Experimental determination of the effect of H2O on the 410-km seismic discontinuity

Due to the high solubility of H2O in the mineral wadsleyite it has been recognized that the presence of H2O in the mantle could influence the pressure and pressure interval of the olivine to wadsleyite phase transformation that is generally considered to be the cause of the 410-km seismic discontinuity. We have performed multianvil experiments in the Mg2SiO4–H2O system in order to quantify the expansion of the wadsleyite stability field as a result of the presence of H2O. At 1200 °C we find that the wadsleyite stability field expands to lower pressures by approximately 1 GPa under H2O-rich fluid-saturated conditions. At 1400 °C the expansion is approximately 0.4 GPa and no expansion can be detected at 1600 °C. A thermodynamic model based on recent H2O solubility measurements and crystallographic observations predicts an expansion of the wadsleyite stability field that is in good agreement with the experimental results. When these results are combined with phase relations in the Mg2SiO4–Fe2SiO4 system it can be shown that significant effects on the pressure and pressure interval of the olivine to wadsleyite transformation, i.e. on the depth and width of the 410-km discontinuity, are only expected at lower than ambient mantle temperatures (< 1400 °C) and for H2O concentrations that are substantially greater than 0.2 wt.%. Areas of the mantle where the 410-km discontinuity appears to occur over a depth interval of over 20 km can only be explained if the temperature in these regions is at or below 1200 °C and water concentrations are close to the level where olivine would become H2O saturated (i.e. > 0.5 wt.%). The presence of ferric Fe in the mantle, however, may also play a role in the broadening of the 410-km discontinuity that is as yet unquantified.