Publication Date:
2016-02-11
Description:
Drop-solution enthalpies (Δ H ° d-s ) of Mg 2 SiO 4 ringwoodite, Mg 2 SiO 4 forsterite, perovskite-type MgSiO 3 (bridgmanite), and MgSiO 3 enstatite were measured using a single batch of 2PbO·B 2 O 3 solvent at 978 K. From the obtained Δ H ° d-s values of Mg 2 SiO 4 ringwoodite, MgSiO 3 bridgmanite, and MgO, an enthalpy of the post-spinel phase transition, Mg 2 SiO 4 ringwoodite = MgSiO 3 bridgmanite + MgO, was determined to be 78.54±2.28 kJ/mol. Thermodynamic calculations using the obtained phase transition enthalpy and available thermochemical and thermoelastic data provided the phase transition pressure of 23.1±1.4 GPa at 298 K. This value is comparable to those at about 2000 K determined by previous experimental and theoretical studies, implying a considerably gentle Clapeyron slope. Thermodynamic calculations of the post-spinel boundary at high temperatures in the anhydrous condition by changing thermochemical and thermoelastic parameters within the uncertainties suggested that the post-spinel transition pressure of Mg 2 SiO 4 at high temperature is lower than the pressure corresponding to the global average depth of the “660-km” seismic discontinuity in the Earth's mantle (~23.5 GPa) estimated from one-dimensional reference Earth models and that a most likely Clapeyron slope is about –1 MPa/K. The post-spinel transition in the hydrous condition with about 2 wt% H 2 O, which shows higher transition pressure and steeper Clapeyron slope than those in the anhydrous condition, gives a plausible explanation for seismic observations on the “660-km” discontinuity, and therefore, hydrous mantle transition zone would be required.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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