FORMING EARTH’S CONTINENTAL CRUST: A STABLE TITANIUM ISOTOPE PERSPECTIVE (Invited Presentation)

Stable titanium (Ti) isotopes have been used in the terrigenous sedimentary record to understand crustal differentiation through Earth’s history as they are tracers of fractional crystallization and are useful indicators of continental crust formation. More recent work has demonstrated the potential for Ti isotope fractionation during physical and chemical weathering and has complicated the interpretation of these records. It was also demonstrated that Ti isotopes are sensitive tracers of magmas produced in different tectonic settings. Specifically, rocks formed in settings with anhydrous (dry) magmas fractionate Ti isotopes more significantly at a given SiO₂ content compared to rocks from hydrous (wet) magmas. It is also likely that Ti isotope fractionation is sensitive to oxygen fugacity and potentially temperature of melt crystallization, however more work is needed to understand the link between melt evolution and crust generation.

Earth’s oldest presently known rocks are found in the Acasta Gneiss Complex (AGC; Slave craton, Canada). These rocks have experienced multiphase metamorphism, deformation, and igneous intrusion, which has resulted in the partial obliteration of primary textures and possibly bulk compositions. Here, I apply the Ti isotope proxy to orthogneisses from the Acasta Gneiss Complex spanning the Hadean to Eoarchean transition. Hadean tonalitic gneisses have Ti isotopic compositions comparable to modern evolved tholeiitic magmas, formed by differentiation of dry parental magmas in plume settings. Younger Eoarchean gneisses have Ti isotopic compositions comparable to modern calc-alkaline magmas produced in convergent arcs. This data documents a unique shift from tholeiitic to calc-alkaline style magmatism between 4.02 and 3.75 billion years ago in the Slave craton.