Fluid–rock interaction, and thus fluid flow plays a fundamental role in geological processes from the shallow crust to mantle depths. In the upper crust, fluid flow is predominantly controlled by brittle features and interconnected porosity. In contrast, at lower crustal conditions, elevated lithostatic pressures are commonly assumed to inhibit fracturing and major porosity, leaving unresolved how fluids migrate at depth. To investigate porosity development during high-pressure metamorphism of initially impermeable mafic rocks, we conducted a series of piston-cylinder experiments that varied reaction duration and fluid availability. Dolerite from the Kerforne dyke (Brittany, France) was used as starting material. Cylindrical samples (2.8 mm diameter, ~3.5 mm length) were characterized prior to experimentation using X-ray micro-computed microtomography (µCT; 3 µm Isovoxel), enabling three-dimensional quantification of mineral fabric, modal proportions, and initial porosity. The starting dolerite consists of ~50–65% plagioclase, 20–30% pyroxene, up to 10% ilmenite, and minor quartz, with an initial porosity of ~0.1%. The fabric is near-isotropic and dominated by randomly oriented plagioclase grains up to 300 µm in length.

Experiments were performed under quasi hydrostatic conditions at 700 °C and 2.4 GPa for varying durations. To evaluate the influence of fluid availability on reaction progress and porosity evolution, three experimental setups were employed: (1) nominally dry conditions without added fluid, (2) addition of paragonite as a source of fluid and sodium, and (3) addition of 5 vol% water (“wet” conditions). Wet experiments were conducted for durations of 1, 7, and 21 days to assess the temporal evolution of reactions.

Following experimentation, all samples were re-imaged using µCT, allowing three-dimensional mapping of reaction progress and porosity development. Largely unreactive Fe–Ti oxides served as internal markers, enabling accurate registration of pre- and post-experimental µCT datasets and direct comparison between the protolith and reaction products. Three-dimensional observations were complemented by high-resolution two-dimensional analyses using electron probe microanalysis and scanning electron microscopy.
Reaction progress increases systematically with fluid availability, from dry to paragonite-bearing to water-added conditions. Under nominally dry conditions, reactions are restricted to narrow zones along pyroxene–plagioclase interfaces and plagioclase grain boundaries, producing predominantly fine-grained zoisite needles (<5 µm). In paragonite-bearing experiments, reaction intensity increases within plagioclase, characterized by the growth of zoisite and phengite, while jadeite forms along pyroxene–plagioclase boundaries. In contrast, wet experiments result in complete replacement of plagioclase within 7 days by an assemblage of zoisite, phengite, amphibole, and minor omphacite and quartz. Pyroxene develops narrow reaction rims (<30 µm wide) marked by increasing Al and Na and decreasing Fe and Ca contents, while garnet occurs as idiomorphic grains in the fine-grained matrix or as coronae surrounding oxides.
Porosity development is closely coupled to reaction progress, and three distinct porosity types are identified: (1) micro- to nanopores within plagioclase reaction products, (2) nanopores within pyroxene reaction rims, and (3) microfractures. The first two porosity types are interpreted to result from volume reduction associated with density increases during metamorphic reactions, whereas microfractures likely form in response to stress concentrations and elevated pore-fluid pressures.  

How to cite: Rogowitz, A., Degenhart, G., Konzett, J., Huet, B., Stoiber, W., and Tropper, P.: High-pressure metamorphism induced porosity in mafic rocks – wet vs. dry, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9041, https://doi.org/10.5194/egusphere-egu26-9041, 2026.

During subduction, progressive heating and burial drive dehydration reactions that release H₂O-rich fluids from altered oceanic crust. Under sub-arc conditions (2–4 GPa), the transition from blueschist- to eclogite-facies mineral assemblages is accompanied by the release of substantial amounts of water (up to ~5 wt.%). If fluid transport occurs on timescales that are short relative to subduction, these fluids migrate through the overlying oceanic crust into the mantle wedge or along the slab–wedge interface. This process is critical for the generation of arc magmas and for their enrichment in fluid-mobile trace elements. Despite the importance of this process, relatively few direct constraints exist on the extent to which such fluids react with the eclogite-facies crust through which they migrate.

Here, we present field, petrological and geochemical observations from a pristine suite of transport veins preserved in mafic eclogites of the Tauern Window, Eastern Alps. The veins are dominated by high-variance, quartz-rich mineral assemblages and are surrounded by well-developed, omphacite-dominated selvages. Phase equilibrium modeling indicates that vein formation occurred at or near peak pressure–temperature conditions of ~2.5 GPa and ~600 °C. A striking feature of the fluid–rock interaction is the near-complete consumption of garnet by the reactive fluid. Trace-element zoning in partially reacted garnet porphyroblasts records a fluid-driven dissolution–precipitation mechanism that mobilized middle and heavy rare earth elements (MREE and HREE). Isocon analysis of the altered eclogite selvages reveals bulk gains in Na and Li, accompanied by losses of REE, Sr, K, Cu, Fe, Al, Y, Mn, Ba, and Cr, while Ni, Sc, and Ti appear to have been conserved.

Pure omphacite layers and seams are commonplace throughout the Eclogite Zone and are interpreted as sealed transport veins. The associated microstructures record embrittlement and fracturing following fluid–rock interaction. Collectively, these observations indicate that reactive fluid flow under eclogite-facies conditions may influence the rheology of subducting oceanic crust.

How to cite: Smye, A., Strobl, L., Forgeng, H., and Fisher, D.: Linking Reactive Fluid Flow to Rheology of Eclogite-Facies Oceanic Crust, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15970, https://doi.org/10.5194/egusphere-egu26-15970, 2026.

Shear zone nucleation in massive rocks commonly exploits pre-existing planar structures, whose presence and type control fluid availability and redistribution. While fluids are widely recognized as key triggers for metamorphic reactions and mineralogical transformations that influence rock rheology, the exact feedback processes between fluid-rock interactions, metamorphism and deformation remain enigmatic. In specific cases, traditional softening mechanisms (e.g. reaction-induced grainsize reduction, crystallization of weak minerals) do not apply or are insufficient to explain shear zone nucleation and strain localization, implying the existence of alternative processes.

