Densification of Polycrystalline Olivine Aggregates by Evacuated Hot-pressing

Cameron Meyers, University of Minnesota

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We have developed an improved method for densifying olivine-rich rocks from powdered gem-quality single crystals, aiming to refine measurements of the kinetic properties of Earth's mantle materials. Convective flow in the upper mantle is accommodated by solid-state deformation of olivine-rich mantle rocks. The measured properties of olivine often are used to approximate upper mantle properties when applied to geophysical problems. Many studies have used conventional hot-pressing to synthesize nearly dense polycrystalline olivine rocks from pulverized gem-quality single crystals. In this case, powders are pressed into a sealed metal can and placed at 300 MPa confining pressure and 1,250 C. We found that conventionally hot-pressed rocks retain a small amount of porosity, containing CO2, measured using Fourier-transform infrared and Raman spectroscopy. In addition, gas-filled voids grew in conventionally hot-pressed material reheated at 1 atm. Voids pin grain boundaries, limiting grain growth. Thus, we developed the evacuated hot-pressing method for generating highly dense polycrystalline olivine rocks. In this case, the pore space of the pressed powder compacts are vented to vacuum as they are heated under confining pressure. This method produces highly dense, green-tinted, transparent specimens (resembling the gem-quality starting material), contrasting the opaque, milky-green conventionally hot-pressed aggregates. CO2 absorptions are not observed using spectroscopic methods. In addition, when reheated at 1 atm, evacuated hot-pressed samples show limited void growth and enhanced grain growth. Our results demonstrate the important effect of grain-boundary pinning in limiting grain growth. The samples we synthesize using this method will be critical to obtaining measurements of the kinetic properties of olivine-rich rocks.

Abstract Author(s): Cameron D. Meyers, David L. Kohlstedt, Mark E. Zimmerman