PICO
Deformation and metamorphism are fundamental processes that act synchronously throughout the Earth; however, their interaction remains unclear. Theoretical models predict that an applied tectonic stress has both a dramatic effect or no effect at all. While the small set of deformation experiments that document the interaction between reaction and deformation are either hard to compare or cannot provide the necessary time resolution to test the various theories. These disagreements of predictions and the gap in data invites new time-resolved experiments to be run that can probe details of model predictions and connect existing datasets from different materials deforming at a range of metamorphic conditions. To this end, we use state-of-the-art time-resolved synchrotron-based x-ray microtomography (4DSµCT) deformation experiments to map out the effect of a differential stress on the kinetics of the dehydration of polycrystalline gypsum samples. Our experiments are highly resolved in space (µm) and time (s), which allows us to track and contrast the emergence of the first small crystals (~100 µm3) and their growth through time in hydrostatic and differentially stressed conditions. We find that the kinetics of a metamorphic reaction are profoundly affected by the addition of deformational energy. Differentially stressed samples transform up to ~90% sooner than in the hydrostatic case, and reaction rates increase by a factor of ~5 with increasing differential stress. Importantly, our findings can be expanded to other published data for reactions occurring in the lower crust and the mantle to show that it is changes in the elastic strain energy that drive accelerated metamorphic kinetics. We find that, when we compare kinetic data from these different reactions and normalise the differential stress to each material’s yield strength, a trend emerges that shows stresses larger than the yield do not contribute to accelerating a reaction. Our results showcase the material-independent effect of a differential stress on metamorphic reactions and support theoretical models which place emphasis on the role of changes in stored energy. Current geodynamic models largely ignore the role of stored energy because it is assumed that it is not relevant at long time scales, our results show that its effect is important and should be accounted for when coupling deformation and metamorphism.
How to cite: Gilgannon, J., Vass Payne, E., Butler, I., Freitas, D., and Fusseis, F.: The material-independent effect of a differential stress on metamorphic kinetics, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6735, https://doi.org/10.5194/egusphere-egu26-6735, 2026.
Prograde metamorphic reactions that reduce solid volume are common in subduction zones and orogenic settings [1]. These reductions are often linked to irreversible deformation such as viscous compaction and deep mantle earthquakes [2]. Viscous compaction involves permanent closure of reaction-generated porosity and fluid release, making porosity transient property of metamorphic reactions [3]. The pore closure observation is often linked to the intuitive loss of the elastic strength of the rock leading to permanent strains [4, 5], but these assumptions are very rarely demonstrated experimentally. In most cases, the nature of field relationships and the design of experiments do not allow for such an assessment to be made. Time-resolved in situ experiments enable the observation of a sample volume undergoing metamorphic transformation to check such assumptions at every stage of the reaction (loading, heating, cooling and unloading).
In this contribution, we provide preliminary visual and quantitative strain mapping during the metamorphic reaction cycle of a rock sample at various stress states and reaction extent.
Using Mjolnir, an X-ray transparent miniature triaxial deformation rig, we performed a series of gypsum (Ca2SO4·2H2O) dehydration experiments into bassanite (Ca2SO4·1/2H2O) at constant confining pressure of 20 MPa, pore fluid pressure (5 MPa) and subjected to similar temperature paths (up to 125ºC). We used a series of differential stresses (radial, hydrostatic and axial principal stresses) to explore how the rock volume responds mechanically while also displaying different reaction fabrics (see [6]). This dehydration reaction produces a solid volume reduction of ~30% [4] enabling the investigation of the evolution of elasticity by unloading fully and partially transformed samples.
Using synchrotron microtomography at the I13-2 beamline of the Diamond Light Source (MG34156), we performed high resolution imaging (1.625 microns/ voxel edge) during gradual unloading to observe and quantify the elastic behaviour both using the mechanical data from the Mjolnir rig [7], sample dimensions (using stitched images; [8]) and digital volume correlation (DVC) techniques (Avizo).
Our results show the complexity of strain distribution and partial preservation in metamorphic rocks with:
- Significant elastic strain preservation during metamorphic reactions and its apparent minimisation during the ultimate stages of the reaction (textural “maturation” via pressure/solution).
- Complex strain distribution influenced by bassanite anisotropy, sample fabric, geometry, and stress state.
These experiments enable to visualise in 4D the grain-scale development of a complex porous network during the reaction. It opens pathways to document the emergence of poro-elasticity (initial solid has very low porosity) and then the release of the elastic strains. This dataset further demonstrates the importance and the complexity of elasticity in metamorphic systems, with complex displacement vector fields under relatively simple boundary conditions.
References:
[1] Brown & Johnson (2019). https://doi.org/ https://doi.org/10.2138/am-2019-6956
[2] van Keken & Wilson (2023). https://doi.org/10.1186/s40645-023-00573-z
[3] Putnis (2015). https://doi.org/10.2138/rmg.2015.80.01
[4] Leclère et al. (2018). https://doi.org/10.1016/j.epsl.2018.05.005
[5] Llana-Fúnez et al. (2012). https://doi.org/10.1007/s00410-012-0726-8
[6] Gilgannon et al. (2024). https://doi.org/10.1130/G51612.1
[7] Butler et al. (2020). https://doi.org/ 10.1107/S160057752001173X
[8] Turpin et al. in prep
How to cite: Freitas, D., Gilgannon, J., Duggins, D., Butler, I., Rizzo, R., Turpin, L., Chen, B., Reinhard, C., and Bourne, N.: The complex evolution of elasticity during metamorphic transformation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4517, https://doi.org/10.5194/egusphere-egu26-4517, 2026.
Strain partitioning in quartzo-feldspathic rock is closely related to the degree of phase mixing. Both quartz and feldspar tend to form the load-bearing framework (LBF) in naturally deformed rocks, provided there are softer phases such as mica, which behaves as the interconnected weak layer (IWL). In cases where mica is absent, the scenario becomes complicated. Quartz in augen gneisses often behaves as the IWL and feldspar takes the role of the LBF. However, the relative degree of weakening in deformed quartz and feldspar depends on their respective deformation mechanisms. As the mechanisms are different, there is a possibility of dissimilar weaking, followed by a strength reversal.
We have studied a deformed quartzo-feldspathic vein from the Bundelkhand Craton in central India. Despite being Archean, this craton experienced long hiatuses between deformation events, which makes the delineation between different events simpler. The sample we collected from this craton is the result of the latest stage of deformation. A high-temperature fluid entered through fractures and softened the granitic country rock. The fluid, being syn-tectonic, allowed the granitic vein to facilitate different deformation mechanisms in quartz and feldspars.
We investigated the crystal-plastic behaviour of quartz and two feldspars in the deformed vein via electron backscatter diffraction. The quartz crystallographic preferred orientation (CPO) and misorientation index (M) is strongest when quartz grains are adjacent to each other. There is no significant difference in CPO strength in feldspars when the proportion of similar neighbouring phases changes. Additionally, a monomineralic quartz layer exhibits a class 3 buckling fold, implying a higher competency than the adjacent matrix, which contains recrystallised feldspar grains. However, the microstructural evidence suggests that the parent feldspar porphyroclasts are stronger than the recrystallised monomineralic quartz bands. From the inverse pole figure of low-angle (2–10°) misorientation axes in quartz, prism <a> activity is observed which is dominant in the temperature range of 500–650°C. Hence, we infer a deformation temperature of at most 650°C, although it can be lower depending upon the water weakening as such weakening activates prism <a> at lower temperatures. Randomised CPO in feldspar suggests strain accommodation via diffusion creep, followed by grain boundary sliding mechanism might have operated in feldspars. These processes could result in greater softening than that in quartz, which deformed by dislocation creep. Isolated quartz grains existing in the triple junctions of feldspars are not part of such pure dislocation creep; rather, it is more likely that they are byproducts of albitic transformation reactions. Hence, higher strength in quartz is limited to the monomineralic bands, which are purely affected by dislocation creep in the deformed quartzo-feldspathic vein of the Bundelkhand Craton.
How to cite: Sikdar, A., Wallis, D., and Misra, S.: Strength Reversal in Recrystallisation: an EBSD-based Study in a Naturally Deformed Granitic Vein, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1143, https://doi.org/10.5194/egusphere-egu26-1143, 2026.
