An investigation into Eoarchean geodynamics via multiple sulfur isotopes

Abstract

The geodynamic processes that gave rise to the first cratons on Earth remain controversial. Whether the first preserved cratons formed through horizontal tectonic processes similar to those operating on Earth today or in a stagnant lid regime dominated by vertical stacking and reworking of crustal material continues to be debated. Central to the question of Eoarchean geodynamics is whether and to what extent material from Earth’s surface was recycled into the magmatic system and mantle. Horizontal tectonic processes similar to those active today would be expected to rework material including sulfur from the Earth’s surface into igneous rocks and the mantle. In contrast, significant reworking of sulfur from the crust into the mantle under proposed vertical tectonic regimes is not expected. Multiple sulfur isotopes provide a valuable window into early Earth processes. They can preserve a record of mass independent fractionation (MIF-S) that took place as a result of photolytic processes in the atmosphere prior to the Great Oxidation Event (GOE). This MIF-S was preserved in sediment and hydrothermal deposits that incorporated sulfur species fractionated in the atmosphere. Large MIF-S signatures are a unique feature of surface deposits formed prior to the GOE, and these signatures are robust enough to survive reworking into the magmatic system. The occurrence of MIF-S in igneous lithologies is therefore an indication that the rocks include material recycled from Earth’s surface in the Archean or before. In this dissertation, the results of multiple sulfur isotope analyses of igneous lithologies from the Eoarchean Itsaq Gneiss Complex (IGC) in southern West Greenland are reported, alongside petrographic observations and additional supporting analyses of the same. Investigated lithologies include tonalite-trondhjemite-granodiorites (TTGs), amphibolites, and peridotites from the IGC. The TTGs were subject to bulk multiple sulfur isotope and petrographic analysis. The majority of measured TTGs were found to contain MIF-S (positive Δ33S values up to +0.30‰), indicating incorporation of surface-derived sulfur dominated by sedimentary material. Elevated Δ36S up to +0.80‰ and δ34S up to +3.36‰ in IGC TTG samples point to additional incorporation of seawater sulfate. The surface-derived material is interpreted to have been incorporated into the TTGs in the context of modern-like arc accretion. Two IGC amphibolites with tholeiite-like compositions were concurrently analyzed for multiple sulfur isotopes. One of the two amphibolites was found to contain positive bulk Δ33S (+0.14‰), similar to other IGC amphibolites reported in the literature. iv These tholeiite-like amphibolites are interpreted to represent the source rocks of the TTGs and to have incorporated sediment-dominated sulfur during subduction in the Eoarchean. Peridotites found in ultramafic enclaves in the IGC were also investigated. These rocks have been well characterized by previous investigations and have been interpreted to represent the oldest remnants of obducted mantle. A mantle origin for these peridotites remains controversial, however, with some researchers arguing that they are ultramafic cumulates. The peridotites were found to contain positive bulk Δ33S values between +0.04‰ and +0.21‰, indicating that they incorporated sulfur dominated by sedimentary material. Trends emerge when plotting the Δ33S and δ34S values of the peridotites against trace and major element concentration data on these rocks from the literature, as well as Hf isotope data published in previous studies. The trends indicate that the peridotites incorporated sediment derived sulfur early in their history, prior to variable melt overprinting that delivered additional sulfur of seawater sulfate origin. The complex history of multiple overprinting events in the Eoarchean is interpreted to have taken place in a mantle wedge. The presence of MIF-S in peridotites originating in the mantle is a significant line of evidence in support of Eoarchean subduction. Additional in-situ sulfur and lead isotope analysis of sulfides in the studied peridotites was conducted by secondary ion mass spectrometry (SIMS), in conjunction with electron microscopy and electron microprobe analysis. The sulfides were predominantly pentlandite and pyrrhotite, typical mantle minerals, and their Δ33S values were consistent with bulk analyses. Amphibole crosscutting the sulfides demonstrates that the sulfides predate amphibolite facies metamorphism the peridotites experienced in the Neoarchean. Highly unradiogenic lead isotope results are consistent with Eoarchean origins followed by partial reequilibration during metamorphism. Because lead is a trace component of the sulfides and sulfur is a major component, this reequilibration is not expected to have influenced Δ33S values. The presence of MIF-S in a majority of studied samples and in representatives of all studied rock types points strongly in the direction of widespread Eoarchean crustal recycling in the IGC. This is consistent with interpretations of Eoarchean geodynamics that include horizontal processes in which plates override one another.