Dissolution-precipitation creep (DPC) is considered to be one of the main processes accommodating localized strain in polymineralic shear zones of the Earth's crust. Extensive field evidence for DPC in natural shear zones supports the importance of this process. The spatiotemporal evolution and the level of compositional heterogeneity that facilitate the nucleation of such polymineralic shear zones remain poorly understood. A series of new experiments was conducted on a granitoid fine-grained (average starting grain size similar to 15 mu m) ultramylonite to different strains at 650 degrees C and 1.2 GPa, with strain rates varying from 10-3 to 10-6 s-1. In Type I experiments, a fracture was induced (prior to reaching the pressure and temperature (P-T) conditions), whereas, in Type II experiments, no initial fracture was induced. Consequently, in the Type I experiments, viscous deformation localized strictly within the previous fracture in an similar to 20 mu m wide zone, with grain sizes being reduced to 150-10 nm. In the Type II experiments, viscous deformation was distributed in the sample, with grain size being reduced locally to 200-50 nm. This study supports two different hypotheses for shear zone nucleation in nature. In brittle-induced strain localization, DPC will be activated and lead to a rapid and strong strain localization, producing a very weak and fast-deforming high-strain zone. In viscously induced strain localization (without main fracture), deformation concentrates in zones distributed through the sample, requiring higher shear strains to reach mechanical and microstructural steady state at slower deformation rates compared to brittle-induced strain localization. In both end-member strain localization scenarios, the dominant viscous deformation mechanism in the shear zones is grain boundary sliding combined with pinning-assisted DPC. Our experiments indicate that chemical potentials in the microstructures in combination with different strain localization types may explain the often-observed concentration of strain in fine-grained polymineralic mylonites such as in granitoids but also other polymineralic rocks (e.g., peridotites, granulites etc.) in nature.

We determine the feedback between fault dynamics and fault gouge structures by examining gouge structures that form during rupture and slip of initially intact granite under upper crustal conditions. Experiments were conducted under quasi-static (3 × 10−5 mm/s), weakly dynamic (0.27 mm/s) and fully dynamic (≫1.5 mm/s) slip conditions, with or without fluids, and limited slip displacement (max. 4 mm). The extent in gouge amorphization positively correlates with deformation rate, and we detect evidence of melting, e.g., magnetite nanograins, associated with the highest deformation rates. Gouge nanostructure is directly correlated to power dissipation rather than total energy input. The presence of amorphous material has no detectable impact on the strength evolution during rupture. We highlight that gouge textures, generally associated with large displacements and/or elevated pressure and temperature conditions, can form during small slip events (Mw < 2) in the upper crust from initially intact materials. A correlation between fault dynamics and fault gouge structure is only striking when investigating gouge structures at the nanoscale. Granite fault gouges undergo extensive amorphization and partial melting during the early stages of seismic slip.

Abstract Coseismic fracturing in the strong, dry, and metastable plagioclase‐rich lower‐crust is an effective mechanism for creating pathways for fluids to infiltrate the host rock, kick‐start metamorphism, and potentially lead to rheological weakening. In this study, we have characterized the damage zone flanking a lower‐crustal pseudotachylyte (solidified frictional melt produced during seismic slip) within an anorthosite to determine the mechanisms of incipient aqueous fluid infiltration and redistribution in a lower‐crustal seismogenic fault. Pulverization‐style fracturing of the host anorthosite resulted in the comminution of the host plagioclase (plagioclase1) grains and the growth of very fine (<20 μm) grained secondary plagioclase neoblasts (plagioclase2) filling the fractures. Fluid‐assisted grain growth accompanied surface‐ and strain‐energy minimization grain growth in the healing and sealing of the fractures. This process was not associated with the densification nor the creation of new reaction‐induced porosity. Fourier transform infrared maps transecting the damage zones show the presence of H2O species along the plagioclase1 and plagioclase2 grain boundary regions, as well as incorporated into plagioclase2 grain interiors. Grain‐size sensitive creep of fine‐grained plagioclase localized along the pseudotachylyte margin where fracturing was most pervasive. In the absence of reaction‐induced porosity, strain localization is determined by repeated occurrences of extreme grain‐size reduction in addition to the mobilization of aqueous fluid to the grain boundary regions, to the extent in which these fine‐grained wet plagioclase2 layers are volumetrically dominant over dry, coarse plagioclase1 fragments. This forms a layer capable of deforming by grain‐size sensitive creep and sustaining the mobility of fluids.

