Titanium and iron isotopic records of granitoid crust production in diverse Archean cratons

Abstract

Archean granitoid provides critical clues on how Earth's felsic crust was established and its geodynamics evolved. In this study, we present Fe-Ti isotope and trace-element data for granitoids (mostly TTGs) from the ∼3.8-3.6 Ga Itsaq Gneiss Complex and ∼3.3 Ga East Pilbara Terrane, Pilbara Craton. TTGs from those localities and several other cratons follow the same Ti isotopic fractionation trend as modern calc-alkaline rocks. This similarity hides important petrogenetic differences as partial melting could have played a much more important role in establishing the felsic character of TTGs compared to modern granites of calc-alkaline affinity. Our thermodynamics-based modeling of isotopic fractionation shows that partial melting of hydrated metabasite can explain the Fe and Ti isotopic compositions of many TTGs but reworking of tonalite crust is needed to explain the most extreme compositions. The same Ti isotopic fractionation trends have been found now in several cratons (West Greenland, Slave, Pilbara, and Kaapvaal cratons) in granitoids formed in different geothermal gradients, meaning that it was likely a global signature of the Archean crust. Sediment provenance studies that use those TTG compositions and point to a primarily felsic crust in the Archean are therefore valid.