Intraplate continental basalts over the past billion years track 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 rheology, composition, and consequently, tectonic behavior of Earth's lithosphere. Although the mantle has likely been cooling since the Archaean eon, how mantle temperature has evolved thereafter is poorly understood. Here, we apply a statistical analysis to secular changes in the alkali index [AI = whole-rock (Na2O + K2O)2/(SiO2 − 35) as weight%] of global sodic intracontinental basalts, a proxy for the degree of mantle melting, to constrain the evolution of mantle potential temperature (TP) over the past billion years. Our results show that, during the early Neoproterozoic, TP remained relatively constant until the beginning of the Cryogenian (720 Ma), when mantle temperature dropped rapidly over the following ∼180 Ma. This remarkable episode of cooling records the onset of modern-style plate tectonics characterized by continuous deep subduction of the oceanic lithosphere, consistent with the widespread appearance of blueschists in the metamorphic rock record.