The study of the evolution of petrophysical properties and alteration of host rock in an active fault system is essential for understanding the mechanisms of deformation localization. The distribution of alterations is closely linked to fluid flow paths, while the formation of new deformation structures depends on the mechanical contrasts induced by these alterations. Our study focuses on granodiorite samples from a borehole drilled in 1996 at Hirabayashi by the Geological Survey of Japan, one year after the Nanbu-Kobe earthquake. Crossing the active Nojima fault, this borehole intersects the fault core at 625 m. The analyses include imaging of samples using neutron and X-ray microtomography (ILL – NeXT) and thin sections, as well as mineralogical quantification by X-ray diffraction (XRD). X-ray diffractograms on oriented slides and Rietveld analyses of XRD data acquired on disoriented powders reveal the presence of secondary mineral phases (e.g., montmorillonite, kaolinite, laumontite, siderite, ankerite), representative of different fluid-rock interaction conditions during the exhumation of the massif. Their proportions, which increase as they approach the fault, reach more than 30% of the volume of a sample at 625 m. Whereas X-ray µ-tomography imaging allows us to observe density contrasts within the samples (e.g., mineral phases and fracture network). On the other hand, neutron imaging allows us to observe the distribution of hydrated mineral phases due to the high neutron absorption coefficient of hydrogen (e.g. for 25 meV neutrons, hydrogen attenuation is 3.44 vs 0.17 and 0.11 for oxygen and silicon, respectively). Neutron and X-ray image registration in the same reference frame allows us to perform joint image segmentation, using gaussian-mixture-model to quantify uncertainties, based on the neutron and X-ray coefficients of absorption of the pre-identified mineral phases. The volumes segmented in this way enable us to (i) quantify in a non-destructive way the volume of secondary mineral phases present in the samples along the fault damage profile and (ii) obtain their spatial distribution and assess the anisotropies of distribution in relation to the deformation structures. This work will subsequently enable us to understand the impact of both the distribution of secondary mineral phases and the network of microfractures on the evolution of the petrophysical and mechanical properties of a seismogenic fault.
How to cite: Jamet, M., Baron, F., Beaufort, D., Dazas, B., Patrier, P., Tengattini, A., Iaquinta, R., Doan, M.-L., Pezard, P., Gibert, B., and Luquot, L.: Neutron and X-ray µ-tomography-based 3D imaging of alteration phases in faulted granodiorite at Nojima (Japan), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7324, https://doi.org/10.5194/egusphere-egu26-7324, 2026.
Earthquake records preserved in rocks provide key insights into the processes that govern crustal deformation and seismic energy dissipation. This manuscript presents new approaches for identifying mineralogical signatures of paleoearthquakes using advanced microstructural analyses, including electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). These techniques enable observations from the millimetre to nanometre scale of features associated with plastic deformation, including crystal reorientation and deformation twinning. Here, we investigate deformation microstructures in monazite, a key geochronometer, with the aim of assessing the impact of deformation on geochronological interpretations, as deformation-induced crystallographic defects can act as high-diffusivity pathways leading to Pb loss. Understanding the deformational behaviour of monazite is therefore critical for interpreting geochronological data. We examine monazite from an eclogite facies mylonite in the Musgrave Province (central Australia) to elucidate mechanisms of seismic deformation under dry (<0.002 wt% H₂O), lower-crustal conditions. The studied monazite grain is directly cross-cut by a pseudotachylite vein, indicating that the observed microstructures formed during the associated seismic event. EBSD and ECCI analyses reveal crystal-plastic deformation in the form of twinning with three distinct orientations: 180° <100>, 180° <001>, and 95° <201>. The latter is associated with dynamic recrystallization via subgrain boundary rotation. ECCI further reveals nanometre-scale (<15 nm) porosity within both parent grains and twins. These microstructures are consistent with those reported in monazite deformed during impact events. Recent studies of shocked monazite have shown that deformation by twinning can liberate Pb during rupture of rare-earth-element–oxygen (REE–O) bonds, enabling rapid diffusion along crystallographic defects and complete expulsion from the crystal, effectively resetting the geochronometer. The new insight provided by these microstructural focussed observations likely accounts for the disparity of electron probe microanalysis (EPMA)-based geochronology on the same monazite grain, which yielded ages of 1309 to 691 Ma. Seismicity in the Musgrave Province is primarily associated with the Petermann Orogeny (~550 Ma), suggesting that the younger EPMA ages were partially reset as a result of the twinning. Our results demonstrate the potential for monazite to record and date seismicity, opening new avenues for reconstructing paleoearthquake histories from deep crustal rocks.
How to cite: Dubosq, R., Camacho, A., and Britton, B.: Seismic twinning in monazite: Microstructural records of deep crustal earthquakes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4322, https://doi.org/10.5194/egusphere-egu26-4322, 2026.
The mechanisms of intraslab earthquakes at depths of > 40 km are fundamentally different from those of shallow earthquakes because the frictional strength of silicate rocks is proportional to the confining pressure. To understand the process triggering intraslab earthquakes, many experimental studies on faulting of slab-forming rocks have been conducted at upper mantle pressures. Previous studies have revealed that shear localization induced by dehydration of hydrous minerals (e.g., Okazaki & Hirth, 2016) or adiabatic shear heating (e.g., Kelemen & Hirth, 2007) is essential for the occurrence of faulting at high pressures. Although acoustic emission (AE) monitoring technique for D-DIA apparatuses enabled us to discuss the process of microcracking at high pressures, mechanical behavior at the onset of faulting is still unclear due to low time-resolution stress/strain measurements using synchrotron X-rays. The cause of bottleneck in stress/strain measurements is a long exposure time required for the acquisition of a two-dimensional X-ray diffraction pattern of minerals. Considering that the timescale of stress drop associating faulting is on the order of 0.01 sec (e.g., Okazaki & Katayama, 2015), a significant improvement for time resolution of stress/strain measurements is required. To improve the time resolution of stress/strain measurements, we installed a series of new devices at BL15XU, SPring-8.
We conducted in situ triaxial deformation experiments on olivine aggregates at pressures of 1-3 GPa and temperatures of 700-1250 K under nominally dry conditions using a D-DIA apparatus, installed at BL15XU, SPring-8. Two-dimensional radial X-ray diffraction patterns and radiographic images were alternately acquired by adjusting sizes of the incident slit and operating a flatpanel detector and a CCD camera using a high-flux pink beam (energy 100 keV) from an undulator source (0.2 s of exposure time for both ones). Pressure and differential stress were determined from the d-spacing of olivine. Strains of deforming samples were evaluated from the distance between platinum strain markers. AEs were recorded continuously on six sensors glued on the rear side of the 2nd-stage anvils, and three-dimensional AE source location were determined.
Stress increased with strain at the beginning of sample deformation, and it reached the yielding point at strains of ~0.1 or less. AEs from the deforming sample were detected when stress exceeded ~1 GPa and the amplitude of AE is positively correlated with the magnitude of stress. At strains higher than 0.1 (i.e., beyond the yielding point), both softening (i.e., decrease in stress and/or increase in strain rate) and a decrease in AE rate were observed prior to the occurrence of faulting. Faulting was observed at 880-1150 K. Most of unstable slips proceeded within 1 s and associated a sudden stress drop (~0.5 GPa) and temporal radiation of large AEs. In contrast, neither stress drops nor AEs were associated with a few “aseismic” unstable slips. Differential stress continuously increased when stable slip proceeded and the stable slip was terminated by the occurrence of another unstable slip. Our observations suggest that unstable slips can be divided into two types (i.e., seismic and aseismic ones) under the P-T conditions of shallow subducting slabs.
How to cite: Ohuchi, T., Higo, Y., Tsujino, N., Kakizawa, S., Ohsumi, H., and Yabashi, M.: In situ observation of faulting in olivine at high pressures and high temperatures using high-flux synchrotron X-rays, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7751, https://doi.org/10.5194/egusphere-egu26-7751, 2026.
When deformed by dislocation creep the dominant slip (Burgers) vectors of olivine dislocations are parallel to [100] or [001]. Dislocations with an [010] Burgers vector component (b dislocations) have been recorded rarely. Here we show an experimentally deformed olivine sample has a substantial population (17%) of b dislocations. Electron Backscatter Diffraction maps of crystal orientations provided information on dislocations from the orientation gradients. Maps show the b dislocations form subgrain walls like those formed by other dislocation types and are interpreted to form similarly by glide and climb, so b dislocations are mobile. To confirm our approach, we used EBSD maps to select an area for Transmission Electron Microscopy imaging, down to an atomic scale image of a b dislocation. Our sample was deformed within range of subduction zone conditions; our approach can be used to investigate the scale and conditions of b slip in the mantle more widely.
How to cite: Wheeler, J., Hunt, S., Eggeman, A., Donoghue, J., Gholinia, A., Li, Y., Tillotson, E., and Haigh, S.: Olivine Deformation: to B Slip or not to B Slip, that is the Question, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11671, https://doi.org/10.5194/egusphere-egu26-11671, 2026.
Olivine is a fundamental rock-forming mineral for which microstructures are closely tied to deformation conditions. However, visualization of olivine deformation has traditionally been limited to two-dimensional observations, ranging from petrographic microscopy at the millimetre– to micrometre scale to electron-based techniques probing crystallographic distortion and ordering at the micro- to nanometre scale (e.g., EBSD and TEM). Here, we introduce dark-field X-ray microscopy (DFXM) and present its first application to geological materials, conducted at beamline ID03 of the ESRF.
Using a focused line beam produced by compound refractive lenses, DFXM enables non- destructive, in-situ imaging with spatial resolution down to ~35 nm. By selectively illuminating a ~500 nm thick volume with the line beam, DFXM allows “slicing” through depth of the crystal volume. By translating the sample through the X-ray beam, the layers can be stacked and reconstructed into full 3D datasets.
