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

Abstract

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.