Paired shear zones developed at the selvages of hydration haoles are a common product of fluid infiltration along hydrofractures, and are a source of information to investigate the rheological effects of the different reaction extents occurring during fluid percolation. Here, we investigate a suite of samples containing eclogitic clinozoisite-filled veins surrounded by omphacite-rich haloes. The sample set includes haloes (a) weakly affected by ductile deformation, preserving pristine metasomatic textures, and (b) displaying paired shear zones at their selvages (Pennacchioni, 1996). The eclogitic host rock foliation, consisting of garnet, clinozoisite, amphibole and omphacite, is only partially obliterated in the hydration halo by the metasomatic overprint, dominated by replacement of clinozoisite by omphacite. EDS major element and in-situ LA-ICP-MS trace element analysis suggests that fluid propagation caused recrystallization, changes in mineral proportions and (re)distribution of major and trace elements, forming a compositional gradient across the halo. Garnet and clinozoisite rims record the gradient with a progressive decrease in the Fe²⁺ content and a progressive increase in LREE and Fe³⁺ concentrations from the vein selvage towards the reaction front, respectively.

Electron backscatter diffraction (EBSD) maps provide evidence for (i) a constant omphacite grainsize across the haloes and at their boundaries in samples weakly affected by ductile deformation, suggesting that metasomatism does not produce textural gradients, (ii) development of very fine-grained monomineralic ribbons of omphacite along the shear zones, suggesting that omphacite is responsible for weakening and localized shearing, (iii) local orientation of these ribbons at 20-30° to the shear zone trace, defining C' bands, and (iii) random orientation of the fine grains. We interpret these observations as evidence for diffusion-assisted grain boundary sliding (GBS) and creep cavitation as the main deformation mechanism active along the shear zones, and for heterogeneous nucleation of very fine-grained omphacite within fluid-filled cavities formed during GBS.

We conclude that, when metasomatic reactions do not directly result in textural gradients (e.g. grainsize decrease) traditionally considered responsible for softening at the propagation front (i.e. halo boundary), shear zones may develop by heterogeneous nucleation of fine grains during fluid-assisted GBS, which further fosters grainsize-sensitive deformation sustaining strain localization within fluid-rich domains.

[1] Pennacchioni, 1996. Journal of Structural Geology, 18, 549-561

How to cite: Cacciari, S., Pennacchioni, G., Cannaò, E., Toffol, G., Scambelluri, M., and Hermann, J.: Shear zone nucleation by fluid-assisted heterogeneous nucleation recorded in texturally homogeneous eclogitized mafic granulites, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16837, https://doi.org/10.5194/egusphere-egu26-16837, 2026.

The magnitude and distribution of stress in Earth’s crust is difficult to quantify, but impacts deformation behavior, phase stability, and metamorphic reactions. Stress is influenced by a variety of factors including compositional heterogeneities, volume changes during ongoing reactions, and the influence of far-field stresses. During metamorphic reactions the stress distribution may be modified, but prevailing stresses may also impact reaction kinetics, or which reactions take place. We studied one of the most impactful reactions within the continental crust; the fluid-induced breakdown of plagioclase at high-pressure conditions. The samples are from former lower-crustal granulites exposed on Holsnøy, western Norway. They preserve a reaction front along which the dry granulite is transformed into an eclogite. Reaction progress is intimately linked to fluid ingress and there is no microstructural evidence of deformation. This lack of deformation indicates that the studied microstructures are entirely related to fluid-induced metamorphic reactions. We measured the residual stress associated with plagioclase breakdown by high-angular resolution electron backscatter diffraction and contrasted the results with compositional variations (scanning electron microscope and electron probe micro analyzer). (Scanning) transmission electron microscopy was conducted on selected sites to link this information with the associated dislocation configuration. We find that intragrain residual stress associated with the breakdown of plagioclase is on the order of hundreds of megapascals, and dominantly caused by the elastic interactions of dislocations. Before the reaction plagioclase contains few, randomly oriented dislocations. Compositional modification of plagioclase during the reaction (increasing albite content) leads to dislocations occurring more frequently in the more albitic part of the plagioclase. In that case, dislocations have a preferred orientation, but no significant long-range increase in dislocation density, i.e., increased organization. Our results thus suggest that as plagioclase breaks down, dislocations are mobilized to accommodate the variations in lattice parameters associated with hundreds-of-megapascal stress variations on the grain scale.

How to cite: Zertani, S., van Schrojenstein Lantman, H. W., Kaatz, L., Chogani, A., Plümper, O., Menegon, L., and John, T.: Grain-scale residual stress distribution associated with fluid-induced plagioclase breakdown , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8177, https://doi.org/10.5194/egusphere-egu26-8177, 2026.