In this work, we reconstruct the 3D microstructures of the mineral olivine across a range of deformation settings, spanning from hydrothermal single crystal olivine, to olivine in Åheim orogenic peridotite which experienced long-term dislocation creep, to olivine in heavily shock-metamorphosed martian basalt with relict crustal strain. We observe individual static dislocations and associated lattice strain field in the hydrothermal olivine single crystal, to arranged low-angle boundaries (LABs) formed by geometrically necessary dislocations (GNDs) in the Åheim peridotite, to chaotic dislocation networks connected by dense, short, and randomized LABs in shocked martian basalts.
By bridging conventional 2D crystallographic observations with volumetric 3D microstructural reconstructions, our work enables robust observations of microstructures developed in distinctive deformation conditions, providing a powerful and advanced 3D imaging technique for geological materials. Our study expands the application of DFXM to Earth and planetary materials and demonstrates the power of multi-scale, three-dimensional imaging for resolving complex deformation histories in geological systems.
How to cite: Li, Y., McCaulsand, P., Flemming, R., Yildirim, C., and Detlefts, C.: Microstructure Across Deformation Regimes: 3D Imaging of Olivine by Dark-Field X-ray Microscopy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15191, https://doi.org/10.5194/egusphere-egu26-15191, 2026.
The deformation and interaction of amphibole grains are crucial for comprehending the rheological behavior and physical properties of middle to lower crust. However, the mechanisms of strain accommodation and grain boundary processes in amphibolites are poorly studied. In this study, we analyzed a naturally deformed amphibolite from an exhumed continental strike-slip shear zone. The amphibole grains can be categorized into two distinct types: type I and type II, with the type II being embedded within type I. Type I amphibole grains exhibit typical plastic deformation behavior, distinguished by the presence of discernible dislocation arrays and formation of subgrains. In contrast, type II amphibole grains predominantly display microfractures in the middle of grains and voids occur in their elongated tails. Meanwhile, we identified three types of low-angle boundaries in amphibole grains with varying microstructural and nanoscale characteristics. Our findings indicate that low-angle boundaries in minerals are not exclusively associated with crystal-plastic deformation. Furthermore, the deformation characteristics in type II amphibole grains are related to grain boundary sliding (GBS) process. To relieve stress concentration during grain boundary sliding in type II amphibole grains, two accommodation mechanisms are proposed: (i) Grain boundary diffusion with elimination of grain boundary irregularities. (ii) Intragranular deformation of adjacent grains through either a brittle or a ductile process. Our findings hold significant implications for understanding the stress concentration and accommodation during deformation process in amphibolite
How to cite: Liu, J., Cao, S., and Cheng, X.: Development of low-angle boundaries in amphibole and their implications for accommodating grain boundary sliding in naturally deformed amphibolite, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20317, https://doi.org/10.5194/egusphere-egu26-20317, 2026.
Serpentinites are key components in subduction zones, acting as primary carriers of water into the deep Earth and critically influencing seismic behavior. Several studies suggest that fluid-saturated deformation in serpentinized subduction channels may control a variety of processes associated with Intermediate-depth seismicity (~50 to 300 km depth) . A key problem in their rheology is the discrepancy between experimentally deformed serpentinites, which exhibit predominantly brittle behavior, and their naturally deformed counterparts, which show ductile fabrics. While the dominant deformation mechanism in subduction settings—whether crystal plasticity, dissolution-precipitation, or a combination— also remains poorly documented. Moreover, there is a lack of constraints on how serpentinites deform during and after partial dehydration at (ultra)high-pressure conditions and transformation to olivine and pyroxene-dominated assemblages. To address these issues, we present an integrated microstructural and geochemical study of serpentinites across a hectometer-scale strain gradient within the Zermatt-Saas meta-ophiolite, documenting the evolution of deformation and major element mobility during subduction and exhumation. In low-strain samples, dehydration forms coarse-grained olivine-diopside-clinohumite-magnetite veins. Host antigorite shows weak crystallographic preferred orientations (CPOs) and twinning. With increasing strain, deformation localizes around these veins, where olivine develops a weak B-type CPO, but with grains showing no evidence of intracrystalline deformation. Progressively, antigorite develops a strong, penetrative foliation with a (001) maximum normal to foliation and grain size reduction, while olivine veins are folded and boudinaged. Low angle grain boundaries are related to fracturing of olivine. In high-strain serpentinite mylonites, transposed olivine veins form isoclinal folds, and S-C' fabrics develop. Antigorite CPO strength increases considerably, something that is not observed for olivine. Whole thin section XRF mapping reveals an increase of Ni and S in the more deformed serpentinites, where pentlandite defines the C' fabric and wraps around olivine porphyroclasts. Antigorite mm thick bands show Cr depletion accompanied by grain size reduction, while Fe-Mn occur normally associated with each other. In the transposed olivine veins there is an increase of Fe content in comparison to the original olivine vein composition. When present, Al-rich phases such as chlorite are mostly undeformed but can breakdown locally and transform into tremolite + magnetite in late shear bands. Our data document a fluid-assisted progression from localized brittle-ductile to distributed ductile deformation. Microstructural and chemical evidence indicate that deformation was primarily controlled by dissolution-precipitation processes, with limited crystal plasticity in antigorite and predominantly brittle olivine deformation. This study provides a rare dataset on metamorphic olivine deformation in subduction zones and highlights the fundamental coupling between element mobility, metamorphic reactions, and strain localization in the subduction interface and mantle wedge.
How to cite: Morales, L. F. G., Muñoz-Montecinos, J., Ceccato, A., Kilian, R., and Volante, S.: Dissolution-Precipitation dominated deformation in (ultra)high-pressure serpentinites from the Zermatt-Saas Meta-Ophiolite, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14234, https://doi.org/10.5194/egusphere-egu26-14234, 2026.
Ultramafic rocks exposed adjacent to mid-ocean ridges in the footwall to large slip oceanic detachment faults provide unique insight into deformation and reaction when transforming from peridotite to serpentinite. In contrast to orogenic serpentinites, oceanic serpentinites have not subjected to superposed metamorphic and/or tectonic overprinting. A suite of samples from mostly fresh peridotites (~20% alteration), with preserved olivine and pyroxene, to completely serpentinized rocks (100% alteration), dominated by serpentine (lizardite) and magnetite, were collected from a ~1.2 km long drill core from IODP Expedition 399 at the Atlantis Massif oceanic core complex.
A combined approach of synchrotron diffraction and electron backscatter diffraction in order to analyze the crystallographic preferred orientation (CPO), and micro X-ray fluorescence mapping and optical microscopy in order to image and analyze the microstructure, is used to explore the variable microstructures.
Magnetite forms polycrystalline aggregates defining a foliation, which ranges from anastomosing to highly parallel. In partially serpentinized, mylonitic peridotites showing olivine grain size reduction and CPO development; magnetite aggregates trace the preexisting mylonitic fabric. Lizardite and magnetite both have a variable CPO strength and different CPO types, suggesting that different processes and parameters influence the formation of these microstructures. Further, late stage deformation, is evident from microfaulting, sheared serpentine veins and dissolution features. The individual contributions of deformation and serpentinization reaction to the final microstructure will be evaluated and discussed.
How to cite: Kühn, R., Schlickum, L., Kilian, R., Morales, L., Parsons, A., John, B., and Deans, J.: Deformation and reaction in the microstructural record of oceanic serpentinites, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18276, https://doi.org/10.5194/egusphere-egu26-18276, 2026.
Dehydration embrittlement is the dominant mechanism proposed to explain deep-focus earthquakes between 100–350 km in depth. Antigorite dehydration was extensively investigated in previous experimental studies, which demonstrated contrasting results regarding the seismic potential of antigorite dehydration. Additionally, microstructural aspects of antigorite dehydration and their implications for deep seismicity are scarce. Localized dehydration, on the other hand, might generate strain weakening, potentially leading to failure at depths relevant to deep earthquakes. Localized antigorite dehydration is demonstrated to occur in nature and the laboratory; however, it is not clear if this is a passive or dynamic process.
To better understand the micro-mechanisms of localized antigorite dehydration, we conducted high-pressure, high-temperature experiments under isostatic and non-isostatic conditions. Experiments were run at 3 GPa and temperatures within and above the antigorite stability field (530 °C–800 °C). Antigorite cores with 2 mm diameter were mounted in cubic assemblies and deformed in a 6-ram multi-anvil press at the Bayerisches Geoinstitute. Pure shear deformation was applied by inserting one pair of anvils while simultaneously removing the remaining two pairs orthogonal to it.
Results show that isostatic dehydration of antigorite at 3 GPa starts at ~530 °C and completes at ~800 °C. Localized dehydration occurs in isostatic and non-isostatic conditions within the antigorite stability field. It is enhanced during deformation experiments, resulting in the formation of nanocrystalline veins and networks containing olivine and pyroxene. These results demonstrate that localized dehydration might occur through passive and dynamic processes with the development of different microstructures.
How to cite: Silva Souza, D., Thielmann, M., Heidelbach, F., and Frost, D.: Localized dehydration of antigorite during experimental deformation at subduction zone conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9434, https://doi.org/10.5194/egusphere-egu26-9434, 2026.
