Hydrothermal seawater-basalt exchange reactions traced by triple oxygen and strontium isotope values of fluids and epidotes

Hydrothermal circulation of seawater at mid-ocean ridges cools the oceanic crust and modulates the oceanic chemistry over multimillion-year time scales. Recent research on mass-dependent fractionation of triple oxygen isotopes allows us to gain a new insight into the seawater-basalt exchange reactions that occur within the oceanic crust. To understand the systematics of triple oxygen isotope exchange, we present a novel combined dataset for Δ¹⁷O and ⁸⁷Sr/⁸⁶Sr isotope values measured in modern seawater-derived vent fluids at the Axial Seamount volcano located on the Juan de Fuca Ridge and oceanic epidotes extracted from altered mid-ocean ridge basalts. Upon reaction with fresh oceanic crust, seawater evolves towards the low Mg compositions characteristic of fluids in equilibrium with basalt. In concert with decreasing Mg content and with decreasing ⁸⁷Sr/⁸⁶Sr, the vent fluids at Axial Seamount shift towards values that are 0.04 ‰ lower in Δ¹⁷O and 2 ‰ higher in δ¹⁸O compared to initial seawater. The ⁸⁷Sr/⁸⁶Sr and Δ¹⁷O values of epidotes extracted from modern hydrothermally altered basalts reveal a trend of isotope exchange similar to the one defined by the fluids. We suggest that epidotes record isotope shifts that were experienced by fluids in the areas of focused flow within the oceanic crust. Both fluids and epidotes display similar trajectories of Δ¹⁷O and ⁸⁷Sr/⁸⁶Sr shifts which are modeled using a Monte-Carlo simulation of reactive transport in dual-porosity medium. These trajectories provide important constraints on the physical complexity of reactive circulation of seawater within the oceanic crust. We show how the contribution of hydrothermal circulation to the isotope budget of seawater can be changed during geologic history and evaluated based on the studies of fragments of ancient oceanic crust.