Deciphering the mechanisms of crustal formation in the Early Earth: Insights from Ti isotopes

Archean tonalite-trondjemite-granodiorite (TTG) plutonic rocks provide a unique, yet imperfectly understood record of crustal formation. Due to the incomplete nature of this record, there are varying interpretations on the prevailing tectonic regime responsible for the formation of juvenile continental crust. Thus, robust geochemical proxies are required to understand the (non-unique) processes of TTG formation. Ti isotopes have recently been utilised to investigate magma evolution in modern tectonic settings. Ti isotope fractionation is driven by the preferential sequestration of light isotopes in Ti-rich minerals (e.g., Fe-Ti oxides, amphibole). Consequently, melts in equilibrium with these phases are enriched in heavy isotopes. Ti isotopes are an ideal tool for tracing the formation of juvenile continental crust due to their sensitivity to amphibole, rutile and (to a lesser extent) garnet crystallization – major phases involved in TTG petrogenesis via dehydration melting of basaltic protoliths.

Preliminary Ti isotope measurements of amphibolites from the 3.7-3.8 Ga Isu Supracrustal Belt (ISB) in SW-Greenland, display d49/47Ti values comparable to mid-oceanic ridge basalts. Ti isotope compositions during TTG generation were modelled by combining this data and constraints from amphibolite partial melting experiments with mineral-melt Ti isotope fractionation factors. Low pressure melting with amphibole as the main Ti-bearing phase produce melts with d49/47Ti ≤ ∼ +0.2‰, whereas models using higher pressure/high H2O experiments containing rutile+amphibole produce melts with d49/47Ti ≤∼ +0.3‰ and higher d49/47Ti at a given SiO2. This suggests different source mineralogy/composition, pressure, and H2O contents can influence d49/47Ti values during partial melting and that TTG formation processes are non-unique.