Elastic Wave Velocity of Polycrystalline Mj80Py20 Majorite to the Mantle Transition Zone Conditions

Abstract:

Laboratory sound velocity measurements on the majorite at high pressures and high temperatures provide the significant information in interpreting the seismological models for the mantle transition zone in the Earth’s interior [1]. The majorite at the mantle transition zone can be approximated by the simplified system majorite (Mg₄Si₄O₁₂) – pyrope (Mg₃Al₂Si₃O₁₂), and its elastic properties will thus contribute significantly to interpreting the seismic models for the mantle transition zone [2-3].

Here we synthesize the well-sintered, translucent, single-phase and fine-grain sized (<5 μm) polycrystalline Mj₈₀Py₂₀ majorite using a Kaiwai-type multi-anvil apparatus (Orange-3000) at the Geodynamics Research Center, Ehime University, Japan. We perform in situ X-ray and ultrasonic measurements on this well-sintered sample at pressures up to 21 GPa and temperatures up to 2000 K in a Kawai-type multi-anvil apparatus at the beamline BL04B1 in SPring-8. The elastic moduli and their pressure and temperature derivatives are determined from the current experimental data: K_S = 161.5(7) GPa, ∂K_S/∂P = 4.42(4), ∂K_S/∂T = -0.0154(2) GPa/K, G = 86.2(2) GPa, ∂G/∂P = 1.28(1), ∂G/∂T = -0.0096(5) GPa/K. The present results together with those of the earlier studies [4-7] on the majorite-pyrope solid solutions suggest the pressure and temperature derivatives of elastic moduli are insensitive to the majorite content in the majorite-pyrope system. The current study demonstrates that the velocity gradients of the majorite-pyrope solid solutions are 3~6 times lower than those required to account for the high seismic velocity gradients observed in the mantle transition zone.

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