Densification and deformation during eclogitization govern the strength and buoyancy of orogenic roots and the stability of mountain ranges over geological timespans. The breakdown of albite to jadeite + quartz represents a key end-member reaction that is associated with densification of about 20%; shear stresses induced by such volumetric changes may cause brittle failure and have been linked to intermediate-depth seismicity (Yamato et al., 2022). Eclogitization is a kinetically sluggish process that requires significant reaction overstepping and may proceed far beyond the equilibrium pressure–temperature conditions, and/or remain largely incomplete – particularly in fluid-deficient felsic crust as evidenced by field observations (Palin et al., 2017) and geophysical constraints (Hetényi et al., 2021). However, the kinetics of the albite = jadeite + quartz reaction is poorly constrained, especially regarding the roles of grain size, pressure–temperature overstepping, and reaction duration. To address this gap, we conducted high-pressure experiments using a piston-cylinder apparatus at the Institute of Geosciences, JGU Mainz. Natural albite crystals were crushed and sieved into grain size fractions between 50 and 500 µm, loaded into Au-capsules, and separated by Au-foils. A subset of experiments involved furnace-drying (~500°C) of the starting materials followed by hot-welding of the capsules to minimize atmospheric moisture contamination. In experimental stage I, pressure was initially set just below (~1 kbar) the albite = jadeite + quartz reaction boundary (Holland, 1980), followed by heating to target temperature. In stage II, pressure was increased at constant temperature to variable target pressures above the reaction to systematically explore the effect of reaction overstepping. Samples where quenched by power shutdown, and reaction progress was quantified using scanning electron microscopy (backscattered electron imaging and cathodoluminescence) based on the relative fractions of reactant albite and products jadeite–quartz. Preliminary results reveal highly variable degrees of reaction progress. Where present, jadeite–quartz occur as finely intergrown symplectites, typically decorating albite grains at the rims, as well as forming within larger albite grains. The latter textures indicate complications arising from fluid inclusions in the starting material. By combining constraints on P(T) overstep, grain size, and experimental run duration, we determine effective reaction rates for albite breakdown. These results provide end-member kinetic constraints on high-pressure transformation in fluid-deficient, coarse-grained felsic rocks, which constitute the bulk of many well-known (U)HP terranes such as the Western Gneiss Region (Norway) and the Dabie–Sulu belt (China).

References

Hetényi, G. et al. (2021). Metamorphic transformation rate over large spatial and temporal scales constrained by geophysical data and coupled modelling. Journal of Metamorphic Geology, 39(9), 1131–1143.

Holland, T. J. (1980). The reaction albite = jadeite + quartz determined experimentally in the range 600–1200°C. American Mineralogist, 65(1-2), 129–134.

Palin, R. et al. (2017). Subduction metamorphism in the Himalayan ultrahigh-pressure Tso Morari massif: an integrated geodynamic and petrological modelling approach. Earth and Planetary Science Letters, 467, 108–119.

Yamato, P. et al. (2022). Reaction-induced volume change triggers brittle failure at eclogite facies conditions. Earth and Planetary Science Letters, 584, 117520.

How to cite: Schorn, S., Yang, Z., Hawemann, F., Buhre, S., Botcharnikov, R., and Moulas, E.: Experimental constraints on the kinetics of the albite = jadeite + quartz reaction depending on grain size and reaction overstepping, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10026, https://doi.org/10.5194/egusphere-egu26-10026, 2026.

A common feature of many hydrothermal ore deposits is that they formed during tectonic switches between extension and shortening on plate boundaries. Several theories to explain this relationship have been proposed but evidence for a mechanism remains elusive. Many of these ore deposits occur within or adjacent to ductile shear zones that changed movement direction during the tectonic switch. Prior to tectonic switches, shear zone structures evolve to orientations optimised to accommodate deformation, which maximises strain rate and creates permeable pathways for fluid migration. But when a tectonic switch occurs the structures are misoriented and must reconfigure to accommodate the new shearing direction. Using numerical models of shear zone evolution, we determined that during tectonic switches the microstructural reconfiguration reduces the strain rate and mean stress, causing fluid influx into the shear zone. To further explore the effect of this microstructural reconfiguration on fluid migration we examined rocks of the Bergen Arc shear zone in Norway in a transition zone where sinistral shearing is progressively overprinted by dextral shearing. We find that during the tectonic switch, accretionary veins of quartz, ankerite and calcite formed in dilatational spaces that opened as the sinistral structures were reconfigured to accommodate dextral shearing. With increasing strain, fluid migration into the shear zone became more pervasive, evidenced by larger vein networks and hydrothermal breccias. Coincident with vein formation there is a statistically significant increase in the water content in quartz as determined by synchrotron FTIR. These data indicate that the microstructural reconfiguration in shear zones during tectonic switching causes fluid influx into shear zones. This process may be responsible for the introduction of ore fluids into the shear zone and the formation of hydrothermal ore deposits during tectonic switching.

How to cite: Finch, M., Knight, B., Tomkins, A., Gomez-Rivas, E., Bons, P., Ribeiro, B., and Olesch-Byrne, A.: Changes in fluid migration in ductile shear zones during tectonic switching may explain the formation of hydrothermal ore deposits, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2868, https://doi.org/10.5194/egusphere-egu26-2868, 2026.

Geological fluids play crucial roles in the stabilisation of minerals and in the mobilization and redistribution of elements during mid- to lower-crustal metamorphism, thereby influencing the chemical evolution of continental crust. The current study demonstrates extensive fluid-induced alterations and formation of a highly peraluminous (A/CNK~30), ferromagnesian yet silica-calcium and alkali-poor rock from the Makrohar granulite belt, part of the Proterozoic mobile belt, Central Indian Tectonic Zone. The study demonstrates a black, garnet-rich, massive rock composed of garnet, cordierite, sillimanite, quartz, and ilmenite, lacking gneissic banding and intruded by multiple veins in the outcrop. Thin microscopic veinlets consist of biotite (80% modal volume) with smaller proportions of quartz±cordierite and fibrolites. Late-stage veins of variable thickness, evident from the outcrop scale, contain coarse-grained sillimanite-quartz-garnet, with large sillimanite grains growing at high angles to the vein boundary, indicating a syntaxial growth. Vein garnets frequently grow inward from the vein wall, often growing on older garnet in the host. Phase equilibrium modelling, coupled with conventional thermobarometry, constrains the P-T conditions of the stabilization of the host rock at approximately 600ºC and 5 kbar. Slightly magnesian cores of the host garnet (X_Mg) yield marginally higher temperatures (~680ºC) than the rim (garnet isopleth yielding ~580ºC).

