Oxidation potential in the Earth's lower mantle as recorded by ferropericlase inclusions in diamond

•Lower-mantle ferropericlase (fPer) inclusions were analyzed by TEM and the flank method.•Exsolved Fe3+-rich non-stoichiometric clusters comprise ∼3.64 vol.% of fPer.•The calculated Δlog⁡fO2 (IW) values for fPer vary from 1.58 to 7.76 (Δ=6.2).•Lower-mantle oxidation potential varies from the IW buffer to the FMQ buffer.

Ferropericlase (fPer) inclusions from kimberlitic lower-mantle diamonds recovered in the Juina area, Mato Grosso State, Brazil were analyzed with transmission electron microscopy, electron energy-loss spectroscopy and the flank method. The presence of exsolved non-stoichiometric Fe3+-enriched clusters, varying in size from 1-2 nm to 10-15 nm and comprising ∼3.64 vol.% of fPer was established. The oxidation conditions necessary for fPer formation within the uppermost lower mantle (P=25 GPa, T=1960 K) vary over a wide range: Δlog⁡fO2 (IW) from 1.58 to 7.76 (Δ=6.2), reaching the fayalite-magnetite-quartz (FMQ) oxygen buffer position. This agrees with the identification of carbonates and free silica among inclusions within lower-mantle Juina diamonds. On the other hand, at the base of the lower mantle Δlog⁡fO2 values may lie at and below the iron-wüstite (IW) oxygen buffer. Hence, the variations of Δlog⁡fO2 values within the entire sequence of the lower mantle may reach ten logarithmic units, varying from the IW buffer to the FMQ buffer values. The similarity between lower- and upper-mantle redox conditions supports whole mantle convection, as already suggested on the basis of nitrogen and carbon isotopic compositions in lower- and upper-mantle diamonds. The mechanisms responsible for redox differentiation in the lower mantle may include subduction of oxidized crustal material, mechanical separation of metallic phase(s) and silicate-oxide mineral assemblages enriched in ferric iron, as well as transfer of fused silicate-oxide material presumably also enriched in ferric iron through the mantle.