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X-ray computed tomography has established itself as a crucial tool in the analysis of rock materials, providing the ability to visualise intricate 3D microstructures and capture quantitative information about internal phenomena such as structural damage, mineral reactions, and fluid–rock interactions. The efficacy of this tool, however, depends significantly on the precision of image segmentation, a process that has seen varied results across different methodologies, ranging from simple histogram thresholding to more complex machine learning and deep-learning strategies. The irregularity in these segmentation outcomes raises concerns about the reproducibility of the results, a challenge that we aim to address in this work. In our study, we employ the mass balance of a metamorphic reaction as an internal standard to verify segmentation accuracy and shed light on the advantages of deep-learning approaches, particularly their capacity to efficiently process expansive datasets. Our methodology utilises deep learning to achieve accurate segmentation of time-resolved volumetric images of the gypsum dehydration reaction, a process that traditional segmentation techniques have struggled with due to poor contrast between reactants and products. We utilise a 2D U-net architecture for segmentation and introduce machine-learning-obtained labelled data (specifically, from random forest classification) as an innovative solution to the limitations of training data obtained from imaging. The deep-learning algorithm we developed has demonstrated remarkable resilience, consistently segmenting volume phases across all experiments. Furthermore, our trained neural network exhibits impressively short run times on a standard workstation equipped with a graphic processing unit (GPU). To evaluate the precision of our workflow, we compared the theoretical and measured molar evolution of gypsum to bassanite during dehydration. The errors between the predicted and segmented volumes in all time series experiments fell within the 2 % confidence intervals of the theoretical curves, affirming the accuracy of our methodology. We also compared the results obtained by the proposed method with standard segmentation methods and found a significant improvement in precision and accuracy of segmented volumes. This makes the segmented computed tomography images suited for extracting quantitative data, such as variations in mineral growth rate and pore size during the reaction. In this work, we introduce a distinctive approach by using an internal standard to validate the accuracy of a segmentation model, demonstrating its potential as a robust and reliable method for image segmentation in this field. This ability to measure the volumetric evolution during a reaction with precision paves the way for advanced modelling and verification of the physical properties of rock materials, particularly those involved in tectono-metamorphic processes. Our work underscores the promise of deep-learning approaches in elevating the quality and reproducibility of research in the geosciences. [ABSTRACT FROM AUTHOR]

Third- and fourth-generation synchrotron light sources with high fluxes and beam energies enable the use of innovative X-ray translucent experimental apparatus. These experimental devices access geologically relevant conditions whilst enabling in situ characterization using the spatial and temporal resolutions accessible at imaging beamlines. Here, Heitt Mjölnir is introduced, a heated miniature triaxial rig based on the design of Mjölnir, but covering a wider temperature range and larger sample volume at similar pressure capacities. This device is designed to investigate coupled thermal, chemical, hydraulic and mechanical processes from grain to centimetre scales using cylindrical samples of 10 mm × 20 mm (diameter × length). Heitt Mjölnir can simultaneously reach confining (hydraulic) pressures of 30 MPa and 500 MPa of axial stress with independently controlled sample pore fluid pressure < 30 MPa. This internally heated apparatus operates to temperatures up to 573 K with a minimal vertical thermal gradient in the sample of <0.3 K mm-1. This new apparatus has been deployed in operando studies at the TOMCAT (Swiss Light Source), I12 JEEP (Diamond Light Source) and PSICHE' (Synchrotron SOLEIL) beamlines for 4D X-ray microtomography with scan intervals of a few minutes. Heitt Mjölnir is portable and modular, allowing a wide range of 4D characterizations of low-grade metamorphism and deformational processes. It enables spatially and temporally resolved fluid-rock interaction studies at conditions of crustal reservoirs and is suitable for characterization of material properties in geothermal, carbonation or subsurface gas storage applications. Technical drawings and an operation guide are included in this publication. [ABSTRACT FROM AUTHOR]

