Iron partitioning in a pyrolite mantle and the nature of the 410-km seismic discontinuity

Pyrolite1 is a hypothetical mixture of distinct minerals which iswidely believed to represent the composition of the Earth's mantle. The main pressure-induced phase transformations of the olivine component of pyrolite occur at about 13.5 GPa (α to β) and 24 GPa (γ to MgSiO3-rich perovskite + magnesiowüstite)2,3, which are thought to be responsible for the seismic discontinuities at 410 and 660 km depths in the mantle. Recent seismological studies, however, have demonstrated that the 410-km seismic discontinuity is sharper in some areas than that expected from the α to β transformation in mantle olivine with a fixed composition4,5,6,7. Moreover, some mineral physics studies suggest that the seismic velocity jump at the 410-km discontinuity is inconsistent with that associated with the α to β transformation in olivine8,9. Here we present a phase equilibria study of a material having pyrolite composition at pressures of 6–16 GPa. We found that the iron content in olivine changes significantly with increasing pressure, as a result of the formation of a relatively iron-rich majorite phase at these pressures. This variation in iron content can overcome, or at least reduce, both of the above difficulties encountered with the pyrolite model of mantle composition, by showing that the component mineral systems cannot be treated as separate.