Garnet grains in the host rock display a distinct positive europium anomaly (Eu/Eu*), likely resulting from garnet growth in the absence of plagioclase. A moderate Gd/Dy ratio in the host garnet indicates stabilization at approximately 4 kbar, supporting low-pressure estimates from conventional barometry and phase-equilibria modelling. Rim-to-rim trace element profiles along host garnet grains show a uniform distribution of Sc, Y, and HREEs in the core, with oscillations and a sharp increase near the rim, suggesting that reverse zoning in HREEs was likely caused by homogenization by intragranular diffusion in the core but remained largely unaffected towards the rim. Whole-rock chemistry of the host, feldspar-free high-variance mineralogy, absence of leucosome and reverse zoning of Y-HREE, positive Eu/Eu* within garnet indicate potential metasomatic alteration of the host itself.

Garnets within the quartz-sillimanite veins exhibit distinct oscillations in trace element concentrations along wall-to-wall line scans, indicating minimal effects of diffusion and grain growth in the presence of vein-fluid. Ca and Mn zoning within vein garnet exactly replicate each other with gradual increase from vein-wall to vein-axis regions of the grains. Y and HREEs show resonating patterns with sharp central peaks in the mid-axis and oscillatory zoning within the vein-wall garnet portions. Sc and MREEs, i.e., Sm, Eu, Gd, Tb still show central peaks along with an annular maxima added with the oscillation within the vein-wall garnets. REE mobilization, at least in micro-scale, is further evident from large monazite clusters observed in sillimanite-quartz-garnet veins. The presence of large sillimanite grains further demonstrates the fluid's capacity to transport aluminium. The absence of any hydrous phases in the vein supports the prevalence of low-H₂O-bearing fluid. XCO2-µK2O and µK2O-µFeO topology further confirms that the intrusion of low-H₂O fluid presumably destabilized the host biotite, producing garnet and quartz in the vein.

How to cite: Chakrabarty, A., Anczkiewicz, R., and Sanyal, S.: Fluid-induced redistribution of REEs within alumino-silicate veins and peraluminous host rock in the Central Indian Tectonic Zone, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7449, https://doi.org/10.5194/egusphere-egu26-7449, 2026.

Gneiss domes are typical tectonic types related to deep crustal exhumation, and their polyphase superimposed deformation characteristics make them ideal natural laboratories for studying deep crustal exhumation processes. The Laojunshan gneiss dome in Yunnan, China, lies at the junction of the Tethyan and Circum-Pacific tectonic domains, as well as the boundary between the South China Block and Indochina Block. Its tectonic setting, deep crustal tectono-thermal evolution and exhumation history are closely linked to the kinematic evolution of the two blocks, making it a key site for investigating the Tethyan tectonic domain’s spatiotemporal evolution and inter-block interactions. Based on systematic field investigations combined with microstructural analysis, stress field inversion, electron backscatter diffraction (EBSD) analysis, geochemical and geochronological analyses, significant findings on the dome’s exhumation-related tectono-thermal evolution are obtained. The Laojunshan tectonic units comprise a core dominated by high-grade metamorphosed-deformed rocks and granites, an arcuate detachment fault system, and a sedimentary cover. Regional stress field inversion reveals two distinct regimes (compressional and extensional), with the latter predominant and radially distributed, reflecting late exhumation tectonics. EBSD analysis of major exposed minerals indicates the core underwent high-temperature plastic deformation (620–710 °C). Mylonite parameters (fractal dimension, differential stress) in the detachment fault zone reflect transitions between high and medium-high temperature deformation. Epidote EBSD constrains late exhumation P-T conditions to 350–500 °C, which, combined with geochemical data, divides late exhumation into three stages: deep compression, uplift transition and shallow extension. Geochronological data show the Caledonian (445–420 Ma) as the main formation period of granitic gneiss protolith (synchronous with coeval magmatism), core leucogranite emplacement at 416–411 Ma, and metamorphic zircons in plagioclase constrain Indosinian high-temperature metamorphism and shortening deformation to 241–220 Ma. An exhumation model is proposed: the dome initiated with early Caledonian (445–420 Ma) regional extension and magmatism, followed by 420–410 Ma compressional orogeny, crustal thickening and anatexis. Indosinian (241–230 Ma) compression induced thrusting, folding and detachment faults. Yanshanian (144–80 Ma) extension and magmatism accelerated exhumation, and Cenozoic (33–21 Ma) strike-slip faulting drove rapid exhumation to the surface.

How to cite: Tian, Y., Cao, S., Zhan, L., Liu, J., Zhou, D., Li, Q., and Tao, L.: Tectono-Thermal Evolution of the Laojunshan Gneiss Dome in Yunnan, China: Constraints from Multi-Mineral Deformation and Composition, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16463, https://doi.org/10.5194/egusphere-egu26-16463, 2026.

Serpentinization is a hydration process that forms distinct serpentine minerals depending on the pressure and temperature conditions prevailing during the fluid-rock reaction. The chemistry and petrology of serpentinite rocks provide constraints on the protolith composition and on the tectonic setting of serpentinization through insights of pressure and temperature conditions.

The Münchberg Massif is a stack of four tectonic nappes of different metamorphic grade, which were emplaced during the Variscan Orogeny. Within the lowermost Prasinit-Phyllit-Serie, several serpentinite bodies are intercalated. Understanding the formation of these serpentinites will add further insights into the tectonic development of the Münchberg Massif.

We present new petrological and chemical data of serpentinites from ten locations along the southeastern margin of the Münchberg Massif, as well as at two locations in the western region of the Massif. The samples are dominated by the serpentine minerals lizardite in the western region and antigorite along the southeastern margin. Furthermore, significant differences in the degree of serpentinization and tectonic strain were observed between the two regions. The petrological and chemical characteristics of the samples indicate distinct protolith material and serpentinization setting. We propose that the protoliths of the western and southeastern serpentinites originated from different structural positions within, or adjacent to, a subduction zone. These findings provide new constraints on the tectonic assembly and metamorphic evolution of the Münchberg Massif.

How to cite: Hasch, M., Klitzke, P., Bagge, M., Koglin, N., and Ostertag-Henning, C.: The formation of the Münchberg Massif: New insights from petrology and chemistry of its serpentinite occurrences., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18428, https://doi.org/10.5194/egusphere-egu26-18428, 2026.