X-ray computed tomography has established itself as a crucial tool in the analysis of rock materials, providing the ability to visualise intricate 3D microstructures and capture quantitative information about internal phenomena such as structural damage, mineral reactions, and fluid-rock interactions. The efficacy of this tool, however, depends significantly on the precision of image segmentation, a process that has seen varied results across different methodologies, ranging from simple histogram thresholding to more complex machine learning and deep learning strategies. The irregularity in these segmentation outcomes raises concerns about the reproducibility of the results, a challenge that we aim to address in this work. In our study, we employ the mass balance of a metamorphic reaction as an internal standard to verify segmentation accuracy and shed light on the advantages of deep learning approaches, particularly their capacity to efficiently process expansive datasets. Our methodology utilises deep learning to achieve accurate segmentation of time-resolved volumetric images of the gypsum dehydration reaction, a process that traditional segmentation techniques have struggled with due to poor contrast between reactants and products. We utilise a 2D U-net architecture for segmentation and introduce machine learning-obtained labelled data (specifically, from random forest classification) as an innovative solution to the limitations of training data obtained from imaging. The deep learning algorithm we developed has demonstrated remarkable resilience, consistently segmenting volume phases across all experiments. Furthermore, our trained neural network exhibits impressively short run times on a standard workstation equipped with a Graphic Processing Unit (GPU). To evaluate the precision of our workflow, we compared the theoretical and measured molar evolution of gypsum to bassanite during dehydration. The errors between the predicted and segmented volumes in all time-series experiments fell within the 2 % confidence intervals of the theoretical curves, affirming the accuracy of our methodology. We also compared the results obtained by the proposed method with standard segmentation methods and found a significant improvement in precision and accuracy of segmented volumes. This makes the segmented CT images suited for extracting quantitative data, such as variations in mineral growth rate and pore size during the reaction. In this work, we introduce a distinctive approach by using an internal standard to validate the accuracy of a segmentation model, demonstrating its potential as a robust and reliable method for image segmentation in this field. This ability to measure the volumetric evolution during a reaction with precision paves the way for advanced modelling and verification of the physical properties of rock materials, particularly those involved in tectono-metamorphic processes. Our work underscores the promise of deep learning approaches in elevating the quality and reproducibility of research in the geosciences. [ABSTRACT FROM AUTHOR]

Earthquake‐induced fracturing of the dry and strong lower crust can transiently increase permeability for fluids to flow and trigger metamorphic and rheological transformations. However, little is known about the porosity that facilitates these transformations. We analyzed microstructures that have recorded the mechanisms generating porosity in the lower crust from a pristine pseudotachylyte (solidified earthquake‐derived frictional melt) and a mylonitized pseudotachylyte from Lofoten, Norway to understand the evolution of fluid pathways from the coseismic to the post‐ and interseismic stages of the earthquake cycle. Porosity is dispersed and poorly interconnected within the pseudotachylyte vein (0.14 vol%), with a noticeably increased amount along garnet grain boundaries (0.25–0.41 vol%). This porosity formed due to a net negative volume change at the grain boundary when garnet overgrows the pseudotachylyte matrix. Efficient healing of the damage zone by fluid‐assisted growth of feldspar neoblasts resulted in the preservation of only a few but relatively large interconnected pores along coseismic fractures (0.03 vol% porosity). In contrast, porosity in the mylonitized pseudotachylyte is dramatically reduced (0.02 vol% overall), because of the efficient precipitation of phases (amphibole, biotite and feldspars) into transient pores during grain‐size sensitive creep. Porosity reduction on the order of >85% may be a contributing factor in shear zone hardening, potentially leading to the development of new pseudotachylytes overprinting the mylonites. Our results show that earthquake‐induced rheological weakening of the lower crust is intermittent and occurs when a fluid can infiltrate a transiently permeable shear zone, thereby facilitating diffusive mass transfer and creep. Plain Language Summary: Earthquakes create fractures and increase the porosity in crustal rocks. These fractures can help transport fluids to newly accessible regions in the crust, which in turn may kickstart metamorphic reactions, and potentially alter the rheology. However, very little is known about the mechanisms, the microstructural context, and the morphology of this increased porosity. We analyzed ancient earthquake‐generated frictional melts (pseudotachylytes) and their immediate damage zone in the host rock, as well as plasticly deformed pseudotachylytes, that have since been exhumed from depth and are now exposed at the surface in Lofoten, Norway. We analyzed these rocks to determine the processes that create porosity and how this porosity evolves with increasing plastic deformation. The pseudotachylyte hosts more porosity than the damage zone immediately flanking the vein, in particular there is a high concentration of porosity around garnets. We interpret this porosity to have formed as a result of the metamorphic growth of garnet. Much of the fracture‐related porosity created during the initial earthquake has been efficiently sealed. Porosity is greatly reduced in the sheared pseudotachylytes because of solution‐precipitation processes that operated during ductile deformation. Porosity reduction may reflect fluid consumption, leading to shear zone hardening and possibly new pseudotachylyte formation. Key Points: Fracture porosity is healed in the wall rock, while porosity from metamorphic recrystallization is preserved in the pseudotachylyte veinPorosity in the pseudotachylyte is poorly interconnected and is concentrated around garnets and in survivor clasts from the wall rockPorosity in the mylonite is greatly reduced because of precipitation of phases in transient pores during grain‐size sensitive creep [ABSTRACT FROM AUTHOR]

