Presentation Information
[R3P-13]Sound velocities of (Mg,Fe)SiO3 bridgmanite at simultaneous high-pressure and high-temperature
*Steeve Greaux1, Donghoon Kim2, Sho Kakizawa3, Yoshio Kono4, Itaru Ohira5, Yoshihiro Inoue4, Jie-jun Jing1, Minsu Jang2, Gunhee Lee2, Ryoichi Nakada6, Noriyoshi Tsujino3, Yuji Higo3 (1. Ehime University, 2. Korea University, 3. JASRI, 4. Kwansei University, 5. Gakushuin University, 6. JAMSTEC)
Keywords:
High pressure and High temperature,Bridgmanite,Iron speciation,Elastic Wave Velocities,Lower Mantle
Bridgmanite (Brg) is the most predominant mineral of the Earth’s lower mantle. A precise knowledge of the elasticity of Brg at the pressure and temperature conditions of the Earth’s lower mantle is therefore essential for interpreting seismic models at depth, which would in turn give important clues about chemical composition and internal structure of our planet’s deep interior. Regardless of the bulk silicate Earth (BSE) model, (Mg,Fe)SiO3 Brg accounts, with (Mg,Fe)O ferropericlase, for the main deposit of Fe in the lower mantle. While previous studies agree that incorporating a few wt.% FeO decreases Brg's adiabatic bulk modulus (KS) (Chantel+ 2012, Fu+ 2018, Mashino+ 2020), its effect on the shear modulus (G) remains debated. Some literature suggests G decreases with Fe incorporation, whereas others conclude it is mostly unaffected (Chantel+ 2012, Fu+ 2018, Criniti+ 2021). Disagreements between previous studies likely stem from the fact that bridgmanite specimens with varying chemical compositions and Fe speciation (e.g. Fe3+/Fetotal) were investigated using different techniques, each carrying its own experimental uncertainties and technical limitations. Ultimately, due to these inconsistencies and a scarcity of simultaneous high-pressure and high-temperature data, the influence of iron on seismic velocities remains poorly constrained.Here we present elastic wave velocities measurements on (Mg1-x,Fex)SiO3 bridgmanite aggregates (x=0.00, 0.04, 0.12) with a Fe3+/Fetotal= 0.25(1), by a combination of in situ ultrasonic interferometry, X-ray radiography and diffraction techniques at high pressure and high temperature in the 1500-ton multianvil press apparatus SPEED1500 at the beamline BL04B1 in SPring-8 (Japan). Results showed VP and VS are generally consistent with previous studies with x = 0.00–0.05 (Criniti+ 2021, Fu+ 2018, Chantel+ 2012). New shear velocity data for x = 0.12 stands in between those of Brg single crystals with x = 0.09 and x = 0.18 (Mashino+ 2020) with differences that may be explained by a varying Fe speciation. Based on our results we will discuss the oxidation state of the lower mantle and propose a thermodynamic model to estimate its bulk Mg/Si composition based on a comparison of seismic models and high-temperature elastic wave velocity measurements.