Precise Comparison of Phase Relations in Pyrolite, MORB and Harzburgite Up To 28 GPa 1600-2200°C Using Multi-sample Cell Technique

Abstract:

It is accepted that 410- and 660-km seismic discontinuities are attributed to phase transitions in pyrolite. MORB, harzburgite and pyrolite constitute a subducting slab, and pyrolite is also major hot plume material. Mantle tomography studies demonstrated that slabs are subducting down to the lower mantle and that some hot plumes are elevating from the deep lower mantle to the upper mantle. Therefore, phase relations of these mantle rocks provide important information to elucidate dynamics of heterogeneous components in the deep mantle. We performed high-pressure high-temperature quench experiments at 12-28 GPa and 1600-2200°C for 2-10 hours using a Kawai-type multianvil apparatus at Gakushuin University. Starting materials were prepared from the oxide mixtures of pyrolite, MORB and harzburgite compositions. The three samples were packed with pressure calibrants (MgSiO₃ enstatite or pyrope) in a Re multi-sample capsule with four holes, and kept at the same P, T conditions to precisely compare the phase relations. Phases of the recovered samples were identified with a microfocus X ray diffractometer and a SEM-EDS. Using the measured compositions, phase proportions were calculated with mass-balance calculation, and densities of the rocks were obtained using thermoelastic data. The density profiles of three rock compositions show that MORB and harzburgite in slabs can subduct to the 660-km discontinuity because the post-spinel transition in pyrolite occurs at lower pressure than that in harzburgite and the post-garnet transition in MORB. Besides, in hot plumes, pyrolite has lower density than average pyrolitic mantle by not only its higher temperature but also different mineral assemblages from the average mantle due to decomposition of ringwoodite to garnet + ferropericlase at the depth around 660-km discontinuity.