Phyllosilicates are generally regarded to have a reinforcing effect on chemical compaction by dissolution–precipitation creep (DPC) and thereby influence the evolution of hydraulic rock properties relevant to groundwater resources and geological repositories as well as fossil fuel reservoirs. We conducted oedometric compaction experiments on layered NaCl–biotite samples to test this assumption. In particular, we aim to analyse slow chemical compaction processes in the presence of biotite on the grain scale and determine the effects of chemical and mechanical feedbacks. We used time-resolved (4-D) microtomographic data to capture the dynamic evolution of the porosity in layered NaCl–NaCl/biotite samples over 1619 and 1932 h of compaction. Percolation analysis in combination with advanced digital volume correlation techniques showed that biotite grains influence the dynamic evolution of porosity in the sample by promoting a reduction of porosity in their vicinity. However, the lack of preferential strain localisation around phyllosilicates and a homogeneous distribution of axial shortening across the sample suggests that the porosity reduction is not achieved by pore collapse but by the precipitation of NaCl sourced from outside the NaCl–biotite layer. Our observations invite a renewed discussion of the effect of phyllosilicates on DPC, with a particular emphasis on the length scales of the processes involved. We propose that, in our experiments, the diffusive transport processes invoked in classical theoretical models of DPC are complemented by chemo-mechanical feedbacks that arise on longer length scales. These feedbacks drive NaCl diffusion from the marginal pure NaCl layers into the central NaCl–biotite mixture over distances of several hundred micrometres and several grain diameters. Such a mechanism was first postulated by. [ABSTRACT FROM AUTHOR]

Phyllosilicates are generally regarded to have a reinforcing effect on chemical compaction by dissolution-precipitation creep (DPC) and thereby influence the evolution of hydraulic rock properties relevant to groundwater resources, geological repositories as well as fossil fuel reservoirs. We conducted oedometric compaction experiments on layered NaCl-biotite samples to test this assumption. In particular, we aim to analyse slow chemical compaction processes in the presence of biotite on the grain scale and determine the effects of chemical and mechanical feedbacks. We used time-resolved (4D) microtomographic data to capture the dynamic evolution of the transport properties in layered NaCl-NaCl/biotite samples over 1619 and 1932 hours of compaction. Percolation analysis in combination with advanced digital volume correlation techniques showed that biotite grains influence the dynamic evolution of porosity in the sample by promoting a reduction of porosity in their vicinity. However, the lack of preferential strain localisation around phyllosilicates and a homogeneous distribution of axial shortening across the sample suggests that the porosity reduction is not achieved by pore collapse but by the precipitation of NaCl sourced from outside the NaCl/biotite layer. Our observations invite a renewed discussion of the effect of phyllosilicates on DPC, with a particular emphasis on the length scales of the processes involved. We propose that, in our experiments, the diffusive transport processes invoked in classical theoretical models of DPC are superseded by chemo-mechanical feedbacks that arise on longer length scales. These feedbacks drive NaCl diffusion from the marginal pure NaCl layers into the central NaCl-biotite mixture over distances of several hundred µm and several grain diameters. Such a mechanism was first postulated by Merino et al. (1983). [ABSTRACT FROM AUTHOR]

An X‐ray transparent experimental triaxial rock deformation apparatus, here named 'Mjölnir', enables investigations of brittle‐style rock deformation and failure, as well as coupled thermal, chemical and mechanical processes relevant to a range of Earth subsurface environments. Designed to operate with cylindrical samples up to 3.2 mm outside‐diameter and up to 10 mm length, Mjölnir can attain up to 50 MPa confining pressure and in excess of 600 MPa axial load. The addition of heaters extends the experimental range to temperatures up to 140°C. Deployment of Mjolnir on synchrotron beamlines indicates that full 3D datasets may be acquired in a few seconds to a few minutes, meaning full 4D investigations of deformation processes can be undertaken. Mjölnir is constructed from readily available materials and components and complete technical drawings are included in the supporting information. [ABSTRACT FROM AUTHOR]

In experiments designed to understand deep shear zones, we show that periodic porous sheets emerge spontaneously during viscous creep, forming a hydro-mechanical anisotropy that influences mass transfer. These findings challenge the current paradigm of viscosity in solid rocks. In particular, they showcase how shear zones may actively focus mass transport and highlight the possibility that viscous rocks could locally transition from flow to fracture. Our work demonstrates that viscosity in solids is not directly comparable to viscosity in fluids and this is consequential for a range of important solid Earth topics, like slow earthquakes, the flow of glacial ice and the tectonics of exoplanets. [ABSTRACT FROM AUTHOR]

