On the Relative Importance of Buoyancy and Thickening of Aging Lithosphere in Mantle Upwelling and Crustal Production Beneath Global Mid‐Ocean Ridge System

Abstract Beneath mid‐ocean ridges, mantle upwelling and decompression melting are fundamental processes contributing to the formation of oceanic crust. Previous geodynamic models have suggested that mantle upwelling driven by separating plates can be intensified by thermochemical buoyancy and the thickening of aging lithosphere, resulting in thicker crust. However, the relative contributions of these factors to crustal accretion remain uncertain. We conducted numerical modeling in three scenarios to investigate this: (a) buoyant flow models incorporating age‐dependent lithospheric thickening as a reference; (b) passive flow models incorporating age‐dependent lithospheric thickening to isolate the effect of buoyancy; and (c) passive flow models that neglect age‐dependent lithospheric thickening to isolate its effect. Models are performed under varying potential mantle temperatures (Tp), viscosity structures, and spreading rates. The model‐predicted crustal thickness suggests that both buoyancy and thickening of aging lithosphere increasingly contribute to crustal production as spreading rate decreases. However, given the range of reference mantle viscosities examined here (10 19 –10 20 Pa s), except in ultraslow spreading rates with lower reference viscosity and warmer Tp, the impact of the thickening of aging lithosphere predominates over buoyancy. Furthermore, the relative importance of buoyancy‐induced crustal production increases as spreading rates decrease. Variations in Tp further amplify the variability in crustal thickness compared to that at fast‐spreading ridges where passive upwelling dominates, when combined with 3‐D effects of buoyant flow on altering axial crustal production at ultraslow‐spreading ridge segments. This 4‐D effects of buoyant flow on melt production may explain the observed more variable crustal thickness at ultraslow‐spreading ridges.