Continental basalts track secular cooling of the mantle and the onset of modern plate tectonics

Abstract The temperature of the convecting mantle exerts a first-order control on the tectonic behaviour of Earth’s lithosphere. Although the mantle has likely been cooling since the Archaean eon (4.0–2.5 billion years ago), how mantle temperature evolved thereafter is poorly understood. Here, we apply a statistical analysis to secular changes in the alkali index (A.I. = whole-rock (Na2O + K2O)2/(SiO2 – 38) as weight%) of intracontinental basalts globally to constrain the evolution of mantle potential temperature (Tp) over the past billion years. During the early Neoproterozoic, Tp remained relatively constant at ~1450 °C until the Cryogenian (720 to 635 million years ago), when mantle temperature dropped by ~50 °C over <180 million years. This remarkable episode of cooling records the onset of modern-style plate tectonics, which has been suggested to have been triggered by a dramatic increase in the supply of sediments to lubricate trenches during the thawing of the Snowball Earth.