Fig. 4

Schematic of Hadean seafloor hydrothermal systems bearing a L/SC-CO₂ pool. The CO₂-rich seawater infiltrates the subseafloor volcanic rocks and reacts with basalts or komatiites, causing carbonation and alteration of basalts or serpentinization of komatiites in the seawater recharge zones. When hydrothermal fluid reaches the reaction zone, it receives abundant degassed CO₂ from an underlying magmatic heat source. During fluid upwelling, phase separation of the CO₂-rich hydrothermal fluid progressively occurs through depressurization, which likely condenses the CO₂ in the vapor phase. If the CO₂-rich vapor phase is physically segregated from the main flow channel of the upwelling brine phase, it is continuously mixed with recharged seawater and/or cooler pore fluid, causing further phase separation by the temperature decrease. CO₂ is then purified in vapor phase. Eventually, these condensation and purification processes generate many SC-CO₂ droplets beneath the seafloor. The SC-CO₂ is less dense than seawater and migrates upward. As it cools during the upward migration, its phase converts to L-CO₂. These processes form a large L/SC-CO₂ pool beneath the seafloor. If the temperature of the bottom seawater is lower than approximately 10 °C, reactions between L-CO₂ and the ambient seawater/pore fluid requisitely generate solid CO₂ hydrates that cement the pores, voids and cavities of the hydrothermal sediments and permeable volcanics. This self-sealing effect by CO₂ hydrates prevents the leakage of L/SC-CO₂ through the seafloor. In contrast, when CO₂ hydrates are absent in the vicinity of warm/hot hydrothermal vents, L/SC-CO₂ leaks from the inner edge of the pool and enters the upwelling hydrothermal fluid just below the seafloor, where it is emitted in the form of bubbles. L-CO₂ containing these bubbles transforms to hydrate-coated L-CO₂ during upward migration through the water column. The bubbles disappear at depths around 500 m because L-CO₂ and CO₂ hydrate are unstable at these depths. Eventually, both CO₂ gas and the organic molecules dissolved in L/SC-CO₂ are released to the ambient seawater. If the hydrothermal vents are black-smoker type, the L-CO₂ bubbles will carry heavy metals upward to the shallower part of the water column