene growth in the decomposing chlorite and demonstrate that an almost isochemical dehydration of the chlorite took place. Accordingly, the appearance of the tremor-like acoustic emissions after crossing a temperature of 600 °C can be linked to a dehydration process related to the chlorite breakdown in the sample. Thermodynamic calculations show that a small amount of released fluids (breakdown of ~1.5 vol.% of a hydrous phase) is enough to trigger seismic signals analogues to tremors. The experiment with additional deformation produced no tremor-like acoustic emission suggesting that the large macroscopic shear stress suppressed the development of the processes that lead to acoustic emissions. We conclude that fluid release during dehydration is the cause of tectonic tremors, whereas shear-stress seems to counteract their development with no occurrence of tremors at high rates of deformation. According to the results from this study, the triggering mechanism can be tentatively interpreted as a fluid propagation front resulting in the vibration of grain boundaries.

Many metamorphic rocks have a fabric. What is often not clear is how much deformational or metamorphic processes contributed to the formation of these fabrics. Are foliations always the result of strain? When does intrinsic crystallographic anisotropy alone lead to the formation of structural elements? Understanding the relative contributions of deformation and metamorphism in rock fabrics is fundamentally important because it is foundational to understanding the role of stress in reacting and deforming rocks.To this end, we make a major advance in our understanding of fabric development in reacting rocks by showing in time-resolved (4D) synchrotron microtomography (µCT) experiments that when a gypsum dehydration reaction occurs in a differentially stressed sample the reaction products develop orthogonally to the largest principal stress. This is an important finding because we can show with our µCT data that this preferred orientation forms early in the reaction and at very small strains (

Recent technological advances allow geoscientists to generate high-resolution (HR) imagery using a variety of different beam-forming mechanisms (e.g. visible light, X-Rays, or charged particles such as electrons and ions). One of the main limitations in producing HR data is the required acquisition time at high magnifications. For example, back-scattered electron (BSE) mapping of a standard petrographic thin section at a resolution of 50nm/pixel takes approximately 60 days and is associated with a storage requirement in the order of 700 GB. Deep-learning methods have proven effective for resolution enhancement in regular photographic images, and in this work we present an integrated image registration and upscaling workflow to enhance image resolution, using real-world BSE datasets.The proposed workflow requires the acquisition of one, or multiple, HR regions within a region that is imaged at low-resolution (LR). Next, close to pixel-accurate image registration is performed by using the successive implementation of two concepts: i) first the precise location of the HR region within the LR region is determined by using a Fast Fourier Transform algorithm (Lewis, 2005), and ii) final image registration is achieved by iteratively calculating a deformation matrix that, using Newton’s method of optimization, is aiming to minimize an error function describing the differences between both images (Tudisco et al., 2017).Subsequently, matching HR and LR image pairs are fed into a Generative Adversarial Network (GAN) that learns to produce HR images from the LR counterparts. A GAN consists of two neural networks, a generator and a discriminator. The generator produces synthetic HR data based on LR input, and the discriminator attempts to classify the data as either real HR or synthetic HR. The two networks are trained together in an adversarial process, with the goal of the generator producing synthetic data that the discriminator cannot distinguish from real data.We demonstrate our method on a variety of large real-world datasets and show that it effectively increases the resolution of full-size BSE maps up to a factor of four, while being able to resolve important features. The upscaling of BSE data, with a factor of four, is associated with a 90% reduction in beamtime and a factor 16 reduction in storage requirements. Image registration, preprocessing, and model training on a high-performance workstation takes 12-24 hours. Having a trained model, inference can be done using a regular laptop.[1] Lewis, J. P. "Fast normalized cross-correlation, Industrial Light and Magic." unpublished (2005).[2] Tudisco, Erika, et al. "An extension of digital volume correlation for multimodality image registration." Measurement Science and Technology 28.9 (2017): 0954

Talc is an important product of several hydration and dehydration reactions in deep faults and subduction zones. The unique weakness of talc along its basal planes makes it an essential component in understanding various fault slip behaviors (e.g., episodic vs continuous slip, seismic vs aseismic) or realistic geodynamic models. A recent experimental study by Boneh et al. (2023) on talc mechanical behavior at high P-T conditions highlighted: (i) talc’s low friction coefficient under all conditions (

Understanding earth materials is essential for the creation of a sustainable, carbon-neutral society. Earth materials control the feasibility of subsurface energy storage, geothermal energy extraction, and are a source of critical elements for future-proof battery technologies. To tackle the current, pressing scientific questions related to sustainable development for a circular economy, there is an urgent need to make multi-scale, multi-dimensional characterisations of earth materials available to a broad spectrum of earth-science disciplines. Besides society-relevant topics, the properties of earth materials determine how the Earth works on the most fundamental level.To overcome this challenge, 15 European institutes joined forces to establish EXCITE, providing free-of-charge access to 24 state-of-the art microscopy and x-ray tomography facilities in Europe. EXCITE can help you gain insight into the processes governing the behavior of the Earth crust through microchemical analyses and 2D- to 4D imaging, and down to nanometer resolution. In particular, the EXCITE strategy integrates joint research programs with networking, training, and trans-national access activities, thereby enabling both academia and industry to answer critical questions in earth-materials science and technology. EXCITE is building a community of highly qualified earth scientists, develops correlative imaging technologies providing access to world-class facilities to particularly new and non-expert users that are often hindered from engaging in problem-solving microscopy of earth-materials. EXCITE is building a community of highly qualified earth scientists, develops correlative imaging technologies providing access to world-class facilities to particularly new and non-expert users that are often hindered from engaging in problem-solving microscopy of earth-materials. Access to EXCITE can be requested by applying to our bi-annual call. Interested? Have a look on the EXCITE website (https://excite-network.eu) – and apply!

Many macroscopic transport properties and physical processes, such as the flow of fluids through a porous medium, are directly controlled by its microstructure, specifically the presence and connectivity of individual pore spaces at micron and submicron scales. Reconstructing and evaluating the material properties of porous media plays a key role across many engineering disciplines from subsurface storage (e.g., CO2 and hydrogen) to geothermal energy and reservoir characterization. As such, the rapid and reliable characterization, evaluation, and simulation of complex pore microstructures is required not only to enhance our understanding of the fundamental processes occurring at the pore scale, but to also better estimate their material behavior on a larger scale.These material behaviors are inherently volumetric and therefore cannot be accurately modelled using two-dimensional (2D) data alone. As a result, the accuracy of reconstruction techniques used to extract these morphological properties and spatial distributions is in part determined by the quality of available three-dimensional (3D) microstructural datasets. However, in comparison to their 3D counterparts, 2D imaging techniques are typically more cost efficient, easier to collect, and higher resolution. Our goal of generating statistically accurate 3D reconstructions of complex pore microstructural distributions based on high resolution 2D datasets is essential to bridging this dimensionality gap.Newly explored 2D-to-3D reconstruction techniques based on deep-learning (DL) algorithms offer an alternative means of generating robust and statistically representative digital 3D rock reconstructions by measuring some spatial morphological properties and statistical microstructural descriptors (SMDs) of porous media samples from high-resolution 2D datasets. These DL models are highly flexible and capable of capturing a variety of complex microstructural features given representative 2D training datasets. In this paper, we implement a newly developed deep Generative Adversarial Network (GAN), known as SliceGAN, to synthesize novel binary digital 3D reconstructions using high-resolution 2D back-scattered electron (BSE) images obtained from thin-sections oriented in the x-, y- & z-direction.Our trained model is capable of accurately reconstructing complex 3D microstructural features of porous media through capturing the underlying (micro-)structural and morphological properties contained in the original sample (2D) thin-sections. To demonstrate the effectiveness of our trained model, we conducted a comparative analysis between the generated 3D reconstructions and real sample datasets by evaluating morphological properties (volume fraction, surface area, equivalent diameter, pore orientations, etc.) as well as the widely popular SMD the two-point correlation function (S2 (r) ). The resulting reconstructions are virtually indistinguishable, both visually and statistically, from the real sample. Our research paves the way for quickly and accurately describing complex heterogenous media for the prediction of transport processes, for example, carbon and hydrogen storage and extraction.

