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1. Natural and Experimental Constraints on a Flow Law for Dislocation‐Dominated Creep in Wet Quartz

   https://doi.org/10.1029/2020JB021302  

   Lusk, Alexander D. J. ; Platt, John P. ; Platt, Jason A. ( May 2021 , Journal of Geophysical Research: Solid Earth)

   We present a flow law for dislocation‐dominated creep in wet quartz derived from compiled experimental and field‐based rheological data. By integrating the field‐based data, including independently calculated strain rates, deformation temperatures, pressures, and differential stresses, we add constraints for dislocation‐dominated creep at conditions unattainable in quartz deformation experiments. A Markov Chain Monte Carlo (MCMC) statistical analysis computes internally consistent parameters for the generalized flow law: = Aσⁿe^−(Q+VP)/RT. From this initial analysis, we identify differenteffectivestress exponents for quartz deformed at confining pressures above and below ∼700 MPa. To minimize the possible effect of confining pressure, compiled data are separated into “low‐pressure” (<560 MPa) and “high‐pressure” (700–1,600 MPa) groups and reanalyzed using the MCMC approach. The “low‐pressure” data set, which is most applicable at midcrustal to lower‐crustal confining pressures, yields the following parameters: log(A) = −9.30 ± 0.66 MPa^−n−r s⁻¹;n = 3.5 ± 0.2;r = 0.49 ± 0.13;Q = 118 ± 5 kJ mol⁻¹; andV = 2.59 ± 2.45 cm³ mol⁻¹. The “high‐pressure” data set produces a different set of parameters: log(A) = −7.90 ± 0.34 MPa^−n−r s⁻¹;n = 2.0 ± 0.1;r = 0.49 ± 0.13;Q = 77 ± 8 kJ mol⁻¹; andV = 2.59 ± 2.45 cm³ mol⁻¹. Predicted quartz rheology is compared to other flow laws for dislocation creep; the calibrations presented in this study predict faster strain rates under geological conditions by more than 1 order of magnitude. The change innat high confining pressure may result from an increase in the activity of grain size sensitive creep.

    

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2. Micromechanics and Strain Localization in Sand in the Ductile Regime

   https://doi.org/10.1029/2022JB024983  

   Shahin, G. ; Hurley, R. C. ( November 2022 , Journal of Geophysical Research: Solid Earth)

   Critical processes including seismic faulting, reservoir compartmentalization, and borehole failure involve high‐pressure mechanical behavior and strain localization of sedimentary rocks such as sandstone. Sand is often used as a model material to study the mechanical behavior of poorly lithified sandstone. Recent studies exploring the multi‐scale mechanics of sand have characterized the brittle, low‐pressure regime of behavior; however, limited work has provided insights into the ductile, high‐pressure regime of behavior viain‐situmeasurements. Critical features of the ductile regime, including grain breakage, grain micromechanics, and volumetric strain behavior therefore remain under‐explored. Here, we use a new high‐pressure triaxial apparatus within‐situx‐ray tomography to provide new insights into deformation banding, grain breakage, and grain micromechanics in Ottawa sand subjected to triaxial compression under confining pressures between 10 and 45 MPa. We observed strain‐hardening at pressures above 15 MPa and strain‐neutral responses at pressures below 15 MPa. Compacting shear bands and grain breakage were observed at all pressures with no significant variation due to grain size, except for minor increases in breakage in less‐rounded sands. Grain breakage emerged at stress levels lower than the assumed yield threshold and more intense breakage was associated with thinner deformation bands. Contact sliding at inter‐grain contacts demonstrated a bifurcation into a bimodal distribution, with intense sliding within deformation bands and reduced but non‐negligible sliding outside of deformation bands, suggesting that off‐band zones remain mechanically active during strain hardening.

    

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3. Test of the Effective Stress Law for Semibrittle Deformation Using Isostatic and Triaxial Load Paths

   https://doi.org/10.1029/2020JB021326  

   Ding, J. ; Chester, F. M. ; Chester, J. S. ( May 2021 , Journal of Geophysical Research: Solid Earth)

