Sulfur and metal cycling in the oceanic crust:
the Samail ophiolite as a proxy for a fast-spreading ridge
Hydrothermal circulation modifies the composition of the oceanic crust by fixating chemical elements during exchange between seawater and basaltic rock. Ultimately, the balance between hydrothermal alteration, submarine volcanic emissions and riverine inputs regulates the composition of the oceans through time and can modify the mantle, during subduction of altered oceanic crust. Within fast spreading ridges, the heat flux from the axial melt lens (AML) powers a vigorous hydrothermal system that represents the roots of Volcanogenic Massive Sulfide (VMS) deposits, where metals are scavenged. Growing evidence suggests that axial hydrothermal circulation penetrates the deep crust. To test this hypothesis and assess metal sources of VMS, a crust-wide profile in the Cretaceous Samail Ophiolite in Oman is being studied. Large S-isotope shifts in the upper crust suggest much larger hydrothermal fluxes in the ophiolitic crust relative to modern in-situ crust, and a complex hydrothermal system reflecting the long-lived dynamics of the AML. Despite near-complete leaching of metals within the “reaction zone” of the hydrothermal system, metals in VMS deposits were mainly sourced from the shallow volcanic footwall. Although the lower crust mostly preserves its magmatic S-isotope signature, sulfates with Cretaceous seawater S-isotope signature preserved within transcrustal oceanic faults support the hypothesis that seawater was introduced in the lower crust via focused fluid flow. The most extreme S-isotope shifts are documented within the Moho Transition Zone where desulfurization during serpentinization and rodingitization was followed by highly confined fluid circulation and formation of an incipient, ultramafic-hosted hydrothermal sulfide assemblage.