Catastrophic failure of brittle rocks is important in managing risk associated with system‐sized material failure. Such failure is caused by nucleation, growth, and coalescence of microcracks that spontaneously self‐organize along localized damage zones under compressive stress. Here we present X‐ray microtomography observations that elucidate the in situ micron‐scale processes, obtained from novel tri‐axial compression experiments conducted in a synchrotron. We examine the effect of microstructural heterogeneity in the starting material (Ailsa Craig microgranite; known for being virtually crack‐free) on crack network evolution and localization. To control for heterogeneity, we introduced a random nanoscale crack network into one sample by thermal stressing, leaving a second sample as‐received. By assessing the time‐dependent statistics of crack size and spatial distribution, we test the hypothesis that the degree of starting heterogeneity influences the order and predictability of the phase transition between intact and failed states. We show that this is indeed the case at the system‐scale. The initially more heterogeneous (heat‐treated) sample showed clear evidence for a second‐order transition: inverse power law acceleration in correlation length with a well‐defined singularity near failure and distinct changes in the scaling exponents. The more homogeneous (untreated) sample showed evidence for a first‐order transition: exponential increase in correlation length associated with distributed damage and unstable crack nucleation ahead of abrupt failure. In both cases, anisotropy in the initial porosity dictated the fault orientation, and system‐sized failure occurred when the correlation length approached the grain size. These results have significant implications for the predictability of catastrophic failure in different materials. Plain Language Summary: When rocks deform, tiny cracks appear, increasing in size and number until the rock breaks completely, often along a narrow plane of weakness where cracks have spontaneously aligned. Sometimes, when the microstructure is complicated, cracking accelerates quickly in a predictable way, giving a good indication of when the rock will break. In other cases, when the microstructure is more uniform, cracking accelerates more slowly and the rock breaks suddenly and early. To understand why failure is predictable in some cases but not others—a major problem in managing risk from material failure (e.g., earthquakes)—we used X‐ray imaging to see how cracks form and interact with each other inside deforming rocks. We found that predictable behavior only arose when cracks aligned themselves asymmetrically. The orientation of this damage zone was governed by the rock's pre‐existing microstructure. We also found distinct changes in crack size and spatial arrangement during alignment, indicating that the rock was approaching failure. However, when cracks did not align asymmetrically, similar changes were not observed and failure was not predictable. Our results are important because they help explain why reliable indicators of catastrophic failure are not always observed and why forecasting may only work in certain cases. Key Points: Micron‐scale crack network evolution and localization are imaged in situ in deforming granite samples with controlled initial heterogeneityHeterogeneity affects order and predictability of the phase transition to failure, which occurs as correlation length approaches grain sizeCorrelation dimension of the crack population is a key indicator of localization, and anisotropy in starting porosity dictates fault strike [ABSTRACT FROM AUTHOR]

Pressure‐solution creep is an important fluid‐mediated deformation mechanism, causing chemo‐mechanical transformations and porosity and permeability changes in rocks. The presence of phyllosilicates, in particular, has previously been hypothesized to further reduce porosity and pore connectivity. Nevertheless, a full characterization of the spatiotemporal evolution of permeability during this process has yet to be reported. A pure NaCl aggregate and a mixture of NaCl and biotite were deformed through pressure‐solution creep while monitoring their microstructural evolution through computed X‐ray microtomography. The evolution of permeability and fluid velocity of the samples were computed by using the pore geometries from the X‐ray microtomography as input for the Lattice‐Boltzmann modeling. The results indicate that, as deformation proceeds, porosity and permeability decrease in both samples. In the salt‐biotite sample pressure solution creep causes the formation of a compaction band perpendicular to the direction of loading, forming a barrier for permeability. Along the other two directions, pore connectivity and permeability are retained in the marginal salt layers, making the sample strongly anisotropic. The presence of biotite controls the way pore coordination number evolves and hence the connectivity of the pathways. Biotite flakes create an enhanced porosity decrease leading to compaction and reduction of pore connectivity. This reduction in porosity affects local stresses and local contact areas, reducing over time the driving force. According to a texture‐porosity process, the reduction in porosity causes salt ions to dissolve in the marginal salt and precipitate within the biotite‐bearing layer, where the bulk volume of salt grains increases over time. Key Points: The presence of biotite prevents permeability along the loading direction and does not affect the permeability of the nearby layersIn the absence of biotite, permeability is retained in pore connectivityPressure‐solution evolves accordingly to a texture‐porosity coupling mechanism [ABSTRACT FROM AUTHOR]

Abstract Pressure-solution creep is one of the most common crustal deformation mechanisms, inducing changes in the porosity and permeability of rocks. For a variety of rock types undergoing pressure solution, it has been shown that the presence of phyllosilicates may significantly enhance the rate of the pressure-solution process. In this experimental investigation, we present 4-dimensional (three dimensions + time) X-ray microtomographic data that contrast deformation by pressure-solution of a pure NaCl aggregate with that of a mixture of NaCl and biotite. The results show that for mixed samples (NaCl+biotite), phyllosilicates induce a marked reduction in porosity and pore connectivity and contribute to an increase in the local strain rates by an order of magnitude over pure NaCl samples. At the same time, phyllosilicates do not induce strain localization in the sample. We discuss various possible explanations for these observations including a possible positive feedback between the porosity distribution and pressure solution. Our study yields novel insights into the local effects of phyllosilicates during pressure-solution creep and provides full 4-dimensional imaging and characterization of the coupled evolution of porosity and pore connectivity over previously unprecedented experimental time scales. Highlights • Local porosity around biotite grains is reduced by ∼100% during pressure-solution. • Biotite increases the local strain rates by an order of magnitude. • Pressure-solution is enhanced due to geochemical self-organization. [ABSTRACT FROM AUTHOR]