Coseismic fracturing of the lower crust is an effective mechanism for creating permeable pathways for fluids to infiltrate and interact with the host rock, thus effectively altering the rheology of otherwise anhydrous and strong lower-crustal rocks. Most of the fracturing and fragmentation that facilitate fluid infiltration occurs in the damage zone of seismogenic faults. In this study, we have focused on characterizing the damage zone adjacent to a lower-crustal pseudotachylyte (solidified frictional melt produced during seismic slip) to understand the fracture generating and healing processes during a seismic event.The Nusfjord East shear zone network (Lofoten, Norway) contains coeval pseudotachylytes and mylonitized pseudotachylytes that formed at lower-crustal conditions within anhydrous anorthosites. We present a micro- and nanostructural analysis of plagioclase grains in the damage zone of a natural pseudotachylyte using focused ion beam (FIB) prepared scanning transmission electron microscopy (S/TEM), electron backscatter diffraction (EBSD) analysis, electron microprobe analysis (EMPA), and SEM-cathodoluminescence (CL) imaging.The damage zone of the host anorthosite is characterized by a network of fractures with minimal offset, consistent with a pulverization-style fragmentation process. CL intensities differentiate primary plagioclase (plagioclase1), from secondary plagioclase neoblasts (plagioclase2) filling some of the fractures. Plagioclase1 grains often exhibit a diffuse CL intensity zonation from bright grain cores to a dark grey in healed cracks, while plagioclase2 have a uniform mid-tone grey CL intensity with dark grain boundaries. CL zonation in the plagioclase1 does not correlate with EMPA major element maps nor EBSD misorientation maps. TEM foils targeted key microstructure domains characterized by CL: (1) bright CL plagioclase1 core, (2) dark CL plagioclase1 in healed cracks, (3) transitional CL from bright to dark across a healed crack, and (4) plagioclase2 neoblast. Results from S/TEM show that the dark CL spanning the healed cracks is associated with a high concentration of crystalline nanoparticles. In contrast, bright CL is associated with a few scattered dislocations, no nanoparticles, and numerous dispersed Ba-Ti-oxide nanograins. The mid-tone grey CL plagioclase2 neoblast have the lowest dislocation density. Follow-up Transmission Kikuchi Diffraction (TKD) and NanoSIMS analyses on the nanoparticles in the healed cracks and the plagioclase1 grains immediately next to these cracks will help further elucidate the origins of the nanoparticles and the CL intensity zonation.

Understanding the interplay of various energy sinks during seismic fault slip is essential for advancing earthquake physics and improving hazard assessment. However, quantifying the energy consumed by major dissipative processes remains a challenge. In this study, we investigate energy partitioning during laboratory earthquakes ("lab-quakes") by performing general shear stick-slip experiments on synthetic granitic cataclasites at elevated confining pressure. Using ultrasound, microstructural, and novel magnetism-based thermal analyses, we independently quantified the energy allocated to seismic radiation, new surfaces, and heat dissipation. These estimates showed good agreement with far-field measurements of mechanical work during the lab-quake. Our findings revealed that under the experimental conditions the majority of the released energy (68%-98%) is dissipated as heat, while seismic radiation accounts for 1%-8%, and the creation of new surfaces consumes <1%-32%. Microstructural observations indicate pre-failure deformation, which includes comminution and development of the principal slip zone, significantly influences energy partitioning. This effect is further evident in the measured shear stress drops, where events with higher stress drops proportionally emitted more energy as seismic waves. This study is the first to constrain the full energy budget of lab-quakes from an observational standpoint, providing critical insights into the dynamics of fault rupture and energy dissipation processes.

