Rheology and microstructural evolution of serpentine

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Serpentine is a group of hydrous minerals that is commonly formed along faults and other interfaces in Earth where water is able to react with other anhydrous minerals. Because serpentine has a number of unusual physical properties, it is hypothesized that serpentine-group minerals play an essential role in the dynamics of boundaries between tectonic plates. Furthermore, serpentine may control where earthquakes can or cannot occur. However, serpentine is only stable through a narrow range of pressure and temperature conditions, which has hindered experimental efforts to unravel its behavior. This study will involve two sets of novel experiments to elucidate how serpentine changes shape or structure when Earth-like deformation conditions are applied. Broader Impacts include training of a graduate student, development of educational and outreach modules in a visualization laboratory, and cross-disciplinary science impact.The objective of this project is to clarify further the rheology and associated deformation microstructures of antigorite, the high pressure and temperature phase of serpentine. Two sets of experiments are proposed. The first set of experiments will investigate microstructural evolution of polycrystalline serpentine as a function of strain, using the Large Volume Torsion (LVT) apparatus at Washington University in St. Louis. The purpose of these experiments is to characterize the crystallographic preferred orientation (CPO) generated by deformation and determine the conditions at which steady-state microstructures are produced. The data that will be obtained on the transient evolution of antigorite CPO are needed to interpret seismic wave-speeds and seismic anisotropy in regions where serpentine is inferred to be present. The second set of experiments will use micromechanical methods, including nanoindentation and micropillar compression testing, to measure precisely the plastic rheology of antigorite up to 600 degrees C. These data will be used to constrain flow-laws for antigorite at conditions relevant to subduction zones, and may be used in future efforts to model deformation at the slab interface.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.