Insights into the behaviour of sulphur in mid-ocean ridge axial hydrothermal systems from the composition of the sheeted dyke complex at Pito Deep

The behaviour of seawater sulphate in hydrothermal systems at intermediate- to fast-spreading ridges is investigated using new analyses of the δ34S, sulphur concentration and Fe2O3/Fe2O3total, combined with existing 87Sr/86Sr, of sheeted dykes from the Pito Deep tectonic window. The Pito Deep sheeted dyke complex has a similar composition to the sheeted dykes drilled at ODP Hole 504B suggesting that the measured compositions are representative of sheeted dyke complexes at intermediate- to fast-spreading ridges. The dykes show only small increases in δ34S which, combined with the rock dominated δ34S of vent fluids, requires the majority of seawater sulphate to be precipitated as anhydrite before the fluid reacts with the sheeted dyke complex. This loss of sulphate from the fluid means that a much higher Fe2O3 in the sheeted dyke complex than in fresh MORB glasses cannot be explained by oxidation due to seawater sulphate reduction during fluid–rock reaction. Instead, oxidation probably occurs due to degassing of reduced species, largely H2, during dyke emplacement and solidification. A mass balance model that accounts for anhydrite precipitation and Sr partitioning into the anhydrite, as well as fitting the concentration and isotopic ratios of S and Sr in the sheeted dykes and vent fluids, suggests water/rock ratios of ∼ 1. For a 1 km thick sheeted dyke complex this is equivalent to a fluid flux of ∼ 3 × 106 kg m− 2, sufficient to remove ∼ 60% of the latent heat of crystallization from the lower crust.