Sequential serpentinization reactions revealed by boron isotope systematics of the Lost City vent fluids

Hydrothermal vent fluids reflect interactions between oceanic lithosphere and seawater, but to date, the processes resulting in unusual hyperalkaline (up to pH 11) fluids venting ultramafic-hosted systems, such as the Lost City Hydrothermal Field, remain enigmatic. To elucidate fluid reaction pathways, we present boron (B) concentration and isotope analyses from fluids venting the Lost City, located off axis atop the serpentinized Atlantis Massif just west of the Mid-Atlantic Ridge at 30°N. Our data reveal that fluid B concentrations are markedly lower and isotopically lighter than that of seawater. These data contrast with the accepted understanding of B isotope systematics that predicts that the progressive removal of B from seawater during serpentine formation should result in low-B fluids with δ11B values higher than that of seawater. We utilize a mechanistic B isotope fractionation model coupled with a Bayesian inversion approach to robustly evaluate fluid-rock interactions and propose that Lost City fluid B compositions reflect a two-step process. Initially, B is progressively removed from seawater during serpentine formation, resulting in the generation of an effectively B-free serpentinizing fluid. Subsequently, B remobilized from previously formed serpentine is transferred to later fluids, resulting in the low-δ11B end-member values analyzed. Such an evolution demonstrates that fluid-rock interactions both add fluid-mobile elements such as B to mantle peridotites as well as remobilize them. Thus, analyzed serpentine δ11B signatures reflect the cumulative impact of B-consuming serpentinization and later B-remobilizing serpentinite recrystallization. Consequently, serpentinite alteration, in addition to serpentinization hydration reactions, actively contributes to chemical exchange between the mantle and the oceans.