Sulfur isotope evidence for surface-derived sulfur in Eoarchean TTGs

Abstract The conditions giving rise to the first evolved melts preserved on Earth remain a subject of debate. These melts formed tonalite-trondhjemite-granodiorites (TTGs) comprising the first cratonic cores. Constraining the nature of the rocks that melted to form these TTGs is crucial to understanding the mechanism that produced the first continents. Previous studies have indicated that TTGs' source rocks are hydrated mafic lithologies similar to modern arc tholeiitic basalts, comparable to amphibolites embedded in greenstone belts within Archean cratons. To elucidate the geodynamic setting of TTG formation, we investigated 3.9-3.6 Ga TTGs and amphibolites from the Itsaq Gneiss Complex (IGC), southern West Greenland using multiple sulfur isotope signatures, as well as textural and compositional analysis of selected sample sulfides. Small but significant nonzero Δ 33 S and Δ 36 S values were measured in the TTGs, with Δ 33 S values from 0.00‰ to +0.30‰, and Δ 36 S values from -0.13‰ to +0.80‰. Amphibolites yielded Δ 33 S values of -0.01‰ and +0.14‰, and Δ 36 S values of +0.08‰ and +0.23‰. These values are consistent with the presence of sedimentary sulfur, likely introduced to the sources of the TTGs' precursor rocks via horizontal tectonics. Sulfur in the TTGs was likely subject to mass independent fractionation on Earth's surface and may have experienced subsequent 34S enrichment during metamorphism in the TTGs' source rocks. Relative enrichment in 34S and 36S in TTGs may be explained by incorporation of fluid sampling hydrothermal deposits within the thickened mafic crust, released during arc accretion. This fluid may have triggered partial melting, forming the TTGs. Hence, the sulfur isotope composition of the TTGs represents a mixture of material derived from the source rocks by melting and from fluid released from other rocks bearing hydrothermally derived sulfur. Our results add weight to existing models of modern-like tectonic processes active in the Eoarchean.