[S-GC36] Solid Earth Geochemistry, Cosmochemistry

[SGC36-05] Effect of iron on the postspinel transition under anhydrous and hydrous conditions

*Rei Torigoe¹, Toru Inoue¹, Yuri Shinoda, Takaaki Kawazoe¹, Masamichi Noda¹, Yasushi Ueno¹ (1.Hiroshima university)

Keywords:The postspinel transition , 660 km discontinuous , Effect of iron , Effect of water

It is known that there are some seismic discontinuities in the Earth's interior, and the seismic discontinuities in the mantle transition zone can be explained by the phase transition of olivine. Among them, the 660 km discontinuity is thought to be caused by the postspinel phase transition, and the FeO and H₂O components should affect the transition pressure, in which ringwoodite (Rw) decomposes into bridgmanite (Brg) and ferro-periclase (fPc). The depth of the 660 km discontinuity has regional dependency of ±17 km (e.g.Houser & Williams, 2010) , and the reason has been often explained by the temperature difference. On the other hand, hydrous Rw in diamond inclusions has been found as the evidence for the presence of water in the mantle transition zone (Pearson et al., 2014), and it contained 1.4-1.5 wt% water. Therefore, it became necessary to consider the effect of water. As for the effect of water on the post spinel phase transition, it has been reported that the phase transition pressure shifts to the high pressure side in the Mg₂SiO₄ component. (Higo et al., 2001). In addition, first-principles calculations have also shown that the phase transition shifts to the high-pressure side by the effect of water (Muir et al., 2021). However, there is no study that has clarified the effect of water in the Mg2SiO4-Fe2SiO4 binary system. In addition, a phase diagram of the (Mg,Fe)₂SiO₄ system under anhydrous conditions has recently been reported, which is quite different from the previous phase transition loops. (Ishi et al., 2019) Therefore, the purpose of this study is to clarify the effects of iron and water on the postspinel phase boundary in the (Mg,Fe)₂SiO₄ system. As a result, it was found that the coexistence loops of Rw and Brg+fPc in the (Mg,Fe)₂SiO₄ system with decreasing phase transition pressure by increasing Fe content were formed. Further details will be presented.