Evolution of Earth’s upper mantle layers in the Magma Ocean Model

Planetary formation theory suggests that the Earth's upper mantle (UM) layers originated from a single silicate proto-mantlea convecting magma ocean (MO).Because of the complexity of the gravitational instability of a solidifying magma ocean, numerical modeling studies have failed to show the formation of a chemically differentiated mantle that can be tectonically active.We present a hypothesis for the evolution of the upper mantle from the magma ocean state, consistent with the known experimental high-pressure studies on peridotitic melts.By theoretically analyzing the gravitational instability of the primordial upper mantle, we show that the peridotitic melt in the upper mantle crystallized to form cumulates of olivine, pyroxene, and garnet, creating a stable solid layerthe lithosphere at ~4.4 Ga.The remaining magma of basaltic composition, containing substantial Ca, Fe, and Al, separated into two partsthe silica-saturated mafic melt over the lithosphere and the olivinecontaining dense mafic melt below.The surface melt crystallized and underwent partial melting by the heat flux from below, creating the TTG melt, thus giving rise to the early (~4.3 Ga) continental crust, which is now found in old cratons.We propose that the partial melting of the transition zone (TZ) material, emplaced at 220-250 km in the gravitational overturn process, created the asthenosphere.The coexisting melt ascended and accumulated below the lithosphere to form the Low-Velocity Zone (LVZ), and the unmoved restite formed the solid asthenosphere.The transformation of olivine into a spinel structure below 410 km and the perovskite structure at 660 km, formed the lower boundary of the upper mantle.