Synchrotron deformation experiments of olivine under the deep upper mantle conditions: Transient creep, plastic anisotropy, and the role of grain-boundary sliding.

Yale University, New Haven CT

Investigators

Abstract

The Earth’s mantle dynamics are governed by the mechanical properties of its constituent materials. These dynamics include geological processes with human impacts, such as surface deformations that occur after earthquakes and ice ages, with consequences on sea-level rise. Olivine is the most abundant mineral in the upper mantle, and it is likely the weakest mineral. Therefore it is olivine’s mechanical properties that control small vertical displacements of the crust and mantle due to the melting of ice caps after the last ice age. This vertical displacement, called post-glacial rebound, is a time dependent deformation arising from the transient creep of the mantle, and is used to estimate mantle viscosity. In contrast, the convection of the mantle is a steady-state phenomenon, frequently resulting in a different effective viscosity. Determining transient creep of olivine will therefore allow accurate constraint of mantle viscosity across time scales, but studies on olivine transient creep are limited. This project will perform deformation experiments of olivine with and without dissolved water under the conditions of the Earth’s upper mantle. The results will be interpreted from a materials science point of view to interpret how the mineral deforms under transient and steady-state creep for a more complete application to geophysical processes such as post-glacial rebound and post-seismic relaxation. This is the first NSF proposal for the PI, who is also the Director of the Yale Earth Materials Characterization Center (EMC2); this project will broaden and support the mission of EMC2 with further outreach, training, and educational opportunities for students and postdoctoral scholars. The PIs actively participate in all levels of STEM training; specifically, this project will train an undergraduate student and a post-doctoral researcher on synchrotron high-pressure deformation experiments, giving them valuable experience in state-of-the-art techniques. In this project, the PI will address geophysically important questions such as post-glacial rebound and post-seismic relaxation which require understanding of transient creep mechanisms at small strains. Deformation in the transient creep regime is controlled by the rate of formation and motion of linear crystal defects produced during deformation. Pressure and water content has been shown to affect the motion of linear defects differently in the steady state, and therefore will most likely also affect transient creep. As creep involves multiple microscopic processes, the rheological properties inferred from short-term deformation may differ from those relevant to long-term deformation. Studies on transient creep have been limited, especially in elucidating the relative roles of inter-granular versus intra-granular deformation mechanisms. This study will provide new deformation experiments on olivine to bridge transient and steady-state creep regimes. This work will include analysis of the roles of inter- and intra-granular deformation mechanisms, and how they are affected by the variations in pressure and water content. The project further includes a set of experiments where grain-boundary sliding will be studied using a bi-crystal to estimate the effect of grain-boundary sliding on viscosity. All results from olivine aggregate and single crystals will be interpreted using materials physics and microstructural characterization, and implications on geophysical processes such as post-glacial rebound and post-seismic relaxation will be determined. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.