Deformation mechanisms and rheology of hydrous minerals (evaporites, phyllosilicates), which are crucial in geodynamics, have mostly been studied with other phases, overshadowing the role of these single phases that may dramatically alter the rheology due to dehydration. My project is designed to experimentally investigate the deformation mechanisms of some of the major crustal hydrous minerals from molecular scale to macroscale under different thermodynamic conditions. The experimental tools to be used are instrumented micro- to nanoindentation coupled with high pressure-temperature (HPT) rock deformation studies and subsequent analyses. I aim to construct the rheology of hydrous minerals across a range of pressures and temperatures and apply the data to understand the mechanics of subduction and mountain building processes.

Mountain building and subduction zone processes appear simple in large scale, however, in small scale numerous interlinked geological processes were and are involved. For example, late-stage activation of fluids (free to trapped, hydrothermal, dehydrated, and partial melt etc.) alter the strength significantly. Phyllosilicates (e.g., talc, serpentine, phengite, Biotite) are common in Ultra High Pressure (UHP) metamorphic zones in active subduction zones and FTBs with evaporites such as gypsum, basanite, etc. These are the product of long-term hydrothermal alterations of mafic and ultramafic rocks of oceanic lithosphere in the presence of Si-rich fluids (Fig. 1). UHP zones are characterized by strong Crystallographic Preferred Orientations (CPOs) as the rocks here accommodate the shear strain and produces seismic anisotropy. As dislocation-dominated creep is pressure-temperature dependent, the interplay between external and dehydrated fluid pressures affects dislocation velocity and deformation mechanism. Though high-pressure experiments on phyllosilicate have been explored, the scale- and rate-depended deformation mechanism of hydrous minerals with coexisting fluid phases are rare. We assume, higher temperatures and deviatoric strains can activate diffusion of mobile elements, resulting in diffusion creep and strain hardening. Therefore, thorough HPT experiments with variable fluid pressure and strain-rate are required to constrain the fluid induced changes in deformation mechanism of mono- and polycrystalline hydrous phases that contribute to subduction dynamics and orogenic processes (Problem – 1).

Phyllosilicates and their dehydrations along the subduction interface trigger the megathrust earthquakes. However, interface properties remain poorly understood. The geophysical and geodetic data suggest changes in friction coefficient at HPT leads to slow-slip and aseismic creeping. A favored explanation for the seismic to aseismic transition with increasing depth along the subduction interface is that effective friction decreases with high pore fluid pressure, lubricating the subduction interface to slide aseismically. However, recent geophysical studies suggest that while fluid pressure is high, it may not be the sole explanation for the observed transitions in deformation. I propose an alternative hypothesis, that phyllosilicates control the seismic to aseismic transition with depth during their dehydration (Fig. 2). Although many recent studies have looked at the shallow frictional properties of clays and phyllosilicates, the thermodynamic stability and frictional properties of these minerals at greater depths are still under question, particularly in the presence of effective shear stress (Problem – 2).

Dehydration, particularly for transversely isotropic hydrous phases such as gypsum or phyllosilicates, controls the mechanical behavior in every possible scale. For example, gypsum shows strain rate-insensitive plastic deformation upon dehydration with a marked decrease in the resistance to plastic deformation beyond a certain temperature at nanoscale (Fig. 3). Therefore, a similar behavior is also expected from other hydrous phases as well. The deformation mechanism that contributes to the aseismicity along a shear interface, therefore, is a probable function of the degree of dehydration and the scale of observation. Hence, a multiscale deformation experiment approach is required to correlate the dehydration induced deformation mechanisms at nanoscale to mesoscale, so that the data can be extrapolated to geodynamic scale for having a better understanding of the origin of aseismic slip in subduction zones (Problem – 3).

To investigate the effect of metasomatism-dehydration coupling on continuously deforming hydrous phases, I will use Talc, Muscovite, Serpentine, and Biotite as representative sample of phyllosilicates, compositionally consistent with a subduction system. I will use commercially available single crystals of phyllosilicates to evaluate the lattice dynamics and monocrystalline deformation mechanisms, and isostatically hot-pressed synthetic sample to characterize the same for polycrystalline phases that are petrographically similar to exposed fault gauge rocks of a classic ophiolite section. I will also use natural monocrystalline and hot-pressed polycrystalline Gypsum sample as a representative hydrous mineral to understand the dehydration-induced nanomechanical evolution of transversely isotropic phases. All sample will be tested under multiscale deformation conditions, starting from nano- to mesoscale range. For nanoscale experiments, sample will be cut along various crystallographic orientations and polished to achieve minimum-asperity surfaces. For HPT tests, cylindrical sample of 10mm diameter and 15mm length will be prepared. I will also prepare thin sections of these samples for petrographic studies. Grain size, crystallographic orientations, initial microstructure, chemical composition, and thermogravimetric nature of the samples will be characterized before and after deformation experiments.
I will employ static micro- to nanoindentation tests with varying strain-rates for nanomechanical characterizations. Experiments will be conducted using in-situ heating mechanism for elastoplastic and viscoelastic rheological evaluation during progressive dehydration of these hydrous phases. Dynamic indentation tests will be carried out to examine the acoustic emission nature of dehydrating surfaces, that will shed light on the frictional rheology. I will also perform high pressure (Pmax=1GPa), temperature (Tmax=1000°C) deformation tests on synthetic sample (with various wt% of fluid) with varying confining pressure and strain-rates that will replicate the strain and metasomatic conditions of the slab-wedge interface of a subduction system. Physicochemical properties of pre- and post-deformed samples will be compared, followed by a detailed microstructural (using optical, scanning electron, and atomic force microscopy) and mechanical analysis and interpretation of mechanical data to track the phase transition events.

I have extensively reviewed the basics of Nanomechanics and its application in mineral rheology. This paper was published in Physics and Chemistry of Minerals. 

 Understanding the dynamics of the lithosphere relies heavily on the scale-dependent rheology of minerals. While quartz, feldspar, and phyllosilicates are the key phases to govern the rheology of the crust and tectonic margins, olivine and other mafic phases control the same in the upper mantle. Phase transition, solid-state substitution, polymorphism, etc. also affect mineral phase rheology. High pressure–temperature deformation tests with natural, synthetic and analog materials have improved our interpretation of the geodynamic state of the lithosphere. However, deforming and studying a single crystal is not easy, because of the scarcity of specimens and laborious sample preparations. Experimental micro- to nanoindentation at room and/or elevated temperatures has proven to be a convenient method over mesoscale compressive testing. Micro- to nanoindentation technique enables higher precision, faster data acquisition and ultra-high resolution (nanoscale) load and displacement. Hardness, elastic moduli, yield stress, fracture toughness, fracture surface energy and rate-dependent creep of mono- or polycrystalline minerals are evaluated using this technique. Here, we present a comprehensive assessment of micro- to nano-mechanics of minerals. We first cover the fundamental theories of instrumented indentation, experimental procedures, pre- and post-indentation interpretations using various existing models followed by a detailed discussion on the application of nanoindentation in understanding the rheology and deformation mechanisms of various minerals commonly occur in the crust and upper mantle. We also address some of the major limitations of indentation tests (e.g., indentation size effect). Finally, we suggest potential future research areas in mineral rheology using instrumented indentation. 

I am currently working on Dehydration-induced phase transition in Gypsum and its mechanical effect using instrumented microindentation and thermogravimetric analysis.