Melt inclusions and host olivine from 3.27 Ga komatiitic lava flows (Barberton Greenstone Belt, South Africa)

The rates of continental crust growth and recycling on early Earth remain controversial because materials in the ancient crust and mantle have been altered, or even erased, by ongoing geodynamical processes. Melt inclusions in minerals are pockets of magma that have been trapped and shielded from the external environment. Where found within Archean high-Mg olivine—the first mineral to crystallize in mantle-derived melts—these inclusions can provide an unaltered glimpse of the geochemical state of the early Earth’s mantle. We discovered an unprecedented unradiogenic Sr mantle source component (87Sr/86Sr=0.69932±0.00024, 95% c.i. here and below) in mantle-derived melts trapped in olivine from ca. 3.27 Ga komatiitic lava flows (Barberton Greenstone Belt, South Africa). This component shows a 4.31±0.19 Ga model age combined with significant chemical fractionation (Nb/U=36.9±1.5, Ce/Pb=16.7±1.1), translating to the extraction, by the late-Hadean, of 80%±16% of the mass of present-day continental crust, assuming whole mantle processing. That agrees with the results of our geodynamic models explaining the Nb/U and Ce/Pb data by the production of 40 to 70% of the present-day continental crust mass during the Hadean in a fluctuating mobile-lid tectonic regime with several tens of million years-long periods of massive subduction induced by mantle plumes.