ORCID Profile
0000-0002-3104-8109
Current Organisation
The University of Edinburgh
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Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-9048
Abstract: & & The localisation of structural damage, in the form of faults and fractures, along a distinct and emergent fault plane is the key driving mechanism for catastrophic failure in the brittle Earth. However, due to the speed at which stable crack growth transitions to dynamic rupture, the precise mechanisms involved in localisation as a pathway to fault formation remain unknown. Understanding these mechanisms is critical to understanding and forecasting earthquakes, including induced seismicity, landslides and volcanic eruptions, as well as failure of man-made materials and structures. We used time-resolved synchrotron x-ray microtomography to image in-situ damage localisation at the micron scale and at bulk axial strain rates down to 10& sup& -7& /sup& s& sup& -1& /sup& . By controlling the rate of micro-fracturing events during a triaxial deformation experiment, we deliberately slowed the strain localisation process from seconds to minutes as failure approached. This approach, originally established to indirectly image fault nucleation and propagation with acoustic emissions, is completely novel in synchrotron x-ray microtomography and has enabled us to image directly processes that are normally too transient even for fast synchrotron imaging methods. Here, we first present the experimental apparatus and control system used to acquire the data, followed by damage localisation and shear zone development in a s le of Clashach sandstone viewed in unprecedented detail. Time-resolved microtomography images demonstrate a strong intrinsic correlation between shear and dilatant strain in the localised zone, with bulk shear strain accomodated by the nucleation and rotation of en-echelon tensile microcracks within a grain-scale shear band. Rotation is accompanied by antithetic to synthetic shear sliding of neighbouring crack surfaces as they rotate. The evolving 4D strain field, measured with incremental digital volume correlation between pairs of recorded x-ray tomographic volumes, independently confirm the correlation between shear and dilatant strain and show how strain localises spontaneously, first through exploration of several competing shear bands at peak stress before transitioning to failure along the optimally-oriented final fault plane. In order to & #8216 ground-truth& #8217 inferences made from bulk measurements and seismic waves (the primary method of detecting deformation at the field-scale where direct imaging of the subsurface is impossible), we (a) compare rupture energy estimates from local slip measurements with those from bulk slip data, and (b) use AE source location estimates to identify in idual cracks and other local changes in the microstucture that may explain the AE source.& &
Publisher: Springer Science and Business Media LLC
Date: 30-01-2015
Publisher: American Geophysical Union (AGU)
Date: 11-2019
DOI: 10.1029/2019JB017750
Publisher: Copernicus GmbH
Date: 18-02-2021
Abstract: Abstract. In experiments designed to understand deep shear zones, we show that periodic porous sheets emerge spontaneously during viscous creep and that they facilitate mass transfer. These findings challenge conventional expectations of how viscosity in solid rocks operates and provide quantitative data in favour of an alternative paradigm, that of the dynamic granular fluid pump model. On this basis, we argue that our results warrant a reappraisal of the community's perception of how viscous deformation in rocks proceeds with time and suggest that the general model for deep shear zones should be updated to include creep cavitation. Through our discussion we highlight how the integration of creep cavitation, and its Generalised Thermodynamic paradigm, would be consequential for a range of important solid Earth topics that involve viscosity in Earth materials like, for ex le, slow earthquakes.
Publisher: SPIE
Date: 17-10-2012
DOI: 10.1117/12.936331
Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-10343
Abstract: & & Shear zones are important conduits that facilitate the bidirectional migration of fluids and dissolved solids across the middle crust. It is a relatively recent revelation that mylonitic deformation in such shear zones can result in the formation of synkinematic pores that are potentially utilised in long-range fluid migration. The pores definitely influence a shear zone& #8217 s hydraulic transport properties on the grain scale, facilitating synkinematic fluid-rock interactions and mass transfer. Our understanding of how exactly various forms of synkinematic porosity integrate with the kinematics and dynamics of shear zones is still growing. Here we show a previously undescribed form of synkinematic porosity in an unweathered, greenschist-facies psammitic ultramylonite from the Cap de Creus Northern Shear Belt (Spain). The sizeable, open pores with volumes & 50k & #181 m3 appear exclusively next to albitic feldspar porphyroclasts, which themselves float in a fine-grained, polymineralic ultramylonitic matrix that likely deformed by grain size-sensitive creep and viscous grain boundary sliding. The pores wrap around their host clasts, occupying asymmetric strain shadows and tailing off into the mylonitic foliation. A detailed analysis using high-resolution backscatter electron imaging and non-invasive synchrotron-based x-ray microtomography confirms that the pores are isolated from each other. We found no evidence for weathering of the s les, or any significant post-mylonitic overprint, unequivocally supporting a synkinematic origin of the pores.& & & & & We propose that this strain shadow porosity formed through the rotations of the Ab porphyroclasts, which was governed by the clasts& #8217 shapes and elongation. The ultramylonitic matrix was critical in enabling the formation of pores in the clast& #8217 s strain shadows. In the matrix, the in idual grains were displaced mostly parallel to the shear direction. As a consequence of clast rotation it can be expected that, in the strain shadows, matrix grains followed erging movement vectors. As a result, phase boundaries in the YZ plane experienced tensile forces, leading to the opening of pores. We infer that this tensile decoupling among matrix grains established a hydraulic gradient that drained the matrix locally and filled the pores with fluid. The fact that the strain shadow pores remained open in our s les suggests a chemical equilibrium with the fluid. Pore shape and volume will have been subject to continuous modification during ongoing matrix deformation and clast rotation.& & & & This form of synkinematic porosity constitutes a puzzling, yet obvious way to maintain surprisingly large pores in ultramylonites whose transport properties are otherwise likely determined by creep cavitation and the granular fluid pump (Fusseis et al., 2009). We envisage that the strain shadow megapores worked in sync with the granular fluid pump in the ultramylonitic matrix and, while the overall porosity of ultramylonites may be small, locally, substantial fluid reservoirs were available to service fluid-rock interaction and fluid-mediated mass transfer. Our findings add another puzzle piece to our evolving understanding of synkinematic transport properties of mid-crustal ultramylonites and fluid-rock interaction in shear zones at the brittle-to-ductile transition.& &
Publisher: Copernicus GmbH
Date: 28-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-8048
Abstract: & & Tectonic-scale features happening at convergent plates are ultimately the outcome of microscopic, grain scale processes. In collision zones, prograde metamorphism occurs by gradual increase of pressure and temperature [1 2]. Among the most important prograde mineral reactions are dehydration reactions, which are characterized by solid volume reduction, porosity creation, fluid release and high pore fluid pressures [3]. Most models linking dehydration and mechanical instabilities [4-6] involve feedback loops between coupled chemical, hydraulic and mechanical processes. Feedbacks control pore fluid pressure build-up and drainage, and provide efficient pathways for the transport of chemical components. Gypsum dehydration is crucial in the formation of detachment faults thin-skinned tectonics [7]. It is also used as a proxy for serpentine dehydration and the generation of intermediate depth seismic events/aseismic slip activity [8].& & & & We performed a set of experimental gypsum dehydrations both at the TOMCAT microtomography beamline at the Swiss Light Source, and in the laboratory. Using a modified version of the Mjolnir triaxial rig [9] that allowed control of pore fluid pressure in the synchrotron microtomography setup enabled us to document how differential stress (& #8710 & #963 ) and pore fluid pressure (P& sub& f& /sub& ) influence the dehydration of Volterra alabaster gypsum to bassanite at a constant confining pressure and temperature in 4D.& & & & We derived data on mineral phase transformation and formation of pore networks by applying a deep-learning algorithm in ORS Dragonfly& #174 software, which reduced data processing times, minimized interpretation biases, and allowed analysing larger volumes. The results exhibit an extremely high accuracy compared to standard procedures. The analysis of phase proportions (gypsum, bassanite and porosity) of segmented volumes correlates very well to theoretical predictions indicating a correct segmentation from the algorithms and self-consistency of the generated datasets. Comparing results obtained& at various & #8710 & #963 and P& sub& f& /sub& to the light of mechanical data and additional in-house experiments allows us to better interpret their effect on reaction duration, magnitude and textural evolution of the rock. Transient phenomena as well as in idual grain transformation and growth are now traceable in a fully automated way.& & & & Our data further our understanding of gypsum dehydration: We found that & #8710 & #963 greatly influences the assemblage of the bassanite needles, which tend to grow nearly vertical at & #8710 & #963 & #8773 0. Increasing & #8710 & #963 significantly increases s le compaction. On the contrary, increasing P& sub& f & /sub& decreases the bulk deformation and slows down the reaction. As pores grow around bassanite needles, the control of the orientation of needles by differential stress can influence the overall pore network and thus introduce anisotropies during transient and final stages of the reaction. Our data confirm that & #8710 & #963 and P& sub& f& /sub& greatly influence transient and final rock texture, which has implications on drainage during nappe emplacements.& & & & & strong& References& /strong& : [1] Hacker et al., 2003, /10.1029/2001JB001129 [2] Peacock, 2001, 10.1130/0091-7613(2001)029& :ATLPOD& .0.CO [3] Llana-Funez et al. 2012, /10.1007/s00410-012-0726-8 [4] Raleigh and Paterson, 1965 /10.1029/JZ070i016p03965& [5] Dobson et al., 2002 /10.1126/science.1075390 [6] Jung et al., 2004 /10.2747/0020-6814.46.12.1089 [7] Hubbert and Rubey, 1959 /10.1130/0016-7606(1959)70[115:ROFPIM]2.0.CO [8] Rutter et al. 2009 /10.1016/j.jsg.2008.09.008 [9] Butler 2020, /10.1107/S160057752001173X.& &