Evidence for the nature of fault slip across the seismic cycle is hard to decipher. Fault-related deformation near the fault surface develops over the seismic cycle, characterized by rapid coseismic slip and intense deformation, followed by slower interseismic slip and stress accumulation. While considerable focus has been placed on characterizing deformation through fracturing and mesoscale structures, the analysis of grain-scale plastic processes has been largely neglected. However, transient temperature increases due to frictional heating, combined with the ability of calcite-bearing rocks to deform plastically at relatively low temperatures, suggest that microstructural damage and subsequent recovery processes could leave diagnostic evidence in carbonate fault rocks. Indeed, Pozzi et al. (2019) demonstrated that shearing gouge at seismic rates (~1 m/s) develops a crystallographic preferred orientation (CPO), accompanied by grain growth and sintering. These observations, however, were confined to nanometer-scale grains, localized at the fault surface.
Here, we present a detailed microstructural analysis of carbonate samples from the North Anatolian Fault Zone. We used Electron Backscatter Diffraction to map the calcite grains' orientations and characterize intragrain deformation and grain-boundary morphologies. We identify three distinct layers extending from the fault surface to a distance of ~4 mm. Layer I, with a thickness of tens of µm to 0.5 mm, exhibits predominantly angular grains with grain sizes ranging from unresolved (<1 µm) to tens of µm. Layer II, with a thickness of 0-200 µm, is comprised of small equant grains (1-5 µm) and some larger grains (10-30 µm), characterized by wavy grain boundaries, suggesting active grain boundary migration. No CPO was observed in layers I and II. Layer III, with a thickness of ~2-3 mm, contains large grains (hundreds of µm) that can be divided into two populations of grains. Rounded grains with wavy grain boundaries indicate the progressive consumption of smaller grains. At the core of the layer, grains contain faceted boundaries and are elongated parallel to the fault surface. This layer is the only one to exhibit a distinct CPO with the c-axis oriented normal to oblique to the slip surface. Importantly, the large grains in layer III also comprise small, isolated ‘islands’ of finer grains.
We infer that deformation mechanisms vary systematically with distance from the fault surface. Layer I records cataclastic flow at the fault surface, whereas layer II, characterized by very small grain sizes, exhibits shearing by grain boundary sliding that resulted in grains with low intragrain misorientation and the absence of CPO. The most striking microstructural record is preserved in layer III, which initially shows strong evidence for recovery processes by abnormal grain growth. We propose that this latter process occurred during or immediately after coseismic frictional heating, resulting in the consumption of previously deformed grains, which maintains the CPO record of deformation and provides a microstructural record of the seismic cycle at millimeter-scale distances from the fault surface.
Pozzi, et al., 2019. Coseismic ultramylonites: An investigation of nanoscale viscous flow and fault weakening during seismic slip. Earth and Planetary Science Letters.
How to cite: Boneh, Y., Levi, T., Nuriel, P., and weinberger, R.: Abnormal grain growth in carbonate samples from the North Anatolian Fault: Microstructural evidence of the seismic cycle, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7796, https://doi.org/10.5194/egusphere-egu26-7796, 2026.
U-Pb geochronology via Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) is a fast and reliable method for in-situ dating of calcite that is used across disciplines in earth science. However, the heterogeneous distribution of U (and Pb) in individual calcite crystals represents a yet unmitigated challenge and identifying zones of sufficiently high U concentrations that can provide precise constraints on timing of calcite precipitation is an inefficient “hit or miss” process. Moreover, it is challenging to confirm that targeted domains of a calcite crystal retain their pristine geochemical signature, given the range of post-crystallization dissolution-reprecipitation and solid diffusion processes that can affect this mineral. There is thus an urgent need to understand the spatial and temporal mechanisms of U incorporation and mobilization in calcite to ultimately improve this key geochronological tool. To determine where specific trace elements are located within calcite crystals, investigate how they are incorporated during crystal growth and how they are affected by post-crystallization fluid-assisted deformation processes, we applied Synchrotron X-Ray Fluorescence Microscopy (XFM) with emphasis on U mapping, Electron Backscatter Diffraction (EBSD), and LA-ICP-MS to a set of calcite veins. Samples were collected from drillcores through the Middle and Upper Jurassic carbonates and marls (max. 85°C) in the Neogene Molasse Basin in central northern Switzerland. By combining high-resolution trace element maps with information on the crystal lattice structure of calcite we show two main textural types of trace element distributions within syntaxial calcite veins: 1) oscillatory crystal growth zonations that reflect preferential incorporation of trace elements into structurally different growth steps and faces of growing calcite crystals during growth and, 2) complete overprint of the initial growth zonation upon potential secondary fluid infiltration and trace element replacement. The anti-correlation between Fe, Mn and Sr, U demonstrates the role of kinetic factors during trace element partitioning between fluid and calcite, pointing to the inhibition of Fe incorporation at higher growth rates. Where the Sr uptake during calcite growth is generally enhanced with growth rate. The results of this project give valuable insights in the complexity of fluid overprint during multi-staged deformation cycles in the modification of trace elements in calcite, with clear implications for the applicability and reliability of U-Pb geochronometer in calcite.
How to cite: Akker, I. V., Schrank, C. E., Jones, M. W. M., Howard, D., Tavazzani, L., and Morales, L.: Trace element mapping in vein calcite with synchrotron XFM: implications for U-Pb geochronology, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9614, https://doi.org/10.5194/egusphere-egu26-9614, 2026.
Bio-cementation is a new, environmentally-friendly soil-reinforcement process. It is used for civil engineering purposes, such as the fabrication of construction materials, as well as the preservation of monuments. This process uses bacterial activity, mainly that of Sporosarcina Pasteurii, that is capable of hydrolysing the urea present in the medium, leading to the precipitation of CaCO3 (calcite) crystals between sand grains, therefore binding them together, and reinforcing the soil. The macro and micro (contact scale) mechanical properties of bio-cememted sand have been extensively studied. However, the microstructure of the precipitated calcite crystals remains undiscovered, which induces mechanical differences under different conditions of cementation. The goal of this study is to investigate the microstructure of biogenic calcite, issued from bio-cementation of sand, and how it varies under different cementation conditions. For this, high resolution synchrotron diffraction imaging at the ESRF was performed, utilizing scanning 3DXRD (s-3DXRD) on ID11 and Dark-Field X-ray Microscopy (DFXM) on ID03. For this, the main experiment was performed on three samples that consist of 3D printed resin cells in which cementation was performed under different conditions, by varying the substrate on which the calcite was grown (between sand grains and glass beads), as well as varying the salinity of the medium. After each cementation cycle, and for each sample, layer measurements were acquired using s-3DXRD. A significant difference was observed between the sand and glass bead cases: the precipitated crystals on the glass beads were much smaller than those precipitated on the sand grains. DFXM measurements showed defects that are only present in the case of high concentration of NaCl in the medium, which could potentially alter the mechanical properties of the material. These two complementary techniques allowed for an in-depth study of the microstructure of the precipitated calcite crystals.
How to cite: Sarkis, M., Detlefs, C., La Bella, M., Naillon, A., Geindreau, C., Emeriault, F., Watier, Y., Ball, J. A. D., and Yildirim, C.: High-resolution microstructural study of calcite crystals precipitated through bio-cementation under different conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11242, https://doi.org/10.5194/egusphere-egu26-11242, 2026.
Spatial phase distributions can be grouped into random, clustered or anticorrelated/distributed based on the probability to encounter a given nearest neighbour. This property can also be probed with respect to intervals of directions, frequently revealing an anisotropy in the spatial phase distribution. In this study, several high temperature ultramylonites with variable composition from felsic to ultramafic as well as coarse grained deformed rocks (e.g. eclogite from Münchberg, Germany) were investigated. Measurements of phase distribution anisotropy frequently manifest in a pronounced direction of phase clustering and one direction of anticorrelation. Especially in the investigated ultramylonites but also in deformed eclogites and amphibolites, those two directions are found not to be orthogonal and not to coincide with finite strain axes (as far as manifested by foliation and stretching lineation). Clustering phases (e.g. qtz, plg or grt, depending on the rock type) form stacks antithetically tilted against the sense of shear. These stacks are separated by phases such as kfs, bt or cpx. Below a certain volume threshold of the stack-forming phase, stacking is not observed.
In addition to the phase distribution, truncated chemical zonations and/or indications of directed growth are frequently observed. On the other hand, there is a lack of microstructures which can reasonabley be associated with steady state dislocation creep.
It is suggested that the observed microstructures in combination indicate deformation by a mechanism best described by dissolution-precipitation accommodated granular flow (or "diffusion creep" in the broadest sense). Stack-forming phases undergo mostly rigid-body rotation and translation temporarily forming transient force chains before being disintegrated again. Since these stacks can be observed in the rock record, the residence time in the force chain position must be greater than in a randomly distributed position, compatible with jamming of particles during granular flow.
The presence of this particular type of anisotropic spatial phase distribution may not only serve as a shear sense indicator but could in general be useful for the identification of deformation mechanisms.
