BUILDING CONTINENTAL CRUST IN A STAGNANT LID TECTONIC REGIME: AN INTERPLANETARY PERSPECTIVE

Archean (4–2.5 Ga) tonalite–trondhjemite–granodiorite (TTG) ‘gray gneiss’ terranes represent Earth’s first continents, which are thought to have formed via high-grade metamorphism and partial melting of hydrated basaltic crust. However, strong debate surrounds the geological environments in which voluminous TTG magmas with appropriate geochemical signatures can be generated, alongside whether they represent primary or hybridized melts. Here, we show via thermodynamic phase equilibrium modeling that precursor mafic crust buried and metamorphosed in a stagnant-lid tectonic regime can produce magmas with geochemical signatures matching natural TTGs, with hybrid magmas formed by accumulation of multiple melt fractions showing the best correlations. These interpreted batch-melting processes are applied to forward modeling generation of felsic crust from Martian and Venusian basalts, which are believed to have formed in non-plate tectonic regime. In all cases, optimal pressure–temperature conditions for generating up to ~30% TTG-like melts lie between ~9–18 kbar and ~800–1000 °C, defining geothermal gradients of 50–100 °C/kbar. A primitive crust with high magnesium oxide content provides the best correlation between the modeled melts and natural TTG compositions. Importantly, the mineral phases present in the solid residuum largely control trace-element distribution in the resulting melts; yet melts in equilibrium with contrasting mineral assemblages can still bear the diagnostic signatures of some natural TTGs. Our results show that a stagnant-lid regime would be suitable for the generation of continental crust, and Earth may have been geodynamically like its inner solar system neighbors soon after planetary formation. The optimal metamorphic conditions for TTG magma genesis require formation at the base of a basaltic crustal column ~30–60 km in thickness, supporting inferences that higher mantle potential temperatures on rocky planets should be associated with thicker overlying crusts.