DI005-0020
Reconciliation of the negative pressure dependence of the 660-km discontinuity depth with the bridgmanite forming reactions in MgO-SiO2 systems
Wednesday, 9 December 2020
Poster
Artem Chanyshev1, Takayuki Ishii1, Dmitry Bondar1, Shrikant Bhat2, Eun Jeong Kim1, Robert J Farla2, Zhaodong Liu1, Lin Wang1, Ayano Nakajima3, Bingmin Yang4, Hu Tang4, Zhen Chen4, Yuji Higo5, Yoshinori Tange5 and Tomoo Katsura1, (1)University of Bayreuth, Bayerisches Geoinstitut, Bayreuth, Germany, (2)Deutsche Elektronen-Synchrotron (DESY), Hamburg, Germany, (3)Tohoku University, Sendai, Japan, (4)Center for High Pressure Science and Technology Advanced Research, Beijing, China, (5)Japan Synchrotron Radiation Research Institute, Hyogo, Japan
Abstract:
A number of seismic studies showed that the 660-km discontinuity (D660) is significantly depressed under subduction zones, which suggests that a mantle phase transition responsible for the D660 has a steep negative Clapeyron slope. Although it is usually considered that the D660 is caused by the dissociation of ringwoodite (Rw) to bridgmanite (Brg) plus periclase (Pc) (hereafter referred to as the Rw–Brg+Pc transition), previous phase-relation studies provided negative but gentle slopes of this transition boundary [e.g. Katsura et al., 2003; Fei et al., 2004; Litasov et al., 2006]. Our recent studies [Ishii et al., submitted] suggested that conventional high-pressure-temperature experiments to determine phase boundaries might contain significant errors, because of drastic change in a sample pressure upon heating and therefore misidentification of a stable phase caused by sluggish kinetics. In order to obtain correct phase relations beyond these problems, we have re-determined phase boundaries of the Rw–Brg+Pc and akimotoite (Ak)–Brg transitions in the MgO-SiO
2 system using a new strategy.
Namely, a stable phase assemblage is determined by observing relative increase/decrease in the ratio of coexisting high- and low-pressure assemblages at spontaneously and gradually decreasing pressure and a constant temperature by means of in situ X-ray diffraction in a multi-anvil press. Since this strategy is strictly based on the principle of phase equilibrium, it excludes problems on determining phase stability caused by sluggish kinetics and surface energy.
Obtained results are summarized as follows. The Rw–Brg+Pc boundary is located at 23.9 GPa, and its slope is zero at temperatures less than 1700 K. The boundary, however, gradually becomes negatively steeper with temperature. At 2000 K, the slope reached -1.3 MPa/K and the phase boundary is located at 23.3 GPa corresponding to the D660 depth. In contrast, the Ak–Brg phase boundary has a very steep boundary at low temperatures. It is located at higher pressures than the Rw–Brg+Pc boundary at temperatures below 1300 K. The Ak–Brg transition therefore controls the D660 depression under cold subduction zones instead of the Rw–Brg+Pc transition responsible for the global D660.
