Low-temperature plastic rheology of calcite plays a significant role in the dynamics of Earth's crust. However, it is technically challenging to study plastic rheology at low temperatures because of the high confining pressures required to inhibit fracturing. Micromechanical tests, such as nanoindentation and micropillar compression, can provide insight into plastic rheology under these conditions because, due to the small scale, plastic deformation can be achieved at low temperatures without the need for secondary confinement. In this study, nanoindentation and micropillar compression experiments were performed on oriented grains within a polycrystalline sample of Carrara marble at temperatures ranging from 23 to 175 °C, using a nanoindenter. Indentation hardness is acquired directly from nanoindentation experiments. These data are then used to calculate yield stress as a function of temperature using numerical approaches that model the stress state under the indenter. Indentation data are complemented by uniaxial micropillar compression experiments. Cylindrical micropillars ∼1 and ∼3 μm in diameter were fabricated using a focused ion beam-based micromachining technique. Yield stress in micropillar experiments is determined directly from the applied load and micropillar dimensions. Mechanical data are fit to constitutive flow laws for low-temperature plasticity and compared to extrapolations of similar flow laws from high-temperature experiments. This study also considered the effects of crystallographic orientation on yield stress in calcite. Although there is a clear orientation dependence to plastic yielding, this effect is relatively small in comparison to the influence of temperature.
Quartz is an abundant mineral in Earth's crust whose mechanical behavior plays a significant role in the deformation of the continental lithosphere. However, the viscoplastic rheology of quartz is difficult to measure experimentally at low temperatures without high confining pressures due to the tendency of quartz (and other geologic materials) to fracture under these conditions. Instrumented nanoindentation experiments inhibit cracking even at ambient conditions, by imposing locally high mean stress, allowing for the measurement of the viscoplastic rheology of hard materials over a wide range of temperatures. Here we measure the indentation hardness of four synthetic quartz specimens and one natural quartz specimen with varying water contents over a temperature range of 23°C to 500°C. Yield stress, which is calculated from hardness but is model dependent, is fit to a constitutive flow law for low‐temperature plasticity to estimate the athermal Peierls stress of quartz. Below 500°C, the yield stresses presented here are lower than those obtained by extrapolating a flow law constrained by experiments at higher temperatures irrespective of the applied model. Indentation hardness and yield stress depend weakly on crystallographic orientation but show no dependence on water content.
more » « less- Award ID(s):
- 1726165
- NSF-PAR ID:
- 10367033
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 126
- Issue:
- 9
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract 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:
= Aσ n e −(Q +V P)/RT . From this initial analysis, we identify differenteffective stress 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 inn at high confining pressure may result from an increase in the activity of grain size sensitive creep. -
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Abstract Relationships between the recrystallized grain size and stress are investigated for experimentally deformed water‐added quartz aggregates. For stresses ≥100 MPa there is a variation in the measured recrystallized grain size for a given stress. This variation correlates with a change in the
c ‐axis fabric in general shear experiments, where samples with larger recrystallized grain sizes for a given stress have dominantly prismc ‐axis fabrics and samples with smaller recrystallized grain sizes for a given stress have dominantly basalc ‐axis fabrics. The dislocation creep flow law also changes at conditions where these twoc ‐axis fabrics form (Tokle et al., 2019,https://doi.org/10.1016/j.epsl.2018.10.017 ). Using the wattmeter model (Austin & Evans, 2007,https://doi.org/10.1130/G23244A.1 ), different piezometric relationships are quantified for samples that develop prism and basalc ‐axis fabrics, respectively. The wattmeter model is sensitive to grain growth kinetics; a new grain growth law for quartz is formulated based on reanalysis of microstructures in samples from previous work. The activation enthalpies and water fugacity exponents for our grain growth law and dislocation creep flow laws are the same within error, suggesting the recrystallized grain size versus stress relationships are nearly independent of temperature and water fugacity, consistent with laboratory observations. The wattmeters successfully predict the recrystallized grain size versus stress relationships of all quartzite samples from experiments with added water. These results support the use and extrapolation of the wattmeter model for both experimental and geologic conditions to investigate the stress state and grain size evolution of quartz rich rocks. -
Debris flows are dense and fast-moving complex suspensions of soil and water that threaten lives and infrastructure. Assessing the hazard potential of debris flows requires predicting yield and flow behavior. Reported measurements of rheology for debris flow slurries are highly variable and sometimes contradictory due to heterogeneity in particle composition and volume fraction ( ϕ ) and also inconsistent measurement methods. Here we examine the composition and flow behavior of source materials that formed the postwildfire debris flows in Montecito, CA, in 2018, for a wide range of ϕ that encapsulates debris flow formation by overland flow. We find that shear viscosity and yield stress are controlled by the distance from jamming, Δ ϕ = ϕ m − ϕ , where the jamming fraction ϕ m is a material parameter that depends on grain size polydispersity and friction. By rescaling shear and viscous stresses to account for these effects, the data collapse onto a simple nondimensional flow curve indicative of a Bingham plastic (viscoplastic) fluid. Given the highly nonlinear dependence of rheology on Δ ϕ , our findings suggest that determining the jamming fraction for natural materials will significantly improve flow models for geophysical suspensions such as hyperconcentrated flows and debris flows.more » « less
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