How to cite: Kilian, R.: Spatial phase distribution and deformation processes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13669, https://doi.org/10.5194/egusphere-egu26-13669, 2026.
X-ray ptychography is a lensless, coherent-diffraction imaging technique developed over the last 20 years that affords 10-nm resolution for optically thick specimens1. It reconstructs the optical transmission function (OTF) of a sample from raster-scanned overlapping 2D transmission diffraction patterns through iterative phase retrieval algorithms1,2. The OTF projects the refractive index of the sample along the incident beam and thus quantifies the phase shift and amplitude attenuation of the transmitted beam2, which in turn relate to the projected electron density of the specimen. X-ray ptychography is therefore an ultramicroscopy technique that is very well suited to mapping nano- and micron-sized objects with significant density differences relative to the bulk such as pores and dense accessory minerals.
In this contribution, we present a primer for the application of X-ray ptychography to nano- and micro-scale studies of rocks. First, we illustrate the underlying physical principles that guide the data processing and interpretation of ptychographs. Then, we show exemplary applications to a wide range of rock samples (e.g., seismogenic brittle fault rocks, mylonites, veins, shale, and micrite) imaged at the XFM beamline of the Australian Synchrotron3 over the last 5 years4,5. Application examples include the measurement of sample surface roughness, imaging of cracks and pores, 3D porosity measurements, and the detection of buried accessory phases.
References
1 Pfeiffer, F. X-ray ptychography. Nature Photonics 12, 9-17, doi:10.1038/s41566-017-0072-5 (2018).
2 Wittwer, F., Hagemann, J., Brückner, D., Flenner, S. & Schroer, C. G. Phase retrieval framework for direct reconstruction of the projected refractive index applied to ptychography and holography. Optica 9, 295-302, doi:10.1364/OPTICA.447021 (2022).
3 Howard, D. L. et al. The XFM beamline at the Australian Synchrotron. Journal of Synchrotron Radiation 27, 1447-1458, doi:doi:10.1107/S1600577520010152 (2020).
4 Jones, M. W. M. et al. High-speed free-run ptychography at the Australian Synchrotron. Journal of Synchrotron Radiation 29, 480-487, doi:https://doi.org/10.1107/S1600577521012856 (2022).
5 Schrank, C. E. et al. Micro-scale dissolution seams mobilise carbon in deep-sea limestones. Communications Earth & Environment 2, 174, doi:10.1038/s43247-021-00257-w (2021).
How to cite: Schrank, C. E., Jones, M. W. M., Kewish, C. M., van Riessen, G. A., Hinsley, G., Berger, A., Herwegh, M., Schwichtenberg, B., Bishop, N. D., Howard, D., Langendam, A. D., and Paterson, D. J.: Nano- and micro-scale imaging of rocks with X-ray ptychography, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8362, https://doi.org/10.5194/egusphere-egu26-8362, 2026.
During the International Ocean Discovery Program (IODP) Expedition 402 in the Tyrrhenian Sea, two of the six drilled sites, the U1613 and U1617, were located on the thinned continental crust of the Cornaglia and Campania terraces, where the deposition of evaporites during the Messinian Salinity Crisis (MSC) had been imaged with seismic data. Expedition 402 recovered Messinian evaporites beneath a relatively thin sedimentary cover at both drill sites. At Site U1613, the Messinian section is extremely thin (a few meters only). In contrast, at Site U1617, a complete 102 m-thick evaporitic sequence ranging from gypsum-enriched terrigenous sediments through anhydrite to halite layers was sampled. This scientific drilling site is the only one in the Mediterranean that penetrated the complete Messinian evaporitic sequence, providing a unique opportunity to study the properties of the so-called Upper, Mobile and Lower units. A series of physical property measurements was performed on these cores on board of the JOIDES Resolution drillship, including P-wave velocity, density, magnetic susceptibility, natural gamma ray and thermal conductivity. In addition, we collected representative discrete samples to measure P-wave velocity (Vp), bulk density, grain density and porosity. These data allowed us to analyze the sealing properties of the halite unit and its interaction with salt-induced tectonics. Furthermore, from Vp and density used as input to calculate reflection coefficients, we generated a 1D synthetic seismogram at Site U1617. We compared this synthetic seismogram with the multi-channel seismic data acquired across the drill site, namely the Medoc 6 line. These new data allowed us to compare the Messinian units recovered in situ with multichannel seismic data and thereby revise seismic interpretation of these units. Thanks to the unique opportunity offered by the IODP Expedition 402, we now have reliable data on the physical properties of Messinian evaporites and we are able to provide new constraints on the interpretation of Messinian facies.
How to cite: Loreto, M. F., Ligi, M., Filina, I. Y., Abe, N., Shuck, B. D., Pezard, P. A., Estes, E. R., Malinverno, A., Ranero, C. R., Yang, L., and Zitellini, N.: A new concept of Messinian Salinity Crisis based on physical properties from the IODP Exp.402 in the Tyrrhenian Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7733, https://doi.org/10.5194/egusphere-egu26-7733, 2026.
Salt caverns are formed in the subsurface during solution mining of salt. After the end of salt production, caverns need to be safely abandoned or may be repurposed for storage of energy carriers such as hydrogen. Salt caverns locally disturb subsurface stresses, leading to creep of the surrounding rock salt. Creep can cause cavern convergence at depth and may result in surface subsidence, with consequences for infrastructure and public safety. Accurate forecasting of cavern stability during abandonment or assessment of suitability for storage requires a deep understanding of the grain scale deformation mechanisms and processes controlling rock salt strength and creep rate. For rock salt, important deformation mechanisms are dislocation creep and pressure solution creep. Laboratory experiments have shown that dynamic recrystallization (DRX) associated with dislocation creep can be activated and contribute to mechanical weakening. However, the weakening effect of DRX is not included in engineering constitutive laws used in salt cavern numerical models. These laws are commonly based on low-strain laboratory experiments, where the influence of DRX is limited, and microstructural data are relatively rarely reported. The aim of this study is to experimentally investigate the dominant DRX process in deforming natural rock salt and its effect on the mechanical behaviour.
Lab experiments have been carried out on natural wet salt samples from the Zechstein formation. We conducted constant strain rate experiments using a triaxial compression apparatus. Experiments were performed at a confining pressure of 20 MPa and a temperature of 125 °C, using constant displacement rates corresponding to strain rates of approximately 5 × 10⁻⁵ s⁻¹ and 5 × 10⁻⁷ s⁻¹, up to 30–40% axial strain. After the experiment, all samples were studied using optical microscopy. Electron backscatter diffraction (EBSD) analysis was performed on the starting material and on two deformed samples, one from each strain-rate condition.
For all samples, we observed an initial transient creep stage followed by a quasi-steady state stage. The transition to quasi-steady occurred at a strain of about 10% for samples deformed at a strain rate of ~5 × 10-7 s-1. For samples deformed at the faster strain rate of ~5 × 10-5 s-1, continuous hardening occurred up to axial strains of 30%, with a gradually decreasing hardening rate approaching steady state. Light optical and EBSD microstructural analysis revealed grains with a dense substructure including subgrain walls, euhedral shape grains with low to no substructure, and grains with irregular shaped grain boundaries including bulges. We infer that the dominant deformation mechanism in the tested natural samples was dislocation creep, providing sufficient local differences in dislocation density to activate DRX dominated by grain boundary migration processes. DRX led to rheological weakening and quasi-steady deformation. We are working on robust understanding of the parameters controlling DRX as this is essential to evaluate the zones prone to weakening by DRX around salt caverns.
How to cite: Dialeismas, E., de Bresser, H., Hangx, S., and ter Heege, J.: An experimental investigation of dynamic recrystallisation processes and their influence on the mechanical properties of natural rock salt samples, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9678, https://doi.org/10.5194/egusphere-egu26-9678, 2026.
Microstructural investigations of halite are essential for understanding deformation mechanisms relevant to salt tectonics and underground storage applications, including radioactive waste disposal and salt caverns. However, the identification of subgrain boundaries, dislocation structures, acting creep mechanisms and fluid-related features remains challenging due to the optical transparency and inherently low defect contrast of halite. Gamma decoration provides a powerful solution by inducing radiation-related colour centers that selectively highlight lattice defects and deformation structures.
At the Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II, TUM), gamma decoration has been implemented since over a decade, recently we re-established and systematically optimized it using older spent fuel elements characterized by comparatively low dose rates. This contribution focuses on methodological developments and parametric studies that enable reliable gamma decoration under these conditions, extending the applicability of the technique beyond high-dose irradiation facilities.
We present results from controlled irradiation experiments on halite thin sections covering a wide range of total doses, irradiation times, and temperatures, combined with post-irradiation optical microscopy, spectroscopy, and digital colorimetry to quantify and optimize suitable optical contrast. Our experimental results from long-term irradiations are compared with theoretical models describing dose-rate-dependent radiation effects on defect formation in natural rock salt. This parametric approach allows identification of threshold conditions required for effective defect visualization, as well as optimization strategies to compensate for reduced dose rates, including extended irradiation times and temperature control.
