Reactions between basalt and CO2-rich seawater at 250 and 350 °C, 500 bars: Implications for the CO2 sequestration into the modern oceanic crust and the composition of hydrothermal vent fluid in the CO2-rich early ocean

Abstract

This study aims to understand how basaltic rocks absorb CO2 in high-temperature alteration zones in the subseafloor, and to reconstruct hydrothermal alteration processes such as carbonatization of Archean greenstones. To this end, we conducted two laboratory experiments, simulating hydrothermal reactions between basalt (synthesized under quartz–fayalite–magnetite oxygen fugacity) and CO2-rich NaCl fluid (pH=6.5 at 25°C) at high temperature and pressure. As the water/rock reactions progressed at 250°C and 350°C, 500bars, total carbonic acid concentration (ΣCO2) reduced from its initial 400mmol/kg to near 0 and 100mmol/kg, respectively, meanwhile calcite was formed in the basalt as an alteration mineral. This indicates that calcite destabilizes as temperature increases in the H2O–CO2–basalt system and that crustal basalts can absorb almost all CO2 in the fluid as calcite, at least at temperatures and initial CO2 concentrations below 250°C and 400mmol/kg, respectively. Although the second aim was realized in the experiments, minerals such as sericite, dolomite, ankerite, and siderite present in Archean greenstones were not identified in the alteration products, possibly because K, Mg, and Fe were lacking in the initial solutions. Steady-state concentrations of SiO2, Mg, and K in the fluids during water/rock reactions were similar to those of high-temperature fluids (>250°C) in modern basalt-hosted hydrothermal systems. However, the final experimental pHin-situ was 6.6 and 7.2 at 250°C and 350°C, respectively, higher than that in modern hydrothermal fluids (approximately 5) and higher than the neutral pH (5.5–5.6) at 250–350°C, 500bars. The results suggest that the presence of abundant CO2 in the initial fluid induced carbonatization of basalt; consequently, pH was buffered by precipitation and dissolution of calcite. Because pHin-situ was elevated, the dissolved Fe and Mn concentrations in the fluid were two to three orders of magnitude lower than those of modern hydrothermal fluids. In modern oceans, high-temperature hydrothermal vent fluids are the second-largest iron source (after riverine input). However, because alkaline, metal-poor hydrothermal fluids are generated in CO2-rich systems, CO2-rich seafloor hydrothermal systems may have behaved as iron sinks in early oceans.