Revising hydrothermal element fluxes at mid-ocean ridges: The role of slow and ultra-slow spreading centers regarding the global hydrothermal element budget.

After nearly 50 years of research on hydrothermal circulation, the global hydrothermal on-axis element turnover is still not well constrained. Existing estimates of hydrothermal element fluxes typically invoke a basalt-hosted black smoker archetype hydrothermal vent fluid that is imposed to be responsible for the global hydrothermal cooling of oceanic lithosphere. The diversity of hydrothermal vent fluid compositions, especially to be found at slow to ultra-slow spreading mid-ocean ridges (due to varying degrees of fluid rock interaction with peridotites), has not been properly addressed yet.

Here we present a study that for the first time considers the diversity of hydrothermal vent fluids by analyzing a global database of hydrothermal vent fluid compositions (MARHYS Database Version 3.0). We derive a proper weighting of these fluid types by analyzing strike lengths and substrate types of the mid-ocean ridge system and estimate the partitioning of these hydrothermal fluid types to improve quantification of hydrothermal element fluxes at mid-ocean ridges. We show that the element‑to‑energy flux ratio in peridotite-hosted (or peridotite-influenced) hydrothermal vent fluids is significantly different to the one in purely basalt-hosted fast spreading ridges. Consequently, for many compounds significantly higher (e.g. H₂, CH₄, Fe) or lower (e.g. H₂S, CO₂) element fluxes are found to be associated with hydrothermal cooling at slow- and ultra-slow spreading ridges. Our results show that, despite their lower power output (compared to fast spreading ridges), slow and ultra-slow spreading centers, with their serpentinization‑derived hydrothermal fluids, play a major role for the element transfer between the ocean crust and the ocean.