The olivine carbonation reaction, in which carbon dioxide is chemically incorporated to form carbonate, is central to the emerging carbon sequestration method using ultramafic rocks. The rate of this retrograde metamorphic reaction is controlled, in part, by the available reactive surface area: as the solid volume increases during carbonation, the feasibility of this method ultimately depends on the maintenance of porosity and the creation of new reactive surfaces. We conducted in situ dynamic X-ray microtomography and nanotomography experiments to image and quantify the porosity generation during olivine carbonation. We designed a sample setup that included a thick-walled cup (made of porous olivine aggregates with a mean grain size of either ~5 or ~80 μm) filled with loose olivine sands with grain sizes of 100-500 μm. The whole sample assembly was reacted with a NaHCO3 aqueous solution at 200°C, under a constant confining pressure of 13MPa and a pore pressure of 10MPa. Using synchrotron-based X-ray microtomography, the three-dimensional (3-D) pore structure evolution of the carbonating olivine cup was documented until the olivine aggregates became disintegrated. The dynamic microtomography data show a volume reduction in olivine at the beginning of the reaction, indicating a vigorous dissolution process consistent with the disequilibrium reaction kinetics. In the olivine cup with a grain size of ~80 μm (coarsegrained cup), dissolution planes developed within 30 h, before any precipitation was observed. In the experiment with the olivine cup of ~5 μm mean grain size (fine-grained cup), idiomorphic magnesite crystals were observed on the surface of the olivine sands. The magnesite shows a near-constant growth throughout the experiment, suggesting that the reaction is self-sustained. Large fractures were generated as the reaction proceeded and eventually disintegrated the aggregate after 140 h. Detailed analysis show that these are expansion cracks caused by the volume mismatch in the cup walls, between the expanding interior and the near-surface which keeps a nearly constant volume. Nanotomography images of the reacted olivine cup reveal pervasive etch pits and wormholes in the olivine grains. We interpret this perforation of the solids to provide continuous fluid access, which is likely key to the complete carbonation observed in nature. Reactions proceeding through the formation of nano- to micronscale dissolution channels provide a viable microscale mechanism in carbon sequestration practices. For the natural peridotite carbonation, a coupled mechanism of dissolution and reaction-induced fracturing should account for the observed self-sustainability of the reaction. [ABSTRACT FROM AUTHOR]

The dispersal of monomineralic quartz domains in a quartzofeldspathic ultramylonite is interpreted to be the result of the emergence of syn-kinematic pores, called creep cavities. The cavities can be considered the product of two distinct mechanisms that formed hierarchically: Zener-Stroh cracking and viscous grain boundary sliding. In initially thick and coherent quartz ribbons deforming by grain size-insensitive creep, cavities were generated by the Zener-Stroh mechanism on grain-boundaries aligned with the YZ plane of finite strain. The opening of creep cavities promoted the ingress of fluids to sites of low stress. The local addition of a fluid lowered the adhesion and cohesion of grain-boundaries and promoted viscous grain boundary sliding. With the increased contribution of viscous grain boundary sliding, a second population of cavities formed to accommodate strain incompatibilities. Ultimately, the emergence of creep cavities is interpreted to be responsible for the transition of quartz domains from a grain size-insensitive, to a grain size-sensitive rheology. [ABSTRACT FROM AUTHOR]

Background: Fossil ticks are extremely rare and Ixodes succineus Weidner, 1964 from Eocene (ca. 44-49 Ma) Baltic amber is one of the oldest examples of a living hard tick genus (Ixodida: Ixodidae). Previous work suggested it was most closely related to the modern and widespread European sheep tick Ixodes ricinus (Linneaus, 1758). Results: Restudy using phase contrast synchrotron x-ray tomography yielded images of exceptional quality. These confirm the fossil's referral to Ixodes Latreille, 1795, but the characters resolved here suggest instead affinities with the Asian subgenus Partipalpiger Hoogstraal et al., 1973 and its single living (and medically significant) species Ixodes ovatus Neumann, 1899. We redescribe the amber fossil here as Ixodes (Partipalpiger) succineus. Conclusions: Our data suggest that Ixodes ricinus is unlikely to be directly derived from Weidner's amber species, but instead reveals that the Partipalpiger lineage was originally more widely distributed across the northern hemisphere. The closeness of Ixodes (P.) succineus to a living vector of a wide range of pathogens offers the potential to correlate its spatial and temporal position (northern Europe, nearly 50 million years ago) with the estimated origination dates of various tick-borne diseases. [ABSTRACT FROM AUTHOR]