Pervasive carbonation of serpentinized peridotite to carbonate-quartz rock (listvenite) due to infiltration of CO2-bearing fluid is a remarkable process because it can be geologically fast and it increases the rock’s carbon content from initially zero to > 30 wt% CO2. This pervasive conversion is related to an overall solid volume increase while at the same time requiring high time-integrated fluid rock ratios with permeability and diffusivity on all scales. Thus, porosity has to be created dynamically during reaction progress as otherwise fluid pathways become clogged by the reaction products carbonate and quartz, which is one of the major obstacles for artificial carbon storage by peridotite carbonation. Processes that can renew porosity and permeability during carbonation are fracturing and veining – in response to tectonic stress [1] or induced by reaction and crystal growth [2], or a combination of both –, reaction-enhanced ductile deformation [3,4], and spatial decoupling of dissolution and precipitation with solute transfer at different length scales.Using SEM, EBSD, TEM and FIB nano-tomography, we investigated the microstructural record of local solute transfer and its role for porosity renewal in natural carbonate-bearing serpentinites, transitional serpentine-carbonate-quartz assemblages and listvenites from the Samail Ophiolite, Oman. The clearest indicators of pervasive replacement accommodated by local solute transfer are pseudomorphic replacement structures where carbonate and quartz occur in distinct microstructures corresponding to different inherited peridotite and serpentinite textures. A common pseudomorphic replacement structure in the Samail carbonated peridotite are quartz-fuchsite intergrowths replacing bastite (pseudomorphs after orthopyroxene) in listvenite with harzburgite protoliths. A local strong crystallographic alignment of quartz in each of the bastite/pyroxene pseudomorphs suggests that the anisotropic porosity structure of bastite serpentine favored oriented, epitaxial growth of quartz. Transitional serpentine-carbonate-quartz assemblages show that the first quartz and carbonate generations precipitate coeval but spatially separated, with distinct crystal habits. FIB nano-tomography, STEM analysis and high resolution SEM on ion-polished samples of a transitional serpentine-dolomite-quartz rock from the carbonation reaction front show nano-porous fluid channels in fibrous serpentine at high angle to a highly serrated carbonate-serpentine replacement contact. These nano-scale fluid channels facilitated bidirectional mass exchange of Ca, C, Mg and Si bearing solutes between sites of preferential replacement, such as the cores of serpentine mesh textures, and larger-scale permeability networks along veins and fractures. These observations imply that massive and pervasive solute transfer through the reacting serpentine matrix is possible on a small scale, without clogging of porosity by immediate co-precipitation of quartz or Mg-silicates. Spatial decoupling of dissolution and precipitation was likely caused by the dynamically evolving composition of the reacting fluid and/or due to the influence of differential stress and volumetric strain – a mechanism that can compensate on a local scale for the volume expansion expected of isochemical carbonation reactions. [1] Menzel et al., Solid Earth, 2022; https://doi.org/10.5194/se-2021-152[2] Kelemen & Hirth, EPSL, 2012; https://doi.org/10.1016/j.epsl.2012.06.018[3] Menzel et al., Nature Communications, 2022; https://doi.org/10.1038/s41467-022-31049-1[4] Kelemen et al., JGR, 2022; https://doi.org/10.1029/2021JB022352 Funding: Junta de Andalucía (Postdoc_21_00791); DFG grants UR 64/20-1, UR 64/17-1; and EU Horizon 2020 Transnational Access EXCITE _C1_2022_34.

The trigger mechanism of intermediate depth earthquakes (30 - 300 km) is a long-standing debate. Many studies showed that these seismic events nucleate along a double-seismic zone within the subducting slab. The seismic events of the lower plane coincide with the depth of major dehydration reactions in the lithospheric mantle. Consequently, it is thought that these events are related to the release of fluids. Several scenarios are currently discussed that might lead to brittle deformation. Among these are dehydration embrittlement and dehydration-driven stress transfer.Antigorite is one of the most important candidates for fluid release due to its high H2O content and stability limits within the lower Wadati-Benioff zone. The release of water through antigorite dehydration can be calculated by equilibrium thermodynamics and is mainly a function of temperature. This does, however, not account for deformation (e.g., stored internal strain energy) leading to local variations in the free energy of minerals. In this study we aim to explore the effect of shear stress on the stability of antigorite.We performed high-pressure and high-temperature experiments in a Griggs rig. We used intact drill cores of two different starting materials for our experiments: a foliated and an isotropic antigorite-serpentinite. Both starting materials did not contain relict olivine and/or orthopyroxene. We run our experiments at 620 to 650 °C with a confining pressure of 1.5 GPa and a strain rate of 10-6 s-2. Subsequent analyses of the experimental runs revealed no dehydration products within the bulk sample. However, we observed the formation of ultra-fine grained (< 100 nm) olivine and orthopyroxene along narrow zones, which are orientated 30 to 40 ° with respect to the compression axis. These zones are similar in all runs and independent of the starting material microstructure. We thus propose that shear stress localization within our cylindrical sample triggered the dehydration. Within subduction zones local variations in stress field due to mineralogical or textural heterogeneities could promote dehydration, eventually leading to seismic events through stress transfer.

In the dry lower crust, earthquake-induced fracturing can increase permeability for fluids to infiltrate and flow, thus facilitating fluid-rock interactions, and potentially altering the strength and rheology of fault systems. Understanding the mechanisms that create and reduce porosity requires a detailed microstructural analysis. Here, we analyze microstructures that have recorded primary and secondary porosity generated by the dynamic rupture propagation of a lower crustal earthquake, and that were subsequently reworked during post- and interseismic viscous creep.An exhumed lower crustal section comprised largely of anhydrous anorthosites cross-cut by a coeval network of pseudotachylytes (solidified melts produced during seismic slip) and mylonitized pseudotachylytes (overprinted during the post- and interseismic viscous creep), is found at Nusfjord, Lofoten, Norway. We study the microstructures using synchrotron X-ray microtomography (SμCT), focused ion beam scanning electron microscopy (FIB-SEM) nanotomography, electron backscatter diffraction (EBSD) analysis, and SEM imaging.SμCT data reveals that porosity is dispersed and poorly interconnected within a pseudotachylyte vein (0.16 vol% porosity overall), and noticeably increased along the grain boundaries of garnet grains (1.07 – 1.87 vol%). The increased porosity around garnet is formed due to a net negative volume change (-DV) during garnet growth, as there is a localized increase in density of ~1.00 g/cm3 when a recrystallizing garnet overgrows a pseudotachylyte matrix (plagioclase + amphibole). Efficient healing of the earthquake damage zone (0.03 vol% porosity) resulted in the preservation of only a few but relatively large interconnected primary pores along fractures in the anorthosite. Fractures were healed by the growth of plagioclase neoblasts nucleated from extremely comminuted fragments of the host anorthosite, and by the precipitation of barium-enriched K-feldspar filling intragranular pores. Fluid-rock interaction was so efficient at sealing the porosity that a FIB-SEM transect along one of these microfractures revealed a myrmekite intergrowth replacing K-feldspar.Porosity is dramatically decreased in the mylonitized pseudotachylyte (0.03 vol% overall), and focused mainly within monomineralic domains of plagioclase (0.07 – 0.11 vol%). These are interpreted as recrystallized and sheared survivor clasts of wall-rock fragments, while the polymineralic domains are primarily derived from the overprint of the original pseudotachylyte veins. The plagioclase grains in both domains are more-or-less equant, very fine grained (< 25 μm), lack a crystallographic preferred orientation, grain boundaries occasionally aligned to form quadruple junctions, and are well-mixed amongst the hydrous phases (polymineralic domain), suggesting that both domains deformed primarily by grain-size sensitive diffusion creep and viscous grain boundary sliding. The polymineralic domain has the least porosity (~0.01 vol%), which reflects the efficient precipitation of phases (amphibole, biotite, and feldspars) into transient pores during creep cavitation.A porosity reduction on the order of 90% from a pristine to a mylonitized pseudotachylyte may eventually result in shear zone hardening, and development of new pseudotachylytes overprinting the mylonites. Therefore, earthquake-induced rheological weakening of the lower crust is intermittent, occurs when a fluid can infiltrate a transiently permeable shear zone, and may stop when the porosity becomes clogged.

