The redox state of the Earth’s lower mantle: combined evidence from natural mineral inclusions in superdeep diamonds and experimental predictions

The redox state of the Earth’s interior (i.e., the oxygen fugacity, fo₂) is related to the Fe speciation (Fe²⁺, Fe³⁺) in mantle rock-forming minerals and controls the speciation of volatiles like carbon at depth. To date, the fo₂ of the lower mantle has been mostly constrained by HP-T experiments, due to the extreme rarity of natural samples represented by mineral inclusions in sub-lithospheric diamonds. Experimental evidence suggests that the lower mantle is reduced and saturated in Fe(-Ni) metal (about 1 wt%). However, coexisting minerals like ferropericlase and bridgmanite are predicted to contain 0.02 and 0.6 of Fe³⁺ /∑Fe, respectively. A slight increase of Fe³⁺ /∑Fe (less than 1 wt%) is expected in the case of fo₂ > iron-wüstite buffer. This would imply the complete oxidation of Fe(Ni) alloys promoted by reduction of carbonates (either fluids or melts). The finding of carbonates trapped in sub-lithospheric diamonds is natural evidence of the (local) oxidative redox state of the deep and inaccessible lower mantle and this is enhanced by the lack of metallic inclusions coexisting with Fe³⁺-poor ferropericlase in sublithospheric diamonds. Moreover, the variation of Fe³⁺ in bridgmanite appears, at least currently, to be better explained by its crystal chemistry while the effect of pressure and fo₂ remains unclear, mainly due to the lack of oxybarometers applicable to lower mantle assemblages.

In this study, we combined an experimental investigation of the Fe³⁺/∑Fe in ferropericlase and bridgmanite equilibrated at known high pressure, temperature and oxygen fugacity conditions with Fe³⁺ /∑Fe measurements conducted on bridgmanite(-like) and ferropericlase inclusions in sublithospheric diamonds from Rio Sorriso and São Luís (Brazil) and Kankan (Guinea). Some inclusions are composite for which the Fe³⁺/∑Fe was determined by in situ synchrotron Mössbauer source spectroscopy and the bulk Fe³⁺ /∑Fe determined.

Our preliminary results show a discrepancy between natural inclusions and experimental products in terms of i) modal abundance of ferropericlase and bridgmanite, likely related to their diverse role in diamond formation (redox) processes; ii) chemical compositions expected for both peridotitic and metabasaltic parageneses; and iii) Fe³⁺ /∑Fe content.