Shear zones channelize fluid flow in Earth's crust. However, little is known about deep crustal fluid migration and how fluids are channelized and distributed in a deforming lower crustal shear zone. This study investigates the deformation mechanisms, fluid-rock interaction, and development of porosity in a monzonite ultramylonite from Lofoten, northern Norway. The rock was deformed and transformed into an ultramylonite under lower crustal conditions (temperature = 700-730 °C, pressure = 0.65-0.8 GPa). The ultramylonite consists of feldspathic layers and domains of amphibole + quartz + calcite, which result from hydration reactions of magmatic clinopyroxene. The average grain size in both domains is <25 mm. Microstructural observations and electron backscatter diffraction analysis are consistent with diffusion creep as the dominant deformation mechanism in both domains. Festoons of isolated quartz grains define C2-type bands in feldspathic layers. These quartz grains do not show a crystallographic preferred orientation. The alignment of quartz grains is parallel to the preferred elongation of pores in the ultramylonites, as evidenced from synchrotron X-ray microtomography. Such C2-type bands are interpreted as creep cavitation bands resulting from diffusion creep deformation associated with grain boundary sliding. Mass-balance calculation indicates a 2% volume increase during the protolith-ultramylonite transformation, which is consistent with synkinematic formation of creep cavities producing dilatancy. Thus, this study presents evidence that creep cavitation bands may control deep crustal porosity and fluid flow. Nucleation of new phases in creep cavitation bands inhibits grain growth and enhances the activity of grain size-sensitive creep, thereby stabilizing strain localization in the polymineralic ultramylonites. [ABSTRACT FROM AUTHOR]

A new modular X-ray-transparent experimental cell enables tomographic investigations of fluid rock interaction under natural reservoir conditions (confining pressure up to 20 MPa, pore fluid pressure up to 15 MPa, temperature ranging from 296 to 473 K). The portable cell can be used at synchrotron radiation sources that deliver a minimum X-ray flux density of 109 photons mm-2 s-1 in the energy range 30-100 keV to acquire tomographic datasets in less than 60 s. It has been successfully used in three experiments at the bending-magnet beamline 2BM at the Advanced Photon Source. The cell can be easily machined and assembled from off-the-shelf components at relatively low costs, and its modular design allows it to be adapted to a wide range of experiments and lower-energy X-ray sources. [ABSTRACT FROM AUTHOR]

The interplay between pore fluid pressure, mineral dehydration and deformation has beensuggested to be of great importance for crustal fault zones in a wide range of settings.Dehydrating hydrous minerals represent one source of metamorphic fluids, and, given theusually positive total volume change caused by the released fluid, the build-up of high porefluid pressures can be expected. During prograde metamorphism, reaction progress, porefluid pressure build-up and rock deformation are hypothesized to be coupled viachemical-hydraulical-mechanical feedbacks. Details of the mechanisms of these feedbacksare undocumented though, which leaves key concepts in tectonics, e.g., the hypothesizedimportance of high pore-fluid pressure on thrusting, insufficiently supported by observationaldata.We investigate chemical-hydraulic-mechanical feedbacks during dehydration of a Volterraalabaster sample at non-hydrostatic stress conditions. A mm-sized monophase,polycrystalline sample cylinder was subjected to 15 MPa confining pressure and constantdifferential stress of 23 MPa. The sample was then heated to 115 ∘C, destabilizing gypsumand leading to the gypsum dehydration reaction CaSO4 ⋅ 2H2O (gypsum) = CaSO4 ⋅0.5H2O (bassanite) + 1.5H2O. Using an X-ray transparent deformation rig, wedocumented the reaction in 240 time-resolved 3D micro-CT scans in order to study1) the influence of an anisotropic stress state on the reaction and its advance inthe sample and, 2) the rheological effects of newly formed bassanite on sampledeformation.Within the limits of the spatial resolution (2μm), the sample shows no evidence of damage,nor evidence of reaction for the first ∼4 hours. Then, weakly localized mode Imicro-fractures start to form parallel to the applied differential load. Roughly simultaneouslywith the formation of the first micro-fractures, the first bassanite grains start to form. At 4h15min, the sample failed by localized shear failure.Bassanite grains align with the largest fractures, though not all fractures trigger localdehydration. After failure, dehydration accelerated significantly in both, the fault and the hostrock. However, the fault rock exhibits higher dehydration rates and markedly smallerbassanite grain- and pore sizes. The main fault continues to creep for 9 minutes, before itarrests and heals. The un-faulted gypsum volume took ∼85 min to dehydrate anddehydration decelerates over that time. Bassanite grains in these domains appearrandomly oriented. No further strain localization can be observed in the reacteddomains.Our data show grain-scale evidence for how damage localization in an anisotropicallystressed sample, drainage and reaction advance are initially coupled. They support a modelwhere an initially subcritically stressed sample destabilizes once the reaction initiates, andstrain localization affects sample drainage. Ultimately, this conclusion is limited by thespatial resolution of our data. However, our data also point to more complexity in the system.At this stage, it remains unclear what controls dehydration of the unfaulted domains aftersample failure. It is further unclear how sample rheology evolves on the grain scale asalabaster turns into pores and comparatively small bassanite grains. Our current analyticalefforts focus on these aspects. [ABSTRACT FROM AUTHOR]

