The Evolution Of Plate Tectonic Style And Its Impacts On The Thermal State Of The Mantle

Earth is the only known planet in the solar system where plate tectonics exists,though plate tectonics was not born with it.The evolution of plate tectonics is closely related to the formation of continental crust,the orogenesis,the evolution of thermal history and the material recycle between Earth’s surface and interior.The mantle temperature in the early Earth was relatively too high to allow the steady subduction of the oceanic slab.With the cooling of the mantle,the regime of Earth’s tectonics has transited from the early plate tectonics to modern plate tectonics,which is characterized by the deep and cold subduction network.The temperature of the convecting mantle exerts a first-order control on the rheology,composition,thickness of Earth’s lithosphere,and consequently,tectonic regime of Earth.Although the mantle has likely been cooling since the Archaean eon（4.0-2.5 billion years ago）,how mantle temperature has 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/（Si O₂–38）as weight%]of global sodic intra-continental basalts,a proxy for the pressure and temperature of magma generation,to constrain the evolution of mantle potential temperature（T_P）over the past billion years.Our results show that,during the early Neoproterozoic,T_P remained relatively constant at ca.1450°C until the Cryogenian（720-635 million years ago）,when mantle temperature dropped by ca.50°C over less than 180 million years.This remarkable episode of cooling records the onset of modern-style plate tectonics characterized by continuous deep subduction of the lithosphere,consistent with the widespread appearance of blueschists in the metamorphic rock record.The emergence of modern plate tectonics is suggested to have been triggered by a huge increase in the supply of sediments to lubricate trenches during the thawing of the Snowball Earth,which rapidly enhanced mantle cooling due to subduction of much larger volumes of cold oceanic lithosphere than previously.The onset of modern plate tectonics and its influence on the subduction of carbonates and deep carbon cycle has not been fully understood.Here we apply statistical analysis on a continental ultramafic-mafic igneous rock database and identify an increased magnitude of nephelinitic volcanism at the end of the Ediacaran.Nephelinitic rocks,a silica-undersaturated high-alkaline rock group,are mostly formed by low-degree melting of carbonated mantle sources.We link their widespread emergence with an enhanced mantle cooling event and a dramatically increased flux of crustal carbonates recycled into the mantle.The rapid cooling of the mantle was ascribed to the onset of modern-style plate tectonics with global-scale cold oceanic and continental subduction since the late Neoproterozoic.The decreased upper-mantle temperature could not only favor the low-degree melting but also allow the subduction of carbonates into the deep mantle without decarbonation at shallow depth.Considering the high oxygen fugacity feature of the nephelinitic rocks and some other high-alkaline volcanism,the establishment of modern plate tectonics and thereafter enhanced mantle cooling and deep carbon cycle might contribute to the high-level atmospheric oxygen content during the Phanerozoic.In summary,the establishment of modern plate tectonics since the late Neoproterozoic contributed to enhanced mantle cooling and deep carbon cycle.