Initiation of plate tectonics from post‐magma ocean thermochemical convection

Abstract

AbstractLeading theories for the presence of plate tectonics on Earth typically appeal to the role of present day conditions in promoting rheological weakening of the lithosphere. However, it is unknown whether the conditions of the early Earth were favorable for plate tectonics, or any form of subduction, and thus, how subduction begins is unclear. Using physical models based on grain‐damage, a grainsize‐feedback mechanism capable of producing plate‐like mantle convection, we demonstrate that subduction was possible on the Hadean Earth (hereafter referred to as proto‐subduction or proto‐plate tectonics), that proto‐subduction differed from modern day plate tectonics, and that it could initiate rapidly. Scaling laws for convection with grain‐damage show that though either higher mantle temperatures or higher surface temperatures lead to slower plates, proto‐subduction, with plate speeds of ≈1.75 cm/yr, can still be maintained in the Hadean, even with a CO2 rich primordial atmosphere. Furthermore, when the mantle potential temperature is high (e.g., above ≈2000 K), the mode of subduction switches to a “sluggish subduction” style, where downwellings are drip like and plate boundaries are diffuse. Finally, numerical models of post‐magma ocean mantle convection demonstrate that proto‐plate tectonics likely initiates within ∼100 Myr of magma ocean solidification, consistent with evidence from Hadean zircons. After the initiation of proto‐subduction, non‐plate‐tectonic “sluggish subduction” prevails, giving way to modern style plate tectonics as both the mantle interior and climate cool. Hadean proto‐subduction may hasten the onset of modern plate tectonics by drawing excess CO2 out of the atmosphere and cooling the climate.