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The debate about early Earth differentiation focuses on the processes responsible for the formation of protocrust(s) and continental crust of felsic (SiO₂ ≥ 55 weight %) composition. One aspect of this debate is how Hadean zircons fit into an ultramafic environment. On the basis of experiments, thermodynamic modeling, and elemental partitioning, we show that felsic melts could have been generated by shallow interaction between primordial serpentinized peridotite and basaltic magmas on Earth and Mars. On the basis of the hafnium isotopic evolution of Hadean detrital zircons worldwide, we infer that these interactions allowed for the formation of extensive Hadean felsic crust (4.4 to 4.5 billion years ago), which, in turn, would account for up to 50% of the present continental crustal mass. A similar process may have occurred on Mars. The serpentinized protocrust had a dual role in the primitive planetary environment: to provide ingredients for the continental crust and to enable life to emerge on water-bearing terrestrial planets. 

Debate regarding early Earth differentiation focuses on the fate, nature, origin, volume, and processes responsible for protocrust(s) and felsic crust formation. One specific aspect of this debate is how Hadean zircons and their felsic parental magmas fit with an expected ultramafic environment. Based on our new experiments, thermodynamic modeling, and elemental partitioning, we infer that felsic liquids could have been generated by shallow (< 20 km) interaction between primordial hydrated peridotite (serpentinite) and basaltic magmas. Felsic melts (SiO2 ≥ 55 wt%) can be generated at a maximum melt fraction of 0.4 when starting serpentinite:basalt mass ratio is high (i.e., higher than 1.5:1). Here we show that felsic melts obtained in our experimental runs can account for the Hf isotope evolutionary array displayed by Hadean detrital zircons worldwide. We propose that open system interactions between serpentinite and basaltic melts at the end of the magma ocean stage after magma degassing and water ocean precipitation allowed the formation of extensive early Hadean felsic crust (4.4 - 4.5 Gy ago). Our calculations indicate that this felsic crust accounts for up to 50% of present-day continental crust mass. The abundant production of primordial felsic crust throughout the Hadean could be due to the impact-induced melting owing to frequent impacts. A similar process could have also occurred on Mars, and other rocky planets, provided that water was abundant at shallow and surficial levels, which would account for the existence of a thick felsic crust. The serpentinised protocrust had a dual role in the primitive planetary environment: to provide the first and most abundant felsic crust and to facilitate the emergence of life in the shallow hydrothermal environments of water-bearing terrestrial planets. 

Active felsic magmatism has been rarely probedin situby drilling but one recent exception is quenched rhyolite sampled during the 2009 Iceland Deep Drilling Project (IDDP). We report finding of rare zircons of up to ∼100 µm in size in rhyolite glasses from the IDDP-1 well products and the host 1724 AD Viti granophyres. The applied SHRIMP U-Th dating for both the IDDP and the Viti granophyre zircons gives zero-age (±2 kyr), and therefore suggests that the IDDP-1 zircons have crystallized from an active magma intrusion rather than due to the 20–80 ka post-caldera magmatic episodes recorded by nearby domes and ridges. Ti-in-zircon geothermometer for Viti granophyre reveals zircon crystallization temperatures ∼800°C–900°C, whereas IDDP-1 rhyolite zircon cores show Ti content higher than 100 ppm, corresponding to temperatures up to ∼1,100°C according to the Ti-in-zircon thermometer. According to our thermochemical model at such elevated temperatures as 1,100°C, rhyolitic magma cannot be saturated with zircon and zircon crystallization is not possible. We explain this controversy by either kinetic effects or non-ideal Ti incorporation into growing zircons at low pressures that start to grow from nucleus at temperatures ∼930°C. High temperatures recorded by IDDP-1 zircon together with an occurrence of baddeleyite require that the rhyolite magma formed by partial melting of the host granophyre due to basaltic magma intrusion. Zr concentration profiles in glass around zircons are flat, suggesting residence in rhyolitic melt for >4 years. In our thermochemical modeling, three scenarios are considered. The host felsite rocks are intruded by: 1) a basaltic sill, 2) rhyolite magma 3) rhyolite sill connected to a deeper magmatic system. Based on the solution of the heat conduction equation accounting for the release of latent heat and effective thermal conductivity, these data confirm that the rhyolite magma could be produced by felsic crust melting as a result of injection of a basaltic or rhyolite sill during the Krafla Fires eruption (1975 AD).