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 H₂ generation potential in komatiite-hosted hydrothermal systems in the early Earth. From the results, the steady-state H₂ 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 H₂ 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 H₂ 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 H₂-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.

Background

Serpentinization of seafloor ultramafic rocks is one of the most important geological processes that could have been involved in the emergence and early evolution of life on Earth. During serpentinization, molecular hydrogen (H₂) is characteristically generated by the reaction of water with ferrous oxide in rocks. This process would have sustained H₂-rich seafloor hydrothermal environments where organic materials necessary for the emergence of life could have been synthesized and preserved (Amend and McCollom 2009), and energetically preferred catabolic and anabolic metabolisms of early life could have been established (Takai et al. 2006; Martin et al. 2008; Shibuya et al. 2010; Russell et al. 2014; Sleep et al. 2011).

In modern oceans, serpentinization of ultramafic rocks and the resulting H₂-rich hydrothermal activity often occur in slow-spreading ridges without sufficient magmatic supply, such as the Mid-Atlantic Ridge (MAR) (Charlou et al. 2002) and the Central Indian Ridge (Kumagai et al. 2008; Nakamura et al. 2009; Morishita et al. 2015) and exceptionally in off-axis mantle-exposed regions such as the Lost City hydrothermal field away from the MAR (Kelley et al. 2001, 2005; Lang et al. 2010). These peridotite-associated hydrothermal systems have been a subject of great research interest because of their geological, geochemical, and biological uniqueness in comparison with conventional basalt-hosted mid-ocean ridge hydrothermal systems (Takai et al. 2006; Nakamura and Takai 2014). Furthermore, an experiment simulating hydrothermal reactions between peridotite and seawater also revealed the great potential of serpentinization to generate H₂-rich hydrothermal fluid (Seyfried et al. 2007). Therefore, such peridotite-associated hydrothermal systems have been perceived as modern analogues of the Hadean ultramafic-hosted systems that could have been possible nurseries for the emergence and early evolution of life.

In the early Earth, the oceanic crust was likely much thicker (approximately three times thicker) than the modern equivalent (6–7 km thickness) owing to the hotter mantle at that time and has become thinner as the temperature of the mantle has decreased with time, as suggested by the geological occurrence of ophiolites (Moores 2002) and the compositional evolution of greenstones derived from mid-ocean ridges (Komiya 2004). Therefore, it has been theoretically hypothesized that the thick lid of oceanic crust probably limited exposure of mantle peridotite on the seafloor; thus, komatiite rather than peridotite would have been the predominant ultramafic rock in the early Archean ocean floor (Takai et al. 2006). Regarding the genesis of komatiite, there was a controversy as to whether komatiites were derived from wet or dry mantle (Arndt et al. 1998; Grove and Parman 2004) since komatiites were first recognized by Viljoen and Viljoen (1969). It is now known that komatiites can be generated not only by high-temperature and high-pressure partial melting in dry mantle (Takahashi and Scarfe 1985) but also under much lower temperature and pressure conditions in hydrous mantle plumes (Inoue and Sawamoto 1992; Inoue et al. 2000). Grove and Parman (2004) even interpreted komatiites as being formed by shallow-level melting of hydrous mantle wedge above subduction zone. However, the low water contents in komatiitic melt inclusions (Shimizu et al. 2001; Berry et al. 2008) and the similar oxygen fugacity of komatiite magmas to the modern normal upper mantle (Canil 1997, 1999; Puchtel et al. 2013) are not consistent with the hydrous melting model. Therefore, most researchers have agreed that komatiites were produced by extraordinarily hot melting in mantle upwellings (plumes) under nearly dry conditions (e.g., Takahashi and Scarfe 1985; Campbell et al. 1989; Herzberg et al. 2010).

Furthermore, Archean komatiites have generally been classified into two types by their Al₂O₃ content, such as Al-depleted (Barberton-type) and Al-undepleted (Munro-type) komatiites, which mainly occur in the early and late Archean greenstone belts, respectively (Nesbitt et al. 1979). Melting experiments under dry conditions revealed that Al-depleted komatiite (ADK) was formed by high-temperature, ultra-deep, high-degree mantle partial melting, whereas Al-undepleted komatiite (AUK) was generated under relatively lower pressure and temperature conditions than ADK (e.g., Wei et al. 1990; Herzberg 1992). Therefore, this compositional change of komatiite likely reflects the decrease in mantle temperature through geologic time (Herzberg et al. 2010).

