Non-standard subduction of gabbroic lithosphere into gabbroic mush ocean

Abstract

Plate tectonics efficiently transports heat on the modern Earth. It will cease about 1 billion years in the future when the mantle becomes too cold to melt extensively at ridge axes. Conversely, the high temperature of the Earth's interior produced thick oceanic crust on the early Earth, which made standard subduction difficult. Yet a non-standard form of subduction may have occurred. The Earth self-organized so that the upper about 100 km was gabbroic mush, capped by gabbroic lithosphere (away from continents). That is, ridge axes resembled very thick modern fast ridge mush chambers. The gabbroic lithosphere, once aged, cooled, and thickened, subducted into the mush. It transformed to eclogite at about 100 km depth and continued to descend. The need for gabbro to transform to dense eclogite buffered the thickness of the mush to 100 km. The mantle temperature was comparable to that of modern hotspots, 200-300 K above modern MORB sources. Melting of peridotite below 100-km depth determined the input composition to the gabbroic layer. The Earth may have alternated between episodes of mush ocean and intervening plate tectonic episodes where the interior heated up from radioactivity. Non-standard subduction is very efficient at generating Si-rich melts because the wedge above the slab is gabbroic mush rather than peridotite. Refractory olivine-rich continental lithosphere may have formed above the slab at greater depths. This hypothesis provides for the coexistence of structures resembling those formed at plate boundaries with voluminous arc magmas formed by direct hydrous melting of sources with gabbroic composition. It also provides for global periods of rapid tectonics and heat loss.