Grain-size reduction – with amorphization or melting as its extreme forms – plays a crucial role in fault-zones dynamics, e.g., the nucleation or arrest of earthquakes. Previous experiments have been mostly conducted on powdered samples and structural investigations of experimentally generated fault-gouge material provide contrasting results when it comes to the initiation of melting during fault slip. In the present study, we deformed four intact Westerly granite samples, to decipher whether there is a correlation between failure mode, i.e., controlled, self-stabilised, or dynamic, and grain-size reduction within the developing fault gouge. Controlled failure took place over several hours, self-stabilised failure occurred within a few seconds and dynamic failure lasted less than a second. To test the influence of aqueous fluids on the grain-size evolution within fault gouges, two runs were performed on samples, dynamically failing either in the presence or absence of pore fluids. All samples were deformed at the same effective pressure of 40 MPa and displacements along the newly created faults were with 1.2 to 2.0 mm in a similar range. We investigated the microstructures of each sample using a scanning electron microscope (SEM) and cut two focused-ion beam (FIB) sections per sample from selected areas, located within the fault gouges, to analyse their nanostructures using a transmission electron microscope (TEM). At low magnification at the SEM, no striking differences between the different fault gouges are visible. Features resembling “cooling cracks” become apparent at the highest magnification at the SEM and are only found in the samples that failed dynamically. Major differences between the samples are only obvious when comparing their nanostructures using TEM imaging. In the high-resolution TEM images as well as with the aid of selected area electron diffraction (SAED), we observe a clear correlation between failure mode or rupture speed and grain-size reduction, with an increase in amorphous material as rupture speed increases. Regardless of the availability of fluids, the samples that underwent dynamic failure reveal similar nanostructures. Both exhibit flow structures created by amorphous material. We believe that latter is the result of melting as we find numerous structural and chemical evidence for melting, e.g., euhedral magnetite crystals of a few tens of nanometer with adjacent depletion halos. Such indicators for melting are absent in samples that failed in a controlled or self-stabilised manner, highlighting the importance of rupture speed on fault gouge melting.

The transition from strong to weak mechanical behavior in the Earth's continental middle crust is always caused by an initiation of viscous deformation. Microstructural evidence from field examples indicates that viscously deforming polymineralic shear zones represent the weakest zones in the crust and may dominate mid-crustal rheology. The results of recent experiments (as in Part 1, Nevskaya et al., 2025) demonstrate that the observed weak behavior is due to the activation of dissolution-precipitation creep (DPC). Formation of fine-grained material and efficient pinning of grain growth are important prerequisites for the formation of a stable deforming microstructure. However, available rheological parameters for fine-grained polymineralic rocks deforming by DPC are insufficient. A series of three types of experiments was conducted on a granitoid fine-grained ultramylonite to different strains at 650-725 degrees C, 1.2 GPa, with strain rates varying from 10-3 to 10-6 s-1. Type I and II experiments are solid natural samples, providing key microstructural evidence for DPC. Type III experiments are general shear experiments performed on coarse- and fine-grained ultramylonite powder. All experiments were combined to estimate rheological parameters for such polymineralic shear zones. A stress exponent n approximate to 1.5 and grain size exponent m approximate to-1.66, with uncertainties, were estimated and coupled with microstructural observations. Extrapolations indicate that, at slow natural strain rates, DPC in polymineralic granitoid fault rocks can occur at lower temperatures than monomineralic quartz. A deformation mechanism map is proposed, indicating a transition in the deformation mechanism from dislocation creep in monomineralic quartz to DPC in weaker polymineralic fine-grained granitoids, based on strain rate and grain size. Most importantly, the polymineralic composition is the determining factor in achieving the fine grain sizes necessary for DPC to become activated. This is due to the presence of additional chemical driving potentials and phase mixing, both of which are absent in monomineralic systems.

