Homogenization of the mantle by convective mixing and diffusion

Mantle convection stirs and homogenizes the subducted oceanic lithosphere with the convecting mantle. Convective mixing stretches and thins the subducted oceanic crust from an original thickness of 6 km to a thickness of 2 cm or less. The thinned, subducted oceanic crust can be observed as pyroxenite bands in high-temperature peridotite massifs. On the scale of centimeters, the bands are destroyed by diffusive processes. In this paper, the homogenization of the subducted oceanic crust with the depleted mantle is modeled by considering the combined problem of thinning and diffusion at a stagnation point. A layer of different composition from the surrounding material is thinned by normal strain until its identity is destroyed by diffusive processes. Thinning dominates the destruction of a layer if ka 2/D > 1, where k is the strain rate, 2a is the initial layer thickness, and D is the diffusivity. Diffusion dominates if ka2/D < 1. Our results indicate that the mantle is homogeneous at the centimeter scale. This conclusion is insensitive to variations in the strain rate and the diffusivity, and it is supported by isotopic studies of high-temperature peridotite massifs. Variations in isotope ratios in MORB can be attributed to the imperfect homogenization of the MORB source region.