Photon Factory Activity Report 2018 #36 (2019)

NE-7A/2017G515

Phase relation in MgSiO3 under mantle conditions
1Shigeaki ONO1,*Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science andTechnology (JAMSTEC), Yokosuka 237-0061, Japan

1 Introduction
As olivine, pyroxene and garnet are major minerals in
the upper mantle, understanding the dynamics andevolution of the mantle requires knowledge of MgSiO3,
which is an end-member of pyroxene. Therefore, phaserelations in MgSiO3 have been repeatedly investigated bya number of authors. However, the transition sequence ofthe MgSiO3 mineral remains as yet unconfirmed. The
discrepancy among researchers is likely due to theaccuracy of phase boundary determinations related withthe stability field of two phases, wadsleyite + stishovite or
ringwoodite + stishovite.
2 ExperimentHigh-pressure experiments were carried out using multianvil high-pressure apparatus installed at the synchrotron
facilities of KEK and SPring-8 in Japan. Experimental
details were described elsewhere [e.g., 1,2]. A mixture of
the powdered MgSiO3 and gold was used. Experimentalpressures were determined from the unit cell volumes ofgold. All recovered samples were investigated by anelectron microprobe analyzer to identify the stable phasein each experimental run.
3 Results and Discussion
Experimental runs were performed at pressures
between 15 and 21 GPa (Fig. 1). Two types of recovered
samples, single (MgSiO3) and two phases (Mg2SiO4 +SiO2), were confirmed. The single phase was highpressure clinoenstatite or akimotoite, and two phases werewadsleyite + stishovite or ringwoodite + stishovite.
According to experimental data, two reaction boundaries
were determined. The reaction boundary between highpressure clinoenstatite and wadsleyite + stishovite has apositive dP/dT gradient, 0.0064 GPa/K [3]. In contrast,
the reaction boundary between ringwoodite + stishovite
and akimotoite has a negative dP/dT gradient, -0.0012
GPa/K [4]. This study indicates that the stability field ofwadsleyite + stishovite expands to a low temperature
region corresponding to the P-T path in the subductedslab. Moreover, a triple point of wadsleyite + stishoviteringwoodite + stishovite-akimotoite is located at atemperature slightly lower than the geotherm (Fig. 2).
These experimental results can reconcile theinconsistency recorded between previous studies
regarding the phase relation in MgSiO3.

Fig. 1: Example X-ray diffraction pattern acquired at 17.1
GPa and 1200 K. Key to label abbreviations: G, gold (Au);E, emission of gold fluorescence; T, TiB2; P, periclase(MgO); and S, unidentified spot peak.

Fig. 2: Phase diagram of MgSiO3 estimated based thepresent and previous studies. Abbreviations: Low-Cen,
low clinoenstatite; Oen, orthoenstatite; Hp-Cen, highpressure clinoenstatite; Wa, wadsleyite; Ri, ringwoodite;St, stishovite; Mj, majorite; Ak, akimotoite; Br,bridgmanite; Pe, periclase.
Acknowledgement
We thank the PF staffs for their helps in carrying out theexperiments.
References[1] Ono et al., Phys. Chem. Minerals, 40, 811-816 (2013).
[2] Ono et al., Phys. Earth Planet. Inter., 264, 1-6 (2017).
[3] Ono et al., Phys. Chem. Minerals, 44, 425-430 (2017).
[4] Ono et al., Am. Mineral., 103, 1512-1515 (2018).
* sono@jamstec.go.jp