Publisher: Wiley
Date: 19-05-2021
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-9992
Abstract: & & The landscape of college and university teaching in the geosciences has changed over the past 20 years.& Research has documented 1) that faculty in the U.S. now spend less time lecturing and more time actively engaging students in the classroom, and 2) that active engagement is more common in geoscience classrooms than it is in biology, chemistry, physics, or engineering. The web sites of & em& Teach the Earth& /em& & and & em& On the Cutting Edge& /em& have thousands of web pages of resources for geoscience faculty who want to more actively engage their students in the classroom. But what if you want to incorporate more active learning but aren& #8217 t sure where to start or how these techniques might work in your courses? Or what if you are looking for new approaches or fresh ideas to add to techniques that you already use?& & & & On-R s are quick-start guides designed to bring you up to speed in effective strategies for engaging students more actively in the classroom. Each 2-page On-R focuses on a particular teaching strategy, rather than on how to teach a particular topic. The current On-R s cover interactive lecture, brainstorming, concept sketches, jigsaws, discussions, quantitative skill-building, just-in-time approaches, case studies, and re-thinking course coverage and linearity. Each On-R includes a simple ex le that illustrates the strategy, why the technique is valuable, implementation tips, additional ex les and modifications, and links to activities, supporting research, and other resources. On-R s will be available at the poster and can also be downloaded as pdfs from nr s/index.html& & & & On-R s originated from the 2018 community vision report to US National Science Foundation on & em& Challenges and Opportunities for Research in Tectonics& /em& , and their development was supported with a grant from NSF. The On-R s writing team is a group of geoscientists at a variety of career levels with specialties across the range of subdisciplines that regularly address tectonic problems. Although ex les currently focus on the broad field of tectonics, On-R s can be easily adapted for courses in other geoscience disciplines at all levels.& & & & & & &
Publisher: Springer Science and Business Media LLC
Date: 31-07-2017
DOI: 10.1038/S41598-017-07160-5
Abstract: Metamorphic reactions influence the evolution of the Earth’s crust in a range of tectonic settings. For ex le hydrous mineral dehydration in a subducting slab can produce fluid overpressures which may trigger seismicity. During reaction the mechanisms of chemical transport, including water expulsion, will dictate the rate of transformation and hence the evolution of physical properties such as fluid pressure. Despite the importance of such processes, direct observation of mineral changes due to chemical transport during metamorphism has been previously impossible both in nature and in experiment. Using time-resolved (4D) synchrotron X-ray microtomography we have imaged a complete metamorphic reaction and show how chemical transport evolves during reaction. We analyse the dehydration of gypsum to form bassanite and H 2 O which, like most dehydration reactions, produces a solid volume reduction leading to the formation of pore space. This porosity surrounds new bassanite grains producing fluid-filled moats, across which transport of dissolved ions to the growing grains occurs via diffusion. As moats grow in width, diffusion and hence reaction rate slow down. Our results demonstrate how, with new insights into the chemical transport mechanisms, we can move towards a more fundamental understanding of the hydraulic and chemical evolution of natural dehydrating systems.
Publisher: American Geophysical Union (AGU)
Date: 05-2015
DOI: 10.1002/2014JB011708
Publisher: Copernicus GmbH
Date: 27-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-5818
Abstract: & & Catastrophic failure in brittle, porous materials initiates when structural damage, in the form of smaller-scale fractures, localises along an emergent failure plane or 'fault' in a transition from stable crack growth to dynamic rupture. Due to the extremely rapid nature of this critical transition, the precise micro-mechanisms involved are poorly understood and difficult to capture. However, these mechanisms are crucial drivers for devastating phenomena such as earthquakes, including induced seismicity, landslides and volcanic eruptions, as well as large-scale infrastructure collapse. Here we observe these micro-mechanisms directly by controlling the rate of micro-fracturing events to slow down the transition in a unique triaxial deformation experiment that combines acoustic monitoring with contemporaneous & em& in-situ& /em& x-ray imaging of the microstructure. The results provide the first integrated picture of how damage and associated micro-seismic events emerge and evolve together during localisation and failure and allow us to ground truth some previous inferences from mechanical and seismic monitoring alone. They also highlight where such inferences miss important kinematically-governed grain-scale mechanisms prior to and during shear failure.& & & & The evolving damage imaged in the 3D x-ray volumes and local strain fields undergoes a breakdown sequence involving several stages: (i) self-organised exploration of candidate shear zones close to peak stress, (ii) spontaneous tensile failure of in idual grains due to point loading and pore-emanating fractures within an emergent and localised shear zone, validating many inferences from acoustic emissions monitoring, and (iii) formation of a proto-cataclasite due to grain rotation and fragmentation, highlighting both the control of grain size on failure and the relative importance of aseismic mechanisms such as crack rotation in accommodating bulk shear deformation. Dilation and shear strain remain strongly correlated both spatially and temporally throughout s le weakening, confirming the existence of a cohesive zone, but with crack damage distributed throughout the shear zone rather than concentrated solely in a breakdown zone at the propagating front of a pre-existing discontinuity.& & & & Contrary to common assumption, we find seismic litude is not correlated with local imaged strain large local strain often occurs with small acoustic emissions, and vice versa. The seismic strain partition coefficient is very low overall and locally highly variable. Local strain is therefore predominantly aseismic, explained in part by grain/crack rotation along the emergent shear zone. The shear fracture energy calculated from local dilation and shear strain on the fault is half of that inferred from the bulk deformation, with a smaller critical slip distance, indicating that less energy is required for local breakdown in the shear zone compared with models of uniform slip.& & & & This improvement in process-based understanding holds out the prospect of reducing systematic errors in forecasting system-sized catastrophic failure in a variety of applications.& &
Publisher: Elsevier BV
Date: 07-2006
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-10164
Abstract: Fatigue and damage accumulation in granitoids are classical, but poorly characterised, rock mechanics problems. In order to explore these phenomena, we consider colliding curling stones as a rock physics experiment. Curling stones are made using granitoids from either Ailsa Craig (Scotland) or Trevor (North Wales), which are chosen for their uniformity, strength, and durability. During a curling game, stones are slid over an ice sheet and made to collide along a circumferential striking band. From a rock physics perspective, the collision of curling stones can be modelled as unconfined uniaxial compression of two convex surfaces under well defined boundary conditions. A curling stone experiences about 2900 collisions per season and is played for 10-15 years before refurbishment, which provides a unique long-term opportunity to study fatigue and damage accumulation under cyclic loading.Here, we first determine the stress magnitudes and strain rates of head-on curling stone impacts using a series of on-ice experiments involving a high speed camera and pressure-sensitive films. We then characterise the observed damage that these collisions produce on the centimetre and micrometre scale using photogrammetry, synchrotron microtomography, optical microscopy, and backscattered electron imaging. We show that during each impact, a curling stone is stressed to at least 300-680 MPa (for a maximum-velocity scenario of 2.9& #177 .1 ms-1), which exceeds the unconfined compressive strength of the rocks (232-395 MPa Nichol, 2001, J. Gemm. 27/5). Over its lifetime, a curling stone thus experiences thousands of impacts that will cause damage. The strain rates of these impacts (24& #177 s-1) most closely resemble seismic magnitudes, suggesting that the impacts are dynamic in nature. This is supported by the type of damage observed in aged curling stones: (1) Hertzian cone fractures, (2) ejection of rock powder during collisions, and (3) minor spalling microcracks. Most s les show damage being confined to macroscopic Hertzian cone fractures and their immediate collet zones in the relatively narrow striking band. Within the striking band, the circumferential density of cone fractures is limited to about 2-2.5 fractures/cm. Surprisingly, damage does not appear to extend beyond about 3-5 cm into the stones along a radial direction.Our observations allow us to formulate a model for damage evolution in curling stones. We infer that high-velocity/high-stress impacts initiate cone fractures up to a specific spatial density. As they mature over repeated impacts in the same regions of the striking band, cone fractures progressively propagate and coarsen with subsequent collisions, concentrating and channelling the accumulating damage. This damage geometry is surprisingly effective in shielding the rest of the stones from the reaching critical stress levels for damage. Our findings are significant for applications where rocks are exposed to large numbers of high-stress impacts and suggest that a relatively narrow damage zone can d en even high-impact stresses over a relatively moderate network of fractures.
