Modeling seawater-basalt interaction at 10–100 °C: Controlling parameters and effects on the composition of the oceanic crust and seawater

Abstract

Chemical reactions and element mobilization during seawater-basalt interaction play a central role in regulating global element fluxes between seawater and the oceanic crust. This study uses kinetic reaction path modeling to investigate basalt alteration by seawater at 10–100 °C, assessing the influence of reaction time, temperature, pH, rock crystallinity and CO₂ concentration on alteration mineral formation and seawater composition during crustal aging and cooling. Early alteration involves dissolution of basaltic glass and primary minerals, releasing Si, Al, Fe, Ca, Mg, Na and K, followed by the formation of oxides, hydroxides, pyrite, Fe(III)- and Fe(II)-bearing clays, carbonates, celadonite and zeolites. At elevated CO₂ concentrations, for example due to deep magmatic degassing, water pH remains buffered at slightly acidic to circumneutral values. In contrast, at lower CO₂ concentrations, corresponding to standard seawater, H⁺ consumption during basalt dissolution increases pH and drives the system towards more reducing conditions, thereby favoring the formation of zeolites, Ca-silicates and Mg-rich clays. Reaction progress, redox conditions, pH and CO₂ concentration are the primary controls on alteration mineralogy and seawater chemistry. Basalt crystallinity mainly influenced reaction rates and timescales while having minor effects on the mineralogical and chemical outcome of alteration. Temperature likewise mostly affected kinetics but also had an additional effect on K fluxes during seawater-basalt interaction, with K being removed from seawater below ∼50 °C, but added to seawater at higher temperatures. The model predicts significant long-term sinks of both K and Mg due to the formation of clay minerals and celadonite over the course of crustal aging and cooling. Calcium behaved variably, being mostly removed from seawater by early carbonate formation but becoming increasingly released over the cause of crustal aging due to basalt leaching. These results emphasize the need to constrain key parameters controlling seawater-basalt interaction when evaluating its role in low-temperature hydrothermal alteration and seafloor weathering.