Our current means to quantify fluid-rock interaction and rock deformation in space and time are limited: Field-centred studies are constrained to the interpretation of fossilized, exhumed and retrograded micro fabrics. Classical laboratory experimental methods can, with few exceptions, only monitor the bulk response of a rock, and interpret samples post-mortem. Both approaches struggle to discern critical developments on the micro-scale. Consequently models to explain fluid-rock interaction are based on an incomplete understanding of the underlying grain-scale processes. The advent of fast x-ray and neutron imaging has opened up new possibilities to study fluid-rock interaction on the grain scale where suitable x-ray/neutron transparent vessels allow reproduction of geological conditions in situ. This contribution gives an overview of a range of devices that we have designed, built and used over the past years. Our goal is to inspire new experiments and motivate imitation to facilitate a wider uptake in experimental geosciences. Our design philosophy aims for modularity, portability and low-cost, and as a consequence our experimental environments share several advantages:1) The advantage of modular designs is that the devices can be easily adopted to specific experiments, and the beam path length of the attenuating pressure vessel can be minimised subject to P, T and load requirements. This allows optimizing of data quality.2) Many x-ray and neutron imaging beam lines are currently specializing on meeting specific experimental requirements. Taking advantage of this development requires the equipment to be transported to the most suitable x-ray and neutron sources for any experiment. As a consequence, each of our cells packs into a single item of airline cabin luggage, and the ancillary laboratory equipment can easily be transported by courier.3) To minimize the manufacturing costs of our cells, we try to use standard and readily available materials and components, in particular with regard to actuators and fluid couplings. All components can be machined in any standard mechanical workshop. Our x-ray/neutron transparent experimental environments have been used in studies of pressure solution creep, dehydration, carbonation, failure in granite, multiphase fluid flow, and more. Our cells have demonstrated their potential to transform our understanding of fluid-rock interaction: Time-resolved (4D) micro-tomographic data allow quantifying the evolution of parameters that describe fluid-rock interaction on the grain scale. These include the proportion of participating minerals and porosity, their spatial distribution, percolation, the size of individual mineral grains and pores, their geometry and orientation and relation to each other. Advanced volume correlation codes further allow the quantifications of local strains and strain rates. Our current efforts aim to significantly increase the temperatures and confining pressures. In the near future, we will be able to conduct confined fluid-rock interaction experiments with mm-sized samples at 300°C. Subject to funding, we will build a prototype of a new triaxial deformation rig that can reach 450°C, 100 MPa Pc and control pore fluid pressure. We are pairing these capabilities with advanced data processing and analyses tools including data mining and machine learning algorithms and numerical modelling to test established theories on 4DμCT data. [ABSTRACT FROM AUTHOR]

Structural Geology courses often follow a rather linear path, where key concepts are taught sequentially. It is thus not always clear to students why they are expected to learn sometimes rather complex content. Here, we present a course concept that reverses the theory-first/application-second approach. Our "Structural Analysis of Rocks and Regions" course confronts 3rd-year students with lectures on deformed regions around the globe and invites them to learn key concepts of structural geology through the exploration of multi-scale data sets. The regions are chosen so that a) in sum, they represent all important deformation processes and styles, b) the students will/may visit them, c) excellent (teaching) material is available to allow for a true multi-scale and integrated assessment and d) the lecturers know them well. In that way students will experience key quantities, concepts and descriptors of rock deformation through a large variety of structural and geospatial datasets. The advantage of this approach is that, at all times, students know why they read a particular chapter in a textbook and immediately see the application of their newly-gained knowledge. This approach requires active student participation. In addition to 4 contact hours/week (two lecture hours/week, a 1-hour practical and a weekly tutorial), students are expected to spend in excess of 10 hours/week with independent studying. To allow students to prepare themselves for lectures, the presentations are available on an online learning platform well in advance and come with recommendations as to which chapters in textbooks and which papers they should read before attending the lectures. Because they revisit structural concepts throughout the semester, coming from different angles each time, their learning is highly networked and non-linear. The tutorials, online discussion forums and individual meetings with lecturing staff give them opportunities to ask questions on theoretical concepts and engage in discussions. In the practicals, they learn how to work with maps and structural data sets to gain the confidence they need to process such data independently. We reinforce this aspect through the course assessment, 50% of which is through an independently produced oral presentation. For this presentation students explore pre-formulated research questions on one of the field areas. Students are expected to independently process and organize tectonic and structural information to address these questions and present their findings in a scientific way at the end of the course. Course feedback we have received in the past years indicates that students are intellectually challenged by the approach. The lack of linearity seems to leave them unclear about the learning aims. Positive comments highlight the benefits of self study, the advantages of learning to integrate different kinds of data sets, and the applicability of the content. From the lecturers' perspective, we learned that students rarely spend the expected hours on independent studying during the semester. As a knock-on effect, they are not in a position to ask reflected questions until just before the exam. We have responded to this by using the tutorials to go through fundamental structural geological and tectonic concepts. [ABSTRACT FROM AUTHOR]