Publisher: Cambridge University Press (CUP)
Date: 23-02-2015
DOI: 10.1017/PAB.2014.17
Abstract: The chambered shell of modern cephalopods functions as a buoyancy apparatus, allowing the animal to enter the water column without expending a large amount of energy to overcome its own weight. Indeed, the chambered shell is largely considered a key adaptation that allowed the earliest cephalopods to leave the ocean floor and enter the water column. It has been argued by some, however, that the iconic chambered shell of Paleozoic and Mesozoic ammonoids did not provide a sufficiently buoyant force to compensate for the weight of the entire animal, thus restricting ammonoids to a largely benthic lifestyle reminiscent of some octopods. Here we develop a technique using high-resolution computed tomography to quantify the buoyant properties of chambered shells without reducing the shell to ideal spirals or eliminating inherent biological variability by using mathematical models that characterize past work in this area. This technique has been tested on Nautilus pompilius and is now extended to the extant deep-sea squid Spirula spirula and the Jurassic ammonite Cadoceras sp. hatchling. Cadoceras is found to have possessed near-neutral to positive buoyancy if hatched when the shell possessed between three and five chambers. However, we show that the animal could also overcome degrees of negative buoyancy through swimming, similar to the paralarvae of modern squids. These calculations challenge past inferences of benthic life habits based solely on calculations of negative buoyancy. The calculated buoyancy of Cadoceras supports the possibility of planktonic dispersal of ammonite hatchlings. This information is essential to understanding ammonoid ecology as well as biotic interactions and has implications for the interpretation of geochemical data gained from the isotopic analysis of the shell.
Publisher: Copernicus GmbH
Date: 13-04-2018
DOI: 10.5194/SE-2018-28
Abstract: Abstract. The olivine carbonation reaction, in which carbon dioxide is chemically incorporated to form carbonate, is central to the emerging carbon sequestration method using ultramafic rocks. The rate of this retrograde metamorphic reaction is controlled, in part, by the available reactive surface area: as the solid volume increases during carbonation, the feasibility of this method ultimately depends on the maintenance of porosity and the creation of new reactive surfaces. We conducted in-situ dynamic x-ray microtomography and nanotomography experiments to image and quantify the porosity generation during olivine carbonation. We designed a s le setup that included a thick-walled cup (made of porous olivine aggregates with a mean grain size of either ~ 5 or ~ 80 μm) filled with loose olivine sands with grain sizes of 100–500 μm. The whole s le assembly was reacted with a NaHCO3 aqueous solution at 200 °C, under a constant confining pressure of 13 MPa and a pore pressure of 10 MPa. Using synchrotron-based X-ray microtomography, the 3-dimensional (3-D) pore structure evolution of the carbonating olivine cup was documented until the olivine aggregates became disintegrated. The dynamic microtomography data show a volume reduction in olivine at the beginning of the reaction, indicating a vigorous dissolution process consistent with the disequilibrium reaction kinetics. In the olivine cup with a grain size of ~ 80 μm (coarse-grained cup), dissolution fractures developed within 30 hours, before any precipitation was observed. In the experiment with the olivine cup of ~ 5 μm mean grain size (fine-grained cup), idiomorphic magnesite crystals were observed on the surface of the olivine sands. The magnesite shows a near constant growth throughout the experiment, suggesting that the reaction is self-sustained. Large fractures were generated as reaction proceeds and eventually disintegrate the aggregate after 140 hours. Detailed analysis show that these are expansion cracks caused by the volume mismatch between the expanding interior and the nearly constant surface. Nanotomography images of the reacted olivine cup reveal pervasive etch-pits and worm-holes in the olivine grains. We interpret this perforation of the solids to provide continuous fluid access, which is likely key to the complete carbonation observed in nature. Reactions proceeding through the formation of nano- to micron-scale dissolution channels provide a viable microscale mechanism in carbon sequestration practices. For the natural peridotite carbonation, a coupled-mechanism of dissolution and reaction-induced fracturing should account for the observed self sustainability of the reaction.
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-3424
Abstract: The 3rd and 4th generation of synchrotron light sources with their high brilliance, fluxes and beam energies allow the development of innovative X-ray translucent rock deformation apparatus that maximise these capabilities. Following on from the development of the Mjolnir triaxial deformation rig (Butler et al., 2020), we present an upscaled design: Heitt Mjolnir, covering a wider temperature range and larger s le volume while operating at similar pressure, enabling a wide range of time-resolved investigations. This device is designed to characterise coupled hydraulic, chemical and mechanical processes, occurring at various temperatures, from the & #181 m to the centimetre scale in cylindrical s les of 10 mm diameter and 20 mm length. Heitt Mjolnir can simultaneously reach confining pressures of & #8804 MPa (hydraulic), 500 MPa of axial stress while the s le& #8217 s pore fluid pressure is controlled in a dedicated fluid channel and can reach 30 MPa. This apparatus has an internal heating system and is able to reach temperatures of 573 K in the s le with a minimal vertical thermal gradient of .5 K/mm. This portable and modular device has been successfully deployed in operando studies at TOMCAT (SLS) and I12 JEEP (DLS) beamlines for 4D X-ray microtomography with scan intervals of a few minutes. Heitt Mjolnir allows the 4D characterisation of low-grade metamorphism, fluid-rock interaction and deformation processes. It enables spatially and temporally resolved fluid-rock interaction studies at a wide range of conditions and, by covering most geological reservoirs, will be particularly valuable for geothermal, carbonation or subsurface gas storage research.
Publisher: Copernicus GmbH
Date: 13-04-2018
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-902
Abstract: & & The rocks used to produce curling stones for international competition are only sourced from two localities in the world: Ailsa Craig, Scotland and Trefor, Wales. Curling stones consist of two components: (1) the running band (the ring-shaped bottom surface of the stone which rests on the ice) and (2) the striking band (the convex band on the profile of stones which collides with those of other stones). With a focus on the striking bands, we aim to document the damage evolution of curling stones using synchrotron microtomography (3D characterisation of pristine s les and 4D damage evolution), optical and scanning electron microscopy (quantitative characterisation of pristine s les and microfracture characterisation of damaged striking bands), and petrophysical testing (fracture characteristics and comparative data). These data will be complemented by an on-ice experiment that will determine the mechanics (e.g., stress distribution, contact area, and velocity) of curling stone impacts. Out of four curling stone varieties (from Ailsa Craig and Trefor), we observe the striking bands of three varieties to show macroscopic, incipient to mature, curvilinear fractures. The curvature of these fractures is consistent and does not vary significantly between in idual stones and between curling stone varieties. However, the degree of macroscopic fracture development differs between aged striking bands of curling stone types: Blue Trefor (macroscopic fractures not observed), Red Trefor (weakly incipient), Ailsa Craig Common Green (incipient to juvenile), and Ailsa Craig Blue Hone (juvenile to mature). Unfortunately, it is not possible to determine the degree of usage (age) of the selected s les and thus it is not possible to normalize these apparent differences in damage. Given that the striking band limits the lifetime of curling stones, understanding the damage evolution of curling stones can contribute valuable information to the maintenance of curling stones. The rock physics of curling stone impacts is linked to dynamic spalling and more broadly to rock failure, as these processes are ultimately related to the initiation and propagation of fractures.& &
Publisher: Copernicus GmbH
Date: 28-08-2017
DOI: 10.5194/SE-2017-96
Abstract: Abstract. The dispersal of monomineralic quartz domains in a quartzofeldspathic ultramylonite is interpreted to be the result of the emergence of syn-kinematic pores, called creep cavities. The cavities can be considered the product of two distinct mechanisms that formed hierarchically: Zener-Stroh cracking and viscous grain boundary sliding. In initially thick and coherent quartz ribbons deforming by grain size-insensitive creep, cavities were generated by the Zener-Stroh mechanism on grain-boundaries aligned with the YZ plane of finite strain. The opening of creep cavities promoted the ingress of fluids to sites of low stress. The local addition of a fluid lowered the adhesion and cohesion of grain-boundaries and promoted viscous grain boundary sliding. With the increased contribution of viscous grain boundary sliding, a second population of cavities formed to accommodate strain incompatibilities. Ultimately, the emergence of creep cavities is interpreted to be responsible for the transition of quartz domains from a grain size-insensitive, to a grain size-sensitive rheology.
