Exploring rheology of Earth’s materials: The marvels of high-temperature high-pressure deformation experiments

The Earth, with its towering mountains, shifting tectonic plates, and dynamic geological processes, has always been a subject of fascination and inquiry for scientists. Amidst the vast array of scientific disciplines, one relatively small yet impactful field, known as experimental rock deformation, plays a crucial role in unraveling the mysteries hidden beneath the Earth’s surface. Though the number of experimentalists is modest, the implications of their work extend far beyond the laboratory, influencing a myriad of scientific applications. This discipline revolves around comprehending mechanical and chemical processes involving deformation across a vast spectrum of temperatures, pressures, and timescales. In this week’s blog post, Subhajit, a specialist in rock deformation experimentation, will explore the importance, scientific principles, and various methods of rock deformation experiments. The blog ends with his introduction to the latest advancements in this field, highlighting their implications in enhancing our understanding of the long-term rheological behavior of Earth’s crust.

From the surface of the Earth to its profound depths and even to other planets, scientists, guided by the principles of materials science, delve into the mysteries of minerals, rocks and ice. For instance, a field geologist measures recrystallised grain sizes through microscopic study, deriving insights to determine paleo stress states. Meanwhile, a geodynamicist seeks realistic mathematical equations (flow laws) to understand long-term flow behaviour of Earth’s crust and mantle through numerical modelling. However, the field of experimental rock deformation isn’t confined to labs and theories; it has real-world applications. Picture a seismologist looking for precursor signs to predict earthquakes or a hydrogeologist assessing the integrity of an aquifer or an engineer monitoring pressure changes associated with waste disposal. The applications are diverse and touch on numerous aspects of our lives, from earthquake risk assessment to extracting energy safely to urban planning for rising sea levels.

In the quest to constrain properties of geological or planetary materials or rocks, information from dominant mineral phases become essential. For instance, quartz and feldspar rheology are important to model the (mid and lower) crustal, while olivine rheology is important to model the mantle processes. Experimentalists adopt two approaches for subsequent laboratory deformation: 1) creating synthetic rock samples in the lab (Ghosh et al., 2021), or 2) collecting natural rock samples from the field. Afterwards, the data collection process involves recording stress (force), and strain (displacement) data, under varying confining pressures, temperatures and hydrous conditions. The results are then fitted into an equation involving the strain rate (˚ε), a dimensionless constant (A), the activation energy for creep (Q), the gas constant (R), absolute temperature (T), differential stress (σ), grain size (d), the water fugacity term  with an exponent (r), stress exponent (n), and grain size exponent (p):Once such a equation (also known as flow law) is established, the lab results can be extraprolated to the natural conditions to model or predict the mechanical behavior of the geological materials.  Moreover, the flow law parameters are good indicators of grain scale deformation mechanisms. Theoretical models of the creep have shown different values of n and p for different creep mechanisms. For example, n = 1 and p = 2 or 3 indicate lattice diffusion (Nabarro-Herring) creep or grain boundary diffusion (Coble) creep (Nabbaro, 1948; Herring, 1950; Coble, 1963), respectively. On the other hand, n ≥ 3 and p = 0 generally correspond to the dislocation creep process. Nonetheless, by identifying grain-scale deformation processes through careful microstructural characterization and quantifying their dependencies on time, stresses, temperatures and pressures, scientists bridge the gap from the laboratory scale to understanding Earth at large.

Conducting these experiments demands cutting-edge technologies, especially considering the large ranges of pressure, temperature, and rate involved. Deformation at elevated pressures is of little practical interest to materials science and engineering communities, consequently apparatuses capable of achieving high pressure conditions are available only at select few universities and institutes in the world. One such apparatus was designed by Prof. David Griggs (Griggs, 1967), which allow experiments at higher confining pressures based on the motor-driven piston-cylinder technology. More recently, the new (third) generation hydraulically-driven Griggs-type apparatus (Précigout et al., 2018) was developed based on the modified (second generation) design by Harry W. Green (Green et al., 1987). This new design allows constant-load or constant-displacement experiments over a wide range of temperatures (20 – 1300 °C) and ‘regulated’ confining pressures (0.3 – 5 GPa), encountered in the lithosphere and in the upper mantle of the Earth. A simple schematic diagram of the sample assembly for coaxial experiments is shown in the inset. The Griggs apparatus present in Orléans, France employs larger sample size, so that microstructures can be better developed. These innovations in laboratory equipment have made it possible to perform new experiments on natural coarse-grained (∼200 μm) quartzite from the Tana quarry, Norway (Nègre et al., 2021; Ghosh et al., 2022). These experiments have yielded new quartz flow law parameters, giving rise to a stress exponent of n ≈ 2 and an activation energy of Q = 110 kJ/mol:Interestingly, these results deviate from the expected predictions of climb-accommodated dislocation creep models with n ≈ 3-5 (e.g., Hirth et al., 2001). The microstructural analysis suggests that the bulk sample strain in those experiments is achieved by crystal plasticity (Nègre et al., 2021), i.e., dislocation glide with minor recovery by recrystallisation and sub-grain rotation, accompanied by grain boundary migration (GBM) (Ghosh et al., 2022). In addition to dynamic recrystallization, micro-cracking helps to nucleate new grains (see figures in Nègre et al., 2021; Pongrac et al., 2022).

Although using optical or electron microscopy, it is very difficult to distinguish between new grains formed either by dynamic recrystallization or micro-cracking. However, cathodoluminescence technique can be a way forward. It is found that dissolution-precipitation in the presence of H₂O facilitates GBM and grain boundary sliding (GBS), which act as accommodation mechanisms to prevent hardening from dislocation glide (Ghosh et al., 2022; Pongrac et al., 2022). Finally, it appears that when aqueous fluid is present along grain boundaries, processes such as dissolution-precipitation can be activated in natural rocks, which may lead to a deviation from the conventional understanding of rate-limiting deformation mechanisms, for example, climb-controlled dislocation creep.

One of the key issues related to rock deformation experiments is justifying the extrapolation of laboratory-derived mechanical data to natural conditions. Therefore, it is important to test the validity of rheological flow laws against natural observations, where geological strain-rate are well constrained. Considering the many uncertainties associated with natural data, the new flow law predicts reasonable strain-rate in excellent agreement with the natural case studies, compared to the previous flow laws (see Table 3 of Ghosh et al., 2022). The stress versus depth profile predicted by the new flow law shows the onset of plasticity (around the brittle to viscous transition) at 9-10 km depth, corresponding to approximately 300 ℃ (at a strain-rate = 10⁻¹² s⁻¹and a geotherm of 30 ℃/km), which is comparable to inferences made by seismological observations (e.g., Scholz, 1998).

In essence, experimental rock deformation serves as an invaluable tool for unraveling the mysteries of our planet’s geological processes. As technology advances and our understanding deepens, experimental rock deformation will continue to serve as a beacon, illuminating the pathways to understanding the Earth’s inner workings.

 

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