The structure and sharpness of (Mg,Fe) 2SiO 4 phase transformations in the transition zone

Abstract

To calculate accurately the pressure interval and mineral proportions (i.e. yields) across the olivine to wadsleyite and wadsleyite to ringwoodite transformations requires a detailed knowledge of the non-ideality of Fe–Mg mixing in these (Mg,Fe) 2SiO 4 solid solutions. In order to constrain the activity–composition relations that describe non-ideal mixing, Fe–Mg partitioning experiments have been conducted between magnesiowüstite and (Mg,Fe) 2SiO 4 olivine, wadsleyite and ringwoodite as a function of pressure at 1400°C. Using known activity–composition relations for magnesiowüstite the corresponding relations for the three polymorphs were determined from the partitioning data. In all experiments the presence of metallic iron ensured redox conditions compatible with the Earth’s transition zone. The non-ideality of the (Mg,Fe) 2SiO 4 solid solutions was found to decrease in the order W wadsleyite FeMg> W ringwoodite FeMg> W olivine FeMg. These partitioning data were used, along with published phase equilibria measurements for the Mg 2SiO 4 and Fe 2SiO 4 end-member transformations, to produce an internally consistent thermodynamic model for the Mg 2SiO 4–Fe 2SiO 4 system at 1400°C. Using this model the pressure interval of the olivine to wadsleyite transformation is calculated to be significantly smaller than previous determinations. By combining these results with Fe–Mg partitioning data for garnet, the widths of transition zone phase transformations in a peridotite composition were calculated. The olivine to wadsleyite transformation at 1400°C in dry peridotite was found to occur over a pressure interval equivalent to approximately 6 km depth and the mineral yields were found to vary almost linearly with depth across the transformation. This transformation is likely to be even sharper at higher temperatures or could be significantly broader in wet mantle or in regions with a significant vertical component of mantle flow. The entire range of estimated widths for the 410 km discontinuity (4–35 km) could, therefore, be explained by the olivine to wadsleyite transformation in a peridotite composition over a range of quite plausible mantle temperatures and H 2O contents. The wadsleyite to ringwoodite transformation in peridotite mantle was calculated to take place over an interval of 20 km at 1400°C. This transformation yield was also found to be near linear.