The link between seawater magnesium concentrations and anhydrite formation in the ocean crust

Subseafloor hydrothermal systems exert a strong control on the chemical composition of the ocean. Likewise, the chemical composition of the ocean impacts the chemical and physical reactions that happen during hydrothermal circulation, although this has been less well considered. We present a 2D model of basalt alteration under hydrothermal conditions, exploring how changes in major seawater ion concentration over geologic time affect anhydrite (CaSO 4 ) formation in the oceanic crust. Anhydrite precipitation plays a key role in influencing the permeability structure of the ocean crust and acts as a sink for sulfur in the global biogeochemical sulfur cycle, one of the major biogeochemical cycles that regulates Earth’s redox state over geologic time. We develop a fully-coupled 2D reactive transport model to simulate the alteration of fresh mid-ocean ridge basalt by circulating seawater. We verify the model by comparing it to measured vent fluid chemistry and associated alteration mineralogy observed in modern drill cores. We then conduct a series of experiments, systematically changing the chemical composition of seawater to evaluate the impact these changes have on alteration and anhydrite formation. Model results support that the largest controls on the amount of anhydrite precipitation in the oceanic crust are sulfate and calcium concentrations in the ocean, and show that magnesium concentrations exert a strong control on the depth and distribution of anhydrite precipitation. The model results suggest that with the chemistry of certain oceans—in particular low magnesium-calcium ratios and higher magnesium and sulfate concentrations—there may have been significantly shallower anhydrite precipitation, with implications for the permeability structure of the crust and therefore extent of hydrothermal alteration. We suggest that the shallowing of the depth of anhydrite precipitation due to higher magnesium concentrations is via the impact of higher seawater magnesium concentrations on clay formation, which also modulates the pH of fluids during hydrothermal circulation. We speculate that, over Earth’s history, changes in the seawater magnesium, sulfate, and calcium concentrations may have influenced the amount and distribution of anhydrite in hydrothermally altered ocean crust, thus affecting crustal permeability structures, with consequences for key global biogeochemical cycles (e.g. sulfur, calcium).