14:45 〜 15:00
[SIT14-16] Post-spinel phase transition in Mg2SiO4-Fe2SiO4 system as an interpretation of the 660-km discontinuity elevation
★Invited Papers
キーワード：Multi anvil experiments, Phase transitions, Mantle dynamics, 660 km discontinuity
Seismic wave velocities increase abruptly at a depth of 660 km almost everywhere in the Earth's mantle, which is referred to as the 660-km discontinuity (D660). As the D660 is thus a ubiquitous feature of the mantle, it should provide fundamental information about the Earth's interior. The D660 is usually attributed to the dissociation of (Mg,Fe)2SiO4 ringwoodite (Rw) into (Mg,Fe)SiO3 bridgmanite (Brg) and (Mg,Fe)O ferropericlase (fPc) which is named post-spinel phase (PSP) transition. The D660 is not flat but undulating. It is elevated up to 17 km in hot regions and depressed down to 90 km in cold regions of the mantle. These observations imply that the pressure of the PSP transition should increase/decrease with decreasing/increasing pressure, respectively, i.e., the dissociation of Rw should have a negative Clapeyron slope. However, the experimental results in Mg2SiO4 obtained using high-precision multi-anvil experiments showed a very gentle or almost zero slope (-0.1~0.9 MPa/K) [1]. Such a gentle slope does not account for the topography of the D660. Our previous study interpreted the depression of D660 in cold regions caused not by the Rw dissociation but by the akimotoite (Akm)-Brg transition [1]. However, as the transition pressure of the Akm-Brg transition is far below the pressure of the D660 (23.4 GPa) in hot regions, the Akm-Brg cannot account for the elevation of the D660. Therefore, we need an alternative interpretation for it. A possible explanation is that the dissociation of Fe-rich Rw has steeper Clapeyron slope and occurs at lower pressures in hot regions.
The experiments were performed at the beamline P61B at DESY (Hamburg, Germany) and at the beamline BL04B1 at SPring-8 using a multi-anvil press with in-situ X-ray diffraction. We used the following strategy to determine transition parameters: (1) the phase stability was determined by the ratio change of already coexisting Rw and Brg+fPc. (2) Two diffraction patterns of the sample were collected at a fixed press load and temperature to observe the growth of a stable phase without a sudden pressure change associated with temperature change. (3) In order to avoid disappearance of either phase and keep the two-phase coexistence, the experimental conditions were kept close to the phase boundary.
The PSP transition was determined for the Mg#(Mg/(Mg+Fe)) = 0.9 and 0.7 in the temperature range of 1300-1800 K. This transition for the Mg# = 0.9 has a negative boundary (-3.9 MPa/K), whereas the transition for the Mg# = 0.7 has a concave curve which gradually changes from -1.6 MPa K-1 at low temperatures to -2.2 MPa K-1 at high temperatures. Mantle rocks are thought to have Mg# = 0.9, and the determined slope for this composition is steeper than that defined for the MgO-SiO2 system; therefore, the PSP transition controls the elevation of the D660 in hot regions of the mantle. Moreover, steep phase boundaries produce large buoyancy forces which affect mantle convection. A positive Clapeyron slope of a phase transition enhances mantle convection, whereas a negative slope impedes it. Therefore, the PSP transition should impede convection in plume zones in the mantle.
[1]. A. Chanyshev, et al. (2022). Depressed 660-km discontinuity caused by akimotoite-bridgmanite transition. Nature, 601(7891), 69-73.
The experiments were performed at the beamline P61B at DESY (Hamburg, Germany) and at the beamline BL04B1 at SPring-8 using a multi-anvil press with in-situ X-ray diffraction. We used the following strategy to determine transition parameters: (1) the phase stability was determined by the ratio change of already coexisting Rw and Brg+fPc. (2) Two diffraction patterns of the sample were collected at a fixed press load and temperature to observe the growth of a stable phase without a sudden pressure change associated with temperature change. (3) In order to avoid disappearance of either phase and keep the two-phase coexistence, the experimental conditions were kept close to the phase boundary.
The PSP transition was determined for the Mg#(Mg/(Mg+Fe)) = 0.9 and 0.7 in the temperature range of 1300-1800 K. This transition for the Mg# = 0.9 has a negative boundary (-3.9 MPa/K), whereas the transition for the Mg# = 0.7 has a concave curve which gradually changes from -1.6 MPa K-1 at low temperatures to -2.2 MPa K-1 at high temperatures. Mantle rocks are thought to have Mg# = 0.9, and the determined slope for this composition is steeper than that defined for the MgO-SiO2 system; therefore, the PSP transition controls the elevation of the D660 in hot regions of the mantle. Moreover, steep phase boundaries produce large buoyancy forces which affect mantle convection. A positive Clapeyron slope of a phase transition enhances mantle convection, whereas a negative slope impedes it. Therefore, the PSP transition should impede convection in plume zones in the mantle.
[1]. A. Chanyshev, et al. (2022). Depressed 660-km discontinuity caused by akimotoite-bridgmanite transition. Nature, 601(7891), 69-73.