Enhanced deep carbon cycle marked by the upsurge of silica-undersaturated nephelinitic magmatism at the Proterozoic-Phanerozoic boundary

Abstract

Abstract The temperature of the upper mantle was a principal factor controlling the style of plate tectonics and influencing magmatism and metamorphism on Earth over geological history. Recent studies emphasized that Earth’s tectonic style has transited into the modern plate tectonics since the late Neoproterozoic, which is characterized by a global network of plate boundaries with deep and cold oceanic plate subduction. However, the consequence of the establishment of modern plate tectonics to Earth’s mantle temperature and deep carbon cycle has not been fully understood. Here we apply statistical analysis on the geochemical data of continental igneous rocks 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 transporting to 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 declined 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.