Publisher: World Scientific Publishing Company
Date: 05-2010
Publisher: American Geophysical Union (AGU)
Date: 2008
DOI: 10.1029/2006JB004833
Publisher: Wiley
Date: 27-03-2017
DOI: 10.1111/JMG.12245
Publisher: American Geophysical Union (AGU)
Date: 08-2023
DOI: 10.1029/2023JB026809
Abstract: Earthquake‐induced fracturing of the dry and strong lower crust can transiently increase permeability for fluids to flow and trigger metamorphic and rheological transformations. However, little is known about the porosity that facilitates these transformations. We analyzed microstructures that have recorded the mechanisms generating porosity in the lower crust from a pristine pseudotachylyte (solidified earthquake‐derived frictional melt) and a mylonitized pseudotachylyte from Lofoten, Norway to understand the evolution of fluid pathways from the coseismic to the post‐ and interseismic stages of the earthquake cycle. Porosity is dispersed and poorly interconnected within the pseudotachylyte vein (0.14 vol%), with a noticeably increased amount along garnet grain boundaries (0.25–0.41 vol%). This porosity formed due to a net negative volume change at the grain boundary when garnet overgrows the pseudotachylyte matrix. Efficient healing of the damage zone by fluid‐assisted growth of feldspar neoblasts resulted in the preservation of only a few but relatively large interconnected pores along coseismic fractures (0.03 vol% porosity). In contrast, porosity in the mylonitized pseudotachylyte is dramatically reduced (0.02 vol% overall), because of the efficient precipitation of phases ( hibole, biotite and feldspars) into transient pores during grain‐size sensitive creep. Porosity reduction on the order of % may be a contributing factor in shear zone hardening, potentially leading to the development of new pseudotachylytes overprinting the mylonites. Our results show that earthquake‐induced rheological weakening of the lower crust is intermittent and occurs when a fluid can infiltrate a transiently permeable shear zone, thereby facilitating diffusive mass transfer and creep.
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-9560
Abstract: & & Long-distance transport along weak basal detachments in thin-skinned tectonics is often accomplished by rheologically weak evaporites. This weakness can be attributed to& the behavior of gypsum and/or halite. While the former dehydrates and the released fluid reduces the effective stress in the system, the latter is known to be extremely weak at the corresponding conditions. Separately, both minerals and their behavior under tectonic loading have been studied in great detail. However, these studies on single minerals are limited in that natural detachments are often not monomineralic and are clearly affected by interdependencies between different mineral species. In evaporitic sequences, two key couplings that can be expected are: 1) the sensitivity of the dehydration reaction to the pore fluid pressure versus the notoriously low permeability of rock salt (a potentially negative feedback), and 2) the exposure of halite to undersaturated water released from the gypsum dehydration reaction, versus the response of the dehydration reaction to lower water activity due to dissolved salt species (a potentially positive feedback).& & & & & Here we present insights from experiments that used time-resolved (4D) synchrotron tomographic microscopy and our x-ray transparent triaxial deformation rig Mj& #248 lnir to document the evolution of layered gypsum-halite s les that were simultaneously deformed and dehydrated. Our data, which were acquired at the TOMCAT beamline at the Swiss Light Source, allow us to visualise chemical-hydraulic-mechanical feedbacks on the grain scale, and quantify the microscale evolution of transport properties. In this contribution, we show that gypsum dehydration affects the capacity of the halite layers to retain the liberated fluids. The reaction itself generates the pore fluid pressure to create permeability in the salt layers through hydraulic fracturing. Dissolved salt significantly accelerates the reaction, and the evolving interconnected porosity facilitates the transport and precipitation of solutes, which contributes to the rheological complexity. These insights have, potentially significant, repercussions on the long-standing assumption about the significance of the gypsum dehydration on thrust fault formation within evaporitic sequences.& &
Publisher: Copernicus GmbH
Date: 19-03-2018
Publisher: Copernicus GmbH
Date: 15-06-2016
Publisher: American Geophysical Union (AGU)
Date: 23-09-2016
DOI: 10.1002/2016GL070834
Publisher: Copernicus GmbH
Date: 19-08-2020
DOI: 10.5194/SE-2020-137
Abstract: Abstract. In experiments designed to understand deep shear zones, we show that periodic porous sheets emerge spontaneously during viscous creep, forming a hydro-mechanical anisotropy that influences mass transfer. These findings challenge the current paradigm of viscosity in solid rocks. In particular, they showcase how shear zones may actively focus mass transport and highlight the possibility that viscous rocks could locally transition from flow to fracture. Our work demonstrates that viscosity in solids is not directly comparable to viscosity in fluids and this is consequential for a range of important solid Earth topics, like slow earthquakes, the flow of glacial ice and the tectonics of exoplanets.
Publisher: Copernicus GmbH
Date: 13-07-2018
Abstract: Abstract. The olivine carbonation reaction, in which carbon dioxide is chemically incorporated to form carbonate, is central to the emerging carbon sequestration method using ultramafic rocks. The rate of this retrograde metamorphic reaction is controlled, in part, by the available reactive surface area: as the solid volume increases during carbonation, the feasibility of this method ultimately depends on the maintenance of porosity and the creation of new reactive surfaces. We conducted in situ dynamic X-ray microtomography and nanotomography experiments to image and quantify the porosity generation during olivine carbonation. We designed a s le setup that included a thick-walled cup (made of porous olivine aggregates with a mean grain size of either ∼ 5 or ∼ 80 µm) filled with loose olivine sands with grain sizes of 100–500 µm. The whole s le assembly was reacted with a NaHCO3 aqueous solution at 200 °C, under a constant confining pressure of 13 MPa and a pore pressure of 10 MPa. Using synchrotron-based X-ray microtomography, the three-dimensional (3-D) pore structure evolution of the carbonating olivine cup was documented until the olivine aggregates became disintegrated. The dynamic microtomography data show a volume reduction in olivine at the beginning of the reaction, indicating a vigorous dissolution process consistent with the disequilibrium reaction kinetics. In the olivine cup with a grain size of ∼ 80 µm (coarse-grained cup), dissolution planes developed within 30 h, before any precipitation was observed. In the experiment with the olivine cup of ∼ 5 µm mean grain size (fine-grained cup), idiomorphic magnesite crystals were observed on the surface of the olivine sands. The magnesite shows a near-constant growth throughout the experiment, suggesting that the reaction is self-sustained. Large fractures were generated as the reaction proceeded and eventually disintegrated the aggregate after 140 h. Detailed analysis show that these are expansion cracks caused by the volume mismatch in the cup walls, between the expanding interior and the near-surface which keeps a nearly constant volume. Nanotomography images of the reacted olivine cup reveal pervasive etch pits and wormholes in the olivine grains. We interpret this perforation of the solids to provide continuous fluid access, which is likely key to the complete carbonation observed in nature. Reactions proceeding through the formation of nano- to micron-scale dissolution channels provide a viable microscale mechanism in carbon sequestration practices. For the natural peridotite carbonation, a coupled mechanism of dissolution and reaction-induced fracturing should account for the observed self-sustainability of the reaction.