On Earth, subduction zones facilitate the cycling of volatiles between the Earth’s surface and interior. Volatile cycling has significant effects on the long-term state of the Earth’s climate and tectono-magmatic events, including volcanism and earthquakes. A key stage in the volatile cycle is the devolatilization of the subducting oceanic lithosphere, in which volatiles can escape the previously hydrated rocks. However, it is not well known how efficiently volatiles are transported at this stage. To better understand how volatiles escape at these conditions, we have analyzed the dehydration-related vein networks of the Erro-Tobbio meta-serpentinites (ET-MS), Italy. The ET-MS display well preserved networks of metamorphic olivine veins. These veins are the result of the dehydration reaction of antigorite and brucite to produce H2O and olivine. However, due to the low permeability of serpentinite at depth, the dehydration reaction requires the formation of self-organizing vein networks to allow the produced fluid to escape [1]. Thus, the metamorphic olivine veins in ET-MS may be used as a proxy for fluid flow pathways. We took a multiscale approach to analyzing the network architectures. For microscale (~16 µm voxel size) and mesoscale (~200 µm voxel size) resolutions, X-ray tomography methods are sufficient to visualize the three-dimensional structure of the networks. However, for large scale observations these methods are inapplicable. To solve this, we apply a novel workflow to analyze outcrop scale (~10 m) network systems in three dimensions using only two-dimensional data. By training a generative adversarial network (GAN) with two-dimensional data conditioned by spatial orientation, we can generate statistically representative three-dimensional networks that mimic those of the ET-MS. These representations also display similar characteristics in their respective pore-network-models. With this method, it is possible to produce reasonable three-dimensional approximations of the ET-MS vein networks using only photogrammetry data of the outcrops. In turn, this allows us to extract metrics, such as permeability, that describe the volatile transport efficiency of the ET-MS, and further, how these characteristics change at a broad range of scales. [1] Plümper et al. (2017) Nature Geoscience 10(2), 150-156.

Fluid flow in subduction zones is one of the essential factors of seismic activity in subduction zones. However, the timescale of fluid flow and fluid flow velocity in subduction zones is unclear. In this study, we report antigorite veins with brucite-rich reaction zones in the crust-mantle transition zone of the Oman ophiolite and estimate the timescale and fluid flow velocity during vein formation.In this study, we observed 28 samples in the lower crust to upper mantle section obtained from the Oman Drilling Project Hole CM1A (Kelemen et al., 2020). The lithology of borehole CM1A consists of lower crust (0-160 m depth), crust-mantle transition zone (160-310 m depth), and mantle section (310-404 m depth), which were mainly composed of altered gabbroic rocks (olivine gabbro, troctolite: 5-95% altered) and completely serpentinized dunite and 70-100% serpentinized harzburgite, respectively. Antigorite-chrysotile (Atg-Ctl) vein network was found in dunite at 160-180 m in Hole CM1A. The matrix of the dunite (lizardite, brucite, and magnetite) is cut by antigorite-chrysotile (Atg-Ctl) vein network. Trace element analysis using LA-ICP-MS revealed that the Atg-Ctl vein is enriched in As and Sb compared to the matrix lizardite, suggesting that the Atg-Ctl veins were formed by fluids interacting with subducting sediments. Some of the Atg-Ctl veins are accompanied by brucite-rich reaction zones. The brucite-rich reaction zone was developed at both sides of the antigorite veins with widths of 0.5 – 4 mm. Elemental mapping of the reaction zone using EPMA and TEM show sharp reaction front at scale from micro- to nano- meter.Mass balance calculations and thermodynamic considerations of the reaction zone suggest that the formation of the reaction zone was caused by the removal of silica from the host rock during the precipitation of antigorite in the veins. Based on a diffusion model, we estimated the fluid activity is short-lived (2.1 × 10–1 to 1.1 × 101 yr), and the fluid flow velocity of 2.7 × 10–3 to 4.9 × 10–2 m s-1, which is much faster than those observed for the intact mantle and crustal rocks. This fluid flow velocity along the fractures within the mantle wedge is similar to the observed propagation velocities of seismic events in subduction zones. These results suggest that fluid flow in the overlying plate occurs as episodic pulses as observed as the migration of seismicity in the present subduction zones.

Pore fluid pressure on subduction zone megathrusts is lowered by fluid drainage into the overlying plate, affecting subduction zone seismicity. However, the spatial and temporal scales of fluid flow through suprasubduction zones are poorly understood. We constrain the duration and velocity of fluid flow through a shallow mantle wedge based on the analyses of vein networks consisting of high-temperature serpentine in hydrated ultramafic rocks from the Oman ophiolite. On the basis of a diffusion model and the time-integrated fluid flux, we show that the channelized fluid flow was short-lived (2.1 × 10 −1 to 1.1 × 10 1 years) and had a high fluid velocity (2.7 × 10 −3 to 4.9 × 10 −2 meters second −1 ), which is close to the propagation velocities of seismic events in present-day subduction zones. Our results suggest that the drainage of fluid into the overlying plate occurs as episodic pulses, which may influence the recurrence of megathrust earthquakes.

To investigate the effect of carbon-bearing phases on the release of fluids in subducted serpentinites, we performed high-pressure multi-anvil experiments on representative ophicarbonate assemblages over a pressure range from 2.5 GPa to 5 GPa and from 450 °C to 900 °C, across the antigorite-out reaction. Parallel experiments were performed on carbonate-free serpentinites. In all experiments, we monitored and/or controlled the oxygen fugacity. The addition of 20 wt. % CaCO3 to a serpentinite assemblage at 2.5 GPa is found to decrease the onset of the serpentine dehydration by over 100 °C, in comparison to carbonate-free assemblages. Similarly, the final disappearance of serpentine is also affected by the presence of CaCO3. For a bulk CaCO3 content of 20 wt. %, this causes a decrease in maximum stability of antigorite by 50 °C. For a bulk CaCO3 content exceeding 25 wt. %, this difference can be as high as 100 °C in warm and 150 °C in cold subduction zones, causing antigorite to be completely dehydrated at 500 °C. This results from the reaction of CaCO3 with serpentine to form clinopyroxene and Mg-rich carbonates. This reaction, however, causes no discernible decrease in the proportion of carbonate, indicating that the amount of released carbon is insignificant. Whilst CaCO3, therefore, influences serpentine stability, there is no significant effect of hydrous phases on the carbonate stability. On the other hand, a MgCO3-bearing system shows no significant effects on the serpentinite stability field. Further experiments and oxygen fugacity calculations indicate that graphite is not stable in typical magnetite-bearing serpentinites. The reduction of carbonates to graphite would require oxygen fugacities that are 1–2 log units below those of magnetite-bearing serpentinites. This confirms earlier studies and indicates that reduction of carbonates can only occur through the infiltration of external H2-rich fluids.

High-magnitude, reaction-induced stresses.