These results establish gamma decoration at FRM II as a robust and versatile experimental method for salt-rock research, providing a valuable link between laboratory testing, microstructural analysis, and mechanical modelling, and ensuring continued applicability of this technique with ageing irradiation infrastructure.
How to cite: Hutanu, V., Li, X., and Schmatz, J.: Gamma decoration at FRM II: recent optimisations and parametric studies for microstructural investigations of halite, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13462, https://doi.org/10.5194/egusphere-egu26-13462, 2026.
The mineralogical-geochemical composition and texture of salt rocks plays an important role for the site selection and construction of a repository for heat-generating, highly active radioactive waste. As the bromide content in halite depends on the degree of evaporation and subsequent processes, Br is frequently analyzed to estimate the genetic history of the rocks (Braitsch 1971). Typically, geochemical analytical methods are applied on powder samples (e.g. ICP-OES, XRF), and textural analyses (e.g. using EBSD) require extensive sample preparation. In this study, first results of diapiric Upper Permian rock salt samples are presented using non-destructive µXRF on polished rock samples.
The µXRF Bruker M4 Tornado Plus (Nikonow & Rammlmair 2016) was used to map and quantify element distributions in rock salt. For calibration, in a first step, a certified reference material (CGL), consisting of mostly halite (NaCl) with minor content of anhydrite (CaSO4), and sylvite (KCl), was pressed into pellets of 2 g and 1 cm diameter. For representativity, three spot measurements and a mapping of the center (1 cm²) were chemically quantified. The µXRF measurements correlate with the certified values yielding an R² of 0.995. In a second step, pressed pellets with a range of defined concentrations of Br in halite and Rb in sylvite were prepared to estimate the concentration ranges measurable by µXRF. For Br, the concentrations range from 1 to 0.005 wt.% Br in halite, and for Rb the concentrations range from 0.4 to 0.005 wt.% Rb in sylvite. Both data sets show a good correlation with R² of 0.99 (n=21 for Br and n=17 for Rb). Therefore, µXRF seems suitable for quantification of Rb and Br in salt rocks.
Furthermore, naturally deformed halite samples were analyzed simultaneously for their geochemically and textural properties, which were previously analyzed using “conventional” methods (ICP-OES and EBSD, respectively; Mertineit et al. 2023). The bromide content in halite is ca. 200 µg/g and thus comparable to the known values. The textural results show misorientations of few degrees within single halite grains and pronounced misorientations at halite grain boundaries, indicating bending of the crystals, but no pronounced texture of the bulk rock.
Although the results are in good agreement with published data, further test should follow, especially on the textural analyses including the misorientation angle resolution and the indexing of the halite crystal axis. However, the application of µXRF on salt rocks offers a fast, non-destructive method providing reliable combined geochemical and textural information.
References
Braitsch 1971. Springer-Verlag, https://doi.org/10.1007/978-3-642-65083-3
Mertineit et al. 2023. Tectonophysics 847, https://doi.org/10.1016/j.tecto.2023.229703
Nikonow & Rammlmair. 2016. Spectrochim Acta B 125, https://doi.org/10.1016/j.sab.2016.09.018
How to cite: Nikonow, W., Mertineit, M., and Schramm, M.: Combined geochemical and textural analyses of halite: First results of non-destructive µXRF measurements, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17303, https://doi.org/10.5194/egusphere-egu26-17303, 2026.
Lithium is a trace component, which is frequently observed in salt deposits and salt solutions collected in salt mines, respectively (Mertineit & Schramm 2019). So far, no naturally formed Li-bearing salt mineral is known, thus, the origin of Li in salt deposits must be related to other sources, e.g. to detrital phyllosilicates (Braitsch 1971). Detailed investigations on the Li content, the occurrence within a mine and the mineralogical composition of specific stratigraphic layers enable the reconstruction of rock-fluid interaction and fluid migration pathways. This is important for the construction, design and dimensions for a repository for radioactive waste in rock salt.
To verify which minerals are Li-hosts, diapiric Upper Permian (Zechstein) samples from the uppermost Staßfurt-Formation and the lower Leine-Formation were investigated for their mineralogical-geochemical composition. The succession contains salt clays, anhydrite and carbonate rocks as these rocks reveal the highest Li content (up to 159 µg/g bulk rock). The samples were previously investigated using ICP-OES, ICP-MS, XRD, SEM and thin section microscopy. Beside typical salt minerals in varying amounts (halite, anhydrite, magnesite, sylvite, carnallite), most samples consist of quartz, illite-muscovite, chlorite (clinochlore) and biotite, all of them with a grain size of ≤100 µm, often <20 µm. Only few samples contain traces of kaolinite, koenenite, hydrotalcite, anatase and tourmaline.
Additionally, µXRF and imaging LIBS (Laser Induced Breakdown Spectroscopy) analyses were performed at the same specimen to obtain detailed information of the element distribution including Li on thick section scale (Nikonow et al. 2019).
The clay containing rocks are intensively deformed by boudinage and subsequent brittle fracturing. The fractures are oriented in different directions and are filled with halite and/or carnallite and single grains of anhydrite and magnesite. Relics of bedding are present, but the phyllosilicates do not show a pronounced shape-preferred orientation. Shear strain is indicated by a slight rotation of single rock fragments. The spatial distribution of Li shows that Li is enriched in certain areas. Li accumulations are observed in single silicate grains, which are unequally distributed in a very fine-grained clay matrix. Furthermore, Li is enriched at the fracture rims, often associated with seams of Fe-bearing phases and probably organic matter.
Depending on the mineralogical composition of the investigated rocks, the Li content varies significantly. Li probably originates from illite-muscovite and a Li-bearing variety of a tourmaline (elbaite). Li was mobilized during brine-host rock interaction and precipitated in fracture infill, probably at reducing geochemical conditions. However, due to the limited spatial resolution of most used methods compared to the very small grain size of the rocks, a distinct relation of Li content to a specific mineral phase requires further analysis.
Braitsch 1971. Springer-Verlag, https://doi.org/10.1007/978-3-642-65083-3
Mertineit & Schramm 2019. Minerals 9, 766; doi:10.3390/min9120766.
Nikonow et al. 2019. Mineralogy & Petrology 113, https://doi.org/10.1007/s00710-019-00657-z
How to cite: Mertineit, M., Schramm, M., Nikonow, W., and Meima, J.: Lithium content and mineralogical composition of fractured salt clay (Upper Permian), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18322, https://doi.org/10.5194/egusphere-egu26-18322, 2026.
The Cation Exchange Capacity (CEC) of clay minerals has been extensively studied in wide applications / purposes, using various imaging techniques to highlight changes of the clay sheet chargeability. Among the clay minerals, swelling clays such as smectite or vermiculite are particularly interesting regarding their adsorption-desorption properties strongly related to their high CEC (80-150 to 100-150 meq/100g respectively). To better monitor and predict cation exchange processes, the CEC has been investigated by different methodological approaches, including X-ray absorption spectroscopy (XAS) and geoelectrical methods. The Spectral Induced Polarization (SIP) is particularly well designed to quantify CEC because its complex conductivity measurements (in phase and quadrature) characterizes the electrical conduction of charge carriers (liquid) and the polarization phenomenon resulting from the local accumulation of electrical charge carriers in the porous medium (mineral interface).
The aim of this work is to investigate in situ how K cations are incorporated within the interlayer of a Ca-montmorillonite by coupling XAS and SIP experimental methods. This novel approach brings multi-scale information at the atomic and clay-sheet levels, providing new insights on enhancing the understanding of CEC mechanisms in terms of time and space and our ability to monitor it with SIP. The experiment was carried out on LUCIA (Soleil synchrotron), at the low energy of potassium K-edge with a microbeam size (2.5 x 2.5 µm²).
We used a 1.7 mm3 cell filled with 0.1 g of Ca-Montmorillonite isolated in a 0.8 µm sieve to avoid flushing of the clay sample during the experiment. The cell was subjected to a solution flux of a few cc per minute with 4 different KCl salinities (0.01, 0.05, 0.1, 1 M of KCl). In situ SIP spectra are compared with XAS to conjointly monitor the CEC exchange at different scales. Preliminary results are shown to test-proof the methods as a new in situ / in operando cross-scale methods for CEC spatio-temporal characterization. Overall,this work is the first step of a technological development project, merging the approaches of geophysicists, mineralogists and physicists to monitor in real time the cation exchange processes of a Ca-montmorillonite by K in swelling clay minerals.
How to cite: Courtin, A., Jougnot, D., Paineau, E., Roy, D., Vantelon, D., Dallaporta, A., and Léger, E.: Combining X-ray absorption and Induced Polarization Spectroscopies for in situ monitoring of Cation Exchange in clay materials, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11961, https://doi.org/10.5194/egusphere-egu26-11961, 2026.
