Collaborative Research: Magnesite Deformation and Potential Roles in the Slip and Seismicity of Subduction Zones

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Intermediate (70-300 km depth) and deep (300-700 km depth) focus earthquakes of great magnitudes are common in subduction zones. Though many ideas have been advanced, the cause of earthquakes at these depths remains as one of the most significant unresolved problems in the earth sciences. Researchers from University of Akron, Texas A&M University, and Brown University are exploring a new mechanism that might explain these enigmatic events: magnesite, a magnesium carbonate commonly observed in altered basalts and peridotite (primary constituents of the subducting plates), could initiate deep focus earthquakes. In this project, the research team is carrying out laboratory experiments in which magnesite is deformed under very high pressure and temperature conditions. Their recent experimental work demonstrated that magnesite is considerably weaker than peridotite, which indicates that veins of magnesite could act as nucleation points for earthquakes. In this project, they will experimentally investigate how two fundamental parameters, grain size and pressure, affect the strength of magnesite and incorporate the results in a computational model of earthquake nucleation. The project would advance desired societal outcomes by potentially shedding light on the causes of earthquakes and providing research experiences for undergraduate and graduate students.The grain-size sensitivity of diffusion creep and the pressure sensitivity of magnesite deformation mechanisms in all three creep regimes (diffusion, dislocation and low-temperature plasticity) need to be quantified in order to apply experimental flow laws to models of creep and shear instability. These parameters are critical considering that diffusion creep is the dominant deformation mechanism in magnesite at many natural conditions and may cause strain localization and possibly seismicity at high pressure in subducting slabs. In this project, the research team will: (1) quantify the grain-size sensitivity of magnesite strength when deforming by diffusion creep and limited plasticity mechanisms; (2) determine the pressure sensitivity of magnesite deformation mechanisms; and (3) model dynamic slip by ductile instabilities in magnesite, applying the shear-heating model. Hydrostatic experiments will be performed to investigate the grain growth kinetics of magnesite. Deformation experiments with different grain sizes over a wide range of pressures will be carried out to determine the grain size and pressure sensitivities of magnesite deformation mechanisms, as identified by scanning and transmission electron microscopy. This will be achieved using state-of-the-art high-pressure deformation apparatuses, such as the D-DIA coupled with X-ray synchrotron radiation for in-situ strain and stress measurements. The results will allow accurate determination of the rheology of magnesite, evaluation of its effects on the bulk rheology of subducting slabs, and prediction of conditions for shear instability where carbonates are subducted.