The relationship between mantle potential temperature and oceanic lithosphere buoyancy

The Earth's mantle potential temperature (T-p) is thought to have cooled by similar to 250 degrees C since the Archean, causing a progressive change in both the structure and composition of oceanic lithosphere. These variables affect the negative buoyancy of subducting slabs, which is known to be an important force in driving plate motions. However, the relationship between T-p and slab buoyancy remains unclear. Here, we model the formation and subduction of oceanic lithosphere as a function of T-p, to investigate how T-p influences the buoyancy of subducting slabs, and by extension how buoyancy forces may have changed through time. First, we simulate isentropic melting of peridotite at mid-ocean ridges over a range of T-p (1300-1550 degrees C) to calculate oceanic lithosphere structure and composition. Second, we model the thermal evolution of oceanic plates undergoing subduction for a variety of scenarios (by varying lithospheric thickness, slab length and subduction velocity). Finally, we integrate the structural, compositional and thermal constraints to forward model subduction metamorphism of oceanic plates to determine downgoing slab density structures. When compared with ambient mantle, these models allow us to calculate buoyancy forces acting on subducting slabs. Our results indicate that oceanic lithosphere derived from hotter mantle has a greater negative buoyancy, and therefore subduction potential, than lithosphere derived from cooler mantle for a wide range of subduction scenarios. With respect to the early Earth, this conclusion supports the viability of subduction, and models of subduction zone initiation that invoke the concept of oceanic lithosphere being primed to subduct. However, we also show that decreases to lithosphere thickness and slab length, and reduced crustal hydration, progressively reduce slab negative buoyancy. These results highlight the need for robust estimates of early Earth lithospheric properties when considering whether subduction was operative at this time. Nevertheless, our findings suggest that subduction processes on the early Earth may have been uniformitarian. (C) 2019 Elsevier B.V. All rights reserved.