Low Fe3+ in Bridgmanite and Possible Existence of an Oxidizing Layer in the Mid Mantle

Abstract:

Earlier multi-anvil experiments suggested a large amount of Fe³⁺ (60-70%) in bridgmanite (Bm) through charge disproportionation of Fe²⁺ to Fe³⁺ and metallic iron at the pressure-temperature (P-T) conditions of the topmost lower mantle (<25 GPa). Similar amount of Fe³⁺ has been recently documented at higher pressures but without redox control. If metallic iron can be stabilized, the lower mantle will have very low oxygen fugacity. However, the inclusions in lower-mantle diamonds suggest large variations in oxygen fugacity to much more oxidizing conditions. We mixed (Mg_0.75Fe_0.2Al_0.05)(Al_0.05Si_0.95)O₃ starting materials with 2-5 wt% metallic iron, in order to ensure reducing conditions during the synthesis of Bm in the laser-heated diamond-anvil cell. X-ray diffraction and Mossbauer spectroscopy were performed at APS. The recovered samples were analyzed using energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) at ASU. We found that 60-70% of iron is Fe³⁺ in Bm at P<40 GPa and P>70 GPa, consistent with previous studies. However, Bm synthesized at 40-70 GPa contains very little Fe³⁺ (~10%), indicating that charge disproportionation does not occur at depths between 1000 and 1700 km in the lower mantle. The low metallic iron layer (LMIL) can be oxidized in early Earth, because of unbalanced Fe³⁺ and metallic iron in lower-mantle flow by core formation. The oxidizing nature of the layer may impact the siderophile element partitioning between metal and silicates if magma ocean was sufficiently deep. The layer can be further oxidized through solid mantle convection because of injection of oxidizing surface materials. The elevated oxygen fugacity will potentially make LMIL a distinct geochemical reservoir. Due to oxidizing conditions, carbon will form diamond instead of FeC in the layer. Strong partitioning of Fe²⁺ into ferropericlase (Fp) and the low content of Fe³⁺ in Bm will enrich Fp with iron (Fe²⁺) in LMIL. Because the Fe³⁺ content changes gradually, the upper and lower boundaries of the layer would not produce seismically detectable discontinuities. The layer may have low thermal conductivity due to the low Fe³⁺ content in Bm, possibly affecting dynamics of the mid mantle.