Hydrogen-rich hydrothermal environments in the Hadean ocean inferred from serpentinization of komatiites at 300 °C and 500 bar

Abstract

Serpentinization of Al-depleted and Al-undepleted komatiites (and olivine for comparison) was experimentally characterized under high-temperature and high-pressure conditions of 300°C and 500 bar to evaluate the H 2 generation potential in komatiite-hosted hydrothermal systems in the early Earth. From the results, the steady-state H 2 concentrations of fluids were estimated to be approximately 20 and 0.05 mmol/kg during the serpentinization reactions for Al-depleted and Al-undepleted komatiites, respectively (60 mmol/kg in the case of olivine). The H 2 concentration of hydrothermal fluid generated from the serpentinization of Al-depleted komatiite is lower than that from olivine but is comparable to that of typical modern peridotite-hosted hydrothermal systems (~16 mmol/kg). The relatively low H 2 concentration from Al-undepleted komatiite is similar to the levels in modern basalt-hosted hydrothermal fluids. Considering that the generation of Al-depleted komatiite melt requires a hotter mantle upwelling (plume) than the generation of Al-undepleted komatiite melt and that the temperature of the mantle has gradually decreased throughout Earth's history, Al-depleted komatiite may have constituted ultramafic volcanism in Hadean oceanic islands/plateaus. Furthermore, it seems unlikely that seafloor exposure of mantle peridotites occurred frequently in the Hadean because the oceanic crust of that time was presumably much thicker than the modern equivalent. Therefore, the serpentinization of Al-depleted komatiites may have been the main process that provided abundant H 2 -rich seafloor hydrothermal environments in the Hadean ocean, which potentially acted as a nursery for the prebiotic chemical evolution and the emergence and early evolution of life on Earth.

Fluid chemistry in the Solitaire and Dodo hydrothermal fields of the Central Indian Ridge

“…A recent review that compiled fluid chemistry of 89 globally distributed vent sites (Nakamura & Takai 2014) has indeed shown evidence and examples of representative chemolithotrophic metabolisms (e.g., hydrogenotrophic, thiotrophic, and methanotrophic) that respond differently to variations in geological background (host rock type and sediment association) and fluid chemistry (concentrations of metabolic reductants such as H 2 , H 2 S, CH 4 , and Fe 2+ ) in deep-sea hydrothermal systems. The key rock-fluid-life relationship in deep-sea hydrothermal systems, argued both empirically and theoretically, has provided important indications for predicting the habitability and metabolic diversity in the hydrosphere of early Earth (Takai et al 2006a;Martin et al 2008;Russell et al 2014;Shibuya et al 2015Shibuya et al , 2016 and extraterrestrial bodies (Hsu et al 2015). However, fluid chemistry of modern deep-sea hydrothermal fields shows significant variation even within a tectonic/geological setting such as sedimentstarved basalt-hosted Mid Ocean Ridge (MOR) system (Von Damm 1995;Butterfield et al 2003;Nakamura & Takai 2014).…”

Weak hydrothermal carbonation of the Ongeluk volcanics: evidence for low CO2 concentrations in seawater and atmosphere during the Paleoproterozoic global glaciation

“…Such CO 2 -rich seawater provided seafloor hydrothermal vent environments that would sustain the emergence and early evolution of life (Takai et al 2006;Russell et al 2010Russell et al , 2014Shibuya et al 2015Shibuya et al , 2016. Furthermore, chemical exchange reactions would have had particular characteristics as the then CO 2 -rich seawater interfaced oceanic lithosphere (e.g., intense CO 2 fixation into the crust) (Nakamura and Kato 2004;Rouchon and Orberger 2008) resulting in the generation of novel high-temperature hydrothermal fluids (e.g.., alkaline, silica-rich, metal-poor fluid, and Mg-rich fluid) (Shibuya et al 2010;Ueda et al 2016).…”

Constraints on the Source of the Martian Magnetic Anomalies Inferred From Relaxation Time of Remanent Magnetization

“…However, the magnetite associated with serpentinization may coalesce as the alteration process continues and often show coarse grain size in terrestrial samples (Oufi et al, 2002). The serpentinization of rock does not always produce magnetite (Shibuya et al, 2015). It is necessary to elucidate a reaction condition producing fine-grained and large amount of magnetite for the serpentinization scenario.…”

“…Because it is difficult to estimate the temperature and pressure dependences of c and V Bark , we assume these parameters as constant values. Shimizu (1960) reported the magnetic viscosity field constant for magnetite with sizes of 1, 2, 100, and 270 μm. Shimizu (1960) reported the magnetic viscosity field constant for magnetite with sizes of 1, 2, 100, and 270 μm.…”

“…The maximum slope of an energy barrier is assumed to be sinusoidal, and the c value at T 0 and ambient pressure is set to be 2∕π (Dunlop & Özdemir, 1997). Shimizu (1960) reported the magnetic viscosity field constant for magnetite with sizes of 1, 2, 100, and 270 μm. Using equation (9) in Shimizu (1960), V Bark for these magnetites are estimated to be (0.45-1.2) ×10 −21 m 3 .…”