Megathrust shear zones are weak interplate faults that accommodate deformation under low effective stress in a fluid-rich environment. The evolution of stress and fluids during megathrusts activity can be reconstructed from syn- and post-tectonic mineral veins in exhumed settings.

The Sestola–Vidiciatico Unit (SVU) in the Northern Apennines is a fossil analogue of a shallow subduction megathrust mélange. It represents the plate boundary shear zone (200–400 m thick) between the Ligurian prism and the underthrusted Adria microplate active during the early–middle Miocene with peak temperatures of ~170 °C. The SVU is composed of kilometre-scale slices of marls, shales and sandstones derived from the Ligurian prism and its sedimentary cover, thrust along a basal décollement onto younger Adria-derived foredeep turbidites.

This study focuses on a well-exposed outcrop along a south-dipping thrust ramp of the basal décollement of the SVU. The footwall consists of sandstones and siltstones of Langhian age, overthrust by a slice of Aquitanian marls, and by an upper slice of Priabonian – Bartonian claystone. We performed structural mapping, microstructural and geochemical analyses (O-C stable isotopes, trace and major element geochemistry), and fluid inclusion studies on calcites filling shear and extensional veins and cementing tectonic breccias.

Marls and claystones within the SVU are bounded by sharp thrust surfaces decorated by multiple generations of shear veins. In the vicinity of the main thrusts, marls and claystone are crosscut by pervasive shear fractures, bounding flattened and elongated lithons defining a foliation at low angle to the thrusts. Deformation in the footwall includes oblique cleavage, bedding-parallel shear planes, and a conjugated set of NNE-SSW left-lateral and N-S trending right-lateral subvertical transtensional faults showing mutually crosscutting relationships with the basal thrust of the SVU. Calcite shear veins mark thrust surfaces, whereas transtensional faults in the footwall are marked by shear and extension veins, as well as calcite-cemented breccias. Two different calcite phases have been observed: an early-stage calcite, rich in host-rock inclusions and a later inclusion-free calcite.

Geochemical and thermometric results point to two groups of distinct mineralizing fluids circulating through the fracture network: (1) diluted seawater precipitating early-stage calcites at low temperatures (< 50 °C up to 70 °C); (2) an external low-salinity fluid precipitating later-stage calcites at higher temperatures (~80-100° C).

Our data suggest a transition from low temperature and low salinity fluids, possibly from mixing of seawater and fluids released from clay dehydration during progressive burial of the SVU, to the ingression of moderately hot fluids (up to 120 °C) external from the system. This indicates that the onset of fluid circulation by faulting is modulated by the embrittlement and seismic ruptures in subduction zones, favoured by a low-stressed environment.

How to cite: Rocca, M., Mittempergher, S., Remitti, F., Molli, G., Gasparrini, M., Hawemann, F., Diamanti, R., Preto, N., and Tesei, T.: Evolving fluid pathways in a shallow mega-thrust shear zone (Northern Apennines, Italy), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10373, https://doi.org/10.5194/egusphere-egu26-10373, 2026.

Deep fluid activities driven by strike-slip fault movement play important roles in the modification of carbonate and hydrocarbon accumulation. The complexities of deep fluid sources and temperature-pressure variations during strike-slip fault movement complicate fluid-rock reactions, diagenetic modification processes and the formation and evolution of reservoirs in deep to ultradeep carbonate strata. To understand the temporal-spatial coupling between strike-slip movement and deep fluid migration, we investigate the migration periods and sources of deep fluids along strike-slip fault belts in the Fuman Area of the Tarim Basin, considering the geometry of the strike-slip faults and analysing laser ablation U-Pb dating, clumped isotopes, REE, and fluid inclusions in diagenetic products such as calcite, chert, and quartz.

U-Pb age results indicate that vug-filling calcites were emplaced between 460.8 ± 3.4 and 448.6 ± 5.3 Ma, and at 335 ± 19 Ma during the early Hercynian orogeny, while the fracture-filling and megacrystalline calcites formed between 364 ± 53 and 282.9 ± 5.4 Ma, and during periods from 324 ± 23 to 300.9 ± 4.8 Ma and from 244.13 ± 13 to 240.5 ± 4.1 Ma, respectively. The latest fracture-filling calcites show a slightly younger U-Pb age of ca. 158 ± 17 Ma. In addition, the U-Pb ages for the chert and quartz in fractures (459 ± 57 Ma, 252 ± 56 Ma, and 174 ± 35 Ma) fall within the middle Caledonian, late Hercynian, and Yanshanian periods.

The combination of geochemical analyses on calcites, including clumped isotopes, d13C, d18O, and 87Sr/86Sr isotopes, REE, and fluid d18O calculation, suggests that these calcites were precipitated from formation fluids mostly of meteoric water origin with some input from hydrothermal fluids. Hydrothermal fluid flow resulted from strike-slip fault activity and volcanism, whereas meteoric water intruded from uplifted areas along the faults during tectonic quiescence. This study shows that the formation of fracture-related cavern reservoirs in the Fuman oil field is related to the early Hercynian, late Hercynian, and Yanshanian tectonic events and their associated fluid activity. The methodologies and outcomes of the present study may guide future hydrocarbon exploration in the Tarim Basin and be applied to other oil fields with similar tectonic backgrounds.

How to cite: Qiao, Z.: Fluid activities Controlled by Intra-craton Strike-slip Faults: A Case Study of Ordovician in Fuman Area in Tarim Basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6851, https://doi.org/10.5194/egusphere-egu26-6851, 2026.

Crustal-to-lithospheric-scale strike-slip faults can act as major pathways for crustal and mantle fluids, with implications for natural resources (e.g., geothermal, ore deposits) as well as natural gas storage and migration. The Betic Cordillera (SE Spain) records a complex geodynamic evolution, from slab retreat, tearing and delamination to the later inversion of a thinned continental margin. This extreme crustal thinning (~15 km) associated with metamorphic dome exhumation during the Miocene was accommodated by crustal to lithospheric-scale faults that are still active today (e.g. the Mw 5.2 Lorca earthquake in 2011).

