Experimental and isotopic insights into the formation of Earth’s first continents

Deciphering the formation of Earth’s earliest continental crust is central to understanding its geodynamic evolution and the establishment of long-term planetary habitability. Archean Tonalite-Trondhjemite-Granodiorites (TTGs) are the preserved remnants of juvenile crust, yet the composition of their protoliths and the tectonic settings in which they formed remain debated.

This talk presents an integrated approach combining titanium (Ti) stable isotopes and trace element systematics from natural samples with high-pressure/high-temperature experiments to constrain TTG petrogenesis. Titanium isotopes are an ideal tool for tracing continental crustformation due to their sensitivity to rutile, ilmenite, amphibole, and (to a lesser extent), clinopyroxene and garnet – the main phases involved in TTG petrogenesis via partial melting of metabasalts.

We focus on Eoarchean metabasalts and TTGs from the Isua supracrustal belt (ISB) in southwest Greenland. Isua metabasalts are enriched in MgO and depleted in Al₂O₃ and TiO₂ compared to modern oceanic basalts. Partial melting experiments conducted at 1–1.8 GPa on synthetic Isua-like metabasalts demonstrate that such high-Mg, low-Al basalts are suitable protoliths capable of producing TTG-like melts.

The resulting experimental melts and mineral phases allow quantification of trace element and Ti isotopic fractionation during partial melting of hydrated mafic crust. Such data is critical for use in thermodynamic modelling of crust formation. Ti isotope data from Archean rocks combined with thermodynamic modelling supports a two-stage model for Archean crust formation: (1) generation of primitive tonalitic melts via polybaric partial melting of low TiO₂ metabasalts (~0.8–1.8 GPa), followed by; (2) fractional crystallisation to yield more evolved TTGs with heavier Ti isotopic compositions. These Ti isotope trends, combined with major and trace element proxies, are consistent with melting under variable pressure–temperature conditions, possibly within a ‘proto’-subduction zone environment.

Together, these results highlight the power of combining experimental petrology with novel isotopic tracers to reconstruct early crust-forming processes and assess their implications for Earth’s early geochemical evolution and planetary habitability.