Olivine‐modified spinel‐spinel transitions in the system Mg<sub>2</sub>SiO<sub>4</sub>‐Fe<sub>2</sub>SiO<sub>4</sub>: Calorimetric measurements, thermochemical calculation, and geophysical application

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<jats:p>The olivine(α)‐modified spinel(β)‐spinel (γ) transitions in the system Mg<jats:sub>2</jats:sub>SiO<jats:sub>4</jats:sub>‐Fe<jats:sub>2</jats:sub>SiO<jats:sub>4</jats:sub> were studied by high‐temperature solution calorimetry. Enthalpies of the β‐γ and a α‐γ transitions in Mg<jats:sub>2</jats:sub>SiO<jats:sub>4</jats:sub> at 975 K and of the α‐γ transition in Fe<jats:sub>2</jats:sub>SiO<jats:sub>4</jats:sub> at 298 K were measured. The γ solid solution showed a positive enthalpy of mixing. Phase relations at high pressures and high temperatures were calculated from these thermochemical data including correction for the effect of nonideality of α, β, and γ solid solutions. The calculated phase diagrams agree well with those determined experimentally by Katsura and Ito very recently. The α − (Mg<jats:sub>0.89</jats:sub>, Fe<jats:sub>0.11</jats:sub>)<jats:sub>2</jats:sub>SiO<jats:sub>4</jats:sub> transforms to β through a region of α+β without passing through the α+γ phase field at around 400 km depth in the mantle with an interval of about 18(±5) km. Temperatures at 390 and 650 km depths are estimated to be about 1673 and 1873 K, respectively, assuming an adiabatic geotherm.</jats:p>

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