Hadean zircon formed due to hydrated ultramafic protocrust melting
Résumé
Hadean zircons, from the Jack Hills (JHZ) and other localities, are currently the only window into the earliest terrestrial felsic crust, whose formation remains enigmatic. Based upon new experimental results, generation of such early crust has been hypothesized to involve the partial melting of hydrated peridotite interacting with basaltic melt at low pressure (<10 km), but it has yet to be demonstrated that such liquids can indeed crystallize zircons comparable to JHZ. Thermodynamic and geochemical modeling are used here to test this hypothesis. The predicted zircon saturation temperatures of <750°C together with the model zircon Th, U, Nb, Hf, Y and rare-earth element (REE) contents at 700°C, δ18OVSMOW signatures and co-crystallizing mineral assemblage are compared to those of the JHZ. This comparison is favorable with respect to crystallization temperature, most trace element contents and mineral inclusions in zircon. Discrepancy in δ18OVSMOW signatures may be explained by hotter conditions of Hadean protocrust hydration. Finally, this work supports the idea that felsic magma generation at shallow depth involving a primordial weathered ultramafic protocrust and local basaltic intrusions is indeed a viable mechanism for the formation of felsic crust on the early Earth.
Domaines
Planète et Univers [physics]Origine | Fichiers produits par l'(les) auteur(s) |
---|