Faults in the upper crust are considered major fluid pathways, raising the need for deformation experiments under wet conditions that focus on the nanoscale interaction between gouge material and pore fluid. Friction experiments on calcite at seismic slip velocities show strong dynamic weakening behaviour attributed to a combination of grain-size reduction and nanoscale diffusion. The resulting syn-deformational physico-chemical interactions between fluid and calcite are key in deciphering deformation mechanisms and rheological changes during and after (seismic) faulting in the presence of a fluid phase. We conducted rotary shear deformation experiments (1 m/s, σn = 2 and 4 MPa) on calcite gouge with water enriched in 18O (97 at%) as pore fluid to track and quantify potential fluid – mineral interaction processes. With our correlative, cross-platform workflow approach, we integrate Raman spectroscopy, nanoscale, and Helium-Ion secondary ion mass spectrometry (nanoSIMS, HIMSIMS), focused ion beam – scanning electron microscope (FIB-SEM) and transmission electron microscopy (TEM) to characterise the nanostructure and analyse isotope distribution. Raman analyses confirm the incorporation of 18O into the calcite crystal structure, as well as the presence of amorphous carbon. We identify three new band positions relating to the possible isotopologues of CO32- (reflecting 16O substitution by 18O). In addition, we detect portlandite (Ca(OH)2), pointing to a hydration reaction of lime (CaO) with water. Raman and NanoSIMS maps reveal that 18O is incorporated throughout the deformed volume, implying that calcite isotope exchange affected the entire fault gouge. Based on oxygen self-diffusion rates in calcite we conclude that solid-state 18O – isotope exchange cannot explain the observed incorporation of 18O into the calcite crystals during wet, seismic deformation. Hydration of portlandite and calcite containing 18O, implies breakdown and decarbonation of the starting calcite and the nucleation of new calcite grains. Our results question the state and nature of calcite gouges during seismic deformation and challenge our knowledge of the rheological properties of wet calcite fault gouges at high strain rates. The observations suggest that the physico-chemical changes are a crucial part of hydrous calcite deformation and have implications for the development of microphysical models that allow us to quantitatively predict crustal fault rheology.

When a fluid is introduced into dry rocks at high-pressure conditions, it acts as a catalyst and facilitates re-equilibration. This often promotes weakening and subsequent ductile deformation. Here, we present a detailed micro-structural and mineral chemical study of eclogitization of initially dry continental crustal rocks in the absence of ductile deformation. The studied sample features an incomplete (fluid-induced) transition from lower crustal granulite to eclogite, and the transition is fully preserved. None of the mineral phases show any signs of ductile deformation, indicating that the transformation was entirely static. Material transport during the reaction was limited to the availability of fluids. Detailed analysis of the local assemblages along the transect reveals that the reaction occurs in three distinct steps: The plagioclase-plagioclase grain boundaries were the first to re-equilibrate followed by clinopyroxene-plagioclase and garnet-plagioclase grain boundaries. Lastly, the grain boundaries that included only garnet and/or clinopyroxene are involved in the transformation. Thermodynamic modelling of local equilibria at dry conditions and with H2O in excess reveals that this stepwise transformation is caused by the varying reactivity of the local assemblages at the prevailing P-T conditions. Those reactions that result in the largest decrease of the Gibbs free energy from the dry case to the case with H2O in excess occur first. Once the reaction is facilitated, this effect is amplified because the density increase is largest at those grains boundaries that have reacted first, creating new fluid pathways through volume reduction. The calculated stable local mineral assemblages are consistent with those present in the sample indicating that element transport is limited, also supported by the observation that the fabric of the granulite is preserved in the eclogite. Our results demonstrate that reactive fluid flow is guided by the local energy budget along the grain boundaries, and that element transport during static re-equilibration is limited to the extent where it is thermodynamically advantageous.

Tectonic-scale features happening at convergent plates are ultimately the outcome of microscopic, grain scale processes. In collision zones, prograde metamorphism occurs by gradual increase of pressure and temperature [1; 2]. Among the most important prograde mineral reactions are dehydration reactions, which are characterized by solid volume reduction, porosity creation, fluid release and high pore fluid pressures [3]. Most models linking dehydration and mechanical instabilities [4-6] involve feedback loops between coupled chemical, hydraulic and mechanical processes. Feedbacks control pore fluid pressure build-up and drainage, and provide efficient pathways for the transport of chemical components. Gypsum dehydration is crucial in the formation of detachment faults thin-skinned tectonics [7]. It is also used as a proxy for serpentine dehydration and the generation of intermediate depth seismic events/aseismic slip activity [8].We performed a set of experimental gypsum dehydrations both at the TOMCAT microtomography beamline at the Swiss Light Source, and in the laboratory. Using a modified version of the Mjolnir triaxial rig [9] that allowed control of pore fluid pressure in the synchrotron microtomography setup enabled us to document how differential stress (∆σ) and pore fluid pressure (Pf) influence the dehydration of Volterra alabaster gypsum to bassanite at a constant confining pressure and temperature in 4D.We derived data on mineral phase transformation and formation of pore networks by applying a deep-learning algorithm in ORS Dragonfly® software, which reduced data processing times, minimized interpretation biases, and allowed analysing larger volumes. The results exhibit an extremely high accuracy compared to standard procedures. The analysis of phase proportions (gypsum, bassanite and porosity) of segmented volumes correlates very well to theoretical predictions indicating a correct segmentation from the algorithms and self-consistency of the generated datasets. Comparing results obtained at various ∆σ and Pf to the light of mechanical data and additional in-house experiments allows us to better interpret their effect on reaction duration, magnitude and textural evolution of the rock. Transient phenomena as well as individual grain transformation and growth are now traceable in a fully automated way.Our data further our understanding of gypsum dehydration: We found that ∆σ greatly influences the assemblage of the bassanite needles, which tend to grow nearly vertical at ∆σ ≅ 0. Increasing ∆σ significantly increases sample compaction. On the contrary, increasing Pf decreases the bulk deformation and slows down the reaction. As pores grow around bassanite needles, the control of the orientation of needles by differential stress can influence the overall pore network and thus introduce anisotropies during transient and final stages of the reaction. Our data confirm that ∆σ and Pf greatly influence transient and final rock texture, which has implications on drainage during nappe emplacements.References: [1] Hacker et al., 2003, /10.1029/2001JB001129; [2] Peacock, 2001, 10.1130/0091-7613(2001)0292.0.CO;2 [3] Llana-Funez et al. 2012, /10.1007/s00410-012-0726-8; [4] Raleigh and Paterson, 1965;/10.1029/JZ070i016p03965 [5] Dobson et al., 2002; /10.1126/science.1075390 [6] Jung et al., 2004; /10.2747/0020-6814.46.12.1089 [7] Hubbert and Rubey, 1959;/10.1130/0016-7606(1959)70[115:ROFPIM]2.0.CO;2 [8] Rutter et al. 2009; /10.1016/j.jsg.2008.09.008 [9] Butler 2020, /10.1107/S160057752001173X.

Isotopic doping is a powerful tool to identify newly formed mineral phases during fluid-mediated mineral transformation reactions. In particular, Raman spectroscopy of isotopically doped minerals can reveal incorporation of the isotopes into the structure of the minerals themselves, rather than enrichment of a fluid within a pore (1). In fluid mediated mineral transformations, dissolution of the reactant mineral enables O isotope exchange between water and dissolved oxyanions (e.g., CO3). Incorporation of the isotopically enriched oxyanions can result in crystals with different isotopic enrichments if the rate of the exchange in the fluid occurs on a similar timescale to the duration of the experiment and the crystals form at different times. This means that the amount of isotopic enrichment can be used as an internal stop clock and demonstrates the relative timings of precipitation in ex-situ analysis (2). In this presentation we will use previous examples to explore how fluid-mediated mineral transformation reactions can be followed using isotopic enrichment traced with Raman spectroscopy, including new data after deformation experiments. Using new data obtained from in-situ analysis of 18O exchange into dissolved carbonate species we will also show the importance of the solution chemistry on exchange kinetics in the fluid. In addition, we will use density functional theory calculations to explore how the mineral structure may influence the isotopic signature obtained from the Raman spectra. (1) King H.E. & Geisler T. (2018) Minerals, 8. 158.(2) King H.E., Mattner D.C., Plümper O., Geisler T., Putnis A., (2014) Crystal Growth & Design, 14, 3910.

