Modern continental growth is dominated by andesitic arc magmatism and is generated when down-going oceanic crust dehydrates, causing flux melting of the overlying mantle. Archean continental crust, in contrast, exhibits bimodal chemical compositions dominated by (meta)basalts and TTG-suite granitoids. This difference is commonly linked to higher mantle temperatures on the early Earth, which may have enabled direct melting of hydrated oceanic crust under amphibolite- to granulite-facies conditions. However, several contentious questions remain regarding (i) the operation of subduction-driven plate tectonics on the early Earth, (ii) the relative contributions of juvenile mantle versus reworked crustal material to continental growth through time (fractional crystallization versus partial melting), and (iii) the mechanisms responsible for continental stabilization over geologic time (“cratonization”).

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Both stable and radiogenic isotopes provide powerful tools for addressing these questions. Dr. Antonelli incorporated Ca isotope fractionations into phase-equilibrium models for TTG petrogenesis, in collaboration with C. Yakymchuk (University of Waterloo), and demonstrated that variations in the Ca isotopic composition of TTGs are primarily controlled by geothermal gradients. The analyzed samples require geothermal gradients of 500–750 °C, comparable to those of modern hot subduction zones, suggesting that TTGs formed through hot subduction throughout the Archean. These results further imply that subduction-driven plate tectonics began prior to ~3.8 Ga (Antonelli et al., 2021, Nature Communications).

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Heat-flow studies indicate that the lower continental crust is depleted in heat-producing elements (U, Th, Rb, K), suggesting either (i) a high proportion of mafic lithologies or (ii) the presence of more evolved felsic rocks that have undergone significant heat-producing element loss. By analyzing ⁴⁰Ca abundances in lower-crustal granulites—derived from the decay of ⁴⁰K, the dominant source of radiogenic heat in the early Earth—Antonelli directly demonstrated that many lower-crustal protoliths lost 60–99% of their initial K contents during Neoarchean metamorphism and partial melting. These findings support the latter interpretation and point to strong coupling between the lower crust and lithospheric mantle during cratonization (Antonelli et al., 2019, Geochemical Perspectives Letters).

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Further analysis of stable Ca isotope compositions in granulites (δ⁴⁴Ca and Δ⁴⁸Ca′) revealed exceptionally large isotopic variations. Antonelli showed that these variations result from diffusive kinetic isotope fractionation, both between coexisting minerals and across adjacent lithological units, providing the first clear demonstration that Δ⁴⁸Ca′ measurements can reliably distinguish between kinetic and equilibrium Ca isotope effects (Antonelli et al., 2019, Earth and Planetary Science Letters). He subsequently developed numerical diffusion models, in collaboration with T. Mittal (MIT), to constrain effective Ca diffusivities and isotopic diffusivity ratios (D44/D40) associated with lower-crustal granulite-facies metamorphism. These models indicate the presence—and subsequent loss—of intermediate silicate melt during granulite-facies metamorphism, challenging previous models in which incompatible element loss is attributed solely to metamorphic dehydration.