Deep Water Cycling and the Multi‐Stage Cooling of the Earth

Abstract

AbstractPaleo‐temperature data indicates that the Earth's mantle has not cooled at a constant rate. The data show slow cooling from 3.8 to 2.5 Ga followed by more rapid cooling until the present. This has been argued to indicate a transition from a single plate mode to a plate tectonics. However, a tectonic change may not be necessary. Multistage cooling can result from deep water cycling coupled to mantle convection. Melting and volcanism removes water from the mantle (degassing). Dehydration tends to stiffen the mantle, which slows convective vigor and plate velocities causing mantle heating. Higher mantle temperature tends to lower mantle viscosity and increase plate velocities. If these two tendencies are in balance, then mantle cooling can be weak. Breaking this balance, via a switch to net mantle rehydration, can cool the mantle more rapidly. We use coupled water cycling and mantle convection models to test the viability of this hypothesis. Within model and data uncertainty, the hypothesis that deep water cycling can lead to a multi‐stage Earth cooling is consistent with data constraints. Probability distributions, for successful models, indicate that plate and plate margin strength play a minor role for resisting plate motions relative to the resistance from interior mantle viscosity.