The interactions between fluid and solids in fault zones are governed by slip, slip rate, and constituent properties. These interactions are recorded by particle shape and size distributions, fracture patterns, and the geochemical composition of material within the deformation zone. The evolution of near-surface sediment microstructures and yielding behaviors under tectonic loading and at variable fluid saturation remains an open question. We collect undisturbed 10 x 40 mm cores from unconsolidated silt-sized sediments (fines) surrounding, and along, a fault strand that slipped while saturated, and likely experienced aseismic slip under variable saturation over the past 300 years. We use X-ray microtomography to analyze voids within the fines and found that they are ellipsoidal, have volume distributions that are best fit by a truncated power-law, orient sub-parallel to the fault strike, and sometimes merge into tabular or irregularly shaped fractures. The volume range for power-law scaling in the distributions separates a smaller population of voids with markedly different distributions in sphericity, tortuosity, aspect ratio, and minor/major axis lengths from a larger population of voids. The power-law truncation is likely due to the finite core size. We interpret the voids as initially small gas bubbles that nucleated where cavities existed within the fines and then grew via diffusion of immiscible gases when saturated, or via brittle/ductile yielding of the fines under variable saturation. Several fractures cross-cut or branch off some voids, indicating multiple deformation events and suggesting that the void boundaries are weak spots within the fines that accommodate tectonic strain. Similar growth mechanisms have been observed in magmatic systems, where ductile yielding of the melt occurs from the merging of bubbles that primarily orient at acute angles from the maximum extension direction. These findings suggest that, in addition to sands, pore structures in finer-grained sediments preserve a record of near-surface aseismic slip and may provide a relative estimate of near-surface strain. The findings further imply that a process akin to ductile yielding deformed the fines and, in turn, the pore voids.
How to cite: Dasent, J., Chang, M., Su, K., Wright, V., and Manga, M.: Memory of brittle-ductile yielding within near surface fault zone sediments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16278, https://doi.org/10.5194/egusphere-egu26-16278, 2026.
The evolution of orogens is steered by complex deformation processes that act at several crustal levels, evolving over time from syn- to post-orogenic. Investigating how strain and deformation localize in the ductile domains of the deep crust and brittle domains of the shallow crust can improve our understanding of the processes ultimately controlling the exhumation of deeply seated rocks. Within this framework, the Sestri-Voltaggio Zone (SVZ) of the Italian Ligurian Alps provides a record of rocks and structures attesting to the complete subduction-exhumation cycle during the Europe-Adria convergence. The SVZ is a mature fault zone characterized by a polyphase tectonic evolution and a high lithological variability, which tectonically juxtaposes high-pressure (HP) metamorphic units to non-metamorphic rocks. It also represents an abrupt structural-metamorphic boundary between the Voltri Massif (an eclogitic domain defining a southern culmination of the Western Alps) to west, and the Northern Apennines units (anchi-metamorphic or non-metamorphic) to east. The exhumation processes that led to the current outcropping units of the SVZ occurred following a multi-stage progression from early ductile to later brittle conditions. However, open questions remain reflecting the generalized lack of systematic descriptions of structural fabrics formed during the exhumation-related events of the SVZ units. In this recently launched study we further explore the exhumation mechanisms of the SVZ by investigating how the pre-existing metamorphic fabrics helped localize the brittle deformation that occurred at later stages at shallow crustal levels. Preliminary field observations and structural analyses document N-S to NNE-SSW-striking brittle faults separating lenses of HP-mafic (metagabbros and metabasalts) and carbonate lithotypes from the enveloping phylladic schists and serpentinites. The enclosed lenses exhibit a pervasive internal schistosity that strikes either parallel or at high angle to the orientation of the main SVZ boundaries. By mapping the orientation of the rock fabric as a function of distance, perpendicular to the main tectonic boundaries, it is possible to identify systematic geometric trends between the metamorphic foliations and the bounding brittle faults. Within the matrix, the metamorphic schistosity wraps around the lenses, varying both in strike and dip. Brittle faults, with dominant oblique kinematics, are characterized by a double behavior: they truncate the metamorphic schistosity when approaching massive lenses; but they tend to rework the schistosity within the phylladic matrix. The overall structural record of the investigated units highlights the distribution of strain localization within the deeply exhumed units, suggesting a distinction between episodic vs. progressive transition from ductile to brittle during exhumation. In this sense, the SVG can be considered a useful example of the deformation history of the Western Alps-Northern Apennines tectonic junction, with noteworthy implications on the first-order mechanisms leading to the exhumation of deeply seated rocks.
How to cite: Generi, A., Viola, G., and Vignaroli, G.: Characterizing ductile-to-brittle exhumation of polymetamorphic units along the Sestri-Voltaggio Zone (Ligurian Alps, Italy)., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13049, https://doi.org/10.5194/egusphere-egu26-13049, 2026.
The fault damage zone of the active Milun Fault in eastern Taiwan exhibits fractured and altered fault-rock textures, including spotted schist, serpentinite, and associated gouge. In the vicinity of the upper boundary of the damage zone, the recovered drill core hosts a non-cohesive, pulverized quartz body (~20-30 cm in length) within the fault rocks. The pulverized quartz is sandwiched between fractured schist and millimetre-scale laminae subparallel to the zone boundary. Microanalytical observations show that the quartz is shattered into a fine powder without an evident shear sense or preferred fracture orientation. No shear-induced amorphous phase is detected, whereas Laue diffraction indicates pronounced lattice distortion and elevated residual stress. The pulverized quartz displays a dense tensile fracture network, a feature commonly reported for seismically pulverized rocks along seismogenic faults, suggesting a dilatational, tensile-dominated fragmentation mechanism rather than progressive shear comminution. We propose that the quartz pulverization resulted from high strain rates associated with transient tensile stresses during coseismic rupture, potentially favoured by specific lithologic conditions.
How to cite: Wu, W.-J., Lien, P., Huang, T.-H., Chiu, W., Chiang, C.-Y., and Kuo, L.-W.: Microanalytic characteristics of extremely fractured quartz in fault damage zone and implications, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15835, https://doi.org/10.5194/egusphere-egu26-15835, 2026.
The origin of the intense damage found in active fault cores is still a matter of debate. We investigated the potential co-seismic contribution to this damage by studying the Nojima fault, which ruptured during the 1995 Kobe earthquake (Mw 6.9). Drilled just a year after the event, the Hirabayashi borehole offers a snapshot of the fault zone’s state shortly after a major rupture.
Working within the French ANR AlterAction, we analyzed drill core samples using X-ray computed tomography (CT) at a resolution of ~50 μm. Instead of relying on complex segmentation of fracture geometries, we applied a 3D fractal analysis to the spatial distribution of voids (empty space) versus the rock matrix. This method allowed us to quantify damage intensity and organization using the fractal dimension D. This metric, ranging from 2 (highly clustered voids) to 3 (homogeneous distribution), tracks the transition from localized fracture networks to diffuse pulverization and correlates well with fracture porosity.
We observed a damage zone extending roughly 70 m on either side of the fault core. While open fracture density generally spikes toward the core, it drops sharply in the immediate vicinity, likely due to rapid post-seismic healing. Our analysis shows D values near 2 in clustered zones, rising toward 3 where damage becomes volumetric. Interestingly, some samples display intense micro-fracturing but lack significant macroscopic deformation, resembling the "pulverized rock" seen at other active faults. This texture suggests high strain-rate loading occurred during the earthquake.
To test the dynamic origin of this damage, we ran Split Hopkinson Pressure Bar (SHPB) experiments on intact borehole samples to reproduce pulverization in the lab. We found a linear link between strain rate and absorbed energy. When combined with the CT data, this relationship helps distinguish two modes of propagation: diffuse pulverization (matching near-fault observations) and sparse, poorly connected networks. Crucially, the fractal dimensions of the experimental samples confirm these contrasting morphologies.
These results suggest that the intense damage in the Nojima fault core likely stems from co-seismic processes, marked by specific fractal patterns associated with high strain rates. We conclude that 3D fractal analysis of void space offers a robust tool, independent of geometry, for identifying the dynamic origins of fault zone damage.
How to cite: Iaquinta, R., Doan, M.-L., and Donze, F. V.: 3D Fractal Analysis of Co-seismic Damage in the Nojima Fault Using X-Ray Tomography and SHPB Experiments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21052, https://doi.org/10.5194/egusphere-egu26-21052, 2026.
Recent advances in microanalytical imaging and machine learning enable quantitative, multiscale characterization of geological materials with direct relevance for subsurface energy storage. This study presents an integrated workflow combining Broad Ion Beam (BIB) sample preparation, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX), and advanced machine learning to quantify pore structures, mineralogy, and their spatial relationships from the micrometre to nanometre scale (Klaver et al. 2021).
High-resolution secondary electron (SE2) and backscattered electron (BSE) imaging, complemented by low-resolution EDX data, provides multimodal datasets for automated analysis. Pore networks are segmented using a pre-trained U-Net deep learning model, enabling efficient and accurate porosity quantification. Mineralogical phases are identified and quantified through a semi-automatic, decision-tree–based segmentation approach. The alignment of SE2 and BSE datasets allows porosity to be directly correlated with specific mineral phases, establishing a robust link between microstructure, mineral composition, and petrophysical properties (Jiang et al, 2021).
