Thermal properties of β‐Mg2SiO4 at mantle pressures derived from vibrational spectroscopy: Implications for the mantle at 400 km depth

Abstract

The pressure dependences of 25 Raman modes of β‐Mg2SiO4 were measured to 235 kbar. The thermodynamic parameters, heat capacity, GV, and entropy, S, of β‐Mg2SiO4 were calculated using a statistical approach from information derived from the spectra. CV and S calculated from the 1‐atm data agree well with the available previous calorimetric data and previous lattice dynamical calculations. The high pressure data were used to calculate the pressure dependence of CV and S. From these data and those from previous spectroscopic measurements on forsterite at high pressures, previous enthalpy data, and the equations of state of both phases, the equilibrium phase boundary was calculated to be P(kbar) = 101 + 0.027 kbar‐K−1×T(K), agreeing exactly with new phase equilibrium data. The Clapeyron slope, ΔS/ΔV, for the forsterite to β‐Mg2SiO4 transition was found to be nearly constant for a large P‐T range, indicating that the pressure and temperature induced volume changes compensate for the pressure and temperature induced entropy changes for the transition. Using the equation of state of both minerals at equilibrium conditions and the thermal expansion systematics of δT = 5.5 ± 0.5, the sound velocity increases across the forsterite to β‐phase transition by approximately 7.5%. Thus, a mantle of ∼ 60% olivine, i.e., a pyrolitic mantle, could account for the average seismic sound velocity change of +4.5% at 400 km depth. In addition, at 92 kbar, strong decreases in the pressure derivative of 7 Raman modes indicate changes in the compression mechanism of the β‐Mg2SiO4 lattice. Similar observations for forsterite at the same pressure imply that similar elastic limits were reached within the respective lattices. This phenomenon is interpreted as a second‐order phase transition.