Subduction and volatile recycling in earth’s mantle

Abstract

The subduction of water and other volatiles into the mantle from oceanic sediments and altered oceanic crust is the major source of volatile recycling in the mantle. Until now, the geotherms that have been used to estimate the amount of volatiles that are recycled at subduction zones have been produced using the hypothesis that the slab is rigid and undergoes no internal deformation after subduction. We consider the effects of the strength of the slab using two‐dimensional calculations of a slab‐like thermal downwelling with an endothermic phase change. Because the rheology and composition of subducting slabs are uncertain, we consider a range of Clapeyron slopes which bound current laboratory estimates of the spinel to perovskite plus magnesiowüstite phase transition and simple temperature‐dependent rheologies based on an Arrhenius law diffusion mechanism. Phase transitions can have two pronounced effects on subducting slab deformation and the resulting geotherms. First, an endothermic phase transformation can inhibit the vertical descent of the slab. If the slab is weak, this can lead to large deformation, even in the upper 200 km of the slab. Second, the phase transformation can slow the subduction velocity (and plate velocity) of the entire slab, a more pronounced effect than the slab deformation. Because the initial thermal structure of the descending lithosphere, the volatile content, and subduction velocity all affect the viscosity of the slab, it is likely that subduction zones may behave differently‐some with more pronounced pile‐up and avalanche periods and some where the subduction velocity is more uniform with time. These mechanisms create a highly uneven distribution of recycled components in the mantle within relatively short periods of time in Earth’s history.