Role of subduction zones and ocean ridges on mantle evolution

Abstract

This study quantitatively explore element redistribution at subduction zones and mid ocean ridges using numerical mass balance models to evaluate the roles of the subduction zone filter and the spreading ridge factory in the Earth's geochemical cycle. Our models of differs from previous works by being internally consistent with geodynamic models of modern arcs and ocean ridges that successfully explains magma geneses and include element fluxes in the residual slab and mantle components. Estimated mantle potential temperature (Tp) was ~1650 °C at 3.5?2.0 Ga and gradually decreased to ~1300 °C today. Hot subduction zones with Tp ~1650 °C have a thermal structure like modern SW Japan where high-Mg andesite is formed that is like continental crust. After 3.0?2.0 Gyr of storage in the mantle, the residual igneous oceanic crust from hot subduction zones can evolve isotopically to the HIMU mantle reservoir, the residual base of the mantle wedge to EMI, the residual sediment becomes an essential component of EMII, and the residual top of the mantle wedge can become the subcontinental lithosphere reservoir. The Common or Focal Zone component is a stable mixture of the first three residues occasionally mixed with early depleted mantle. Slab residue that recycled earlier (~2.5 Ga) can form the DUPAL anomaly in the southern hemisphere, whereas more recent recycling (~1.7 Ga) focused in the northern hemisphere. The east-west heterogeneity of the depleted upper mantle involves sub continental mantle except in the Pacific. We also examined Sr-Nd-Hf-Pb isotopic evolution of the Earth's depleted mantle using a melting model for mid ocean ridges coupled with subsequent isotopic evolutions of the residual mantle. The two-stage mantle evolution model, including primitive mantle formation occurred at ~4.57 Ga followed by MORB melt depletion at ~2.0 Ga (average age), accounted for generation of the depleted MORB source mantle today. The high-mantle Tp age by ~2.0 Ga coincides with formation period of the majority continental crust and enriched mantle reservoirs from the hot Archean subduction zones. All above results suggest changes in mode of mantle convection, vigorous before ~2.0 Ga and gradually sluggish after that time. This change may characterize the thermochemical evolution of the Earth's mantle.