To determine the geochemical origin of fluids, their migration pathways, and fault-controlled permeability through time, we analyzed noble gases (He, Ne, Ar) as inert, non-reactive geochemical tracers in both paleo- and present-day fluids. Noble gases in paleo-fluids were analyzed in quartz and calcite minerals associated with faults (INGV, Palermo). We also analyzed noble gases dissolved in water discharged from thermal springs at ~20 to 53°C (Eawag/ETH Zürich).

In calcite and thermal water, low ³He/⁴He ratios (R/Ra ≤ 0.5) indicate mixing between a dominantly crustal component and a mantle contribution or a mixed crustal-atmospheric origins. Quartz samples show stronger atmospheric contamination than in calcite, although ⁴⁰Ar/³⁶Ar ratios may suggest deep input (mantle vs crust; ⁴⁰Ar/³⁶Ar values between 490 and 1215). He-Ne isotopes in paleo-fluids reveal two areas that show a likely mantle-derived noble gas signature: Sierra Elvira, with ~3% mantle contribution suggested, and the Carboneras Fault Zone, with ~6%. In contrast, present-day fluids could reflect a ~4% mantle contribution in the northeastern Betics at Mula and Archena. We infer that mantle-derived signatures detected in paleo-fluids are not preserved in the same locations in present-day fluids. For instance, along a single fault system (e.g., the Carboneras Fault), paleo-fluids display up to four times higher mantle contributions (Rc/Ra ≈ 0.5) than present-day fluids (Rc/Ra ≈ 0.1). This contrast opens new questions regarding potential changes in mantle fluid sources or migration pathways during the evolution of the Betic Cordillera, the impact of tectonic inversion on deep fault permeability, the residence time of fluids in the crust, and the role of fault geometry in controlling fluid pathways.

How to cite: Cateland, B., Battani, A., Mouthereau, F., Brennwald, M. S., Caracausi, A., Lefeuvre, B., and Pujol, M.: Migration pathways of crustal and mantle fluids during the formation of the Betic Cordillera (SE Spain)., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12738, https://doi.org/10.5194/egusphere-egu26-12738, 2026.

The analysis of fault-related mineralization, with particular emphasis on fluid inclusions (FIs) trapped in syn-kinematic minerals, provides crucial insights into fluid circulation modality and fluid–rock interactions, so furnishing new tools to investigate the relationship between fluids and active tectonic. This study investigates the genesis, microstructural characteristics, and geochemical signatures of calcite veins associated with dip-slip faults in the Irpinia region (southern Apennines, Italy), a seismically active area located very close to the epicentral zone of the 1980 Mw 6.9 Irpinia earthquake. A comprehensive approach combining field observations, petrographic and microstructural analyses, fluid inclusion microthermometry, and geochemical profiling based on isotopic (δ¹³C and δ¹⁸O) and rare earth element (REE+Y) data reveals that the calcite veins precipitated from low-salinity H₂O–NaCl fluids, derived from the mixing of shallow and deep groundwater. These fluids, rich in CO₂ and coming from deep crustal reservoirs (8–12 km), migrated episodically through fault zones and were modified by mixing with post-depositional fluids produced during carbonate diagenesis, under varying thermal conditions (100–320 °C). Our study also proposes a computational model that reconstructs the isotopic evolution of the mineralizing fluids, capturing the sequential processes of fluid equilibration with dolostones, interaction with aquifer waters, and CO₂ degassing prior to calcite precipitation forming the mineralization. The good agreement between model predictions and measured isotopic data demonstrates the robustness of the model and highlights the dynamic fluid mixing processes within the fault zone. Furthermore, these findings highlight the role of episodic fluid migration, driven by fault-valve processes, in promoting calcite oversaturation and precipitation during seismic events. The integration of structural, geochemical, and modelling data refines our understanding of CO₂-rich fluid ascent, fault-related mineralization, and their link to fluid–rock interaction processes. This multidisciplinary approach offers new insights into fault mechanics and seismo-genesis, with implications for seismic hazard assessment and geochemical monitoring in active fault systems

How to cite: Zummo, F., Alvarez-Valero, A. M., Billi, A., Buttitta, D., Carnevale, G., Marchesini, B., Pibiri, I., Sinisi, R., Smeraglia, L., Caracausi, A., Agosta, F., and Paternoster, M.: Fault-related fluid circulation in the seismically active Irpinia region (southern Italy): insights from fluid inclusions and calcite veins, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19908, https://doi.org/10.5194/egusphere-egu26-19908, 2026.

Large active faults tend to differ from younger, fresh faults. They are the loci of the greatest earthquakes, but they often creep. The damage zone thickness tends to saturate with large displacement. Deep scientific drilling in large active faults systematically reveals differentiated fault gouge surrounded by a halo of fracturing and alteration. Despite its importance for fault evolution, the process of alteration in large active faults remains poorly understood.

After the devastating M_w 6.9 Nanbu-Kobe earthquake of 1995, a 750 m deep borehole was drilled into the Nojima fault, reaching the fault core at 625 m. The Hirabayashi borehole was drilled ~1 year after the earthquake and provided an extensive dataset on the structure of the fault where the earthquake originated. Continuous coring and borehole geophysics conducted within the borehole showed that the granodiorite protolith experienced several stages of alteration, including fault-related alteration that produced for example laumontite (Ca-rich zeolite) and smectite at T>150°C.

The ANR project AlterAction is revisiting this data with a multidisciplinary team to better understand the interplay between alteration and fault deformation, with 3D imaging of the core samples (see presentations by Romain Iaquinta and Maxime Jamet) and systematic petrophysical characterization.

