Chemically homogeneous spin transition zone in Earth's lower mantle

► Fe-Mg partitioning between (Mg,Fe)SiO3 perovskite and (Mg,Fe)O ferropericlase is measured in situ up to 115 GPa and 2300 K by combined X-ray absorption near edge spectroscopy and X-ray diffraction. ► The spin transition zone within the Earth's lower mantle is shown to be homogeneous. ► The effect of the spin transitions in lower mantle perovskite and ferropericlase on the morphology of plumes is estimated.