The applicability of this technology-driven approach is demonstrated through two case studies. Case study 1 investigates geological hydrogen storage in underground salt caverns, focusing on the impact of biotic and abiotic reactions on anhydrite. Flow-cell experiments combined with cryogenic BIB-SEM analyses enable early detection of microstructural, mineralogical, and pore-space changes induced by hydrogen, hydrogen sulfide, and microbial sulfate reduction. Despite slow reaction kinetics, microstructural observations reveal the substantial onset of chemical alteration, biofilm formation, and evolving pore connectivity at the submicron scale, providing essential constraints for geochemical and hydraulic models (Berest et al., 2024).
Case study 2 examines fault sealing in mechanically layered limestone–marl successions. Oriented transfer samples from normal fault systems were analysed using multiscale microanalytical workflows to capture marl smearing, mechanical mixing, fracturing, and cementation processes. High-quality microstructural datasets serve as ground truth for training machine learning algorithms for efficient interpretation of 2D image data. The results show that fault cores are composed of recurrent structural building blocks whose distribution and sealing capacity are strongly controlled by the presence and properties of marly interbeds (Schmatz et al., 2022).
Overall, the integrated microscopy–machine learning framework provides a transferable, data-driven approach for quantifying coupled structural, hydraulic, and geochemical processes in complex geological systems.
References
Berest et al.,2024. Risk assessment of hydrogen storage in a conglomerate of salt caverns in the Netherlands. KEM-28 report. https://www.kemprogramma.nl/documenten/2024/04/03/kem-28-project-rapportfinal-report-kem-28-h2c3-240403_v2
Jiang et al., 2021.Workflow for high-resolution phase segmentation of cement clinker from combined BSE image and EDX spectral data. Journal of Microscopy, 1-7.
Klaver et al., 2021. Automated carbonate reservoir pore and fracture classification by multiscale imaging and deep learning. 82nd EAGE Annual Conference & Exhibition, Oct 2021, Volume 2021, p.1 – 5.
Schmatz et al., 2022. Prediction of Fault Rock Permeability With Deep Learning: Training Data from Transfer Samples of Fault Cores. 83rd EAGE Annual Conference & Exhibition, Jun 2022, Volume 2022, p.1 – 5.
How to cite: Schmatz, J., Jiang, M., and Schmitz, J.: Advanced Microscopy and Machine Learning for Multiscale Analysis of Porosity and Mineralogy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17416, https://doi.org/10.5194/egusphere-egu26-17416, 2026.
The investigation of microstructural characteristics in concrete constitutes a fundamental basis for advancing its performance in civil engineering construction. Existing expertise in digital rock physics (DRP), developed for natural rock materials, is transferred and adapted for concrete. DRP utilizes non-destructive X-ray computed tomography (XRCT) to examine the internal microstructure of concrete, allowing for the visualization of features such as phase distributions, pore space, and microcracks. In this study, high-resolution digital concrete twins are created to capture and quantify internal microstructural changes induced by external mechanical loading. To overcome limitations in phase and microstructure identification caused by the restricted resolution of XRCT, these digital investigations are complemented by detailed microstructural analyses using standard polarization microscopy and scanning electron microscopy (SEM). The results show that externally applied stresses significantly influence the microstructural response of concrete and thus affect the accuracy of physical measurements conducted under high-pressure conditions.
XRCT datasets with varying spatial resolutions were acquired under in-situ confining pressures ranging from 0.1 MPa to 46 MPa. CT images of concrete in unloaded and mechanically loaded states were subsequently analyzed and compared to identify stress-induced microstructural changes, with particular emphasis on the segmentation workflow. Here, particular focus is on large and small concrete aggregates, grain/phase boundaries within the aggregates, (micro-)porosity, and especially the interfacial transition zone (ITZ), which represents a major source of uncertainty in phase assignment during segmentation.
Image quality was first assessed by identifying artifacts and evaluating grayscale histograms. Subsequently, global thresholding was applied for phase assignment and initial segmentation, which was iteratively refined using complementary microscopic analyses of thin sections, including SEM, as reference data. The resulting segmentation of the concrete subvolume (600x600x769) distinguishes large and small aggregates (<80 % quartz, ca. 20 % phyllosilicates), pore space, phyllosilicate-composed matrix, silica-composed matrix, and inclusions (mainly rutile, zircon, apatite, iron oxides). Small changes can be seen in the distribution of the individual phases at the different pressures. With increasing pressure, the porosity decreases, and partially areas with characteristic phase arrangements arise along the large aggregates, potentially indicating the influence of the ITZ.
However, the quantitative determination of the interfacial transition zone remains challenging using XRCT data, and microcracks are likewise difficult to reliably resolve and segment. Therefore, the high-resolution microstructural investigations are also required to adequately capture these features. Overall, the study highlights the necessity of detailed microstructural characterization for the reliable interpretation of XRCT data and the assessment of stress-induced changes in concrete.
How to cite: Beiers, L. M., Balcewicz, M., Lebedev, M., and Saenger, E. H.: Digital Concrete Physics – Microstructural Characterization of Concrete under Confining Pressure: Insights from X-ray Computed Tomography and Microscopy , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17609, https://doi.org/10.5194/egusphere-egu26-17609, 2026.
The properties of fault slip surfaces, gouge characteristics, and fluid-rock reactions are tightly coupled and control earthquake mechanics. To visualise and quantify the role of this coupling, we have developed a new operando imaging approach that allows the documentation of fast direct-shear deformation experiments in time-resolved 2- and 3-dimensional image data at low single-digit micrometer resolution. A direct-shear inset developed for the X-ray transparent Heitt Mjölnir triaxial deformation apparatus enables experiments at 20 MPa normal stress under fluid-pressurised conditions and allows real-time permeability measurements.
We apply this platform to three fault systems: 1) Slip surfaces in basaltic rocks, imaged while sliding at 1 mm.s-1, reveal how asperities, phenocrysts, and surface roughness control stick-slip behavior and damage localization during fast slip. 2) Reactive quartz-gypsum gouges imaged during velocity stepping and healing experiments, enable the direct linking of evolving frictional properties to microphysical developments. 3) A shearing, dehydrating gypsum gouge provides insights into transient rheologies and the resulting strain distributions.
These datasets demonstrate that 4D imaging resolves coupled mechanical, chemical, and hydraulic fault evolution in real time. Our approach allows documenting microphysical processes underlying the frictional properties of faults and thereby constitutes a potent tool for studying faults in a variety of tectonic settings.
How to cite: Jayawickrama, E., Harpers, N., Schwichtenberg, B., Bell, A., Ng, A., Rizzi, R., Cordonnier, B., Herwegh, M., and Fusseis, F.: Recent Developments in 4D X-ray Tomography for Real-Time Observation of Fault Slip and Gouge Evolution, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18619, https://doi.org/10.5194/egusphere-egu26-18619, 2026.
Direct observations of geological processes are often limited by available observation time, the spatial resolution of imaging techniques or the accessibility of active sites. These limitations also apply to fault healing, whereby faults progressively regain strength throughout the interseismic phase of the earthquake cycle. Here, conventional approaches either capture a static snapshot of the final microstructure in exhumed natural fault rocks or focus on the bulk mechanical behaviour through slide–hold–slide or direct shear experiments. However, these approaches generally fail to resolve the dynamic evolution of the microstructural record, and the associated chemo-mechanical feedback that controls a rock’s hydraulic properties. To overcome these limitations and constrain the spatiotemporal coupling between mechanical, chemical, and hydraulic processes in healing fault gouges, we conducted a series of direct shear experiments on analogue fault gouges composed of a quartz–hemihydrate mixture. We then monitored their microstructural evolution using operando 4D synchrotron-based X-ray CT imaging.
Our experiments, performed at constant shear rates of 0.3–1 µm/s, were designed to mimic gouge-rich faults in the uppermost continental crust during the interseismic phase. In the presence of a reactive pore fluid, we simulated chemical fault healing through dissolution-reprecipitation and cementation, which are associated with the hydration reaction of CaSO₄ hemihydrate to gypsum. In our deforming samples, these time-dependent healing processes compete with mechanical weakening processes, such as frictional granular flow.
Our novel approach combines an innovative experimental setup [1, 2] with high-resolution 4D imaging and advanced image analysis techniques, including digital volume correlation (DVC). In this contribution we discuss the benefits of integrating micromechanical data with high-resolution 4D imaging by linking active deformation mechanisms to the evolving mechanical and hydraulic response of the simulated fault gouge. Further, we demonstrate a shear-rate-dependent competition between time-dependent healing processes and mechanical weakening.
[1] Freitas, D. et al. (2024): Heitt Mjölnir: a heated miniature triaxial apparatus for 4D synchrotron microtomography. Journal of Synchrotron Radiation 31, 150-161. doi.org/10.1107/S1600577523009876
[2] Jayawickrama, E. et al. (2026): Recent Developments in 4D X-ray Tomography for Real-Time Observation of Fault Slip and Gouge Evolution, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18619.
How to cite: Schwichtenberg, B., Fusseis, F., Jayawickrama, E., Cordonnier, B., Harpers, N., and Herwegh, M.: Probing active deformation - Fault healing through the lens of 4D operando imaging, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21418, https://doi.org/10.5194/egusphere-egu26-21418, 2026.