Induced subsidence and seismicity caused by the production of hydrocarbons in the Groningen gas field (the Netherlands) is a widely known issue facing this naturally aseismic region (Smith et al., 2019). Extraction reduces pore-fluid pressure leading to accumulation of small elastic and inelastic strains and an increase in effective vertical stress driving compaction of reservoir sandstones.Recent studies (Pijnenburg et al., 2019a, b and Verberne et al., 2021) identify grain-scale deformation of intergranular and grain-coating clays as largely responsible for accommodating (permanent) inelastic deformation at small strains relevant to production (≤1.0%). However, their distribution, microstructure, abundance, and contribution to inelastic deformation remains unconstrained, presenting challenges when evaluating grain-scale deformation mechanisms within a natural system. Traditional methods of mineral identification are costly, labor-intensive, and time-consuming. Digital imaging coupled with machine-learning-driven segmentation is necessary to accelerate the identification of clay microstructures and distributions within reservoir sandstones for later large-scale analysis and geomechanical modeling.We performed digital imaging on thin-sections taken from core recovered from the highly-depleted Zeerijp ZRP-3a well located at the most seismogenic part of the field. The core was kindly made available by the field operator, NAM. Optical digital images were acquired using the Zeiss AxioScan optical light microscope at 10x magnification with a resolution of 0.44µm and compared to backscattered electron (BSE) digital images from the Zeiss EVO 15 Scanning Electron Microscope (SEM) at varying magnifications with resolutions ranging from 0.09µm - 2.24 µm. Digital images were processed in ilastik, an interactive machine-learning-based toolkit for image segmentation that uses a Random Forest classifier to separate clays from a digital image (Berg et al., 2019).Comparisons between segmented optical and BSE digital images indicate that image resolution is the main limiting factor for successful mineral identification and image segmentation, especially for clay minerals. Lower resolution digital images obtained using optical light microscopy may be sufficient to segment larger intergranular/pore-filling clays, but higher resolution BSE images are necessary to segment smaller micron to submicron-sized grain-coating clays. Comparing the same segmented optical image (~11.5% clay) versus BSE image (~16.3% clay) reveals an error of ~30%, illustrating the potential of underestimating the clay content necessary for geomechanical modeling.Our analysis shows that coupled automated electron microscopy with machine-learning-driven image segmentation has the potential to provide statistically relevant and robust information to further constrain the role of clay films on the compaction behavior of reservoir rocks. References:Berg, S. et al., Nat Methods 16, 1226–1232 (2019).(NAM) Nederlandse Aardolie Maatschappij BV (2015).Pijnenburg, R. P. J. et al., Journal of Geophysical Research: Solid Earth, 124 (2019a).Pijnenburg, R. P. J. et al., Journal of Geophysical Research: Solid Earth, 124, 5254–5282. (2019b)Smith, J. D. et al., Journal of Geophysical Research: Solid Earth, 124, 6165–6178. (2019)Verberne, B. A. et al., Geology, 49 (5): 483–487. (2020)

Hydration of upper mantle rocks leads to serpentinization with drastic consequences for the geophysical and geochemical properties of the Earth’s lithosphere. Serpentinization takes place via a dissolution-precipitation process in which the fluid phase plays a key role both in the transport of dissolved constituents and in the supply of reactants. A limiting factor for serpentinization is the likelihood of pore-space clogging due to the large solid-volume increase and potential negative repercussions on reaction-induced fracturing [1]. However, small-angle neutron scattering [2] has shown that porosity remains abundant at the nanometer-scale ensuring that aqueous fluids can penetrate to the reaction front allowing serpentinization to progress. To further determine the nature of serpentinite nanoporosity and explore the consequences of fluids confined to nanometric dimensions, we couple multi-scale correlative electron microscopy to molecular dynamics simulations. In the analytical part, we combined electron backscatter diffraction (EBSD) with focused-ion beam scanning electron microscopy (FIB-SEM) nanotomography and transmission electron microscopy (TEM) to investigate two partially serpentinized peridotites from the mid-Atlantic ridge (ODP Leg 209) and the Røragen ultramafic complex, Norway. We determined the crystallographic orientation of the host olivine grains to constrain any potential orientational relationship between the host and the reaction-induced porosity within serpentine. No apparent correlation was found. Based on the EBSD maps, we excavated 23 FIB-SEM cross-sections across serpentine veins and the serpentine-olivine interface. At a pixel resolution of 3 nm, only three out of 23 cross-sections showed apparent pore space. Subsequent, FIB-SEM nanotomography of these three regions showed that vein porosity (average φFIB-SEM 10 nm. A higher porosity within brucite-bearing domains is also apparent at the SEM-scale, where we observe larger brucite-rich veins with a high density of nanopores. Hence, our microstructural investigations suggest that continued fluid transport to the reaction interface in the potential absence of reaction-induced fracturing could be sustained through a combination of a nanoporous serpentine network, porous brucite-rich veins, and a highly nanoporous brucite-layer at the olivine reaction interface. Overall, our observations that serpentinite porosity is constrained to the nanoscale have first-order implications for fluid transport and behaviour, because critical physicochemical properties, such as the dielectric constant ε, differ significantly in nanoscale-confined fluids when compared to their bulk counterparts. Initial molecular dynamics simulations of aqueous fluids confined in brucite nanochannels indicate that with the reduction of the width of the nanochannel, the perpendicular component of ε drops drastically which likely has a profound impact on minerals solubility hence overall reaction progress.[1] Plümper et al. Geology (2012) 40(12): 1103-1106.[2] Tutolo et al. Geology (2016) 44(2): 103-106.

Fractures derived from earthquakes can create permeable conduits for fluids to flow, enhancing fluid-rock interactions, and potentially altering the strength and rheology of fault systems. In the dry lower crust, numerous field examples show mutually overprinting pseudotachylytes (solidified melts produced during seismic slip) and mylonitized pseudotachylytes (produced during the post- and interseismic viscous creep). The mylonites contain hydrous mineral assemblages, suggesting episodic pulses of fluids infiltration and rheological weakening triggered by the earthquake. Here, our aim is to understand the porosity generating mechanisms during the earthquake cycle and characterize the intermittent evolution of porosity.The Nusfjord East shear zone network (Lofoten, Norway) is an exhumed lower crustal section composed largely of anhydrous anorthosites that contain mutually overprinting pseudotachylytes and mylonitized pseudotachylytes. We present a microstructural analysis focusing on the mechanisms generating, maintaining, and destroying porosity from an exhumed network of lower crustal coeval pseudotachylytes and mylonites using synchrotron X-ray microtomography (SμCT), focused ion beam scanning electron microscopy (FIB-SEM) nanotomography, electron backscatter diffraction (EBSD) analysis, and SEM imaging.In the pristine pseudotachylyte, SμCT data show that porosity is concentrated within the pseudotachylyte vein (0.16 vol% porosity), especially around framboidal garnet clusters and single garnet grains. SEM observations reveal that garnets within the vein often contain an asymmetric rim of barium-enriched K-feldspar. The damage zone of the host anorthosite on the other hand is efficiently healed (0.03 vol% porosity) primarily with the growth of plagioclase neoblasts nucleated from pulverized fragments of the host anorthosite, and secondly with the precipitation of barium-enriched K-feldspar found lining intragranular microfractures. A FIB-SEM transect along one of these microfractures shows a myrmekite microstructure formed during fluid-rock interaction that completely sealed the porosity.In the mylonitized pseudotachylyte, SμCT data show a porosity of 0.03 vol%, mainly concentrated within monomineralic domains of plagioclase, which are interpreted as recrystallized, sheared survivor clasts of wall-rock fragments. EBSD analyses indicate that deformation in these monomineralic domains was accommodated by diffusion creep and grain boundary sliding. Polymineralic domains along the mylonitic foliation, which primarily derive from the overprint of the original pseudotachylyte veins, also deformed by diffusion creep and grain boundary sliding. However, unlike in the monomineralic domains, they lack detectable porosity. We interpret these observations to reflect the efficient precipitation of hydrous phases into the pores during creep cavitation.Dynamic fracturing during earthquakes is the primary mechanism for porosity generation in the lower crust. Our study shows that porosity is further reduced by up to 90% when a pristine pseudotachylyte is viscously re-worked under deformation conditions promoting grain-size sensitive creep and grain boundary sliding. We suggest that such porosity reduction eventually results in shear zone hardening, which may evolve in the development of new pseudotachylytes overprinting the mylonites, as frequently observed in Nusfjord. Thus, earthquake-induced rheological weakening of the lower crust is intermittent, and occurs only as long as the fluid can infiltrate in the shear zone, thereby facilitating diffusive mass transfer.

