Abstract
Earth has likely cooled by several hundred degrees over its history, which has probably affected subduction dynamics and associated magmatism. Today, the process of compositional buoyancy driven upwelling, and subsequent underplating, of subducted materials (commonly referred to as “relamination”) is thought to play a role in the formation of continental crust. Given that Archean continental crust formation is best explained by the involvement of mafic material, we investigate the feasibility of mafic crust relamination under a wide range of conditions applicable to modern and early Earth subduction zones, to assess if such a process might have been viable in an early Earth setting.Our numerical parametric study illustrates that the hotter, thicker-crust conditions of the early Earth favour the upward relamination of mafic subducting crust. The amount of relaminating subducting crust is observed to vary significantly, with subduction convergence rate having the strongest control on the volume of relaminated material. Indeed, removal of the entire mafic crust from the subducting slab is possible for slow subduction (∼2 cm/yr) under Archean conditions. We also observe great variability in the depth at which this separation occurs (80–120 km), with events corresponding to shallower detachment being more voluminous, and that relaminating material has to remain metastably buoyant until this separation depth, which is supported by geological, geophysical and geodynamical observations. Furthermore, this relamination behaviour is commonly episodic with a typical repeat time of approximately 10 Myrs, similar to timescales of episodicity observed in the Archean rock record. We demonstrate that this relamination process can result in the heating of considerable quantities of mafic material (to temperatures in excess of 900 °C), which is then emplaced below the over-riding lithosphere. As such, our results have implications for Archean subduction zone magmatism, for continental crust formation in the early Earth, and provide a novel explanation for the secular evolution of continental crust.
Highlights
The mechanisms that have led to the formation of the felsic continental crust (CC) from the mantle throughout Earth’s history are still poorly understood, but the presence of subduction systems seems to be an essential component
We show that under Archean conditions, relamination of the mafic subducting crust is viable under a wider range of subduction conditions
Two default models apply to a modern day subduction system and the other two apply to an Archean setting, in which the mantle is made 200 K hotter and the crust is 15 km thick (Abbott et al, 1994) rather than 7 km thick
Summary
The mechanisms that have led to the formation of the felsic continental crust (CC) from the mantle throughout Earth’s history are still poorly understood, but the presence of subduction systems seems to be an essential component. The composition of today’s subduction related magmas shares many similarities with the CC (Rudnick, 1995; Jagoutz and Kelemen, 2015). This applies to the most ancient CC (>2.5 Ga), there are systematic differences (Taylor and McLennan, 1995). A commonly accepted mechanism explaining the structure and composition of the CC is a basalt input model, where mantle derived basaltic arc magmas are differentiated into a felsic component, leaving an ultramafic to mafic cumulative counterpart that delaminates and is recycled back into the mantle (Rudnick, 1995).
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