Ferropericlase breakdown drives mantle weakening in super-Earths

Prevailing models of super-Earth interiors are commonly built on scaled-up versions of Earth’s mineralogy. This framework, however, may neglect a critical possibility: key minerals such as ferropericlase, a major iron-bearing mantle mineral, may become unstable under the extreme conditions of more massive rocky worlds. Here we show that ferropericlase segregates into Fe-rich and Fe-poor oxide domains near 200 GPa, based on laser-heated diamond-anvil cell experiments up to 3300 K, combined with X-ray diffraction, recovered-sample chemical analysis, and first-principles calculations. In super-Earths above ~ 1.5 Earth masses, this demixing may promote the formation of an FeO-enriched, ultra-low-viscosity layer in the deep mantle, a feature absent on Earth. Ferropericlase breakdown may therefore produce distinct mineralogical and rheological structures by weakening the lowermost mantle and modifying mechanical coupling between the mantle and core, with implications for the interior dynamics and long-term evolution of massive rocky exoplanets. This study shows that ferropericlase, a major lower-mantle mineral, becomes unstable and undergoes phase separation into Fe-rich and Mg-rich domains near 200 GPa, with implications for deep interior structure and the formation of mechanically weak layers in massive super-Earth mantles.