Understanding earth materials is critical to creating a sustainable, carbon-neutral society. Earth materials control the feasibility of subsurface energy storage, geothermal energy extraction, and are a source of critical elements for future-proof battery technologies. Perturbations to geological systems can also result in hazards, such as human-induced earthquakes. If we want to tackle the current, pressing scientific questions related to sustainable development for a circular economy, there is an urgent need to make multi-scale, multi-dimensional characterisations of earth materials available to a broad spectrum of earth-science disciplines. In addition to the society-relevant topics, the properties of earth materials determine how the Earth workson the most fundamental level.To overcome this challenge, 15 European facilities for electron and X-ray microscopy join forces to establish EXCITE. EXCITE is a Horizon Europe infrastructure project, and enables access to high-end microscopy facilities and to join the knowledge and experience from the different institutions. By doing so, EXCITE will develop community-driven technological imaging advancements that will strengthen and extend the current implementation of leading-edge microscopy for earth-materials research. In particular, the EXCITE strategy is to integrate joint research programmes with networking, training, and trans-national access activities, to enable both academia and industry to answer critical questions in earth-materials science and technology. As such, EXCITE builds a community of highly qualified earth scientists, develops correlative imaging technologies providing access to world-class facilities to particularly new and non-expert users that are often hindered from engaging in problem-solving microscopy of earth-materials.This presentation gives an overview EXCITE, its activities and open calls, and the progress of the first year of the project.

Subduction zones are principal pathways for the cycling of volatiles such as hydrogen and carbonfrom the Earth’s surface to the mantle and back to the atmosphere. This cycling has significant long-term effects on Earth’s climate. However, the processes that lead to volatile release during subduction and total volatile fluxes are poorly understood. In our study, we will quantify and characterize the network architecture of dehydration pathways exhibited as mineralized olivine-bearing metamorphic veins in the exhumed meta-serpentinites from the Erro-Tobbio unit, Italy [1]. Applying network analytical methods and graph theory both macroscopically and microscopically can provide the mode of propagation and describe the controlling factors affecting the evolution of these dehydration networks. Furthermore, multiscale observations can confirm the scalability of the vein network and if quantitative results such as permeability or volatile flux can be extrapolated to larger scales.Along with 2-D network analysis, these vein networks will be analyzed in 3-dimensions using X-ray tomography and sophisticated machine-learning methods, such as generative adversarial networks. The results of both will be compared, which can then assure whether current machine-learning methods can effectively create statistically equivalent copies of these networks. Lastly, the synthesis of 2-D and 3-D multiscale results should provide meaningful parameters for accurate calculations of volatile flux during the dehydration of subducting slabs. [1] Plümper et al. (2017) Nature Geoscience 10(2), 150-156.

It is generally accepted that subduction zones are important sites for element recycling into the Earth’s mantle. This does in particular also include carbon, which is transported in the form of organic carbon and carbonates. While organic carbon is expected to effectively fix carbon in the slab, carbonates are often entitled as an important CO2 source for arc magmatism. The exact composition of the total subducted carbon load, in terms of oxidised and reduced carbon material, changes between different slabs and consequently the total released carbon varies significantly among suduction zones. An important mechanism for carbon release is the dissolution of carbonates in aqueous fluids. Ophicarbonates, containing both serpentine and carbonate minerals, are thus of special interest: The fluid released through serpentine dehydration reactions interacts with carbonates and causes the release of carbon. However, to better constrain the carbon release it is essential to understand the release of fluid in carbonated systems.In this study we present a detailed experimental analysis on the effect of carbonates on the fluid release from serpentinites. We performed multi-anvil experiments on model ophicarbonates. Our starting material was a mixture between natural antigorite and Ca-carbonate and/or graphite. We also conducted thermodynamic calculations on various serpentinite-carbonate systems. Our experimental results show that serpentine dehydrates at temperatures 3 the dehydration of serpentine thus takes place at 50 - 60 km depth. In the absence of CaCO3 the fluid is released at 60 - 70 km depth. In cold subduction zones this shift in dehydration depth is even more extreme: In a carbonated system the serpentine was found to dehydrate at 80 - 110 km depth, in comparison to 110 - 130 km depth in the uncarbonated system. We found that this shift is mainly due to Ca-Mg exchange reactions between the carbonate and silicate fraction. The experimental run products show distinct dehydration mineralogy, forming Ca-silicates and Mg-bearing carbonates. In combination with mass balance calculations we show that the total carbonate-fraction does not decrease over the whole experimental temperature range. In conclusion, serpentinites with a high Ca-carbonate content are expected to dehydrate earlier in the subduction zones, whereas the carbon remains in the slab. The presence of Ca-carbonate thus has the potential to prevent subduction of water into deeper levels of the Earth’s mantle.

It is well established that effective and macroscopic properties of geological materials are controlled by the geometry and physical properties at small scales, i.e., by their microstructures. Recent progress in imaging technology has enabled us to visualize and characterize the microstructures at different length scales and dimensions. As Earth materials are often heterogeneous with a certain degree of randomness, such a characterization must be of statistical nature – and one approach to this end is performed by computing n-point correlation functions known as statistical microstructural descriptors. These microstructural descriptors can, in principle, then be directly employed in upscaling to predict the macroscopic behaviours of the system as a whole. Alternatively, once microstructural descriptors are inferred from one or more samples, they can be used to generate new, statistically-equivalent structures having a larger size and additional dimension – this process is known as reconstruction. While several approaches have been proposed in the past decades, advanced machine-learning based image processing methods have shown to be promising for reconstructing microstructures of chosen representative sizes. Here, we train a deep-convolutional generative adversarial network (GAN) to reconstruct two-dimensional electron microscopy images of two chemically-altered rock samples. We show that employing a Wasserstein-loss with a gradient penalty, instead of common binary cross entropy, results in improved training stability and high-quality reconstructed microstructures. To quantitatively evaluate how reconstruction performs in retrieving patterns with high-order spatial correlations, n-point polytope functions are calculated in both reconstructed and original microstructures, and mean square error (MSE) between them is used as a quality metric. These n-point polytope functions, which are a subset of n-point correlation functions, provide statistical information about symmetric higher-order geometrical patterns in microstructures. Furthermore, we compare our model with a benchmark reconstruction method based on a two-point correlation function and stochastic optimization by simulated annealing (SA). Our findings indicate that although showing the same two-point statistics, two microstructures can be morphologically and structurally different, emphasizing the need for coupling higher-order correlation functions with reconstruction methods. We also show that GANs are naturally able to capture higher-order correlation functions at short and long range scales due to the convolutional layers which can learn to extract complex structural features, leading to realistic image reconstructions. This is of critical importance for future schemes that aim to exceed the limits of current imaging techniques by reconstructing the higher-order geometry in complex heterogeneous systems and couple microstructures to macroscopic phenomena.

