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

• Sound velocity of (Mg 0.9 Fe 0.1 ) 2 SiO 4 ringwoodite was measured at mantle conditions. • V P and V S are 2.5% and 1.5% higher, respectively, than previous studies at 300 K. • PVT-V P -V S data were fitted globally to derive thermos-elastic parameters. • Pyrolite and harzburgite can’t explain seismic features at the 660 km discontinuity. 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 0.9 Fe 0.1 ) 2 SiO 4 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.% H 2 O, 5.6 ± 1.2% Fe 3+ /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 V 0 = 39.69(2) cm 3 /mol, K 0 = 183(4) GPa, K 0 ′ = 5.5(3), G 0 = 125(2) GPa, G 0 ′ = 1.3(1), q 0 = 0.3(3), γ 0 = 1.27(4), θ 0 = 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.