Geochemical time series of hydrothermal fluids from the slow-spreading Mid-Atlantic Ridge: Implications of medium-term stability

Abstract

Time series investigations of hydrothermal fluids from different geological settings at the slow-spreading Mid-Atlantic Ridge (MAR) indicate a surprising medium-term stability of their chemical composition over up to 20 years. This does not only apply to tectonically controlled systems with exposure of mantle rocks where magmatic activity is limited, such as the Logatchev-1 hydrothermal field at 14°45′N. We also found the 407 °C hot and extremely metal-rich fluids at the 5°S site Turtle Pits (2986 m water depth) to discharge at constant composition indicating supercritical phase separation for at least 4 years, which contrasts with rather dynamic volcanically driven hydrothermal systems at the fast-spreading East-Pacific Rise (EPR). Possibly, fluid pathways and pressure-temperature conditions are more stable in tectonically controlled settings at MAR than at the EPR where frequent volcanic activity affects permeability within the hydrothermal systems. Despite some spatial variability, hydrothermal fluids discharging at the shallower (ca. 810–870 m) Menez Gwen hydrothermal field at 37°50′N seem to represent a stable discharge of phase-separated vapor-type fluids over 16 years. This demonstrates that medium-term stability of hydrothermal fluid composition seems to be a general feature at slow-spreading ridges. As a consequence, hydrothermal vent organisms at the MAR may live in a more stable chemical environment compared to vent systems at fast-spreading ridges. Considering the fact that slow-spreading ridges account for more than half of the submarine ridge system, such a constant emanation of hydrothermal fluids makes hydrothermal vent systems on slow spreading ridges a medium-term stable source of Fe and other micronutrients into the ocean.