The spatial separation of macroscopic rheological behaviors has led to independent conceptual treatments of frictional failure, often referred to as brittle, and viscous deformation. Detailed microstructural investigations of naturally deformed carbonate rocks indicate that both frictional failure and viscous mechanisms might operate during seismic deformation of carbonates. Here, we investigate the deformation mechanisms that were active in two carbonate fault zones in Greece by performing detailed slip-system analyses on data from automated crystal-orientation mapping transmission electron microscopy and electron-backscatter diffraction. We combine the slip-system analyses with interpretations of nanostructures and predictions from deformation mechanism maps for calcite. The nanometric grains at the principal slip surface should deform by diffusion creep but the activation of the (0001)< (Formula presented.) 2 (Formula presented.) 0> slip system is evidence for a contribution of crystal plasticity. A similar crystallographic preferred orientation appears in the cataclastic parts of the fault rocks despite exhibiting a larger grain size and a different fractal dimension, compared to the principal slip surface. The cataclastic region exhibits microstructures consistent with activation of the (0001)< (Formula presented.) 2 (Formula presented.) 0> and {10 (Formula presented.) 4}< (Formula presented.) 021> slip systems. Postdeformational, static recrystallization, and annealing produce an equilibrium microstructure with triple junctions and equant grain size. We propose that repeated introduction of plastic strain and recrystallization reduces the grain size and offers a mechanism to form a cohesive nanogranular material. This formation mechanism leads to a grain-boundary strengthening effect resulting in slip delocalization which is observed over 6 orders of magnitude (μm-m) and is expressed by multiple faults planes, suggesting cyclic repetition of deformation and annealing.

Faulting in seismically active regions commonly involves the deformation of unconsolidated to poorly lithified sediments at shallow to near-surface depths. When compared to classic crustal strength profiles that predict a velocity-strengthening behaviour for the first few km of depth, the propagation of seismic rupture to the surface appears counterintuitive. Rock deformation experiments have shown an inverse relationship between normal stress and displacement needed to the onset of dynamic weakening during seismic slip, meaning that for a seismic rupture to be able to propagate towards the surface, displacements should be large enough to counter the progressive decrease of normal and confining stresses.In this contribution, we document the occurrence of mirror-like faults that formed within 20-30 m-thick, unconsolidated colluvium fan deposits at the hanging wall of the active Vado di Corno Fault Zone (VCFZ) in the Central Apennines, Italy. The deposits lie in direct contact with the master normal-fault surface, are Late Pleistocene to Holocene in age, and consist of angular carbonate clasts with grain size ranging ~0.1-10 mm derived from the dismantling of the adjacent VCFZ footwall. Field observations of cross cutting relationships and marker layer displacements suggest a maximum formation depth of the faults of c. 20-30 m and slip accommodated along single faults on the order of few cm. Faults are organised in three sets: subvertical, N-S and NE-SW trending faults, and WNW-ESE striking faults, synthetic and antithetic to the VCFZ master fault surface (N195/55°). Faults are commonly lineated with a dip-slip to slightly oblique kinematic.Detailed microstructural analysis of the mirror faults shows extreme strain localization on a 2-5 µm thick principal slip zone composed of calcite nanograins ranging 10s-100s nm in size with amorphous material and phyllosilicates occurring along grain boundaries and within intragranular porosity. Locally, aggregates of nanograins coalesce and transition to µm-sized polygonal, larger grains. Calcite nanograins are mostly equant, with straight grain boundaries, 120° dihedral angles, and negligible porosity. These microstructures strongly resemble high temperature recrystallization structures documented along seismic faults exhumed from >5 km of depth, where stresses are significantly larger. In our case, field constraints show that deformation occurred in very confining stress conditions and with limited displacement.Collectively, our observations provide new documentation on the conditions for the formation of mirror faults and new insights into the mechanics of faulting and strain accommodation in the shallowest part of the crust (< 1 km).

The overall rates of multi-component reaction networks are known to be controlled by feedback mechanisms. Feedback mechanisms represent loop systems where the output of the system is conveyed back as input and the system is either accelerated or regulated (positive and negative feedback respectively). In other words, feedback mechanisms control the rate of a reaction network without external influences. Feedback mechanisms are well-studied in a variety of reaction networks (e.g. bio-chemical, atmospheric); however, in fluid-rock interaction systems they are not researched as such. Still, indirect evidence, theoretical considerations and direct observations attest to their existence [e.g. 1, 2, 3]. It remains unknown how mass and energy transport between distinct reaction sites affect the overall reaction rate and outcome through feedback mechanisms. We propose that feedback mechanisms are a missing critical ingredient to understand reaction progress and timescales of fluid-rock interactions. We apply the serpentinization of ultramafic silicates as a relatively simple reaction network to investigate feedback mechanisms during fluid-rock interactions. Recent studies show that theoretical timescale-predictions appear inconsistent with natural observations [e.g. 4, 5]. The ultramafic silicate system is ideal for investigating feedback mechanisms as it is relevant to natural processes, is reactive on timescales that can be explored in the laboratory, and natural peridotite typically consists of less than four phases. Our preliminary observations indicate a feedback between pyroxene dissolution and olivine serpentinization. Olivine serpentinization appears to proceed faster in the presence of pyroxene. Furthermore, the bulk system reaction rate increases with increasing fluid salinity, which is opposite to the salinity effect on the monomineralic olivine system. Dunite (>90% olivine) is rare, which is why it is crucial to explore the more common pyroxene-bearing systems. The salinity effect is important to investigate due to the inevitable increase in fluid salinity from the boiling-induced phase separation and OH-uptake in the formation of serpentine. Here we present preliminary textural and chemical observations, which will subsequently be used for kinetic modelling of feedback.[1] Ortoleva P., Merino, E., Moore, C. & Chadam, J. (1987). American Journal of Science 287, 997-1007.[2] Centrella, S., Austrheim, H., & Putnis, A. (2015). Lithos 236–237, 245–255.[3] Nakatani, T. & Nakamura, M. (2016). Geochemistry, Geophysics, Geosystems 17, 3393-3419.[4] Ingebritsen, S. E. & Manning, C. E. (2010). Geofluids 10, 193-205.[5] Beinlich, A., John, T., Vrijmoed, J.C., Tominaga, M., Magna, T. & Podladchikov, Y.Y. (2020). Nature Geoscience 13, 307–311.

Amorphous materials are frequently observed in natural and experimentally produced fault rocks. Their common occurrence suggests that amorphous materials are of importance to fault zone dynamics. However, little is known about the physico-chemical impact of amorphous materials on fault rheology. Here we present deformation experiments on mafic fault rock, where amorphous material forms due to intense mechanical wear during the experiments. The experiments are run at temperatures from 300 to 600 °C, confining pressures of 0.5 or 1.0 GPa, and at constant displacement rates of (d˙ax) 2 ·10−7, 2 ·10−8 or 2 ·10−9 ms−1, resulting in bulk strain rates (γ˙) of ≈3 ·10−4, 3 ·10−5 and 3 ·10−6 s−1. At these conditions, the mafic rock material undergoes intense brittle deformation and cataclastic flow, but sample strength significantly decreases with increasing temperatures – a feature commonly attributed to viscous deformation processes. Microstructural analyses show that after an initial stage of homogeneous cataclastic flow, strain localizes into narrow (2–10 μm wide) ultra-cataclastic bands that evolve into amorphous shear bands. With the data presented in this research paper, we argue that the temperature sensitivity recorded in the mechanical data is caused by viscous deformation of the amorphous material. We suggest that with the formation of amorphous materials during brittle deformation, fault rheology becomes significantly temperature-sensitive. This has important implications for our understanding of fault strength and weakening due to the presence of amorphous materials. In addition, weak material along faults will lead to stress concentrations that may trigger seismic rupture.

