Using a scann ing electron microscope (SEM) and an image analyzer, we have developed a technique for counting and measuring cracks in rocks which is more eff i cient than traditional techniques in which an operator performs all image analysis functions. The key aspect of the technique i s that black-on-white tracings of fresh cracks, which can be made rather rap idly by an operator, are measured and digitized by an image analyzer. The most time-consuming step in the process has now become the generation of SEM micrographs and pertinent chemical (mineralogical) information, not the quantification of crack structure. The technique has been appli ed to two studies involving nuclear waste isolation in a granitic rock, Climax Stock (Nevada Test Site) quartz monzonite, a Cretaceous age rock which is structura ll y very inhomogeneous. One study detected a relationship between crack structure and distance from a hammer-drilled borehole; the other st udy was unable to detect a relationsh i p between crack structure and gamma irradiation treatment in rocks l oaded to near failure. KEY WORD S: crack counting, image analysis, waste isolation, secondary vs backscattered electron imaging, damage in rocks, granites, Climax Stock quartz monzonite *Address for Correspondence: W.B. Durham, L-201 U.C.L.L.N .L. Livermore, CA 94550 Phone Number: 415 422-7046

Plastic faulting is a brittle‐like failure phenomenon exhibited by water ice and several other rock types under confinement. It is suspected to be the mechanism of deep earthquakes and extreme cases of shear localization in shallow rocks. Unlike ordinary Coulombic failure, plastic faulting is characterized by a pressure‐independent failure strength and fault plane oriented 45° to maximum principal stress. To research the question of how the instability initiates, we conducted over 50 constant‐displacement‐rate experiments on polycrystalline ice (phases Ih and II) near the brittle‐to‐ductile (B‐D) transition, at confining pressures P = 0–300 MPa, applied strain rates ε· = 5 × 10−5 – 7 × 10−3 s−1, temperatures T = 105–233 K, and mean grain sizes d = 0.25–1.18 mm. We find that (1) the width of the B‐D transition in variable space is vanishingly narrow, to the point of appearing as a crossover, (2) a plastic fault plane, once formed, is not a zone of subsequent weakness, (3) distributed ice I→II phase transformation in small amounts (<1 vol%) shows no causal relationship to subsequent failure, and (4) plastic faulting also occurs in ice II. We hypothesize that the elusive nucleating “trigger” parallels that of metals and ceramics undergoing severe plastic deformation, wherein transient local structural rearrangement occurs, in turn causing material strength to drop to a level sufficiently low, in a volume sufficiently large, that adiabatic instability is nucleated. Our results do not require and often are inconsistent with phase transformation. Plastic faulting may therefore be available to all solids undergoing severe deformation, and its appearance in so few is simply the result of insufficiently extreme conditions.

Kinking is a common process in materials with a strong visco-plastic anisotropy but its impact on the plastic deformation and recrystallisation of crystal aggregates is still poorly constrained. Here, kinking is studied via constant stress experiments on polycrystalline ice under relatively low temperature (240 K) and high differential stress (13.3 MPa and 2.7 MPa, with 50 MPa confining pressure). EBSD data analysis shows that samples comprise large and small grains, interpreted as remnant and recrystallised. Boundary trace analyses and misorientation data of kinked remnant grains reveal that kink boundaries are best characterised with rotation axes within the basal plane of the ice crystal. Inferred slip directions within the basal plane are variable and include <11–20>, <10-10> and intermediate directions. Shorter grain boundaries with rotation axes outside the basal plane surround kink boundaries and indicate that non-basal dislocations or ripplocations play a role in accommodating local kink deformation. More kink boundaries per grain and a larger boundary misorientation are generally found in remnant grains in the high-stress sample. At an aggregate level, kinking as a key facilitator for the dynamic recrystallisation process is represented by many straight grain boundaries with basal rotation axes in the remnant and recrystallised grain population. The statistically preferred slip direction within the basal plane is <21–30>, consistent with coupled slip on two crystallographic a-axes with uneven contributions.

