Hadean zircon formed due to hydrated ultramafic protocrust melting

Hadean zircons, from the Jack Hills (Western Australia) and other localities, are currently the only window into the earliest terrestrial felsic crust, the formation of which 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 Jack Hills zircon. We used thermodynamic and geochemical modeling to test this hypothesis. The predicted zircon saturation temperatures of <750 degrees C, together with the model zircon Th, U, Nb, Hf, Y, and rare earth element (REE) contents at 700 degrees C, delta O-18(VSMOW) (Vienna standard mean ocean water) signatures, and co-crystallizing mineral assemblage were compared to those of the Jack Hills zircon. This comparison was favorable with respect to crystallization temperature, most trace-element contents, and mineral inclusions in zircon. The discrepancy in delta O-18(VSMOW) signatures may be explained by hotter conditions of Hadean protocrust hydration. Our work supports the idea that felsic magma generation at shallow depths involving a primordial weathered ultramafic protocrust and local basaltic intrusions is indeed a viable mechanism for the formation of felsic crust on early Earth.

Automated summary

The study suggests that the formation of early crust may involve partial melting of hydrated peridotite interacting with basaltic melt at low pressure, providing a potential mechanism for the formation of Hadean zircons. Comparison of thermodynamic and geochemical modeling results with Jack Hills zircon suggests favorable similarities in crystallization temperature, trace-element contents, and mineral inclusions, supporting the hypothesis.