Philip Skemer

The rheology of geologic materials is crucial for understanding the dynamics of planetary interiors. However, there have been few experimental studies of geologic materials subjected to large cyclical strain, as might be imposed by periodic ice‐sheet loading or tidal forcing. To investigate the effect of cyclical strain on microstructural evolution we conducted deformation experiments with the same absolute shear strain magnitude ( γ _abs = 4, 8 or 16), same net strain ( γ _net = 0), and different strain paths. We deformed Carrara Marble at high temperature (845 ± 25°C) and high pressure (1.5 ± 0.1 GPa), at a constant shear strain rate of 5 × 10 ⁻⁵ s ⁻¹ , in the Large Volume Torsion Apparatus at Washington University in St. Louis. Thin sections and EBSD maps were used to characterize deformation microstructures, including grain‐size, the fraction of material that is dynamically recrystallized, and crystallographic preferred orientation. There is poor correlation between the areal fraction of recrystallized grains and either the net or the absolute shear strain imposed by cyclical loading. However, there is excellent correlation between the percentage of recrystallized grains and the maximum strain for any deformation stage ( γ _max ). We conclude that microstructural evolution during complex cyclical loading is controlled by the largest strain increment, regardless of when it occurs during the deformation history. Comparison of microstructures to single‐stage deformation experiments indicates that, despite nearly identical total deformation work, the resulting recrystallized fraction vary substantially. It demonstrates that strain‐path history, rather than total amount of deformation, exerts stronger control on evolution of recrystallized fraction.

Many of the outer Solar System's icy satellites feature known or suspected subsurface oceans, at least some of which are likely situated atop rocky interiors. Water-rock interactions at and beneath these seafloors might support active chemoautotrophic habitats, with subseafloor fluid flow facilitated by active faulting and hydrothermal systems. Absent such phenomena, however, any attainment of chemical equilibrium between the seafloor and ocean might limit the availability of chemical energy for life. Here, we characterise the stress state of the seafloor of Jupiter's moon Europa, and thus the prospect for fracturing and associated sub-seafloor fluid flow there. We consider stresses from tidal forcing, global contraction, mantle convection, and serpentinisation. We find that none of these mechanisms is likely able to drive slip along even weak, pre-existing fractures in the present. Ocean water-rock reactions taking place today are therefore probably restricted to fluid flow through only the upper few hundred metres of the seafloor. Any processes able to sustain habitable conditions at the Europan seafloor today must therefore be independent of ongoing tectonic activity.

Earth's particular style of plate‐tectonics—characterized by localized deformation along dynamic plate boundaries and long‐lived stable plate interiors—appears to be unique among rocky objects in the solar system. However, it is entirely unknown how common plate tectonics and related lithospheric phenomena are among the vast population of exoplanets discovered astronomically or assumed to exist throughout the Universe. In this study, we explore the effect of planetary composition on mylonitization—a set of microphysical processes that is commonly associated with shear localization and plate boundary deformation on Earth. A model for planet compositions, based on stellar spectroscopy, is used to define a plausible range of theoretical mineral abundances in the mantles of rocky Earth‐sized exoplanets. These mineral abundances, along with experimental rock rheology, are used to model microphysical evolution with two‐phase mixing. The model is then used to determine the effect of composition on the time‐scales for shear zone formation. We demonstrate that lithospheres composed of sub‐equal proportions of two mineral phases will form shear zones over relatively short time‐scales, a more favorable condition for forming Earth‐like plate boundaries. In contrast, lithospheres that are nearly monomineralic may require unrealistically long time‐scales to form plate boundary shear zones. Using this approach, we identify specific nearby stars with the optimal range of compositions to be targeted by future astronomical missions, including the Habitable Worlds Observatory.

For planets to develop narrow, dynamic plate boundaries that resemble Earth’s, the rocks that make up the lithosphere must be able to localize deformation. Decades of field studies have shown that plate boundary deformation manifests as frictional faults at shallow depths and mylonitic ductile shear zones below the brittle-plastic transition, with individual strands as narrow as 10-100s of meters. The physical mechanisms that produce mylonites from a primary lithosphere are of considerable interest since it is presumably impossible to create or sustain Earth-like plate tectonics without them. Experimental studies demonstrate that the characteristic microstructures in mylonites form through the serial processes of dynamic recrystallization and phase mixing. However, the rapidity with which this occurs depends on temperature, grain-size, and composition, and the volume fraction and viscosity contrast between constituent mineral phases. As such, the mineralogical composition of a rocky planet will determine whether the planet can (a) localize deformation, and (b) initiate and sustain Earth-like plate tectonics. This contribution will review experimental evidence for the onset of mylonitization and show the results of models that predict the time scales (and therefore ease) with which planets of different compositions can localize deformation. Drawing on data from the Hypatia catalog of exoplanets, these models identify specific stars with exoplanets that may be most amenable to forming Earth-like plate tectonics.