The strength of the lithosphere is typically modelled based on constitutive equations for steady-state flow. However, strain hardening may cause significant evolution of strength in the colder load-bearing portion of the lithosphere. Recent rheological data from low-temperature deformation experiments on olivine suggest that strain hardening occurs due to the presence of temperature-independent back stresses generated by long-range elastic interactions among dislocations. These interpretations provided the basis for a flow law that incorporates hardening by the development of back stress. Here, we test this dislocation-interaction hypothesis by examining the microstructures of olivine samples deformed plastically at room temperature either in a deformation-DIA apparatus at differential stresses of <= 4.3 GPa or in a nanoindenter at applied contact stresses of >= 10.2 GPa. High-angular resolution electron backscatter diffraction maps reveal the presence of geometrically necessary dislocations with densities commonly above 10(14) m(-2) and intragranular heterogeneities in residual stress on the order of 1 GPa in both sets of samples. Scanning transmission electron micrographs reveal straight dislocations aligned in slip bands and interacting with dislocations of other types that act as obstacles. The resulting accumulations of dislocations in their slip planes, and associated stress heterogeneities, are consistent with strain hardening resulting from long-range back-stresses acting among dislocations and thereby support the form of the flow law for low-temperature plasticity. Based on these observations, we predict that back stresses among dislocations will impart significant mechanical anisotropy to deformed lithosphere by enhancing or reducing the effective stress. Therefore, strain history, with associated microstructural and micromechanical evolution, is an important consideration for models of lithospheric strength. The microstructural observations also provide new criteria for identifying the operation of back-stress induced strain hardening in natural samples and therefore provide a means to test the applicability of the flow law for low-temperature plasticity. (C) 2020 The Author(s). Published by Elsevier B.V.

The strength of the lithosphere is typically modelled based on constitutive equations for steady-state flow. However, models of lithospheric flexure reveal differences in lithospheric strength that are difficult to reconcile based on such flow laws. Recent rheological data from low-temperature deformation experiments on olivine suggest that this discrepancy may be largely explained by strain hardening. Details of the mechanical data, specifically the effects of temperature-independent back stresses stored in the samples, indicate that strain hardening in olivine occurs primarily due to long-range elastic interactions between dislocations. These interpretations provided the basis for a new flow law that incorporates hardening by development of back stress. Here, we test this hypothesis by examining the microstructures of olivine samples deformed plastically at room temperature either in a deformation-DIA apparatus at differential stresses of ≤ 4.3 GPa or in a nanoindenter at applied contact stresses of ≥ 10.2 GPa. High-angular resolution electron backscatter diffraction maps reveal the presence of geometrically necessary dislocations with densities commonly above 10^14 m^-2 and intragranular heterogeneities in residual stress on the order of 1 GPa in both sets of samples. Scanning transmission electron micrographs reveal straight dislocations aligned along slip bands and interacting with dislocations of other types that act as obstacles. The stress heterogeneities and accumulations of dislocations along their slip planes are consistent with strain hardening resulting from long-range back-stresses acting between dislocations. These results corroborate the mechanical data in supporting the form of the new flow law for low-temperature plasticity and provide new microstructural criteria for identifying the operation of this deformation mechanism in natural samples. Furthermore, similarities in the structure and stress fields of slip bands formed in single crystals deformed at low temperatures and those formed at high temperatures suggest that similar hardening processes occur in both regimes, providing a new constraint for models of transient creep at high temperatures.

Plastic deformation of olivine at relatively low temperatures (i.e., low‐temperature plasticity) likely controls the strength of the lithospheric mantle in a variety of geodynamic contexts. Unfortunately, laboratory estimates of the strength of olivine deforming by low‐temperature plasticity vary considerably from study to study, limiting confidence in extrapolation to geological conditions. Here we present the results of deformation experiments on olivine single crystals and aggregates conducted in a deformation‐DIA at confining pressures of 5 to 9 GPa and temperatures of 298 to 1473 K. These results demonstrate that, under conditions in which low‐temperature plasticity is the dominant deformation mechanism, fine‐grained samples are stronger at yield than coarse‐grained samples, and the yield stress decreases with increasing temperature. All samples exhibited significant strain hardening until an approximately constant flow stress was reached. The magnitude of the increase in stress from the yield stress to the flow stress was independent of grain size and temperature. Cyclical loading experiments revealed a Bauschinger effect, wherein the initial yield strength is higher than the yield strength during subsequent cycles. Both strain hardening and the Bauschinger effect are interpreted to result from the development of back stresses associated with long‐range dislocation interactions. We calibrated a constitutive model based on these observations, and extrapolation of the model to geological conditions predicts that the strength of the lithosphere at yield is low compared to previous experimental predictions but increases significantly with increasing strain. Our results resolve apparent discrepancies in recent observational estimates of the strength of the oceanic lithosphere.

