Collaborative Research: Transformation plasticity as a transient creep mechanism in Earth's crust and mantle

The minerals that comprise Earth's crust and mantle undergo phase transformations. Changes in their crystal structure occur in response to changes in temperature and pressure. With increasing pressure, olivine — the main constituent of the upper mantle — transforms into denser minerals; much like graphite transforms into diamond. As one mineral transforms into another, crystal defects, known as dislocations, are produced. Dislocations allow crystals to deform plastically, i.e., permanently. These dislocations can cause temporary weakening that speeds up crust and mantle flows. Transformation-induced weakening — 'transformation plasticity' — may impact the mantle flows driving plate tectonics. It may also provide an explanation for the earthquakes occurring at great depths in the mantle. These deep earthquakes are still poorly understood. Despite its importance, transformation plasticity has seldom been observed in the laboratory. This is because it involves complex processes, difficult to quantify by conventional methods. Here, The researchers conduct experiments at the extreme pressures and temperature prevailing in the mantle. They study the plasticity of fayalite, olivine Fe-rich end member, and quartz which is an important constituent of the continental crust. They use a state-of-the-art high-pressure deformation device set up at a national synchrotron facility. There, powerful X-rays allow imaging the minerals and analyzing their properties during their transformations. The multidisciplinary project, at the intersection of geophysics, materials science, and crystallography, aims to expand the understanding of Earth's interior dynamics. It supports the professional development of an early career scientist. It also provides training for one graduate and several undergraduate students.

Transformation plasticity may play a central role in: 1) mantle decoupling and the development of layered convection; 2) slab ponding in Earth's mantle transition zone; 3) shear zone nucleation during orogenesis; 4) mantle plume upwelling; and 5) the nucleation of deep-focus earthquakes. However, few studies have examined transformation plasticity in major rock-forming minerals. This is because of the difficulty to quantify complex transient behaviors at high temperatures (>1000 K) and pressures (> 1 GPa). Here, the team explores the quartz-coesite and olivine-spinel phase transformations — under hydrostatic and non-hydrostatic conditions — using a Deformation-DIA (D-DIA) apparatus located at the Advanced Photon Source (APS) at Argonne National Laboratory. This experimental setup is uniquely suited to examining transformation-related phenomena in situ at mantle pressures and temperatures. Stresses and transformation kinetics are quantified via energy-dispersive X-ray diffraction. Axial and volume strains are measured using X-ray radiography. Constitutive laws describing transformation plasticity are derived, benchmarked against experimental data, and extrapolated to Earth conditions. Run product microstructures are also investigated in detail — using high-resolution electron backscatter diffraction — to interrogate the physical processes responsible for transformation-induced weakening. Ultimately, the team seeks to determine the importance of transformation plasticity in Earth's crust and mantle.

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