Chemical compaction is a relevant factor influencing groundwater resources, geological repositories for waste and CO2 sequestration as well as oil and gas reservoirs. In this context sheet silicates are hypothesized to reinforce chemical compaction by dissolution-precipitation creep (DPC) and thereby influence the hydraulic properties as well as the direction of fluid flow. To test these ideas we designed uniaxial compaction experiments in x-ray transparent oedometers, using samples where a layer of a salt/biotite mixture is sandwiched in between two marginal salt layers.Here we present time-resolved (4D) microtomographic data collected with our in-house microCT scanner over 1600 hours of compaction. These data allow us to analyse slow chemical compaction processes on the grain scale, as well as explore the effects of chemical and mechanical feedbacks on the evolving material. The samples' dynamically evolving transport properties can be characterised via pore segmentation and a percolation analysis. Using advanced quantitative volume correlation techniques we determine a locally resolved displacement field, calculate strain tensors on the grain scale and track separated biotite grains during progressive shortening. Our analyses show that biotite grains influence the dynamic evolution of porosity in the sample, clearly causing a reduction of porosity in their vicinity. However, they do not cause strain localization and axial shortening is homogeneously distributed within our sample. This means that porosity reduction is not achieved by pore collapse but by precipitation of salt that has been transported into the biotite layer.Our observations invite a renewed discussion of of the effect of phyllosilicates on DPC, with a particular emphasis on the length scales of the processes involved. We claim that, in our experiments, the diffusive transport processes invoked in classical theoretical models of DPC are superseded by chemo-mechanical feedbacks that arise on longer length scales. These feedbacks drive salt diffusion from the marginal pure salt layers into the central salt-biotite mixture over distances of several hundred μm and several grain diameters. Such a mechanism was postulated by Merino et al. (1983, Contrib Mineral Petrol 82, 360-370). [ABSTRACT FROM AUTHOR]

Detailing the relationship between stress and reactions in metamorphic rocks has been controversial, and much of the debate has centered on theory. Here, we add to this discussion and make a major advance by showing in time-resolved synchrotron microtomography experiments that a reacting and deforming sample experiencing an elastic differential stress produces a fabric orthogonal to the largest principal stress. This fabric forms very early in the reaction and can be shown to be unrelated to strain. The consequences of this are significant because a non-hydrostatic stress state is a very common geological occurrence. Our data provide the basis for new interpretations of the classical, and enigmatic, serpentine fabrics of Val Malenco, Italy, and Cerro del Almirez, Spain, where we relate the reported fabrics to transient, and cyclical, differential stresses from magma intrusion and the earthquake cycle. [ABSTRACT FROM AUTHOR]

Manganese (Mn) dendrites are a common type of mineral dendrite that typically forms two-dimensional structures on rock surfaces. Three-dimensional (3-D) Mn dendrites in rocks have rarely been reported, and so their growth implications have largely escaped attention. Here, we combined high-resolution X-ray and electron-based data with numerical modeling to give the first detailed description of natural 3-D Mn dendrites (in clinoptilolite tuffs) and elucidate their growth dynamics. Our data show that 3-D dendrite growth occurred by accretion of Mn-oxide nanoparticles formed when Mn-bearing fluids mixed with oxygenated pore water. The geometry of the resulting structures is sensitive to ion concentrations, the volume of infiltrating fluid, and the number of fluid pulses; thus, 3-D dendrites record the hydrogeochemical rock history. [ABSTRACT FROM AUTHOR]

Plaster, made of calcium sulfate hemihydrate (CaSO 4 •½H 2 O) and/or γ -anhydrite (CaSO 4) is obtained by dehydration of gypsum (CaSO 4 ·2H 2 O) at 100–200 °C. When mixed with water, it dissolves while new gypsum crystals precipitate. Plaster's microstructures affect its reactivity and setting properties, but are poorly understood due to confusion between crystals and polycrystalline structures in the literature. Through a multi-scale approach combining X-ray microtomography, optical and scanning electron microscopy, we revealed size, morphology and orientation of pores and crystals formed by dehydrating single crystals at T = 105–130 °C and p H2O = 0–40 hPa. The expulsion of water from the (010) interlayers results in a three-level fragmentation: cracks form parallel to (010), then the solid is divided into [101] polycrystalline needles (5–10 x 30–300 μm) formed of micron-sized crystals arranged in a fishbone pattern. The same behavior is observed whatever T and p H2O , however the crystallites size increases with p H2O. Gypsum dehydration-induced microstructure: the fishbone pattern is the smallest feature in a multi-scale system of cracks induced by strains in the crystal structure. [Display omitted] • First study of gypsum single crystal dehydration using X-ray absorption tomography. • Evidence and mechanism of a multi-scale crack pattern due to structure change. • How stress-induced shearing generates the "fishbone" patterns. [ABSTRACT FROM AUTHOR]

The permeability of the upper mantle controls melt segregation beneath spreading centers. Reconciling contradictory geochemical and geophysical observations at ocean ridges requires a better understanding of transport properties in partially molten rocks. Using x-ray synchrotron microtomography, we obtained three-dimensional data on melt distribution for mantle peridotite with various melt fractions. At melt fractions as low as 0.02, triple junctions along grain edges dominated the melt network; there was no evidence of an abrupt change in the fundamental character of melt extraction as melt fraction increased to 0.2. The porosity of the partially molten region beneath ocean ridges is therefore controlled by a balance between viscous compaction and melting rate, not by a change in melt topology. [ABSTRACT FROM AUTHOR]