This presentation will focus on the reanalysis of geophysical logs. Sonic velocities, electrical resistivities, and lithodensities progressively decrease when approaching the fault, starting to deviate from the protolith rock. In the granitoid rocks composing the borehole, variations in gamma-ray may reflect changes in the protolith rather than alteration. This fault zone starts at 370 m (255 m above the fault core) in the hanging wall, with a more pronounced decrease below 540m (85 m above the fault core, corresponding to a zone of thickness ~8 m, given the well trajectory, which is almost tangent to the fault dip). In the footwall, the strong decrease extends down to 680 m (55 m long zone), with lower velocities, resistivities and densities between 625 and 635m, below the fault core. Crossplotting the logging dataset shows the same trends, whether in the footwall or the hanging wall, regardless of the distance to the fault. This suggests that most of the fault zone is affected by the same interplay between alteration and damage. The fault core (623-626m) is singular owing to its relatively large sonic velocities, suggesting that sealing was strongly localized and effective in the fault core one year after the earthquake.

How to cite: Doan, M.-L., Jamet, M., Iaquinta, R., Gibert, B., Patrier, P., and Lucas, Y.: Interplay between alteration and damage at the Nojima fault zone (Japan) revealed by borehole geophysics, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15103, https://doi.org/10.5194/egusphere-egu26-15103, 2026.

Significant breakthroughs have been made recently in petroleum exploration within ultra-deep (burial depth > 6,000 m) carbonates in the Tarim Basin, northwestern China. The discovery of several large-scale oil and gas accumulation (e.g., Shunbei and Fuman) in these deeply buried, highly fractured and vuggy carbonates highlights the crucial role of strike-slip faults in reservoir development. However, the formation mechanisms of these ultra-deep, fault-controlled carbonate reservoirs remain poorly understood. It is therefore essential to conduct experimental simulations investigating the controlling factors and evolutionary trends governing the impact of deep CO₂-rich fluids on carbonate rocks.

For this reason, experiments were performed by using an ultra-deep, multi-tectonic-stage, high-temperature and high-pressure reservoir simulation system. This study focused on two key aspects, the dissolution mechanisms of dolomite in CO₂-saturated solutions, and the evolutionary trends of pore structure in carbonate rocks with different initial pore types during dissolution. Overall, two major findings were obtained. First, within temperature of 40–220 °C and pressure of 10–132 MPa, the saturated dissolution capacity of dolomite in CO₂-rich fluids exhibited an initial increase that was followed by a decrease, with the maximum dissolution occurred approximately at 60–110°C. This provides the theoretical basis for predicting favorable depth intervals where large-scale secondary pores may be formed in dolomite by deep CO₂-rich fluids. Second, influenced by the deep CO₂-rich fluid dissolution, both pore-dominated and fracture-dominated limestones tend to transform into fracture-vug reservoirs. Dissolution preferentially occurred along major fractures, gradually enhancing reservoir space and percolation capacity, ultimately becoming concentrated within these main fracture systems.

These results led to the construction of a genetic model for the development of fault-controlled, fracture-vug carbonate reservoirs. When deep CO₂-rich fluid activity coincides with fault development periods, fluids preferentially migrate into main faults, leading to dissolution-enlarged porosity along fault planes. When fluids migrate to fault intersections, they stagnate and induce dissolution and connectivity to form vugs. The fluids continue to expand along multiple sets of pre-existing faults, stagnating at new fault intersections to create more vugs. Such dissolution cycles are controlled by the episodic regional tectono-fluid activity. Ultimately, early-formed fracture-vug systems may become merged to formwell-connected fracture-vug reservoirs with superior reservoir performance. This model effectively explains the differences in dissolution and modification effects observed in different segments of strike-slip faults and clarifies the underlying mechanisms.

How to cite: She, M., Qiao, Z., and Liu, Y.: Influence of Deep CO₂-Charged Fluids on the Development of Carbonate Reservoirs in Fault-Controlled, Ultra-Deep burial setting: Insights from Water-Rock Interaction Experiment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6340, https://doi.org/10.5194/egusphere-egu26-6340, 2026.

In deep sedimentary basins, the formation and evolution of fracture-vein systems are critical for understanding fluid migration and overpressure history. This study investigates antitaxial fibrous illite veins in the Upper Triassic Xujiahe Formation of the Wubaochang area, Sichuan Basin, to decipher the mechanisms of fluid overpressure and diagenetic evolution. A multi-proxy approach was employed, combining detailed petrography, SEM, and micro-XRD. Crucially, we applied Optical Photothermal Infrared Spectroscopy (O-PTIR)  to achieve sub-micron resolution mapping of organic functional groups within single illite fibers, alongside in-situ REE and C-O isotope analysis. Detailed petrography and SEM analysis reveal that these veins exhibit typical antitaxial growth characteristics, where mineral fibers grow from a median plane toward the host rock, recording the continuous opening and synchronous filling of fractures. In-situ rare earth element (REE) and carbon-oxygen (C-O) isotope analyses identify two distinct fluid evolution stages: Stage I reflects an external, deep-circulating basin fluid system driven by regional tectonic stress, characterized by significant water-rock interaction with host rocks. Conversely, Stage II represents localized diagenetic and hydrocarbon-generated fluids, where isotopic signatures shift toward organic-derived carbon sources, indicating a transition to hydrocarbon-generation-induced overpressure. To definitively address the timing of fluid injection, sub-micron resolution O-PTIR (Optical-Photothermal Infrared) analysis was conducted, revealing the simultaneous presence of organic acid (1720cm^-1), aromatic (1600cm^-1), and aliphatic (1450cm^-1) functional groups coexisting with the illite lattice vibration (1034 cm^-1) within single fibrous crystals. The high ratio of organic acids to mineral signals indicates that organic acids directly mediated water-rock reactions and mineral precipitation rather than being late-stage infiltrations. Our findings demonstrate that these fibrous veins are coupled products of tectonic-induced fracturing and organic-acid-mediated mineral growth. This study highlights the power of O-PTIR as a novel tracer for deciphering organic-inorganic interplays, offering new insights into the mechanisms of fluid overpressure and hydrocarbon expulsion in deep, complex basin systems.

