Experimental redetermination of the post-spinel transition boundary in Mg2SiO4 by means of in-situ X-ray diffraction in a multi-anvil press
Abstract
The 660-km seismic discontinuity (D660) is key to understanding the structure and dynamics of the Earth's mantle. It has been accepted that the D660 is caused by the post-spinel (Psp) transition, which is decomposition of ringwoodite (Rw) to bridgmanite (Brg) + ferropericlase. Previous high-pressure experiments showed that the phase boundary of the post-spinel transition have a negative slope, although different studies showed different magnitudes of the slope. Seismological studies have suggested that the 660-km discontinuity is depressed under subduction zones, which are expected to have lower temperature than ambient mantle. Thus, results of high pressure experiments have been generally in agreement with seismic observations. However, our recent study (Ishii et al., 2018) demonstrated that previous high-pressure experiment gave incorrect transition pressures due to pressure drop upon heating. Therefore, previously obtained Clapeyron slopes could be also incorrect. For this reason, we have therefore determined the transition pressure in Mg2SiO4 at 1500 K by avoiding the pressure drop upon heating in this study.
In-situ X-ray diffraction using a multi-anvil press was adopted in this study. A fine-grained mixture of forsterite, enstatite and periclase (Pc) and an MgO pressure marker were placed at the center of a furnace to alternatively conduct phase identification and pressure determination, respectively. We have found that the transition pressure is located at a pressure of 23.2 23.3 GPa at a temperature of 1500 K based on the equation state of MgO based on the 3rd-order Birch-Murnaghan equation of state proposed by Tange et al (2009). On the other hand, Ishii et al. (2018) gave a transition pressure of 23.63 GPa at a temperature of 1700 K. Combination of these two results suggests that the Clapeyron slope of the Psp transition is +2 MPa/K. This positive slope is completely against previous investigations. The reason for the negative slopes given by previous studies is probably because dissociation of Rw to Brg + Pc requires very high excess pressure due to sluggish kinetics at low temperatures, causing overestimation of the transition pressures at low temperatures. The reason for disagreement of seismic observations with the Clapeyron slope obtained in this study is currently unclear.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMDI31C0032I
- Keywords:
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- 3924 High-pressure behavior;
- MINERAL PHYSICSDE: 3621 Mantle processes;
- MINERALOGY AND PETROLOGYDE: 7208 Mantle;
- SEISMOLOGYDE: 8124 Earth's interior: composition and state;
- TECTONOPHYSICS