Publisher: Springer Berlin Heidelberg
Date: 2009
Publisher: Elsevier BV
Date: 2021
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-7933
Abstract: Catastrophic failure in brittle, porous materials initiates when structural damage, in the form of smaller-scale fractures, localises along an emergent failure plane or 'fault' in a transition from stable crack growth to dynamic rupture. Due to the extremely rapid nature of this critical transition, the precise micro-mechanisms involved are poorly understood and difficult to capture. However, these mechanisms are crucial drivers for devastating phenomena such as earthquakes, including induced seismicity, landslides and volcanic eruptions, as well as large-scale infrastructure collapse. Here we observe these micro-mechanisms directly by controlling the rate of micro-fracturing events to slow down the transition in a unique triaxial deformation experiment that combines acoustic monitoring with contemporaneous in-situ x-ray imaging of the microstructure. The results [1] provide the first integrated picture of how damage and associated micro-seismic events emerge and evolve together during localisation and failure and allow us to ground truth some previous inferences from mechanical and seismic monitoring alone. They also highlight where such inferences miss important kinematically-governed grain-scale mechanisms prior to and during shear failure.The evolving damage imaged in the 3D x-ray volumes and local strain fields undergoes a breakdown sequence involving several stages: (i) self-organised exploration of candidate shear zones close to peak stress, (ii) spontaneous tensile failure of in idual grains due to point loading and pore-emanating fractures within an emergent and localised shear zone, validating many inferences from acoustic emissions monitoring, (iii) formation of a proto-cataclasite due to grain rotation and fragmentation, highlighting both the control of grain size on failure and the relative importance of aseismic mechanisms such as crack rotation in accommodating bulk shear deformation. Dilation and shear strain remain strongly correlated both spatially and temporally throughout s le weakening, confirming the existence of a cohesive zone, but with crack damage distributed throughout the shear zone rather than concentrated solely in a breakdown zone at the propagating front of a pre-existing discontinuity.Contrary to common assumption, we find seismic litude is not correlated with local imaged strain large local strain often occurs with small acoustic emissions, and vice versa. The seismic strain partition coefficient is very low overall and locally highly variable. Local strain is therefore predominantly aseismic, explained in part by grain/crack rotation along the emergent shear zone. The shear fracture energy calculated from local dilation and shear strain on the fault is half of that inferred from the bulk deformation, with a smaller critical slip distance, indicating that less energy is required for local breakdown in the shear zone compared with models of uniform slip.This improvement in process-based understanding holds out the prospect of reducing systematic errors in forecasting system-sized catastrophic failure in a variety of applications.[1] Cartwright-Taylor et al. 2022, Nature Communications 13, 6169, 0.1038/s41467-022-33855-z
Publisher: American Geophysical Union (AGU)
Date: 09-2012
DOI: 10.1029/2012GC004085
Publisher: Springer Science and Business Media LLC
Date: 27-06-2009
Publisher: Elsevier BV
Date: 04-2022
Publisher: Copernicus GmbH
Date: 28-08-2017
Publisher: Geological Society of America
Date: 02-06-2023
DOI: 10.1130/G51128.1
Abstract: Mylonitic shear zones are important fluid conduits in the Earth’s crust. They host transient and permeable porosity that facilitates fluid transfer and controls fluid-rock interaction. Here we present microstructural observations from a mid-crustal ultramylonite with very large pores that occupy the strain shadows of albite porphyroclasts. Our non-invasive three-dimensional X-ray microtomographic data show that the largest of these strain shadow megapores have substantial volumes of as much as ∼1.7 × 105 µm3. Given that the s le shows no signs of retrogressive overprint or weathering, these pores must be synkinematic. Importantly, the close proximity of the pores to creep cavities in dynamically recrystallized quartz ribbon grains suggests a potential hydraulic link between fluid in the strain shadow megapores and fluid in the creeping rock matrix. The evolving megapores constitute very large syndeformational local fluid reservoirs in mylonites that likely fed into the granular fluid pump established by the dynamically evolving creep cavities. Our findings add to an emerging picture of the dynamic transport properties of ultramylonitic shear zones, where the formation and destruction of porosity are intrinsically linked to microscale deformation processes. They also suggest that despite many studies on porphyroclast systems, open questions remain, especially concerning the interaction of clasts with their matrix.
Publisher: American Association for the Advancement of Science (AAAS)
Date: 04-2011
Abstract: As mantle rocks melt, an interconnected network of liquid drives the ascent of magma to the sea floor.
Publisher: Elsevier BV
Date: 10-2008
Publisher: Elsevier BV
Date: 11-2018
Publisher: Copernicus GmbH
Date: 21-06-2021
Publisher: Copernicus GmbH
Date: 10-01-2022
Abstract: Abstract. Phyllosilicates are generally regarded to have a reinforcing effect on chemical compaction by dissolution–precipitation creep (DPC) and thereby influence the evolution of hydraulic rock properties relevant to groundwater resources and geological repositories as well as fossil fuel reservoirs. We conducted oedometric compaction experiments on layered NaCl–biotite s les to test this assumption. In particular, we aim to analyse slow chemical compaction processes in the presence of biotite on the grain scale and determine the effects of chemical and mechanical feedbacks. We used time-resolved (4-D) microtomographic data to capture the dynamic evolution of the porosity in layered NaCl–NaCl/biotite s les over 1619 and 1932 h of compaction. Percolation analysis in combination with advanced digital volume correlation techniques showed that biotite grains influence the dynamic evolution of porosity in the s le by promoting a reduction of porosity in their vicinity. However, the lack of preferential strain localisation around phyllosilicates and a homogeneous distribution of axial shortening across the s le suggests that the porosity reduction is not achieved by pore collapse but by the precipitation of NaCl sourced from outside the NaCl–biotite layer. Our observations invite a renewed discussion of the effect of phyllosilicates on DPC, with a particular emphasis on the length scales of the processes involved. We propose that, in our experiments, the diffusive transport processes invoked in classical theoretical models of DPC are complemented by chemo-mechanical feedbacks that arise on longer length scales. These feedbacks drive NaCl diffusion from the marginal pure NaCl layers into the central NaCl–biotite mixture over distances of several hundred micrometres and several grain diameters. Such a mechanism was first postulated by Merino et al. (1983).
Publisher: Virtual Explorer Pty Ltd.
Date: 2002
Publisher: Springer International Publishing
Date: 12-07-2018
Publisher: Copernicus GmbH
Date: 08-03-2012
DOI: 10.5194/SE-3-71-2012
Abstract: Abstract. We conducted an in-situ X-ray micro-computed tomography heating experiment at the Advanced Photon Source (USA) to dehydrate an unconfined 2.3 mm diameter cylinder of Volterra Gypsum. We used a purpose-built X-ray transparent furnace to heat the s le to 388 K for a total of 310 min to acquire a three-dimensional time-series tomography dataset comprising nine time steps. The voxel size of 2.2 μm3 proved sufficient to pinpoint reaction initiation and the organization of drainage architecture in space and time. We observed that dehydration commences across a narrow front, which propagates from the margins to the centre of the s le in more than four hours. The advance of this front can be fitted with a square-root function, implying that the initiation of the reaction in the s le can be described as a diffusion process. Novel parallelized computer codes allow quantifying the geometry of the porosity and the drainage architecture from the very large tomographic datasets (20483 voxels) in unprecedented detail. We determined position, volume, shape and orientation of each resolvable pore and tracked these properties over the duration of the experiment. We found that the pore-size distribution follows a power law. Pores tend to be anisotropic but rarely crack-shaped and have a preferred orientation, likely controlled by a pre-existing fabric in the s le. With on-going dehydration, pores coalesce into a single interconnected pore cluster that is connected to the surface of the s le cylinder and provides an effective drainage pathway. Our observations can be summarized in a model in which gypsum is stabilized by thermal expansion stresses and locally increased pore fluid pressures until the dehydration front approaches to within about 100 μm. Then, the internal stresses are released and dehydration happens efficiently, resulting in new pore space. Pressure release, the production of pores and the advance of the front are coupled in a feedback loop.
