Eoarchaean Tectonics: New constraints from high pressure-temperature experiments and mass balance modelling

Four billion years ago the formation and preservation of the Earth’s first continental crust marked a new stage in the long-term geochemical evolution of our planet. This continental material is predominantly composed of plagioclase-rich granitoids that form Eoarchaean Tonalite and Trondjhemite (ETT) rock suites. There are two plausible overarching models for generating ETT: by a variety of intraplate melting mechanisms or through the onset of primitive subduction. Here we use mass balance modelling to show that ETT can only be derived from the hydrous melting of mafic rocks in convergent margins. These mafic source regions could be composed of relatively undepleted (oceanic plateau-like) non-arc metabasalts with or without residual rutile. Melts derived from these metabasalts subsequently mix with a slab-related component to form ETT magmas. Additionally, ETT can be formed from partial melting of metabasalts with geochemical affinities similar to modern island arc basalts. We propose that a primitive form of plate tectonics began on the surface of Hadean magma ocean(s), in a similar way to the plate-like motions observed on modern lava lakes, and subsequently evolved into the plate tectonics operating today. Hydrous melting of subducted basaltic crust and the formation of the proto-continental ETT crust would have followed the onset of accumulation of liquid water on the Earth’s surface.