Melting Behavior of B1 FeO Up To 186 GPa: Existence of FeO‐Rich Melts in the Lowermost Mantle

FeO is an important component in both mantle silicates and core iron alloys. Understanding its melting behavior and physical properties is crucial for exploring the chemistry and physics of our planet. Here we report the melting curve of FeO up to 186 GPa from laser‐heating experiments in a diamond‐anvil cell coupled with synchrotron X‐ray diffraction (XRD) techniques. In‐situ observations of both temperature plateau and changes in XRD patterns were used as primary melting criteria. The ex‐situ examination of a recovered sample shows consistent melting temperatures of FeO with in‐situ determinations. Our melting curve of FeO agrees with existing low‐pressure data within uncertainties and is much lower than earlier experimental results above 100 GPa including those extrapolated by Lindemann's law. Our results indicate that FeO‐rich materials could be present as melts coexisting with surrounding solids in the lowermost mantle, providing plausible explanations to the seismically observed ultra‐low velocity zones. Plain Language Summary As an endmember of mantle silicates, the melting behavior of FeO is critical to understand the early magma ocean crystallization as well as present mantle signatures. However, the question on the high‐pressure melting curve of FeO remains because of the use of different melting criteria in earlier experiments. In this study, we conducted melting experiments on FeO up to 186 GPa in a laser‐heated diamond‐anvil cell using both in‐situ synchrotron X‐ray diffraction techniques and supplementary ex‐situ analyses of a recovered sample. Our results show that the melting temperature of FeO is 3,440 ± 250 K at 135 GPa, the Earth's core‐mantle boundary (CMB) pressure. The MgO‐FeO‐SiO2 melting phase diagram shows that fractional crystallization of mantle silicate melts would end up with a cotectic ternary minimum containing up to 95 mol% FeO, which would have a similar or slightly lower melting temperature than FeO. Considering the temperature at present‐day CMB, we anticipate that FeO‐rich materials, possible products of late‐stage magma ocean crystallization, would still exist as melts with surrounding solids at the bottom of the mantle. Due to its liquid nature and high density, the existence of FeO‐rich melts above the CMB could provide important insights for explaining the seismologically observed ultra‐low velocity zones. Key Points The melting curve of FeO was determined up to 186 GPa based on in‐situ observations with supplementar