• Jing, Z.
• Shibazaki, Y.
• Fei, Y.
• Nestola, F.
• Wang, Y.
• Stagno, Vincenzo
• Ohuchi, T.
• Higo, Y.

Abstract

It is commonly assumed that enstatite and SiO2-rich glass inclusions in diamonds with low nickel contents are representative of former high-pressure MgSiO3 polymorphs, such as perovskite. In fact, natural MgSiO3 perovskite has been never found as inclusion in diamonds since the kinetic reactions would not allow perovskite to reach the Earth's surface. In order to understand the nature of MgSiO3 inclusions, we need to investigate possible amorphization of perovskite as function of pressure and temperature and obtain kinetics data of the perovskite-enstatite back transformation that can be used to estimate the average ascent rate of diamonds throughout the Earth's interior. Available data on the activation energy associated with the perovskite-enstatite back-transformation would suggest that perovskite could revert to enstatite in very short time (3 to 100 years) at upper mantle conditions. However, the thermodynamic data used in such estimates have been debated because amorphization of perovskite was observed by previous studies on phase equilibria in the MgSiO3 system at high pressure. We present preliminary time-resolved in situ synchrotron x-ray diffraction data at high pressure and high temperature in multi-anvil experiments that are aimed to improve our current knowledge regarding the metastability and the rate of transformation of MgSiO3 perovskite at pressure and temperature of the Earth's upper mantle. Polycrystalline perovskite samples were employed as starting material, synthesized from MgSiO3 glass using multi-anvil at 26 GPa and 2000 Kelvin. Time-resolved x-ray diffraction data in energy dispersive mode were collected at the 13-ID-D beamline of GSECARS sector (Advanced Photon Source, Argonne) using a 1000 ton press and a T-25 multi-anvil module. The synthetic perovskite was loaded in a graphite capsule. A mixture of Au and MgO was placed at the top of the sample to serve as pressure markers. Each sample was first compressed to the target pressures (3 and 10 GPa) at room temperature using WC anvils, and then temperature was also increased rapidly to induce the phase transformation in runs at 900-1400 °C. Diffraction data were collected every 60 to 300 seconds at constant temperature and pressure. Further measurements were performed at high pressure and high temperature using the 2D angle-dispersive x-ray diffraction system at BL04B1 beamline (SPring-8) in SPEED-1500 multi-anvil press. To ensure quality of diffraction patterns obtained from the entire Debye-Scherrer two downstream inner WC anvils were replaced by diamond-SiC composite anvils transparent to x-rays. We also performed experiments with Mg(Fe)SiO3 perovskite to investigate the possible compositional effect on kinetics controlling this back-transformation. Additional quenched experiments were performed to obtain the transformation rate from perovskite to enstatite in each run. Phase identification and textural information of the run products were then investigated by Raman and Scanning Electron Microscopy. We analyze the data to obtain the activation energy and volume as function of pressure and temperature. We also compare the results with ambient-pressure data available in literature and evaluate the pressure effect on the back-transformation. Finally, we discuss the ascent rate of diamonds from the lower mantle to the surface based on the derived thermodynamic dataset.

Topics

• perovskite
• impedance spectroscopy
• surface
• nickel
• inclusion
• phase
• scanning electron microscopy
• x-ray diffraction
• experiment
• glass
• glass
• composite
• activation