Pressure enhances sodium solubility in magnesiowüstite

Sodium is present in the deep Earth in subducted oceanic crust and deep fluids at percent levels, and its concentration and cycling in the Earth's lower mantle are relevant to mixing and generation of chemical heterogeneity and the deep water and carbon cycles. Experiments have demonstrated that ~1% sodium is incorporated in the (Mg,Fe)O oxide phase in pyrolite compositions at shallow lower mantle conditions. However, mechanisms of sodium incorporation and solubility at deeper pressures have not been systematically investigated. We carried out experiments and complementary density functional theory calculations on partitioning and solubility of sodium in bridgmanite and magnesiowüstite at lower mantle conditions. Experiments examined MgO sandwiched between Na2CO3 and (Fe,Mg)2SiO4 fayalite compositions sandwiched between NaCl insulating medium using the laser-heated diamond anvil cell to reach 33-128 GPa and temperatures 1600-3000 K. Synchrotron X-ray diffraction at APS 13-ID-D demonstrated the oxide phase adopted the cubic B1 structure. Scanning transmission electron microscopy with energy-dispersive X-ray spectroscopic mapping determined that recovered magnesiowüstite in equilibrium with bridgmanite was not only enriched in iron, but also in sodium. Measured sodium concentrations in magnesiowüstite are ~2 mol% in samples laser-heated at 30-70 GPa, but in samples heated at higher pressures Na-content increases dramatically to 4-12 mol% (higher than the concentration of Mg in some samples). While Na is concentrated within the oxide phase, Cl is observed to accumulate at grain boundaries in an accessory chloride phase. Possible mechanisms for incorporating Na in magnesiowüstite include substitution as NaFe3+O2, Na2O (oxygen vacancy), and NaOH.