Vertebrates use the phosphate mineral apatite in their skeletons, which allowed them to develop tissues such as enamel, characterized by an outstanding combination of hardness and elasticity. It has been hypothesized that the evolution of the earliest vertebrate skeletal tissues, found in the teeth of the extinct group of conodonts, was driven by adaptation to dental function. We test this hypothesis quantitatively and demonstrate that the crystallographic order increased throughout the early evolution of conodont teeth in parallel with morphological adaptation to food processing. With the c-axes of apatite crystals oriented perpendicular to the functional feeding surfaces, the strongest resistance to uniaxial compressional stress is conferred along the long axes of denticles. Our results support increasing control over biomineralization in the first skeletonized vertebrates and allow us to test models of functional morphology and material properties across conodont dental diversity. Conodonts, early vertebrates, are thought to have evolved complex tooth tissue as an adaptation for feeding. Here, the authors use Electron Backscatter Diffraction to show increasing dental crystallographic order through conodont evolution, in parallel with dietary adaptations.

To comprehend the rheology of the Earth's crust and the relevant rock properties, one key approach is to deform rocks and minerals at elevated pressures and temperatures and then extrapolate the measured stress and strain rate values to natural conditions using constitutive equations. Laboratory experiments are mostly conducted on monomineralic rocks, with quartz being considered as the weakest constituent of the middle continental crust. However, field observations suggest that this is an oversimplification, and polymineralic fault rocks may be weaker than monomineralic quartz rocks. This study presents the first experiments on fine-grained, solid, natural rock samples, containing their natural homogeneities and inhomogeneities, demonstrating that granitoid rocks may be weaker than quartz at mid-crustal conditions. It also highlights the importance of pre-existing faults and polymineralic fine-grained zones for strain localisation and proposes values for extrapolation to natural conditions and their use in numerical models of the deformation of the granitoid crust.Cylindrical granitoid ultramylonite samples, composed of qtz + ab + K-fsp + bt + ep, with grain sizes of 125-15 μm are deformed in a Grigg’s type apparatus at T=650°C, confining P=1.2 GPa, strain rates=10-3 to 10-5s-1, and 0.2 wt% H2O added. Mechanical data are combined with light microscope, SEM, TEM, and quantitative image analysis to connect microstructures with stress and strain evolution. We show that polymineralic granitoid rocks deform through other mechanisms than monomineralic quartz aggregates at pressure and temperature conditions characteristic for the middle crust: Ultra-fine grain size reduction down to <50nm is developed by nucleation and growth of new grains in a polymineralic mixture. Grain size remains small because of pinning processes. We therefore refer to the deformation mechanism as pinning-controlled dissolution-precipitation creep (P-DPC).Furthermore, we establish a new constitutive equation for this P-DPC, based on an exponential diffusion creep flow law, to model our experiments and tackle the extrapolation to various natural conditions. This flow law is supported by the microstructural evidence for the deformation mechanisms. Extrapolations show that the shear zones of the granitoid middle crust may be magnitudes weaker than extrapolated so far, and deformation may occur at magnitudes faster rates. The brittle to viscous transition may be shifted to shallower levels. This may have implications for the seismogenic zone and/or stress fields below geothermal reservoirs. Most importantly, we show the necessity to take polymineralic rocks into consideration for various numerical model applications.

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

The rheological properties of olivine influence large‐scale, long‐term deformation processes on rocky planets. Studies of the deformation of olivine at low temperatures and high stresses have emphasized the importance of a grain‐size effect impacting yield stress. Laboratory studies indicate that aggregates with finer grains are stronger than those with coarser grains. However, the specific interactions between intracrystalline defects and grain boundaries leading to this effect in olivine remain unresolved. In this study, to directly observe and quantify the mechanical properties of olivine grain boundaries, we conduct nanoindentation tests on well characterized bicrystals. Specifically, we perform room‐temperature spherical and Berkovich nanoindentation tests on a subgrain boundary (13°, [100]/(016)) and a high‐angle grain boundary (60°, [100]/(011)). These tests reveal that plasticity is easier to initiate if the high‐angle grain boundary is within the deformation volume, whereas the subgrain boundary does not impact the initiation of plasticity. Additionally, the high‐angle grain boundary acts as a barrier to slip transmission, whereas the subgrain boundary does not interact with dislocations in a measurable manner. We suggest that the distribution of grain‐boundary types in olivine‐rich rocks might play a role in generating local differences in mechanical behavior during deformation.