Olivine is the most abundant and among the weakest phases in Earth's upper mantle, and thus, its rheological properties play a critical role in governing thermal structure and convective flow in the upper mantle. A persistent obstacle to constraining the in situ flow properties of olivine by laboratory experiment has been the difficulty in resolving the effect of pressure, which is weak within the 0‐ to ~2‐GPa pressure range of conventional laboratory deformation instruments but potentially strong over the 1‐ to ~14‐GPa range of the upper mantle. Using a deformation‐DIA, one of a new generation of bonafide deformation devices designed for operation to ≥10 GPa, we have deformed dry, polycrystalline San Carlos olivine in high‐temperature creep with the singular intent of providing the best achievable measurement of activation volume V* and a comprehensive statement of uncertainty. Under strictly dry conditions, at constant temperature (1,373 K) and strain rate (1 × 10−5 s−1), varying only pressure (1.8 to 8.8 GPa), we measure V* = 15 ± 5 cm3/mol. We have reproduced the well‐known mechanism change from [100]‐slip to [001]‐slip near 5 GPa and determined that, whatever the change in V* associated with the change in slip system, the effective value of 15 ± 5 cm3/mol is still accurate for modeling purposes in the 2‐ to 9‐GPa pressure range. This is a substantial pressure effect, which in the absence of a temperature gradient would represent a viscosity increase from the top to bottom of the upper mantle of 5 ± 2 orders of magnitude.

Abstract Ice in both terrestrial and planetary settings often contains rock particles. Here we present an experimental investigation of the influence of intergranular particles on the rheological behavior of ice. Experiments were performed on samples fabricated from 10‐μm ice powders +1‐μm graphite or 0.8‐μm alumina particles and subjected to elevated confining pressures. A critical particle fraction, ∼6%, was observed, below which samples behave like pure ice and deform by both grain boundary sliding (GBS) and dislocation creep, and above which GBS creep is impeded. Above this critical fraction, ice grains occur in particle‐free clusters surrounded by bands of particles mixed with fine‐grained ice, resulting in the impedance of GBS in the bands as well as sliding between the ice clusters. Our results imply that South Polar Layered Deposits and midlatitude lobate debris aprons on Mars must contain >94% ice and that the shallow subsurface of Ceres could contain >90% ice.

In situ resource utilization (ISRU) activities have been identified by the National Research Council as one of the most important scientific endeavors of the coming decade. The failure of the MUPUS-PEN experiment on Rosetta's Philae lander to penetrate the surface of comet 67P demands knowledge of the mechanical properties of extraterrestrial materials in such environments. To such an end we conducted laboratory strength measurements at cryogenic conditions of two terrestrial rocks, Bishop tuff and Indiana limestone as potential cometary and asteroid analogs, respectively. We measured failure strength of dried and fully saturated samples under constant displacement rate compressional loading at temperatures of 77 K-295 K and (for most) confining pressure of 5 MPa. The strength of saturated samples increased dramatically with decreasing temperature below the ice point: saturated limestone from 30 MPa at 295 K to > 200 MPa at 150 K and below, and saturated tuff from 26 MPa at 240 K to 160 MPa at 150 K. Some ductility or distributed fracturing was evident in both saturated rock types above 150 K. The saturated tuff was stronger than pure ice at all conditions but its transition from brittle to ductile deformation with temperature closely paralleled that of ice. The results of this study will be useful to future sample retrieval missions or ISRU maneuvers. The fully saturated state of our samples and use of Bishop tuff as an unconsolidated cometary analog likely means our results are upper estimates for substrates we may encounter in our solar system. Pending corroboration and refinement of these preliminary measurements, the large increase in compressive strength we observed in the saturated materials at cryogenic temperatures suggests that future missions must prepare technology with the energetic and mechanical capability to penetrate very hard substrates.

Constraints on the state of stress in the lithosphere are fundamental to understanding a breadth of geological phenomena. Paleo-stresses are generally estimated using microstructural elements for which there are experimentally calibrated relationships with applied stress, with an emphasis on recrystallised grain-size piezometers. However, it is often difficult to clearly distinguish newly recrystallised grains from the relict matrix. Furthermore, these grain-size piezometers are only applicable to rocks consisting of a single mineral.

Plastic deformation of olivine at relatively low temperatures (ie, by exponential creep) likely controls the strength of the lithospheric mantle during flexure under volcanic or glacial loads, during bending of slabs at subduction zones, and after seismic events in the ductile roots of major fault zones. Unfortunately, laboratory estimates of the strength of olivine deforming by low-T plasticity exhibit considerable variation from study to study, limiting confidence in extrapolation to geological conditions. Recent work based on nanoindentation of olivine single crystals suggests that the strength of olivine deforming by low-T plasticity should depend on the grain size. Furthermore, that work suggests that a grain-size dependence of strength could explain the inconsistencies among previous studies. Here we test this hypothesis using a set of experiments specifically designed to investigate the effect of grain size on low-T plasticity …

