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 CO 2 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 CO 2 -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, 500 bars, total carbonic acid concentration (ΣCO 2 ) reduced from its initial 400 mmol/kg to near 0 and 100 mmol/kg, respectively, meanwhile calcite was formed in the basalt as an alteration mineral. This indicates that calcite destabilizes as temperature increases in the H 2 O–CO 2 –basalt system and that crustal basalts can absorb almost all CO 2 in the fluid as calcite, at least at temperatures and initial CO 2 concentrations below 250 °C and 400 mmol/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 SiO 2 , 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 pH in-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, 500 bars. The results suggest that the presence of abundant CO 2 in the initial fluid induced carbonatization of basalt; consequently, pH was buffered by precipitation and dissolution of calcite. Because pH in-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 CO 2 -rich systems, CO 2 -rich seafloor hydrothermal systems may have behaved as iron sinks in early oceans.