&lt;p&gt;Pervasive carbonation of serpentinized peridotite to carbonate-quartz rock (listvenite) due to infiltration of CO2-bearing fluid is a remarkable process because it can be geologically fast and it increases the rock&amp;#8217;s carbon content from initially zero to &gt; 30 wt% CO2. This pervasive conversion is related to an overall solid volume increase while at the same time requiring high time-integrated fluid rock ratios with permeability and diffusivity on all scales. Thus, porosity has to be created dynamically during reaction progress as otherwise fluid pathways become clogged by the reaction products carbonate and quartz, which is one of the major obstacles for artificial carbon storage by peridotite carbonation. Processes that can renew porosity and permeability during carbonation are fracturing and veining &amp;#8211; in response to tectonic stress [1] or induced by reaction and crystal growth [2], or a combination of both &amp;#8211;, reaction-enhanced ductile deformation [3,4], and spatial decoupling of dissolution and precipitation with solute transfer at different length scales.&lt;/p&gt; &lt;p&gt;Using SEM, EBSD, TEM and FIB nano-tomography, we investigated the microstructural record of local solute transfer and its role for porosity renewal in natural carbonate-bearing serpentinites, transitional serpentine-carbonate-quartz assemblages and listvenites from the Samail Ophiolite, Oman. The clearest indicators of pervasive replacement accommodated by local solute transfer are pseudomorphic replacement structures where carbonate and quartz occur in distinct microstructures corresponding to different inherited peridotite and serpentinite textures. A common pseudomorphic replacement structure in the Samail carbonated peridotite are quartz-fuchsite intergrowths replacing bastite (pseudomorphs after orthopyroxene) in listvenite with harzburgite protoliths. A local strong crystallographic alignment of quartz in each of the bastite/pyroxene pseudomorphs suggests that the anisotropic porosity structure of bastite serpentine favored oriented, epitaxial growth of quartz. Transitional serpentine-carbonate-quartz assemblages show that the first quartz and carbonate generations precipitate coeval but spatially separated, with distinct crystal habits. FIB nano-tomography, STEM analysis and high resolution SEM on ion-polished samples of a transitional serpentine-dolomite-quartz rock from the carbonation reaction front show nano-porous fluid channels in fibrous serpentine at high angle to a highly serrated carbonate-serpentine replacement contact. These nano-scale fluid channels facilitated bidirectional mass exchange of Ca, C, Mg and Si bearing solutes between sites of preferential replacement, such as the cores of serpentine mesh textures, and larger-scale permeability networks along veins and fractures. These observations imply that massive and pervasive solute transfer through the reacting serpentine matrix is possible on a small scale, without clogging of porosity by immediate co-precipitation of quartz or Mg-silicates. Spatial decoupling of dissolution and precipitation was likely caused by the dynamically evolving composition of the reacting fluid and/or due to the influence of differential stress and volumetric strain &amp;#8211; a mechanism that can compensate on a local scale for the volume expansion expected of isochemical carbonation reactions.&lt;/p&gt; &lt;p&gt;&amp;#160;&lt;/p&gt; &lt;p&gt;[1] Menzel et al., Solid Earth, 2022; https://doi.org/10.5194/se-2021-152&lt;/p&gt; &lt;p&gt;[2] Kelemen &amp; Hirth, EPSL, 2012; https://doi.org/10.1016/j.epsl.2012.06.018&lt;/p&gt; &lt;p&gt;[3] Menzel et al., Nature Communications, 2022; https://doi.org/10.1038/s41467-022-31049-1&lt;/p&gt; &lt;p&gt;[4] Kelemen et al., JGR, 2022; https://doi.org/10.1029/2021JB022352&lt;/p&gt; &lt;p&gt;&amp;#160;&lt;/p&gt; &lt;p&gt;Funding: Junta de Andaluc&amp;#237;a (Postdoc_21_00791); DFG grants UR 64/20-1, UR 64/17-1; and EU Horizon 2020 Transnational Access EXCITE _C1_2022_34.&lt;/p&gt;

