Determination of thermal expansion coefficient of the major mantle minerals at high pressures and temperatures and estimation of adiabatic geotherm in the upper part of the mantle

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We have measured unit cell volumets of the major mantle minerals, namely, olivine, wadsleyite and perovskite at high pressures and temperatures by means of in situ X-ray diffraction using a large-volume high-pressure and high-temperature apparatus installed at the synchrotron radiation facility, SPring-8. We have constructed the Mie-Grueneisen-Debye equations of state of these minerals by using the obtained data. In this procedure, we have used the bulk modulus, its pressure derivative and Debye temperature from the literatures, and estimated Grueneisen parameter gamma0 and its logarithmic volume dependence q. The estimated parameters are as follows: gamma0 = 1.40(1) and q = 2.2(1) for olivine, gamma0 =1.64(1) and q = 1.5(1) for wadsleyite. Gamma0 = 2.66(2) and q = 1.9(1) for perovskite. We have estimated thermal expansion coefficient of the major mantle minerals in the Earth's mantle using these parameters and those given by Katsura, et. Al. (2004). By assuming the adiabatic temperature gradient, the thermal expansion coefficient of olivine largely decreases with depth to 2.6 x 10(-5) /K at the 410-km depth. The thermal expansion coefficient increases associated with the olivine-wadsleyite-ringwoodite-perovskite transitions and decreasing with depth. As a result, the thermal expansion coefficient is nearly constant in the upper part of the mantle. The behavior of the adiabatic gradient is similar, and is nearly constant, that is, 0.4 K/km at the upper part of the mantle.