Native sulfur at the seafloor: Composition and origin

Mineralogy, geochemistry and sulfur (S) isotope composition of native sulfur (S⁰) precipitated at intraoceanic and intracontinental back-arc rift, submarine and subaerial volcanic arc, sediment and sediment-free mid-ocean ridge, hot spot, accretionary wedge, and submarine and continental cave settings were investigated with a main goal to understand the mode of formation of all the types of native S at the modern seafloor. Native S occurs as various depositional forms: chimneys, colloform flows, liquid ponds, slabs; fills in cavities and pore space of the sediment, vesicles and cracks in volcanic rocks; cements and coats the sediment; stains the conduits or fills in pores of the sulfide chimneys; forms fine-grained layers within the sediment; coats the walls, stalactites and stalagmites in the caves. Mineralogically, the native S from the seafloor is pure rhombic S⁰ with negligible impurities of metal sulfides, aluminosilicates, and carbonates inferred from the chemistry data. Mineral interrelations and S isotope data suggest that native S from different geologic settings has different origin. In the sulfide chimneys and mounds at the mid-ocean ridges, native S appears to be a result of oxidative alteration of primary pyrrhotite. The native S from back-arc rifts, volcanic arcs and hot spots can be a result of either disproportionation of magmatic SO₂ (δ³⁴S < 0 ‰), or synproportionation of magmatic SO₂ and H₂S (δ³⁴S > 0 ‰). The native S from the sediments in anoxic brine-filled deeps (accretionary wedge setting) is a result of bacterial sulfate reduction and consequent sulfide (δ³⁴S < 0 ‰) oxidation. The native S coating the cave walls and forms also has a bacterial origin (δ³⁴S < 0 ‰).