Unravelling crustal formation in the Early Earth through the mass-dependent stable Ti isotope composition of Archaean Tonalite-trondhjemite-granodiorites (TTGs)

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Rocks from the so-called Tonalite-Trondhjemite-Granodiorites (TTG) series, a type of sodic granitoid that is largely restricted to the Archean, are pivotal to our understanding of continental crust formation in the early Earth. While most agree TTG formed as a result of a multi-stage process that is initiated by the partial melting of hydrous mafic lithologies, controversy remains about the exact geodynamic setting that presided over this partial melting event. Hypothesis range from the melting of thickened altered oceanic crust and eclogite in a subduction-like setting, to the partial melting of mafic crust in an oceanic plateau setting. Through 10-25% of TTG melt extraction dense residues which contain accessory phases like ilmenite and rutile are left behind. Later during the differentiation of the TTG melts in the magma chamber cumulates form during crystallization processes. These contain ilmenite and hornblende. The involvement of Ti-bearing minerals in either magmatic process (i.e. melting and later crystallization), likely results in resolvable Ti isotope fractionation. Moreover, delamination of the dense restites into the mantle may contribute to the sources of later komatiite melts, possibly carrying their unique Ti isotope signature. By measuring Paleoarchean komatiites, the fate of these restites can potentially be traced. Moreover, to place constraints on the effect of fractional crystallization, we plan to investigate samples from an Archean layered anorthosite complex for Ti isotopes. In comparison to the contemporary rock record, which clearly shows that plume-related magmatic samples, and those associated with island arcs, display disparate behaviour concerning their Ti isotope composition during magmatic differentiation, Ti isotopes may provide insights into the exact geodynamic environment where TTG formed. With this proposal we aim to carry out a detailed analytical and experimental campaign, dealing with the main aspects of TTG petrogenesis, starting from the partial melting of their hydrous mafic precursors to the fractional crystallization and differentiation of their parental magmas. To do this we will combine Ti isotope data of well characterized TTG, rocks from layered anorthosite intrusions from southern West Greenland and komatiite samples from South Africa, with similar data that results from piston cylinder experiments dealing with TTG petrogenesis. Both datasets will then be combined to unravel the exact conditions that lead up to TTG formation. This proposal may establish Ti isotopes as a new tool to reconstruct the earliest formation history of Earth’s continental crust and may contribute to our understanding of Archean geodynamics.