Search

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

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

Mineral carbonation, a process that binds CO2in the form of carbonates by silicate weathering, is widespread on the Earth's surface. Because of the abundance of silicate rocks and the permanence of the carbonated solids, sequestering CO2via mineral carbonation has generated lots of interests. However, it is unclear how the fluid‐rock reaction proceeds to completion in spite of an increasing solid volume. We conducted a mineral carbonation experiment in which a sintered olivine aggregate reacted with a sodium bicarbonate solution at reservoir conditions. Time‐resolved synchrotron X‐ray microtomographic images show cracks in polygonal patterns arising in the surface layers and propagating into the interior of the olivine aggregate. The nanotomography data reveal that the incipient cracks intersect at right angles. We infer that stretching due to nonuniform volume expansion generates polygonal cracking of the surfaces. Our data shed new lights on the processes that control hydration and carbonation of peridotite. We demonstrate that fracturing of the host rock can be achieved without additional energy inputWe identify that nonuniform volume expansion is key to reaction‐induced cracking in olivineWe provide experimental evidence that the positive volume‐changing olivine carbonation reaction can be self‐sustaining

The mechanism of shear zone formation in lower crustal, relatively “dry” rocks is still poorly understood. We have studied the high‐temperature deformation of the Hasvik gabbro (northern Norway) which commences by fracturing. The 10–20 µm wide fractures show little displacement. The fine‐grained plagioclase and orthopyroxene in the fractures lack a crystallographic preferred orientation (CPO) or a systematic crystallographic orientation with respect to the host grains. Fractures grade into narrow shear zones, which are composed of fine (10–20 µm), equant grains of recrystallized plagioclase, amphibole, and pyroxene. Recrystallized plagioclase and pyroxene have compositions different from the magmatic grains, suggesting that they have formed by nucleation and growth. Based on conventional plagioclase‐amphibole thermobarometry, the shear zones have formed at temperatures and pressures of 700–750°C and 0.5–0.6 GPa. The observed primary minerals cut by fractures suggest high‐temperature fracturing in the absence of high pore pressures, which implies a high strength of the lower crustal gabbros and high stresses at fracturing. The shear zones are characterized by the lack of CPO and a small grain size, suggesting that the mechanism of deformation of the fine‐grained plagioclase and orthopyroxene has been grain boundary sliding accommodated by diffusive mass transfer. The amphibole grains have strong CPOs, which most likely result from oriented growth and/or rigid body rotations during deformation. The process that initiated the fracturing and subsequent viscous creep in the Hasvik gabbro may have resulted from a process of coseismic loading followed by creep during decaying stress in the lower crust. Shear zones formed by fracturing and subsequent creep are found in gabbroDeformation occurred at the lower crustal conditionsShear zones are related to a process of coseismic loading followed by creep

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