High-Fe (Mg, Fe)O inclusion in diamond apparently from the lowermost mantle

•This (Mg, Fe)O inclusion in diamond has high Fe:Mg ratio and is enriched in Fe3+.•Despite high Fe3+, the magnesiowustite crystal also contains Fe-Ni metal.•Much of Fe3+ precipitated as high pressure polymorph of magnesioferrite.•Simplest interpretation of observations suggests diamond from base of lower mantle.

The only direct samples of Earth's lower mantle are rare inclusions in diamonds. The most abundant are (Mg, Fe)O, showing highly magnesian compositions. Here we report a transmission electron microscopy study of an exceptional inclusion with composition ∼(Mg0.35Fe0.65)O, with 5-7 vol% nanometer-sized magnesioferrite spinel precipitated on dislocations and the interface with diamond. The magnesioferrite apparently precipitated as a high-pressure polymorph from the (Mg, Fe)O host that contained an extraordinarily large concentration of Fe3+. Paradoxically, the crystal also contains blebs of Fe-Ni alloy. These observations plus the recently documented very low seismic velocities of high-Fe (Mg, Fe)O and other experimental results suggest that this inclusion may have originated in one of the ultra-low velocity zones at the base of the mantle and come to the surface through diapiric upwelling culminating in kimberlite eruption.