Publisher: International Union of Crystallography (IUCr)
Date: 16-10-2020
DOI: 10.1107/S160057752001173X
Abstract: An X-ray transparent experimental triaxial rock deformation apparatus, here named `Mjölnir', enables investigations of brittle-style rock deformation and failure, as well as coupled thermal, chemical and mechanical processes relevant to a range of Earth subsurface environments. Designed to operate with cylindrical s les up to 3.2 mm outside-diameter and up to 10 mm length, Mjölnir can attain up to 50 MPa confining pressure and in excess of 600 MPa axial load. The addition of heaters extends the experimental range to temperatures up to 140°C. Deployment of Mjolnir on synchrotron beamlines indicates that full 3D datasets may be acquired in a few seconds to a few minutes, meaning full 4D investigations of deformation processes can be undertaken. Mjölnir is constructed from readily available materials and components and complete technical drawings are included in the supporting information.
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-5250
Abstract: & & We revisit large shear strain deformation experiments on Carrara marble and observe that anisotropic porous domains develop spontaneously during shearing. Specifically, as s les are deformed periodic porous sheets are documented to emerge and are found to transfer mass. These results imply that viscous shear zones may naturally partition fluids into highly anisotropic bands. As this hydro-mechanical anisotropy is produced by creep, each porous sheet is interpreted to represent a transient dynamic pathway for fluid transport. It is unclear how long each porous domain is uniquely sustained but it is clear that sheets are persistently present with increasing strain. Our results forward the idea that viscous shear zones have dynamic transport properties that are not related to fracturing or chemical reaction. We believe these new results provide experimental foundation for changing the paradigm of viscosity in rocks to include dynamic permeability. In our view making this change in perspective could alter many classical interpretations in natural banded mylonite zones, for ex le shear zone parallel syn-kinematic veining may be the result of& ore sheet instability and ductile fracturing.& &
Publisher: Elsevier BV
Date: 08-2014
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-6483
Abstract: Many metamorphic rocks have a fabric. What is often not clear is how much deformational or metamorphic processes contributed to the formation of these fabrics. Are foliations always the result of strain? When does intrinsic crystallographic anisotropy alone lead to the formation of structural elements? Understanding the relative contributions of deformation and metamorphism in rock fabrics is fundamentally important because it is foundational to understanding the role of stress in reacting and deforming rocks.To this end, we make a major advance in our understanding of fabric development in reacting rocks by showing in time-resolved (4D) synchrotron microtomography (& #181 CT) experiments that when a gypsum dehydration reaction occurs in a differentially stressed s le the reaction products develop orthogonally to the largest principal stress. This is an important finding because we can show with our & #181 CT data that this preferred orientation forms early in the reaction and at very small strains ( %). Using a simple kinematic model we can demonstrate that it cannot have formed because of reorientation during mechanical compaction. It remains to be established if it is nucleation or growth of bassanite that is being affected by the stress or both. Our experiments suggest that metamorphic transformations may be inherently anisotropic when reacting under the influence of a non-hydrostatic stress state.& The consequences of this are many. For ex le, there will be cases in natural rocks where the interpretation of a lineation, foliation or crystallographic preferred orientation as formed by strain may be incorrect. Moreover, the physical properties (e.g. hydraulic and mechanics) of metamorphic rocks could also be significantly anisotropic from early in a transformation. Mass transport pathways might initialise as channelled or partitioned conduits which would have an impact during subduction and in thin-skinned tectonics. Our data reveal a critical new finding related to the very common geological occurrence of reacting rocks experiencing a differential stress.
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-10254
Abstract: & & & span& In most university geosciences curricula, structural geology and tectonics (SGT) form a core part of teaching. While only a small percentage of Earth science graduates will become structural geologists, many will someday use structural concepts and techniques to solve problems in fields such as nuclear waste storage, the geology of growing urban environments,& geohazards, unconventional reservoirs, geothermal energy, CO2 sequestration, energy storage and more. A basic understanding of structural geology is thus part of a critical knowledge foundation in Earth sciences and many related disciplines. In addition, new tools and data are becoming available at a rapid pace, and enable more integrated, multi-dimensional assessments of the geosphere and our societal interfaces with it. All of this provides new opportunities and challenges for STG courses. & /span& & & & & & span& In April 2019, a pre-EGU two-day workshop (TeachSGT21) was organized during which strengths and weaknesses of, and threats to current SGT curricula were analyzed. Participants of the workshop covered 11 European and 2 overseas countries, and came from academia as well as industry. On the basis of the workshop, we now outline educational demands from industry and research and discuss the role and significance of field training. Further, we review initiatives that use innovative tools and techniques in teaching. While not claiming to represent all aspects of modern SGT teaching, we expect that our observations can stimulate reflection on degrees and approach and may help making choices in curriculum renewal.& /span& & &
Publisher: Copernicus GmbH
Date: 14-08-2023
DOI: 10.5194/EGUSPHERE-2023-1819
Abstract: Abstract. X-ray computed tomography has established itself as a crucial tool in the analysis of rock materials, providing the ability to visualise intricate 3D microstructures and capture quantitative information about internal phenomena such as structural damage, mineral reactions, and fluid-rock interactions. The efficacy of this tool, however, depends significantly on the precision of image segmentation, a process that has seen varied results across different methodologies, ranging from simple histogram thresholding to more complex machine learning and deep learning strategies. The irregularity in these segmentation outcomes raises concerns about the reproducibility of the results, a challenge that we aim to address in this work. In our study, we employ the mass balance of a metamorphic reaction as an internal standard to verify segmentation accuracy and shed light on the advantages of deep learning approaches, particularly their capacity to efficiently process expansive datasets. Our methodology utilises deep learning to achieve accurate segmentation of time-resolved volumetric images of the gypsum dehydration reaction, a process that traditional segmentation techniques have struggled with due to poor contrast between reactants and products. We utilise a 2D U-net architecture for segmentation and introduce machine learning-obtained labelled data (specifically, from random forest classification) as an innovative solution to the limitations of training data obtained from imaging. The deep learning algorithm we developed has demonstrated remarkable resilience, consistently segmenting volume phases across all experiments. Furthermore, our trained neural network exhibits impressively short run times on a standard workstation equipped with a Graphic Processing Unit (GPU). To evaluate the precision of our workflow, we compared the theoretical and measured molar evolution of gypsum to bassanite during dehydration. The errors between the predicted and segmented volumes in all time-series experiments fell within the 2 % confidence intervals of the theoretical curves, affirming the accuracy of our methodology. We also compared the results obtained by the proposed method with standard segmentation methods and found a significant improvement in precision and accuracy of segmented volumes. This makes the segmented CT images suited for extracting quantitative data, such as variations in mineral growth rate and pore size during the reaction. In this work, we introduce a distinctive approach by using an internal standard to validate the accuracy of a segmentation model, demonstrating its potential as a robust and reliable method for image segmentation in this field. This ability to measure the volumetric evolution during a reaction with precision paves the way for advanced modelling and verification of the physical properties of rock materials, particularly those involved in tectono-metamorphic processes. Our work underscores the promise of deep learning approaches in elevating the quality and reproducibility of research in the geosciences.
