Sulfur transfer processes in Archaean mantle rocks

Geodynamic processes operating during the Eoarchean (3.6 to 4.0 Ga) most likely were different to those operating today. Current discussions mostly concern the onset of plate tectonics and subduction, and the processes that produced the first continental crust. Previous models have suggested that continental crust formation initially took place by melting of thickened, altered oceanic crust in a setting similar to today’s subduction zones or alternatively by melting of mafic crust within an oceanic plateau. At the same time, it is rather unclear how the atmosphere interacted with the oceans, how extensive water-rock interaction was during the Eoarchean, and how ocean-atmosphere-crust-interaction impacted the chemical evolution of Earth’s interior. An ideal tracer for studying the interaction between atmosphere, ocean and crust are multiple sulfur isotopes (³²S, ³³S, ³⁴S, ³⁶S). Multiple sulfur isotope signatures are particularly useful for the processes that took place in the Archean as prior to the great oxidation event (~2.45 Ga), photolytic processes in the Earth’s atmosphere caused mass independent fractionation (MIF) of sulfur species (mostly SO₂) released by volcanic degassing. This MIF is detectable in Δ³³S and Δ³⁶S signatures and as mass independent fractionation processes only take place on the Earth’s surface, Δ³³S and Δ³⁶S signatures can be used as a tracer for surface-derived sulfur and allow us to trace subduction-like processes.

In this project we investigate 3.9 to 3.6 Ga old altered oceanic crust and mantle, metasomatically overprinted mantle peridotites, as well as tonalites-trondhjemites and granodiorites (TTGs), which represent early continental crust and are a product of partial melting of thickened, hydrothermally altered oceanic crust. Our major objectives are to i) provide new insights into the sulfur cycling within Eoarchean hydrothermal systems with emphasis on the source of sulfur, ii) trace Eoarchean mantle metasomatism in peridotites, iii) investigate the source rocks of Eoarchean TTGs to elucidate the geodynamic setting in which they formed, and iv) provide new constraints on the early magmatic sulfur cycle as an example for the incorporation and recycling of volatiles into the Eoarchean mantle and crust.