Detailed microstructural investigations of naturally deformed carbonate rocks are of interest for unraveling potential co-seismic deformation mechanisms. The spatial separation of macroscopic rheol...

Deformation and failure mode of carbonate rocks depend on the confining pressure. In this study, the mechanical behaviour of a limestone with an initial porosity of 14.7 % is investigated at constant stress. At confining pressures below 55 MPa, dilatancy associated with micro-fracturing occurs during constant stress steps, ultimately leading to failure, similar to creep in other brittle media. At confining pressures higher than 55 MPa, depending on applied differential stress, inelastic compaction occurs, accommodated by crystal plasticity and characterized by constant ultrasonic wave velocities, or dilatancy resulting from nucleation and propagation of cracks due to local stress concentrations associated with dislocation pile-ups, ultimately causing failure. Strain rates during secondary creep preceding dilative brittle failure are sensitive to stress while rates during compactive creep exhibit an insensitivity to stress indicative of the operation of crystal plasticity, in agreement with elastic wave velocity evolution and microstructural observations.

Molecular dynamics simulations of diffusive properties of stressed water films in quartz and clay grain contactsFloris S.R. Teuling1, Marthe G. Guren2, François Renard2, Martyn R. Drury1, Suzanne J.T. Hangx1, Helen E. King1, Oliver Plümper1, Henrik A. Sveinsson2Utrecht University, Department of Earth Sciences, Princetonlaan 8a, 3584 CB Utrecht, the Netherlands University of Oslo, Departments of Geosciences and Physics, The Njord Centre, box 1048, Blindern, 0316 Oslo, Norway 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.AcknowledgementsThis project received funding from the DeepNL programmePijnenburg, R. P. J., Verberne, B. A., Hangx, S. J. T., & Spiers, C. J. (2019). Intergranular clay films control inelastic deformation in the Groningen gas reservoir: Evidence from split‐cylinder deformation tests. Journal of Geophysical Research: Solid Earth, 124. Cygan, R. T., Liang, J. J., & Kalinichev, A. G. (2004). Molecular models of hydroxide, oxyhydroxide, and clay phases and the development of a general force field. The Journal of Physical Chemistry B, 108(4), 1255-1266.

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.

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.

Metamorphic reactions involving hydration and dehydration frequently occur during orogenic cycles, for example, when ambient pressure and temperature conditions change due to subduction and subsequent exhumation, or when fluids infiltrate metastable mineral assemblages at constant ambient conditions. Such (de)hydration reactions can be associated with significant volume changes, which may cause significant differential stresses in the rock, potentially leading to fracturing. The impact of (de)hydration reactions on the rock’s stress state and on the magnitudes of associated differential stresses is still controversially debated. One reason for the debate is due to the different theoretical models used to quantify and simulate (de)hydration reactions coupled with rock deformation. In many models, the rock deformation is frequently simplified, by either completely ignoring rock deformation or by considering volume deformation only. Additionally, the fluid flow is often simplified, by for example considering constant porosity. Here, we present a method to derive a system of governing equations to describe coupled Hydro-Mechanical-Chemical processes, which is suitable to quantify rock deformation coupled to (de)hydration reactions. Reactions are mainly treated as density changes whereby the density changes are determined by tabulated densities from thermodynamic Gibbs free energy minimizations in pressure, temperature and composition space. The rock deformation is quantified by the continuum mechanics force balance equations, here the Stokes equations. Considered flow laws describe either linear viscous deformation or dislocation and diffusion creep. Equations for reactions and rock deformation are coupled by several equations for the conservation of mass, such as total mass or mass of solid components stored in the solid. The governing system of equations is solved with a pseudo-transient finite difference method. For simplicity, we apply the numerical model here to several Brucite – Periclase (de)hydration reactions and show results of models with different levels of coupling, for example, constant or variable porosity. We also quantify the differential stresses associated with the (de)hydration reactions. Furthermore, we compare the modelled stresses with microstructural observations and stress estimates from high-resolution EBSD measurements in natural rock.

Strong dynamic weakening at seismic slip velocities in experiments on calcite has been attributed to a combination of grain-size reduction and nanoscale diffusion. However, these experiments were performed mostly dry and it is unknown how fluid-rock interactions affect the deformation mechanisms. The resulting physico-chemical interactions are key in deciphering deformation mechanisms and rheological changes during and after (seismic) faulting in the presence of a fluid phase. It is the interaction of the nanoscale of granular fault materials with fluids that may drive changes in rheological behaviour and fault stability. Considering that faults in the upper crust are major fluid pathways, there is a particular need for deformation experiments under wet conditions that focus on the nanoscale interaction between gouge material and pore fluid.In order to track and quantify potential fluid – mineral interaction processes in carbonate faults, we have conducted deformation experiments on calcite gouge with water enriched in 18O (97 at%) as pore fluid. The fault gouge was deformed in a rotary shear apparatus at v = 1 m/s and a normal load of σn = 2 and 4 MPa. Raman spectroscopy and nanoscale secondary ion mass spectrometry (nanoSIMS) were used to analyse isotope distribution in the post-experiment samples. The nanostructure was characterised in electron transparent thin foils, prepared in a focused ion beam – scanning electron microscope (FIB-SEM), using transmission electron microscopy (TEM).Raman analyses confirm the incorporation of 18O into the calcite crystal structure, as well as the presence of amorphous carbon. We identify three new band positions relating to the possible isotopologues of CO32- (reflecting 16O substitution by 18O). In addition, we detected portlandite (Ca(OH)2), pointing to the hydration reaction of lime (CaO) with water. Raman and NanoSIMS maps reveal that 18O is incorporated throughout the deformed volume, implying that calcite breakdown and isotope exchange affected the entire fault gouge.Considering the oxygen self-diffusion rates in calcite (Farver, 1994) we conclude that solid-state 18O – isotope exchange cannot explain the observed incorporation of 18O into the calcite crystals during wet, seismic deformation. The hydration of portlandite and, calcite containing 18O implies the breakdown and decarbonation of the starting calcite and the nucleation of new calcite grains. Our results question the state and nature of calcite gouges during seismic deformation and challenge our knowledge of the rheological properties of wet calcite fault gouges at high strain rates. The observations suggest that the physico-chemical changes are a crucial part of the deformation mechanism and have implications for the development of microphysical models that allow us to quantitatively predict fault rheology. ReferencesJohn R. Farver, Oxygen self-diffusion in calcite: Dependence on temperature and water fugacity, Earth and Planetary Science Letters, Volume 121, Issues 3–4, 1994, Pages 575-587, doi:10.1016/0012-821X(94)90092-2.

We investigated the microstructures and geochemistry of pseudotachylytes found within the ophiolitic peridotites and gabbros from the Moncuni area (Lanzo Ultramafic Massif, Western Alps). The pseudotachylytes preserve dry glass and pristine microlites. The presence of garnet and the static development of eclogite-facies assemblages in local hydrated domains indicate that pseudotachylytes experienced subduction conditions of 600 °C and 2.1 GPa. Samples were investigated using optical and scanning electron microscopic imaging as well as electron backscatter diffraction, Raman and IR spectroscopy, single-crystal X-ray diffraction and microprobe analysis. Based on the analysis of the pseudotachylytes and associated wall-rocks, we suggest that the Moncuni body can be regarded as representative for the rheological behaviour during subduction of seismically active, dry, lithosphere. The data is organised in 8 folders termed Figure X, where X is the figure number in the primary publication. One folder is titled Supplements containing 5 subfolders equivalent to the supplementary material in the primary publication. Detailed information about the files and methods used is given in a readme file.