How to cite: Wang, Z., Zeng, L., and Gasparrini, M.: Fluid overpressure and diagenetic evolution recorded by antitaxial fibrous illite veins in deep coal-bearing strata: Insights from sub-micron O-PTIR and in-situ geochemistry, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14497, https://doi.org/10.5194/egusphere-egu26-14497, 2026.

Monazite from placer deposits along the southwestern coast of Taiwan was previously exploited as a source of rare earth elements (REEs). However, the formation mechanisms of the distinct monazite types remain debated and are commonly attributed to fluid-related processes. Due to the extremely high denudation rates of rivers in Taiwan, sediments undergo rapid transport, allowing the protolith characteristics of their provenance to be preserved. In this study, we examine river sediments from the Zengwen, Ailiao, and Laonong Rivers, which drain contrasting tectonostratigraphic units within each catchment. We characterize the occurrence and elemental distributions (e.g., La, Th, Nd) of monazite and examine the REE geochemical behavior of the bulk sediments. This study provides a comprehensive mineralogical and geochemical assessment of monazite associated with specific tectonostratigraphic units, offering constraints on sediment provenance.

Preliminary results summarize the variations in monazite occurrence and alteration across different tectonostratigraphic units. In the sediments of the Zengwen River, which primarily drains the Western Foothills, we observed a predominance of detrital monazite (<10 µm), as well as monazite associated with TiO₂, apatite, and clay minerals. These features suggest an origin primarily from the physical weathering of detritus or minor fluid precipitation, differing significantly from the occurrences of monazite found on the southwest coast.

Sediments from the Ailiao and Laonong rivers, which drain low-grade metamorphic rocks of the Central Range, exhibit evidence for variable degrees of low-temperature alteration affecting primary monazite. This includes inclusion-hosted, morphologically black monazite comparable to that observed along the southwestern coast of Taiwan. In the Laonong River, which additionally drains the Western Foothills, monazite occurs either within quartz grains or within the interlayers of clay minerals, similar to that observed in the Zengwen River. Furthermore, in rivers originating from the Slate Belt, in addition to monazite as a heavy mineral, we identified pyrite spherules comparable to those in the slate host rocks, as well as xenotime associated with thorite. Overall, these observations reveal distinct patterns in monazite occurrence and alteration among different tectonostratigraphic settings, with implications for sediment provenance in high-denudation river systems.

How to cite: Yeh, H. L., Wang, Y. T., Huang, C. C., and Chen, Y. H.: Monazite occurrence and low-temperature alteration in river sediments from contrasting tectonostratigraphic units in southwestern Taiwan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16117, https://doi.org/10.5194/egusphere-egu26-16117, 2026.

We investigate infiltration of an aqueous fluid into granitic rocks by means of numerical models at the field scale. Our methodology is based on a finite difference approach for solving the transport problem in combination with lookup tables generated from precomputed thermodynamic equilibria covering the compositional space. We also compare results to an approach involving on-the-fly calculation of local equilibrium between fluid and rock. Porosity and density evolution is predicted based on mass conservation. The ability to predict porosity evolution is valuable to better understand applications such as enhanced geothermal systems (EGS). The prediction of reaction zone sequences is also helpful in the understanding of ore deposit formation. We demonstrate how sensitive the metasomatic zoning sequences are to varying rock and fluid composition. As an example, we model metasomatic zone sequences observed in topaz-greisen to show how metasomatic sequences comprising multiple lithologies can be formed in one event with constant incoming fluid composition as boundary condition. Lithological zones formed along fractures do not necessarily imply temporal changes in the fluid composition of the source.

How to cite: Vrijmoed, J. C. and John, T.: Numerical modelling of an aqueous F-Cl-Na-K-Al bearing fluid in local equilibrium with granitic rocks with relevance to enhanced geothermal systems and ore deposit formation., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7508, https://doi.org/10.5194/egusphere-egu26-7508, 2026.

Interactions between minerals and reactive fluids in porous rocks and building materials often result in crystallization of new minerals. The forces exerted by minerals growing under confinement on the surrounding matrix can be large enough to cause fracturing. Fractures expose new reactive surfaces, leading to progressive disintegration of the material. These processes can result in severe damage to cultural heritage and modern infrastructure, as well as changes in the rheological properties and weathering of natural rocks. Understanding and controlling volume-expanding mineral replacement reactions in pore spaces is thus an important objective to address both societal and geological issues. While crystallization pressures have been measured at larger scales, nanoscale force evolution during confined mineral growth remains poorly constrained.

Here, we investigate volume-expansive mineral reactions in pores spaces by studying the hydration of MgO (periclase) in the Surface Forces Apparatus (SFA). Hydration of MgO to Mg(OH)­₂ (brucite) causes a volume increase to 220%, yielding high crystallization pressures. We use MgO thin films (~90 nm) prepared by atomic layer deposition as reactive surfaces in our experiments, which are performed at the Flow Laboratory, Njord Center, University of Oslo. With the SFA, we measure distance-resolved adhesive and repulsive forces acting between two MgO surfaces under variable external load and how these change over time as the reaction progresses. Preliminary results indicate evolution of forces from strongly adhesive to repulsive during the hydration reaction, likely due to the presence of amorphous, gel-like precursors in the early stages of the reaction. As a reference for the SFA experiments, we study the hydration of isolated MgO surfaces with Atomic Force Microscopy (AFM). This allows us to compare nucleation and growth rates, as well as microstructure and porosity of Mg(OH)­₂ grown in SFA and AFM.

How to cite: Trautner, V., Rogowska, M., Plümper, O., Nilsen, O., and Dziadkowiec, J.: How hard do crystals push when growing under confinement? Real-time measurements of surface forces during hydration of periclase to brucite, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7493, https://doi.org/10.5194/egusphere-egu26-7493, 2026.