Redox evolution of the Earth caused by a multi-stage formation of its core

A model of multi-stage formation of the Earthʹs core is presented allowing consistent solution to the problem of deficiency of heat sources in the Earthʹs thermal balance and problem of redox evolution of the Earthʹs mantle. The additional heat yield and oxidative evolution of the mantle are coordinated with continued growth of the core. It is suggested that after early formation of the major part of the core (ca. 95%) – within the first 100 million years of Earthʹs life – the growth of the core slowly continued. Following the exhaustion of native metal and sulfide iron, the core build-up proceeded at the expense of the mantle FeO. As partial disproportionation of iron in Fe2+O to Fe0 and Fe23+O3 may occur in the deep mantle and at the core–mantle boundary, this brought about the gradual oxidization of the mantle. This stage lasted the next 150–300 million years. By the end of this period the mantle redox potential transformed so that the state of oxidization of the upper mantle approached the level of QFM buffer. At the same period the composition of the atmosphere must have been changed from a reduced (containing CH4, CO, NH3) to a neutral one (containing CO2, N2). The final and the longest stage of the core build-up did not involve significant changes in its redox potential and is still under way. The slow core growth is implemented due to the mantle resources in FeO. The process of dissolution of FeO, which is delivered to the core boundary by the mantle convective downflow, ensures generation of heat maintaining the super-adiabatic temperature gradient in the mantle and eliminating apparent deficiency of heat sources in the Earthʹs thermal balance. Some other geological consequences of the model are considered.