Stability of the MgSiO3 analog NaMgF3 and its implication for mantle structure in super-Earths

Abstract

First-principles calculations on MgSiO(subscript 3) suggested a breakdown into MgO + SiO(subscript 2) at pressure above 1000 GPa with an extremely large negative Clapeyron slope, isolating the lowermost mantles of larger super-Earths (∼10M ⊕) from convection. Similar calculations predicted the same type of breakdown in NaMgF(subscript 3) to NaF + MgF2 at 40 GPa, allowing for experimental examination. We found that NaMgF(subscript 3) is stable to at least 70 GPa and 2500 K. In our measurements on MgF(subscript 2) (an SiO(subscript 2) analog), we found a previously unidentified phase (“phase X”) between the stability fields of pyrite-type and cotunnite-type (49–53 GPa and 1500–2500 K). A very small density increase (1%) at the pyrite-type → phase X transition would extend the stability of NaMgF3 relative to the breakdown products. Furthermore, because phase X appears to have a cation coordination number intermediate between pyrite-type (6) and cotunnite-type (9), entropy change (ΔS) would be smaller at the breakdown boundary, making the Clapeyron slope (dP/dT = ΔS/ΔV) much smaller than the prediction. If similar trend occurs in MgSiO(subscript 3) and SiO(subscript 2), the breakdown of MgSiO(subscript 3) may occur at higher pressure and have much smaller negative Clapeyron slope than the prediction, allowing for large-scale convection in the mantles of super-Earth exoplanets.