Publisher: Frontiers Media SA
Date: 26-11-2019
Publisher: Geological Society of America
Date: 03-2015
DOI: 10.1130/G36307.1
Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-15691
Abstract: & & Catastrophic failure is a critical phenomenon present in Earth systems on a variety of scales, and is associated with the evolution of damage leading to system-size failure. Laboratory testing of rock failure permits characterization of fracture network evolution at the micro-scale to understand the interaction of cracks, pores and grain boundaries to an applied stress field, and the relationship between deformation and seismic response. Previous studies have relied on acoustic emissions (hearing) or X-ray imaging (seeing) to study the process of localization, which involves spontaneous self-organization of smaller cracks along faults and fractures on localised zones of deformation. To combine hearing and seeing of the microscopic processes and their control of system-sized failure, a novel x-ray transparent cell was used for deformation experiments of rock s les, which permits integration of acoustic monitoring with fast synchrotron x-ray imaging. To increase temporal characterization of damage beyond the temporal resolution of the fast 3D synchrotron system, acoustic emission (AE) feedback control was used to regulate the applied stress and slow down the deformation processes. As a result, there is increased temporal resolution of the incremental deformation between successive x-ray scanned states allowing synchronized comparison of acoustic emissions to x-ray scans. Here, we present the seismic analysis used to characterize the velocity evolution of the rock s les, and the location and characteristics of in idual AE events in relation to microscopic deformation processes. Time-lapse velocity measurements are linked to internal stress changes and structural damage corresponding to seismic and aseismic deformation processes, while acoustic emissions are a direct indication of local cracking.& We show that we can successfully locate AE events in 3D using only two sensors on either end of the s le, based on ellipsoid mapping, and x-ray image to event correlation. We explore temporal and spatial statistics of AE signatures and how those are linked to the strain field in the s les measured with incremental digital volume correlation between pairs of recorded x-ray tomograms. The direct observation of AE and X-ray images enables quantification of relevant seismic (local cracking leading to AE) to aseismic (elastic loading and silent irreversible damage) processes, with information extracted over fine temporal resolution throughout the deformation process through the AE-feedback control.& &
Publisher: Frontiers Media SA
Date: 14-01-2020
Publisher: Copernicus GmbH
Date: 19-12-2017
Abstract: Abstract. Establishing models for the formation of well-mixed polyphase domains in ultramylonites is difficult because the effects of large strains and thermo-hydro-chemo-mechanical feedbacks can obscure the transient phenomena that may be responsible for domain production. We use scanning electron microscopy and nanotomography to offer critical insights into how the microstructure of a highly deformed quartzo-feldspathic ultramylonite evolved. The dispersal of monomineralic quartz domains in the ultramylonite is interpreted to be the result of the emergence of synkinematic pores, called creep cavities. The cavities can be considered the product of two distinct mechanisms that formed hierarchically: Zener–Stroh cracking and viscous grain-boundary sliding. In initially thick and coherent quartz ribbons deforming by grain-size-insensitive creep, cavities were generated by the Zener–Stroh mechanism on grain boundaries aligned with the YZ plane of finite strain. The opening of creep cavities promoted the ingress of fluids to sites of low stress. The local addition of a fluid lowered the adhesion and cohesion of grain boundaries and promoted viscous grain-boundary sliding. With the increased contribution of viscous grain-boundary sliding, a second population of cavities formed to accommodate strain incompatibilities. Ultimately, the emergence of creep cavities is interpreted to be responsible for the transition of quartz domains from a grain-size-insensitive to a grain-size-sensitive rheology.
Publisher: Springer Science and Business Media LLC
Date: 06-2009
DOI: 10.1038/NATURE08051
Abstract: The feedback between fluid migration and rock deformation in mid-crustal shear zones is acknowledged as being critical for earthquake nucleation, the initiation of subduction zones and the formation of mineral deposits. The importance of this poorly understood feedback is further highlighted by evidence for shear-zone-controlled advective flow of fluids in the ductile lower crust and the recognition that deformation-induced grain-scale porosity is a key to large-scale geodynamics. Fluid migration in the middle crust cannot be explained in terms of classical concepts. The environment is considered too hot for a dynamic fracture-sustained permeability as in the upper crust, and fluid pathways are generally too deformed to be controlled by equilibrium wetting angles that apply to hotter, deeper environments. Here we present evidence that mechanical and chemical potentials control a syndeformational porosity generation in mid-crustal shear zones. High-resolution synchrotron X-ray tomography and scanning electron microscopy observations allow us to formulate a model for fluid migration in shear zones where a permeable porosity is dynamically created by viscous grain-boundary sliding, creep cavitation, dissolution and precipitation. We propose that syndeformational fluid migration in our 'granular fluid pump' model is a self-sustained process controlled by the explicit role of the rate of entropy production of the underlying irreversible mechanical and chemical microprocesses. The model explains fluid transfer through the middle crust, where strain localization in the creep regime is required for plate tectonics, the formation of giant ore deposits, mantle degassing and earthquake nucleation. Our findings provide a key component for the understanding of creep instabilities in the middle crust.
Publisher: Elsevier BV
Date: 07-2016
Publisher: Springer Science and Business Media LLC
Date: 10-10-2016
Publisher: Copernicus GmbH
Date: 21-06-2021
DOI: 10.5194/SE-2021-83
Abstract: Abstract. Phyllosilicates are generally regarded to have a reinforcing effect on chemical compaction by dissolution-precipitation creep (DPC) and thereby influence the evolution of hydraulic rock properties relevant to groundwater resources, geological repositories as well as fossil fuel reservoirs. We conducted oedometric compaction experiments on layered NaCl-biotite s les to test this assumption. In particular, we aim to analyse slow chemical compaction processes in the presence of biotite on the grain scale and determine the effects of chemical and mechanical feedbacks. We used time-resolved (4D) microtomographic data to capture the dynamic evolution of the transport properties in layered NaCl-NaCl/biotite s les over 1619 and1932 hours of compaction. Percolation analysis in combination with advanced digital volume correlation techniques showed that biotite grains influence the dynamic evolution of porosity in the s le by promoting a reduction of porosity in their vicinity. However, the lack of preferential strain localisation around phyllosilicates and a homogeneous distribution of axial shortening across the s le suggests that the porosity reduction is not achieved by pore collapse but by the precipitation of NaCl sourced from outside the NaCl/biotite layer. Our observations invite a renewed discussion of the effect of phyllosilicates on DPC, with a particular emphasis on the length scales of the processes involved. We propose that, in our experiments, the diffusive transport processes invoked in classical theoretical models of DPC are superseded by chemo-mechanical feedbacks that arise on longer length scales. These feedbacks drive NaCl diffusion from the marginal pure NaCl layers into the central NaCl-biotite mixture over distances of several hundredμm and several grain diameters. Such a mechanism was first postulated by Merino et al. (1983)
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-9462
Abstract: & & Deformed eclogites often reveal interconnected layers of omphacite and intercalated elongated garnet clusters. The evolution of such fabrics is usually associated with strain localization and rheological weakening. To better understand the onset of strain localization in eclogite, we experimentally investigate the strain-dependence of microfabrics in omphacite-garnet aggregates. Eclogites were synthesized by hot-pressing omphacite-garnet powders (with a volume fraction of 25% garnet) in a piston-cylinder press at 3 GPa and 1100 & #176 C for 24 h. These synthetic eclogites were then axially shortened by ~3.5%, ~4.7%, ~17% and ~40% in a Griggs-type deformation apparatus at 2.5 GPa, 900& #176 C, and a strain rate of 6.4& #183 & sup& -6& /sup& s& sup& -1& /sup& . The low-strain experiments document microstructures developing near the material& #8217 s yield point at ~4% axial strain, whereas the highly strained s les reached nearly mechanical steady state with minor strain weakening. The recovered s les were analyzed using an X-ray microtomography instrument (& #956 CT) which provides quantitative volume, shape and spatial arrangement data in three dimensions. By utilizing optical light microscope, scanning electron microscope and electron backscatter diffraction analyses in combination with the & #956 CT data we identified the dominant deformation mechanisms operating at different strains and linked them to the microfabric. At low strain, omphacite exhibits a weak shape preferred orientation (SPO) and garnet tends to form clusters. The highly strained s les show a strong foliation with elongated omphacite crystals exhibiting a pronounced SPO and garnet clusters being arranged into elongated layers perpendicular to the maximum compressive stress. A reduction in grain size and an increase in density of low-angle grain boundaries with increasing strain indicate deformation of omphacite by dislocation creep. Elongated garnet clusters show brittle deformation in the form of micro-cracking. Evidence for minor crystal-plastic deformation in garnet occurs locally at the proximity of the grain boundaries where high differential stresses tend to localize resulting in increased misorientation. Similar to naturally deformed eclogites, we observe a layering of omphacite and garnet in our experimental s les, in which omphacite generally accommodated most of the strain while garnet grains behaved essentially like rigid bodies.& &
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-8015
Abstract: Stylolites are common microstructures in rocks, where mineral dissolution in a fluid localises in more or less discrete seams. Stylolite formation strongly affects rock porosity, pore connectivity and thus fluid flows. However, details of porosity evolution associated with stylolite nucleation, propagation and growth remain unclear, leading to the debate whether stylolites are conduits or barriers for fluids.In this contribution, we use an exceptionally large high-resolution SEM-BSE mosaic (102,600 & #215 18,239 pixels, 0.17& #181 m ixel) to investigate the detailed microstructures of stylolites from the Eilean Dubh limestone of the NW Highlands in Scotland. Advanced image analyses indicate that porosity self-organises systematically around stylolites suggesting a four-stage growth process for stylolites: In stage 1, primary pyrites in the rock matrix concentrate stress and dissolution, leading to the formation of porosity. In stage 2, the dissolution porosity self-organises, triggering a chemical-hydraulic-mechanical feedback loop that facilitates further dissolution. In stage 3, the porous zones enlarge and grow along the direction of the smallest principal stress (process zone), concentrating the synkinematic precipitation of pyrites from an external fluid in the core (core zone). In stage 4, the isolated domains connect forming a stylolite with a core zone in the center (highest porosity) surrounded on both sides by a process zones (higher-than-matrix porosity), suggesting a conduit phase during stylolite formation. Since the stylolite-hosting Eilean-Dubh limestones were episodically imbricated below the Ullapool thrust and the stylolites formed sub-parallel to the internal thrust planes, we speculate that synkinematic fluids during the stylolite formation were pumped episodically into the rock during activity of the Ullapool thrust. The detailed stylolite microstructures and the proposed process for stylolite growth suggest that stylolite in carbonate may act as a conduit for a fluid flow.