Laboratory studies of rock rheology rely on purpose-built devices that can apply planetarily relevant pressures, temperatures, and non-hydrostatic stresses. Generating these pressures and stresses requires the application of large forces over small specimen areas. However, because rocks are generally polymineralic and deformation microstructures form across many length scales, it is advantageous to study relatively large (millimetric) specimens. In addition, many microstructures continue to evolve with progressive strain, so it is vital that some apparatus are able to generate enough shear strain to study these deformation phenomena. This contribution describes two new rock deformation apparatus—the Large Volume Torsion apparatus—at Washington University in St. Louis, which are capable of deforming geological specimens at high pressure and temperature (P = 3 GPa; T = 1800 K). Deformation is imposed in a torsional geometry, which enables the generation of extremely large shear strains (γ > 100) relevant to Earth’s plate boundaries and convecting mantle. A large specimen (diameter up to 4.2 mm) permits detailed postmortem microstructural analysis. Apparatus design, calibration, experimental procedures, and some examples of applications are reviewed.

Talc is commonly found in the cores of exhumed faults and may be important to the dynamics of slip in active fault zones. To understand the rheology of talc at conditions relevant to subduction zones, we conducted torsional deformation experiments at high pressure (1 GPa) and temperatures (450–500°C). Scanning Transmission Electron Microscope imaging revealed a marked decrease in grain size with increasing strain, in addition to the development of grain kinking and nanoporosity. The similarity of these microstructures to talc deformed in natural faults and low‐pressure experiments indicates that the dominant deformation mechanisms of talc are similar across a wide range of depths. We conclude that frictional processes remain an important control on talc rheology even under high normal stresses. However, deformation‐induced porosity could enhance the percolation of high‐pressure or reactive fluids through talc‐rich lithologies.

Talc is a hydrous magnesium silicate with an extremely low coefficient of friction.  In recent years, the recognition that talc is present in many fault systems has led to the suggestion that talc strongly influences the strength of faults.  To understand the role of talc in the seismic cycle, we conducted high pressure and temperature torsional deformation experiments on specimens of natural talc at shear strain rates relevant to slow-slip earthquakes (~10-4 s-1).  Scanning transmission electron microscopy revealed decreasing talc grain sizes (from ~3-5 mm to <100 nm), alongside delamination and kinking of individual talc grains.  This microstructural evolution with progressive strain greatly increases the density of planar defects (including grain-boundaries), and is consistent both with observations of natural, talc-rich faults, and prior experimental work.  Nanoindentation tests at room temperature were performed on deformed specimens to assess precisely whether the observed microstructural changes also affect rheology. At these conditions, nanoindentation is assumed to produce deformation predominantly by intercrystalline frictional slip.  However, bulk hardness data determined from nanoindentation show that there is no change in indentation hardness with increasing strain or defect density, both for indents made parallel to and perpendicular to the shear plane.  Although the talc grains become increasingly damaged with strain, the overall strength of deformed talc does not change.  This suggests that accumulated slip on talc-bearing faults does not change their mechanical response or hazard potential.

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.

Ultramylonites—intensely deformed rocks with fine grain sizes and well‐mixed mineral phases—are thought to be a key component of Earth‐like plate tectonics, because coupled phase mixing and grain boundary pinning enable rocks to deform by grain‐size‐sensitive, self‐softening creep mechanisms over long geologic timescales. In isoviscous two‐phase composites, “geometric” phase mixing occurs via the sequential formation, attenuation (stretching), and disaggregation of compositional layering. However, the effects of viscosity contrast on the mechanisms and timescales for geometric mixing are poorly understood. Here, we describe a series of high‐strain torsion experiments on nonisoviscous calcite‐fluorite composites (viscosity contrast, η_ca/η_fl ≈ 200) at 500°C, 0.75 GPa confining pressure, and 10⁻⁶–10⁻⁴ s⁻¹ shear strain rate. At low to intermediate shear strains (γ ≤ 10), polycrystalline domains of the individual phases become sheared and form compositional layering. As layering develops, strain localizes into the weaker phase, fluorite. Strain partitioning impedes mixing by reducing the rate at which the stronger (calcite) layers deform, attenuate, and disaggregate. Even at very large shear strains (γ ≥ 50), grain‐scale mixing is limited, and thick compositional layers are preserved. Our experiments (1) demonstrate that viscosity contrasts impede mechanical phase mixing and (2) highlight the relative inefficiency of mechanical mixing. Nevertheless, by employing laboratory flow laws, we show that “ideal” conditions for mechanical phase mixing may be found in the wet middle to lower continental crust and in the dry mantle lithosphere, where quartz‐feldspar and olivine‐pyroxene viscosity contrasts are minimized, respectively.