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

&lt;p&gt;Talc is an important product of several hydration and dehydration reactions in deep faults and subduction zones. The unique weakness of talc along its basal planes makes it an essential component in understanding various fault slip behaviors (e.g., episodic vs continuous slip, seismic vs aseismic) or realistic geodynamic models. A recent experimental study by Boneh et al. (2023) on talc mechanical behavior at high P-T conditions highlighted: (i) talc&amp;#8217;s low friction coefficient under all conditions (&lt;0.14), with thermal weakening down to &amp;#181;~0.01 at 700 &amp;#176;C. (ii) Grain-scale microstructures demonstrate a component of fracturing and microcracking under all conditions tested. And (iii) pressure-dependence of talc strength decreases at higher temperatures, where there is also a greater tendency&amp;#160;for localization. A vital part of depicting mineral rheology is the understanding of their underlying mechanisms of deformation associated with the observed bulk mechanical and microstructural behavior. To reveal the underlying deformation mechanism/s we analyzed the deformed samples through high-resolution transmission electron microscopy (TEM) at Utrecht university of samples prepared using a focus ion beam (FIB). Five talc samples were examined &amp;#8211; an undeformed sample, and samples deformed at 400, 600, and 700&amp;#176;C under 1 GPa, and at 400 &amp;#176;C under 1.5 GPa.&lt;/p&gt; &lt;p&gt;Seven FIB lamellae sampled areas adjacent to the main fracture (if exists) or high damage zones. The starting material shows talc flakes with a thickness of ~100-400 nm without a sample-scale preferred alignment. The sample deformed under 400&amp;#176;C and 1.5 GPa exhibits distributed deformation with opening cracks along talc basal planes and pervasive kinking normal to the basal planes. The sample deformed at 400&amp;#176;C and lower pressure (1.0 GPa) exhibits thin lamination (~50 nm) well oriented with the orientation of the main fracture plane. The sample deformed at 600&amp;#176;C exhibits crystal delamination along the basal cleavage (forming grain fragments &lt;10 nm in width) along the main fracture. The sample deformed at 700&amp;#176;C exhibits more areas of high damage, possibly due to the similar basal-cleavage delamination. A key incentive is to relate the observed nano-scale crystal defects with the bulk mechanical behavior and with processes that might promote the localization of deformation. Pressure-dependent strength can be accounted for by kinking and kinking-induced porosity while thermal weakening can be related to temperature-dependent mobility of crystal defects leading to delamination along the basal cleavage. We will discuss possible physical mechanisms of talc deformation and the prospect of extrapolating the mechanical behavior of talc achieved at the lab to the range of conditions expected in natural settings.&lt;/p&gt;

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

Abstract A new method to determine fluid flux at high pressures and temperatures has been developed and used to study serpentinites at subduction zone conditions. Drill cores of a natural antigorite‐serpentinite with a strong foliation were used in multi‐anvil experiments in the range of 2–5 GPa and 450–800°C. Fluids released upon dehydration are fixed by the formation of brucite in an adjacent fluid sink. The amount and distribution of brucite serves as a proxy for fluid flow. In our specific setup the sample reacted with the surrounding fluid sink to form an additional layer of olivine, which has the potential to limit fluid flux within our experiments. For conditions prior to serpentine dehydration we used Al(OH)3 as fluid source. Fluid in this experiment did not migrate through the serpentinite, indicating that serpentine has a low diffusivity. The experiments also show that small deviatoric stresses have an influence on the fluid flux and can cause an anisotropic fluid flux. Comparison between the time scales of the determined fluid flux with fluid production rates indicates fluid pressure buildup during dehydration reactions. Adjacent less permeable layers can inhibit fluid flux and cause fluid pressure buildup even at conditions when an interconnected pore space formed.

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