Hafnium isotopes suggest a volcanically quiescent Hadean Earth

How the early Earth lost its internal heat remains a subject of debate. Early Earth may have been characterized by extensive magmatism due to a hot mantle or been more volcanically quiescent, where heat conduction through the lithosphere dominates. The primary mode of heat loss would then strongly influence crust formation and thermal evolution. Here, we show a novel approach to evaluate rates of magmatism involved in the generation of felsic crust by integrating numerical simulations of crustal evolution with a Hf isotopic dataset from zircons. We focus on continental crust production before ~3.6 Ga, when geochemical indicators suggest genesis via melting within a thick mafic crustal plateau. Our analysis indicates that in such a setting mantle melt production and extrusive volcanism remained limited before 3.6 Ga. We find this quiescent volcanism is necessary to produce negative εHf signatures, through preservation and remelting of an early formed protocrust. Our models indicate that heat carried by eruption could not have been the primary mode of heat loss (< 10 mW/m ² ). Rather than a heat-pipe dominated by rapid volcanic resurfacing, the early Earth was likely more quiescent, with heat loss by conduction across the lithosphere.