Subsolidus phase relations for Mg2SiO4 at mantle transition zone conditions

Abstract:

Detailed knowledge of phase relations for (Mg,Fe)₂SiO₄ is required for a better understanding of the thermochemical structure across the 660-kilometer boundary between Earth’s transition zone and lower mantle. The phase diagram of Mg₂SiO₄ at mantle transition zone pressures has been extensively investigated up to temperatures of ~2000 K, while the wadsleyite ↔ bridgmanite boundary from ~2000 K to the solidus as well as the triple point at which wadsleyite (wd), ringwoodite (rw) and bridgmanite/periclase (bdg/per) co-exist are still poorly constrained. In this study, we report new experimental constraints on the phase relations for Mg₂SiO₄ at pressures between 17 and 23 GPa and temperatures from 1600 to 2500 K, with an exploration of the effect of iron on the phase transitions, using synchrotron X-ray diffraction collected in high-pressure and high-temperature laser-heated diamond anvil cell experiments. In combination with previously published datasets, we determined the Clapeyron slope of the wd ↔ bdg + per boundary and the location of the wd–rw–bdg/per triple point based on a logistic regression model. Our statistical analysis suggests that the wd ↔ bdg + per reaction occurs at 20.7 GPa and 2100 K and 20.4 GPa and 2300 K, with a Clapeyron slope of –1.8 MPa/K; the wd–rw–bdg triple point is located at 20.8 GPa and 2060 K. The combined effects of the multiple phase transitions in the system MgO–FeO–SiO₂, including the wd ↔ bdg + per reaction and the wd–rw–bdg/per triple point, influence the proportions of stable mineral phases in the mantle rock assemblages and their thermo-physical properties across the 660-km boundary, which eventually controls the vigor and style of the mantle convection through this upper-lower mantle interface of Earth.