Sound velocity measurements of γ-(Mg0.91Fe0.09)2SiO4 show that the ringwoodite to bridgmanite and ferropericlase phase transformation does not produce the seismically observed 660 km discontinuity

Abstract

The canonical model of Earth’s interior directly links the global 410 and 660 km seismic discontinuities to olivine’s high pressure phase transitions in a pyrolite mineral assemblage. However, previous studies observe that the expected sound velocities of pyrolite are too low to match 1-dimensional seismic models in the lower mantle transition zone (MTZ). In this study, we report measurements of the elastic properties of (Mg_0.9Fe_0.1)₂SiO₄ ringwoodite (Rw90), the dominant component of pyrolite between 520 km and 660 km depths, using pulse-echo ultrasonic interferometry combined with synchrotron X-radiation in the multi-anvil press up to 21 GPa and 1650 K. Our results show that V_P and V_S of anhydrous Rw90 (0.003–0.035 wt.% H₂O, 5.6 ± 1.2% Fe³⁺/Fe^tot) are both higher than predictions at MTZ conditions from previous studies, with a smaller increase for V_S. Simultaneous fitting of PVT-V_P-V_S data yields global fit equation of state (EoS) parameters of V₀= 39.69(2) cm³/mol, K₀ = 183(4) GPa, K₀′ = 5.5(3), G₀ = 125(2) GPa, G₀′ = 1.3(1), q₀ = 0.3(3), γ₀ = 1.27(4), θ₀ = 1100(100) K and η_S0 = 3.5(2). Combining results with literature data, our predicted V_S for pyrolite produces a sharp discontinuity consistent with seismic models, although V_S is slightly lower than observed in the lower MTZ. In contrast, we find that pyrolite would not produce a large and sharp V_P discontinuity at 660 km as the jump in V_P caused by the decomposition of Rw90 is too small. A homogenous assemblage of pyrolite cannot, therefore, currently explain the seismic features in the lower MTZ. We also find that neither an accumulation of harzburgite nor basaltic crust immediately above or below the 660 km discontinuity explains the observations. Instead, either a heterogeneous mixture of some other chemically distinctive components is required or more prosaically, the elastic properties of all the other mantle phases need reevaluation.