Diamonds reveal subducted slab harzburgite in the lower mantle

Characterizing compositional heterogeneity in Earth's lower mantle is critical to under-standing its dynamics. Three low-nitrogen diamonds from Koffiefontein (South Africa), con-taining inclusion assemblages of ferropericlase +/- orthopyroxene +/- magnesite, constrain dia-mond formation in an Mg-rich lower-mantle environment. Ferropericlase inclusions have Mg# 82.7-88.5 and orthopyroxene inclusions (retrogressed bridgmanite) have Mg# 95.0-95.1 and mantle-like delta 18O of +5.6 parts per thousand +/- 0.2 parts per thousand. Magnesite included in one diamond implicates carbonated fluids in diamond formation. High Mg# and low Ca, Al, and Na of the assemblage indicate a melt-depleted meta-harzburgitic environment, in contrast to more fertile compositions ex-pected for primitive lower mantle. Extremely low Ca in orthopyroxene inclusions may reflect a combination of melt depletion and low equilibration temperatures at the time of trapping. Inclusion compositions implicate subducted oceanic slab meta-harzburgite as the host for diamond growth. Mantle-like delta 18O of the orthopyroxene inclusions indicates unaltered oce-anic lithosphere. Similar melt-depleted characteristics in lower-mantle inclusion assemblages worldwide support that meta-harzburgite is the dominant host of lower-mantle diamonds.