Formation of oxidized sulfur-rich magmas in Neoarchaean subduction zones

Oxidized, sulfur-rich arc magmas are ubiquitous in modern subduction-zone environments. These magmas are thought to form when the fluids released during prograde metamorphism of subducting oceanic crust and overlying sediments oxidize and hydrate the asthenospheric mantle. In contrast, Archaean arc-type magmas are thought to be relatively reduced and sulfur poor, owing to the lower concentrations of marine sulfate and limited oxidative seafloor alteration in the anoxic ocean before the Great Oxidation Event some 2.4 billion years ago (Ga). Here we measure the total sulfur concentration and relative abundances of S 6+ , S 4+ and S 2− in zircon-hosted apatite grains from sodic and potassic intrusive rocks from the ~2.7 Ga southeastern Superior Province, Canada. We find that, rather than being reduced and sulfur poor, the sulfur budget of the Neoarchaean magmas was dominated by S 6+ and abruptly increased to concentrations comparable to Phanerozoic arc magmas following the interpreted onset of subduction at approximately 2.7 Ga, coincident with the first global pulse of crust generation. These findings indicate that oxidized, sulfur-rich magmas formed in subduction zones independent of ocean redox state and could have influenced oceanic–atmospheric and metallogenic evolution in the Neoarchaean. Neoarchaean arc magmas in Superior Province, Canada, were relatively oxidized and sulfur rich, reaching compositions comparable to modern subduction zones by approximately 2.7 Ga, according to analysis of sulfur speciation in zircon-hosted apatite grains.