We have developed a high-resolution technique based on micro Raman spectroscopy to measure hydrogen isotope diffusion profiles in ice Ih. The calibration curve for quantitative analysis of deuterium in ice Ih was constructed using micro Raman spectroscopy. Diffusion experiments using diffusion couples composed of dense polycrystalline H2O and D2O ice were carried out under a gas confining pressure of 100MPa (to suppress micro-fracturing and pore formation) at temperatures from 235K to 245K and diffusion times from 0.2 to 94 hours. Two-dimensional deuterium profiles across the diffusion couples were determined by Raman imaging. The location of small spots of frost from room air could be detected from the shapes of the Raman bands of OH and OD stretching modes, which change because of the effect of the molar ratio of deuterium on the molecular coupling interaction. We emphasize the validity for screening the impurities utilizing the coupling interaction. Some recrystallization and grain boundary migration occurred in recovered diffusion couples, but analysis of two-dimensional diffusion profiles of regions not affected by grain boundary migration allowed us to measure a volume diffusivity for ice at 100MPa of (2.8±0.4)×10-3exp-57.0±15.4kJ/molRTm2/s (R is the gas constant, T is temperature). Based on ambient pressure diffusivity measurements by others, this value indicates a high (negative) activation volume for volume diffusivity of −29.5cm3/mol or more. We can also constrain the value of grain boundary diffusivity in ice at 100MPa to be <104 that of volume diffusivity.

Introduction: The influence of embedded particles on the rheological behavior of polycrystalline ice is not well known. Although numerous experimental studies on ‘dirty ice’ have been conducted, these studies often yield inconsistent to contradictory results regarding the effect of particles on flow behavior [1]. This knowledge gap is compounded by the recent discovery that ice deforms not by a single deformation mechanism, as classically assumed, but rather by multiple creep mechanisms, each of which dominates the flow behavior of ice over different regimes of grain size, temperature and stress [2,3]. Given the different microphysics of the primary deformation mechanisms in ice dislocation creep and grain boundary sliding (GBS) creep – the presence of particles will affect these mechanisms differently, depending on their size, fraction, and distribution within the ice. Intergranular particles may slow the rate of GBS creep, whereas intragranular particles may impede the motion of lattice dislocations, slowing both dislocation creep and GBS creep. To help fill this knowledge gap, we conducted experiments on particle-bearing ice samples and deformed them in both creep regimes. Methods: Samples with ice grain sizes of ~10 μm containing intragranular graphite particles were fabricated from powders created by misting a water/graphite slurry into liquid N2 using a pneumatic nozzle. Samples with ice grain sizes of ~10 μm containing intergranular particles were fabricated by mechanically mixing ice powders (created by misting pure water into liquid nitrogen) with graphite particles. In both cases, loose powders were packed into and sealed within cylindrical indium jackets and subsequently isostatically hot pressed into fully dense creep specimens at a confining pressure P=20 MPa and a temperature T=236 K in a high-pressure gas-medium apparatus. For the samples prepared by spraying ice + particles, particle sizes of 0.1 and 1 μm were used, in concentrations of up to 4.4 vol.%. Mechanically mixed samples were prepared for a particle size of 1 μm, in concentrations of up to ~7%. After hot pressing, samples were removed from the high-pressure apparatus, measured, and placed back in the apparatus for mechanical testing. Samples were deformed at nominally constant strain rate in axial compression at P=20 MPa and T=236 K in the gas apparatus. Strain-rate steps were conducted on a given sample to determine the value of the stress exponent n, where ε̇ ∝ σ, ε ̇is strain rate, and  is differential stress. After each rate step, deformation was continued until a roughly constant stress was obtained before stepping to a higher strain rate. After deformation, samples were quenched in liquid nitrogen for long-term storage. A subset of the deformation samples were analyzed in a field emission gun scanning electron microscope (SEM) to determine the ice grain size and the location and fraction of particles present in the ice. Results: For samples containing an intragranular dispersion of <1% of either 0.1-μm or 1-μm graphite, a value of n=1.6-1.8 is observed at lower stresses (<10 MPa) and a value of n=4 is observed at higher stresses. These values of n, and the magnitudes of the stress at a given strain rate, are consistent with GBS creep and dislocation creep of pure water ice. In Fig. 1, for example, data for two samples of ice containing 0.4 vol.% of 0.1-μm graphite particles are shown to be in

The goal of this collaborative research effort between W.B. Durham at the Massachusetts Institute of Technology (MIT) and D.L. Kohlstedt and S. Mei at the University of Minnesota (UMN) was to exploit a newly developed technology for high-pressure, high-temperature deformation experimentation, namely, the deformation DIA (D-DIA), to determine the deformation behavior of a number of important upper mantle rock types including olivine, garnet, enstatite, and periclase. Experiments were carried out under both hydrous and anhydrous conditions and at both lithospheric and asthenospheric stress and temperature conditions. The result was a group of flow laws for Earth’s upper mantle that quantitatively describe the viscosity of mantle rocks from shallow depths (the lithosphere) to great depths (the asthenosphere). These flow laws are fundamental for modeling the geodynamic behavior and heat transport from depth to Earth’s surface.-