   For brittle friction and rock deformation, the coefficientαin the general effective stress relationσ^e = σ − αP_pcan be approximated as unity with sufficient accuracy. However, it is uncertain ifαdeviates from unity for semibrittle flow when both brittle and intracrystalline‐plastic deformation is involved. We conducted triaxial and isostatic compression experiments on synthetic salt‐rocks (∼300 ppm water) at room temperature to test the effective stress relation in the semibrittle regime using silicone oil and argon gas as pore fluids. Confining and pore pressures were cycled while their difference (differential pressure) was kept constant, such that changes in the mechanical behavior would indicate deviation ofαfrom unity. Microstructural observations were used to determine the dependence ofαon true area of grain contact from asperity yielding. In triaxial compression experiments, semibrittle flow involves grain boundary cracking and sliding, and intragranular dislocation glide and cracking. Flow strength remains constant for changes in pore fluid pressure of more than two orders of magnitude. In isostatic compression experiments, samples show combined processes of microcracking, grain boundary sliding, dislocation glide, and fluid‐assisted grain boundary migration recrystallization. Volumetric strain depends directly on the differential pressures (i.e.,αequals one). Analysis of grain‐contact area in both experiments indicates thatαis independent of the true area of contact defined by plastic yielding at grain boundaries. The observation ofαeffectively equals one may be explained by operation of pressure‐independent intracrystalline‐plastic mechanisms and transmission of pore pressure at grain boundaries through thin fluid films.

    

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4. The Brittle–Ductile Transition and the Formation of Compaction Bands in the Savonnières Limestone: Impact of the Stress and Pore Fluid

   https://doi.org/10.1007/s00603-022-02963-z  

   Sari, Mustafa ; Sarout, Joel ; Poulet, Thomas ; Dautriat, Jeremie ; Veveakis, Manolis ( January 2022 , Rock Mechanics and Rock Engineering)

   Abstract Carbonate sediments play a prominent role on the global geological stage as they store more than $$60\%$$ 60 % of world’s oil and $$40\%$$ 40 % of world’s gas reserves. Prediction of the deformation and failure of porous carbonates is, therefore, essential to minimise reservoir compaction, fault reactivation, or wellbore instability. This relies on our understanding of the mechanisms underlying the observed inelastic response to fluid injection or deviatoric stress perturbations. Understanding the impact of deformation/failure on the hydraulic properties of the rock is also essential as injection/production rates will be affected. In this work, we present new experimental results from triaxial deformation experiments carried out to elucidate the behaviour of a porous limestone reservoir analogue (Savonnières limestone). Drained triaxial and isotropic compression tests were conducted at five different confining pressures in dry and water-saturated conditions. Stress–strain data and X-ray tomography images of the rock indicate two distinct types of deformation and failure regimes: at low confinement (10 MPa) brittle failure in the form of dilatant shear banding was dominant; whereas at higher confinement compaction bands orthogonal to the maximum principal stress formed. In addition to the pore pressure effect, the presence of water in the pore space significantly weakened the rock, thereby shrinking the yield envelope compared to the dry conditions, and shifted the brittle–ductile transition to lower effective confining pressures (from 35 MPa to 29 MPa). Finally, permeability measurements during deformation show a reduction of an order of magnitude in the ductile regime due to the formation of the compaction bands. These results highlight the importance of considering the role of the saturating fluid in the brittle–ductile response of porous rocks and elucidate some of the microstructural processes taking place during this transition. 

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5. Reaction between Hydrogen and Ferrous/Ferric Oxides at High Pressures and High Temperatures—Implications for Sub-Neptunes and Super-Earths

   https://doi.org/10.3847/PSJ/acab03  

   Horn, H. W. ; Prakapenka, V. ; Chariton, S. ; Speziale, S. ; Shim, S. -H. ( February 2023 , The Planetary Science Journal)

   Sub-Neptune exoplanets may have thick hydrogen envelopes and therefore develop a high-pressure interface between hydrogen and the underlying silicates/metals. Some sub-Neptunes may convert to super-Earths via massive gas loss. If hydrogen chemically reacts with oxides and metals at high pressures and temperatures (P−T), it could impact the structure and composition of the cores and atmospheres of sub-Neptunes and super-Earths. While H₂gas is a strong reducing agent at low pressures, the behavior of hydrogen is unknown at theP−Texpected for sub-Neptunes’ interiors, where hydrogen is a dense supercritical fluid. Here we report experimental results of reactions between ferrous/ferric oxides and hydrogen at 20–40 GPa and 1000–4000 K utilizing the pulsed laser-heated diamond-anvil cell combined with synchrotron X-ray diffraction. Under these conditions, hydrogen spontaneously strips iron off the oxides, forming Fe-H alloys and releasing oxygen to the hydrogen medium. In a planetary context where this reaction may occur, the Fe-H alloy may sink to the metallic part of the core, while released oxygen may stabilize as water in the silicate layer, providing a mechanism to ingas hydrogen to the deep interiors of sub-Neptunes. Water produced from the redox reaction can also partition to the atmosphere of sub-Neptunes, which has important implications for understanding the composition of their atmospheres. In addition, super-Earths converted from sub-Neptunes may contain a large amount of hydrogen and water in their interiors (at least a few wt% H₂O). This is distinct from smaller rocky planets, which were formed relatively dry (likely a few hundredths wt% H₂O).

    

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