The conditions and mechanisms that led to formation of the earliest felsic (Si- and Al-enriched) crust on Earth and Mars are the subject of debate (e.g., Kemp et al., 2010; Burnham and Berry, 2017; Harrison et al., 2017; Turner et al., 2020). In terms of general context,
- one possibility is that the Hadean Eon was very similar to the modern Earth; i.e., a voluminous granitic continental crust shaped by plate tectonics (Armstrong, 1981; Harrison, 2009).
- A contrasting scenario is a single-plate basaltic protocrust that formed after the solidification of the global magma ocean (Kramers, 2007).
In either case, various scenarios have been proposed to account for Hadean felsic crust formation, either through processes resembling those of Archean tonalite-trondhjemite-granodiorite (TTG) formation, such as melting of a garnet-bearing igneous protolith at depth (Burnham and Berry, 2017), or through shallow-level hydration of an igneous mafic protolith before subsequent melting to generate Hadean felsic crust (e.g., Drabon et al., 2021). For example, Kemp et al. (2010) suggested that a thin KREEP-like crust accumulated at Earth's surface just after crystallization of the magma ocean and interacted with the hydrosphere (where KREEP indicates residuum with high abundance of incompatible elements such as K, rare earth elements [REEs], and P). This hydrated protocrust was subsequently buried by later komatiitic and basaltic lava flows and finally
remelted due to radioactive self-heating to generate the magmas that crystallized the Hadean zircons. As an alternative scenario, Borisova et al. (2021) assumed that a primitive protocrust quenched on top of the magma ocean, as observed in a permanent lava lake (Helz, 2009).
In the case of the Hadean, this protocrust would have been of peridotite composition and become rapidly hydrated (e.g., serpentinized) in its uppermost part by the readily available liquid hydrosphere (Valley et al., 2002; Elkins-Tanton, 2012), as proposed by Albarède and Blichert-Toft (2007). The residual part of the magma ocean then evolved beneath the early quenched crust to yield basaltic magmas, likely enriched in incompatible elements, with some similarity to lunar KREEP basalts (e.g., Cronberger and Neal, 2018). Borisova et al. (2021) performed experiments on the interaction between serpentinite and basaltic magma that demonstrated that tonalite to granodiorite compositions can be formed in this way at shallow depths (<10 km). In this scenario, partial melting is triggered by dehydration driven by intrusion of basaltic magmas or by meteorite impacts.
The most common approach to address the question of the origin of the felsic Hadean crust is to investigate the possibility to crystallize zircons analogous to the Hadean zircons, the only mineral remnants of Earth's Hadean crust identified so far. Most samples are detrital zircons from the Jack Hills (JHZ), Western Australia (Harrison, 2009), but other localities are also known (Harrison et al., 2017), including a recent discovery in South Africa (Drabon et al., 2021). While the Hadean zircons provide valuable information about their crystallization environment, no clear consensus exists for the genesis conditions of their parental magmas. In any case, mechanisms for the generation of Earth's earliest felsic crust should be compatible with the mineralogical and geochemical features of the Hadean detrital zircons.
Our study tested the hypothesis that an early felsic crust may have formed by low-pressure, fluid-present melting of hydrated peridotite interacting with basaltic magma through assessment of the capacity of experimental felsic melts (Borisova et al., 2021) to crystallize zircon (referred to here as “model zircon”) and then comparison of the temperature conditions, co-crystallizing mineral assemblages, and isotopic and trace-element patterns of these model zircons with data from detrital Hadean zircons.
