FORMATION & EVOLUTION OF CONTINENTAL CRUST
Field work in the Archean-aged Yellowknife Greenstone Belt. Photo Credit: M. Antonelli
Mapping exercise in the Islands of Great Slave Lake, NWT, Canada. Photo Credit: M. Antonelli
Mapping exercise in the Islands of Great Slave Lake, NWT, Canada. Photo Credit: F. Pilecki
Modern continental growth is dominated by andesitic arc magmatism and generated when down-going oceanic crust dehydrates, causing flux melting of overlying mantle. Archean continental crust, on the other hand, has bimodal chemical compositions dominated by (meta)basalts and TTG-suite granitoids. This difference is linked to the higher mantle temperatures on the early Earth, which may have allowed for direct melting of hydrated oceanic crust at amphibolite- to granulite-facies conditions. However, many contentious questions remain concerning (i) the operation of subduction-driven plate tectonics on the early Earth, (ii) the addition of juvenile mantle vs. reworked crustal material to the continents through time (fractional crystallization vs. partial melting), and (iii) the mechanisms responsible for continent stabilization over geologic time (“cratonization”).
Both stable and radiogenic isotopes are useful for addressing these questions, for example I incorporated Ca isotope fractionations into phase-equilibrium models for TTG petrogenesis (in collaboration with C. Yakymchuk, U Waterloo) and found that variations in the Ca isotopic composition of TTGs are mainly controlled by geothermal gradients. The analyzed samples require geothermal gradients of 500-750°C, which are similar to modern-day (hot) subduction zones, suggesting that TTGs formed through hot subduction throughout the Archean, and in turn, that subduction-driven plate tectonics started prior to ~3.8 Ga (Antonelli et al., 2021, Nat Comms).
Heat-flow studies indicate that the lower continental crust is poor in heat-producing elements (U, Th, Rb, K), suggesting that it either (i) contains a high proportion of mafic rocks or that it (ii) contains more evolved felsic rocks that underwent heat-producing element depletion. Analyzing 40Ca abundances in lower crustal granulites (from 40K decay: the most important source of radiogenic heat in the early Earth) I directly confirmed that many lower-crustal protoliths lost 60% to 99% of their initial K contents during Neoarchean metamorphism & partial melting, supporting the latter interpretation and promoting coupling between the lower crust and lithospheric mantle (“cratonization” Antonelli et al., 2019, Geoch Persp Lett). Analyzing stable Ca isotope abundances in granulites (δ44Ca and Δ48Ca’), I found very large stable isotope variations and proved that they were the result of diffusive kinetic isotope fractionations, both between co-existing minerals and adjacent lithological units, demonstrating for the first time that Δ48Ca’ measurements can be used to reliably distinguish between kinetic and equilibrium Ca isotope effects (Antonelli et al., 2019, EPSL). I then developed numerical diffusion models (in collaboration with T. Mittal, MIT) to constrain the effective Ca diffusivities and isotopic diffusivity ratios (D44/D40) associated with lower crustal granulite-facies metamorphism. These indicate the presence (and subsequent loss) of intermediate silicate melt during granulite-facies metamorphism, contrary to previous models where incompatible element loss is explained solely through metamorphic dehydration.
Antonelli, M.A., Kendrick, J., Yakymchuk, C., Guitreau, M., Mittal, T., Moynier, F. (2021) “Calcium isotope evidence for early Archaean carbonates and subduction of oceanic crust” Nature Communications 12, 2534.
Antonelli, M.A., Schiller, M., Schauble, E.A., Mittal, T., DePaolo, D.J., Chacko, T., Grew, E.S., Tripoli, B. (2019) “Kinetic and equilibrium Ca isotope effects in high-T rocks and minerals” Earth and Planetary Science Letters 517, 71-82.
Antonelli, M.A., DePaolo, D.J., Chacko, T., Grew, E.S., Rubatto, D. (2019) “Radiogenic Ca isotopes confirm post-formation K depletion of lower crust” Geochemical Perspectives Letters 9, 43–48.