Effect of Fe3+ on Phase Relations in the Lower Mantle : Implications for Redox Melting in Stagnant Slabs
Journal of Geophysical Research - Solid Earth
Recent studies have revealed that Earth's deep mantle may have a wider range of oxygen fugacities than previously thought. Such a large heterogeneity might be caused by material subducted into the deep mantle. However, high-pressure phase relations are poorly known in systems including Fe 3+ at the top of the lower mantle, where the subducted slab may be stagnant. We therefore conducted highpressure and high-temperature experiments using a multi-anvil apparatus to study the phase relations in a
... hase relations in a Fe 3+ -bearing system at 26 GPa and 1573-2073 K, at conditions prevailing at the top of the lower mantle. At temperatures below 1923 K, MgSiO 3 -rich bridgmanite, an Fe 3+ -rich oxide phase, and SiO 2 coexist in the recovered sample. Quenched partial melt was observed above 1973 K, which is significantly lower than the solidus temperature of an equivalent Fe 3+ -free bulk composition. The partial melt obtained from the Fe 3+rich bulk composition has a higher iron content than coexisting bridgmanite, similar to the Fe 2+ -dominant system. The results suggest that strong mantle oxygen fugacity anomalies might alter the subsolidus and melting phase relations under lower mantle conditions. We conclude that (1) a small amount of melt may be generated from an Al-depleted region of a stagnant slab, such as subducted former banded-iron-formation, and (2) Fe 3+ is not transported into the deep part of the lower mantle because of its incompatibility during melting.