Current theories suggest that the first continental crust on Earth, and possibly on other terrestrial planets, may have been produced early in their history by direct melting of hydrated peridotite. However, the conditions, mechanisms and necessary ingredients for this crustal formation remain elusive. To fill this gap, we conducted time-series experiments to investigate the reaction of serpentinite with variable proportions (from 0 to 87 wt%) of basaltic melt at temperatures of 1,250–1,300°C and pressures of 0.2–1.0 GPa (corresponding to lithostatic depths of ∼5–30 km). The experiments at 0.2 GPa reveal the formation of forsterite-rich olivine (Fo90–94) and chromite coexisting with silica-rich liquids (57–71 wt% SiO2). These melts share geochemical similarities with tonalite-trondhjemite-granodiorite rocks (TTG) identified in modern terrestrial oceanic mantle settings. By contrast, liquids formed at pressures of 1.0 GPa are poorer in silica (∼50 wt% SiO2). Our results suggest a new mechanism for the formation of the embryonic continental crust via aqueous fluid-assisted partial melting of peridotite at relatively low pressures (∼0.2 GPa). We hypothesize that such a mechanism of felsic crust formation may have been widespread on the early Earth and, possibly on Mars as well, before the onset of modern plate tectonics and just after solidification of the first ultramafic-mafic magma ocean and alteration of this primitive protocrust by seawater at depths of less than 10 km.

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Zircon (ZrSiO4) is the most frequently used geochronometer of terrestrial and extraterrestrial processes. To shed light on question of zircon survival in the Earth's shallow asthenosphere, high-temperature experiments of zircon dissolution in natural mid-ocean ridge basaltic (MORB) and synthetic haplobasaltic melts have been performed at temperatures of 1250–1300 °C and pressures from 0.1 MPa to 0.7 GPa. Zirconium measurements were made in situ by electron probe microanalyses (EPMA) at high current. Taking into account secondary fluorescence effects in zircon-glass pairs during EPMA, a zirconium diffusion coefficient of 2.87E-08 cm2/s was determined at 1300 °C and 0.5 GPa. When applied to the question of zircon survival in asthenospheric melts of tholeiitic basalt composition, the data are used to infer that typical 100 mm zircon crystals dissolve rapidly (~10 h) and congruently upon reaction with basaltic melt at pressures of 0.2–0.7 GPa. We observed incongruent (to crystal ZrO2 and SiO2 in melt) dissolution of zircon in natural mid-ocean ridge the basaltic melt at low pressures <0.2 GPa and in the haplobasaltic melt at 0.7 GPa pressure. Our experimental data raise questions about the origin of zircon crystals in mafic and ultramafic rocks, in particular, in shallow oceanic asthenosphere and deep lithosphere, as well as the meaning of the zircon-based ages estimated from these minerals. The origin of zircon in shallow (ultra-) mafic chambers is likely related to the crystallization of intercumulus liquid. Large zircon megacrysts in kimberlites, peridotites, alkali basalts, and carbonatite magmas suggest fast transport and short interaction durations between zircon and melt. The origin of zircon megacrysts is likely related to metasomatic addition of Zr into the mantle as an episode of mantle melting should eliminate them on geologically short timescales.

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 °C, together with the model zircon Th, U, Nb, Hf, Y, and rare earth element (REE) contents at 700 °C, δ18OVSMOW (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 δ18OVSMOW 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.

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