Early Earth zircons formed in residual granitic melts produced by tonalite differentiation

Abstract

Laurent et al. (2021) suggested that zircons from the Paleoarchean Barberton tonalite-trondhjemite-granodiorites (TTGs) did not crystallize from TTG melts, but from low-temperature (near-solidus), residual granitic melts resulting from extensive fractional crystallization and cooling of a parental tonalitic magma. This is partly based on their previous interpretation that the Barberton TTGs and associated volcanic rocks do not represent magma compositions (Laurent et al., 2020). They further suggested that detrital Hadean zircons from the Jack Hills (Australia) have similar Ti and Hf contents to the Barberton TTG zircons, so might have also crystallized from such residual melts. These interpretations have far-reaching implications for the origin of TTGs and thus continental crust in the early Earth. However, a detailed examination indicates that these interpretations are not conclusive. We wish to discuss three points