Multiple Sulfur Isotope Insights on the Tectonic Evolution of the 4.02-2.94 Ga Acasta Gneiss Complex

The Acasta Gneiss Complex (AGC) of the Canadian Northwest Territories represents the oldest known evolved crust, with zircon U-Pb ages up to 4.03 Ga. Collectively, the AGC captures more than a billion years of crustal generation, spanning several metamorphic episodes, with diverse lithologies dominated by tonalitic and granitic gneisses, but also containing amphibolites and metamorphosed ultramafic rocks. The tectonic processes that sustained such long-lived magmatism are a subject of active study, and a shift in tectonic regime from plume-like to subduction-like processes around 3.6 Ga has been proposed. Multiple sulfur isotopes (MSI) have the potential to inform this debate since they are a unique tracer of Archean atmospheric sulfur (with Δ33S ≠ 0) and can thus identify surficial inputs to magmatic systems. Here we present whole rock MSI data on low-strain Acasta felsic gneisses spanning 4.0-2.9 Ga. We find small but statistically significant variations in δ34S and Δ33S values from 4.0 to 3.5 Ga, with δ34S increasing from 1.0 ± 0.1 to 3.5 ± 0.1 ‰ and D33S decreasing from 0.00 ± 0.01 to -0.04 ± 0.01 ‰ (relative to V-CDT). This may be explained by kinetic mass-dependent fractionation of S isotopes during magmatic differentiation or by small degrees of mixing with a negative Δ33S component. S isotope signatures show a clear shift after 3.5 Ga, with positive Δ33S values up to +0.08 ± 0.02 ‰ suggesting input of surface-derived material. Ultramafic units occurring within the 4.0-3.6 Ga gneisses also feature small positive Δ33S signatures, offering a potential complementary sink to the felsic gneisses. We suggest that ultramafic rocks represent magmatic residues of basaltic parental magmas, with tonalites and granites representing the evolved melts. A narrow range in δ34S values of 4.0-3.6 Ga rocks centered around 1-1.5‰ suggests that extraction of Earth's first crust from the primordial mantle was accompanied by S isotope fractionation, encouraging discussion over the origin of the non-chondritic S isotope signature (δ34S ≈ -1‰) of the depleted mantle.