Geodynamic models of early Earth crust formation: Stagnant-lid, plate tectonics, or something in between?
Bradford Foley
Pennsylvania State University
The style of tectonics that operated on the Hadean and Archean Earth is poorly constrained and highly debated. One key feature of the early Earth geologic record that can shed light on this issue is the predominance of the trondhjemite-tonalite-granodiorite suite of felsic rocks, or TTGs. TTGs were formed commonly through the Archean, making up the bulk of early Earth continental crust, but are rare at more modern times. TTGs have been proposed to form via multiple mechanisms, including remelting at the base of a thick crustal pile, a mechanism that does not require subduction and plate tectonics, or melting of downgoing crust in a subduction zone. I present a range of geodynamic models combined with geochemical observations to test these different scenarios. Hf isotopes recorded in zircons from TTGs provide information on the residence time at the surface for the mafic precursor crust, prior to its remelting to form TTGs. Using this constraint combined with a simple model of the geotherm in a thick crustal pile, I show that only very slow rates of volcanism and crustal burial are consistent with the geologic record during Earth’s earliest phase of continental crust building, prior to ~3.6 Ga. Previously proposed “heat pipe” models with very rapid rates of extrusive volcanism and crustal burial are not consistent with the Hf isotope observations. Later in the Archean, geochemical indicators are more broadly consistent with TTG generation via subduction. However, recent geodynamical models have called into question whether subducting crust can melt as required for a subduction model of TTG genesis. I show that when grain size evolution is considered as a mechanism for forming plate boundaries, the early Earth is characterized by sluggish, drip-like subduction due to plate boundaries being effectively stronger than on the modern Earth. This sluggish form of subduction allows sinking crust sufficient time to heat up, melt, and produce TTGs.
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