Thermochemical differentiation and the emergence of Precambrian cratons

Abstract:

The record of the lithosphere emergence in the early Earth ranges from large melting and complete recycling, in the Hadean, to later stabilisation and reworking, in the Archaean. Here, we propose a mechanism for the formation of large volumes of melt and depleted lithospheric mantle and their recycling, followed by progressive differentiation and stabilisation to become cratonic lithospheric mantle (CLM). Numerical models show large decompression melting of a hot, early Earth mantle beneath stretching lithosphere, where melt extraction leaves large volumes of depleted residue at depth. The dehydrated, stiffer mantle resists further deformation, forcing strain migration and cooling, thereby assimilating depleted mantle into the lithosphere. The negative feedback between strain localisation and stiffening sustains long-term diffused extension and emplacement of large amounts of depleted CLM. The formation of CLM at low-pressure and its deeper re-equilibration reproduces the evolution of Archaean lithosphere constrained by depth-temperature conditions, while large degrees of depletion and melt volumes in Archaean cratons are best matched by models with lower lithospheric strength. Under these conditions, otherwise viable for plate tectonics, thermochemical differentiationeffectively prevents yielding and the formation of margins: rifting and subduction are short-lived and embedded in the cooling CLM as relict structures, reproducing environments for recycling and reworking found in Archaean cratons. While prone to major melting and extensive recycling during an early stage of ~500 Myr, modelled lithospheres progressively differentiate and stabilise, then recycling and reworking become episodic. Early major melting and recycling hold the key for production and loss of primordial Hadean lithosphere and crust, whereas later stabilisation and episodic reworking provides a context for the making of continental cratons in the Archaean.