Publisher: Springer Science and Business Media LLC
Date: 02-10-2018
Publisher: Wiley
Date: 22-02-2013
Publisher: Springer Berlin Heidelberg
Date: 2013
Publisher: American Geophysical Union (AGU)
Date: 29-07-2020
DOI: 10.1029/2020JB019642
Abstract: Catastrophic failure of brittle rocks is important in managing risk associated with system‐sized material failure. Such failure is caused by nucleation, growth, and coalescence of microcracks that spontaneously self‐organize along localized damage zones under compressive stress. Here we present X‐ray microtomography observations that elucidate the in situ micron‐scale processes, obtained from novel tri‐axial compression experiments conducted in a synchrotron. We examine the effect of microstructural heterogeneity in the starting material (Ailsa Craig microgranite known for being virtually crack‐free) on crack network evolution and localization. To control for heterogeneity, we introduced a random nanoscale crack network into one s le by thermal stressing, leaving a second s le as‐received. By assessing the time‐dependent statistics of crack size and spatial distribution, we test the hypothesis that the degree of starting heterogeneity influences the order and predictability of the phase transition between intact and failed states. We show that this is indeed the case at the system‐scale. The initially more heterogeneous (heat‐treated) s le showed clear evidence for a second‐order transition: inverse power law acceleration in correlation length with a well‐defined singularity near failure and distinct changes in the scaling exponents. The more homogeneous (untreated) s le showed evidence for a first‐order transition: exponential increase in correlation length associated with distributed damage and unstable crack nucleation ahead of abrupt failure. In both cases, anisotropy in the initial porosity dictated the fault orientation, and system‐sized failure occurred when the correlation length approached the grain size. These results have significant implications for the predictability of catastrophic failure in different materials.
Publisher: IEEE
Date: 10-2012
Publisher: International Union of Crystallography (IUCr)
Date: 05-12-2013
DOI: 10.1107/S1600577513026969
Abstract: A new modular X-ray-transparent experimental cell enables tomographic investigations of fluid rock interaction under natural reservoir conditions (confining pressure up to 20 MPa, pore fluid pressure up to 15 MPa, temperature ranging from 296 to 473 K). The portable cell can be used at synchrotron radiation sources that deliver a minimum X-ray flux density of 10 9 photons mm −2 s −1 in the energy range 30–100 keV to acquire tomographic datasets in less than 60 s. It has been successfully used in three experiments at the bending-magnet beamline 2BM at the Advanced Photon Source. The cell can be easily machined and assembled from off-the-shelf components at relatively low costs, and its modular design allows it to be adapted to a wide range of experiments and lower-energy X-ray sources.
Publisher: Elsevier BV
Date: 03-2017
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-7041
Abstract: Fluid-rock interaction relies on the fluid& #8217 s ability to migrate through rocks, utilising permeable pore space. While we understand permeability in rocks that interact with fluids to evolve dynamically, e.g. in dehydration or carbonation reactions, we have very little quantitative information on these dynamics, as direct measurements of permeability in reacting rocks are inherently difficult.Here, we present a series of permeability measurements that capture the evolving fluid transport properties of dehydrating gypsum s les. To derive these measurements, we used an X-ray transparent deformation rig to document gypsum dehydration in 4-dimensional & #181 CT datasets and then modelled the permeability evolution for a segmented sub-volume numerically. In doing so, we were able to characterise the grain-scale porosity and permeability evolution of a dehydration reaction for the first time. We present analyses from two experimental time-series run at a fixed confining pressure, temperature and pore fluid pressure (Pc = 20 MPa T = ~125 C Pf = 5 MPa) but contrasting stress states: one with the largest principal stress (& #916 & #963 = 16.1 MPa) parallel to the s le cylinder axis and another the largest principal stress (& #916 & #963 = 11.3 MPa) being radial. In both cases, as pore space formed due to the negative change in the solid molar volume during the reaction, permeability evolved and increased congruently with porosity in time until ultimately reaching average values of 3.14E-13 m& #178 and 4.55E-13 m& #178 , respectively. A clear spatial heterogeneity of fluid flow develops at the grain-scale along with the fabrics in the s les. Importantly,& the calculated permeability tensors are anisotropic from the onset, but& develop over different spatiotemporal trajectories and have different preferred orientations in the two experimental geometries: If the anisotropy is expressed as 1-(min_eigenvalue/max_eigenvalue) of the permeability tensor (where isotropy = 0), then the experiment with the largest principal stress applied radially has a final anisotropy of 0.45, with fluid flow efficiently focussed into a vertical lineation. In the case with an axial largest principal stress, the final anisotropy of permeability is 0.30 with fluid flow being channelled along a foliation that developed orthogonally to & #963 .Our results suggest that the spatial and temporal developments of permeability during a dehydration reaction are controlled by the orientation and relative magnitudes of the principal stresses of a tectonic environment, and that these two parameters exert a strong control on the efficiency of drainage and thus reaction progress. This has consequences for our understanding of fluid movements in thrust tectonics and subduction zones, but also in applications such as the in-situ carbonation of ultramafic rocks.
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-7120
Abstract: X-ray tomographic imaging has become a very valuable tool for the analysis of (rock) materials, both for visualising complex 3D microstructures and for imaging internal features such as damage, mineral reaction, and fluid/rock interactions quantitatively. The validity of the results derived from X-ray tomography, however, hinge on the& accuracy of the image segmentation. There are many methods for image segmentation (from simple manual thresholding to machine learning and deep learning approaches), which can produce a high range of variability in the segmentation results. Accuracy of segmentation results is seldom checked and thus calling the reproducibility of the results into question. In this contribution we show how metamorphic reactions themselves can be used to constrain accuracy and highlight the benefits of deep learning methods to extend this over many large datasets efficiently.Here, we demonstrate a methodology that uses deep learning to achieve reliable segmentation of time-series volumetric images of gypsum dehydration reaction, on which standard segmentation approaches fail due to insufficient contrast. We implement 2D U-net architecture for segmentation, and, to overcome the limitations of training data obtained experimentally through imaging, we show how labelled data obtained via machine learning (i.e., Random Forest Classification) can be used as input data and enhance the neural network performances. The developed deep learning algorithm proves to be incredibly robust, as it is able to consistently segment volume phases within the whole suite of experiments. In addition, the trained neural network exhibits short run times ( minutes for ~250 MB of image volumes) on a local workstation equipped with a GPU card.& & To confirm the precision achieved by our workflow, we consider the theoretical and measured molar evolution of gypsum (CaSO4.2H2O) to bassanite (CaSO4.& #189 H2O) during the dehydration. Within all time-series experiments, errors between the predicted theoretical and the segmented volumes fall within the 5% confidence intervals of the theoretical curves. Thus, the segmented CT images are very well suited for extracting quantitative information, such as mineral growth rate and pore size variations during the reaction. To our knowledge, this is the first time an internal standard is used to unequivocally measure the accuracy of a segmentation model.& Being able to accurately and unambiguously measure the volumetric evolution during a reaction enables high-level modelling and verification of the physical (hydraulic and mechanical) properties of rock materials involved in tectono-metamorphic processes.
Location: Australia
Location: Germany
Location: United Kingdom of Great Britain and Northern Ireland
Start Date: 2020
End Date: 2022
Funder: Natural Environment Research Council
View Funded ActivityStart Date: 2017
End Date: 2021
Funder: Natural Environment Research Council
View Funded ActivityStart Date: 2017
End Date: 2022
Funder: Natural Environment Research Council
View Funded ActivityStart Date: 2011
End Date: 2011
Funder: Swiss National Science Foundation
View Funded ActivityStart Date: 2012
End Date: 2012
Funder: Australian Research Council
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