Of course, komatiite-associated hydrothermal activity does not occur in the modern ocean; thus, the H₂ generation potential of komatiites during serpentinization can only be experimentally estimated through simulated hydrothermal fluid–rock reactions under high-temperature and high-pressure conditions, as previously conducted to reconstruct modern (Seyfried 1987; Seyfried et al. 2007; McCollom et al. 2010; Kato et al. 2013; Suzuki et al. 2015a, b), ancient (Yoshizaki et al. 2009; Lazar et al. 2012; Shibuya et al. 2013), and even extraterrestrial subseafloor hydrothermal systems (Hsu et al. 2015; Sekine et al. 2015). Yoshizaki et al. (2009) confirmed hydrogen generation by a preliminary, ongoing experiment using komatiite and pure water (Yoshizaki et al. 2009). However, the H₂ generation potential of not only ADK but also AUK, during serpentinization, has not yet been evaluated by completed experiments.

In this study, we conducted experiments to simulate the reactions between komatiite and seawater (NaCl solution) at 300 °C and 500 bar, using a batch-type (closed system) hydrothermal reactor (Yoshizaki et al. 2009). For these experiments, both ADK and AUK were prepared to ascertain the H₂ generation potential in komatiite-hosted hydrothermal system through geologic time. The results allowed us to estimate the H₂ concentration of hydrothermal fluids in Hadean komatiite-hosted hydrothermal systems, which can be used to obtain further insights into the possible energetics and kinetics of prebiotic chemical evolution and the emergence and early evolution of life on the early Earth.

Methods

Preparation of the starting solid materials

Olivine (San Carlos) and two types of synthetic komatiite were prepared for use as the starting solid materials in the experiments. Two types of komatiites were synthesized from reagents. The composition of the reagent powders was adjusted to produce synthetic Al-depleted (Al₂O₃ = c. 5 wt%) and Al-undepleted (Al₂O₃ = c. 10 %) komatiites based on the compositions of natural ADK and AUK (Arth et al. 1977; Wei et al. 1990) (Table 1). Approximately 30 g of sample powder was placed in a Pt-Rh crucible and heated at 1000 °C for 1 h in an electric furnace to decarbonate the initial reagents. Subsequently, the sample was fused at 1600 °C for 1 h while regulating oxygen fugacity at a QFM (quartz–fayalite–magnetite) buffer under an H₂–CO₂ mixed gas atmosphere. To create the spinifex texture of olivine, the temperature was lowered to 1450 °C over 1.5 h and then to 1350 °C over 18 h under QFM conditions, after which the sample was immediately quenched, yielding a fresh komatiite.

Table 1 Composition of the starting materials used in the experiments and the natural Al-depleted and Al-undepleted komatiites (wt%)

The San Carlos olivine crystals and synthesized olivine-spinifex komatiite (with minor spinel phase) was crushed in a tungsten mortar and sieved to obtain the <90 μm fraction. To remove possible contamination by organic matter during sample preparation, the powdered komatiite was ultrasonically washed with acetone and pure water several times and then dried at 50 °C for 12 h prior to the experiment. The composition of olivine was determined using an electron probe microanalyzer, and the composition of synthetic komatiites was also confirmed using X-ray fluorescence.

Experimental system

An autoclave based on Seyfried et al. (1979) was used for the hydrothermal reaction experiment in this study. The autoclave is made of Inconel-alloy, which is corrosion-resistant and possesses adequate strength at elevated temperatures and pressures of up to 600 °C and 600 bar, respectively (Fig. 1; modified after Yoshizaki et al. 2009). The reaction cell is made of a gold bag with a titanium head because these materials are inert with respect to high-temperature water. In addition, as gold is flexible, the water inside the reaction cell can be pressurized by the surrounding water. Although the Ti head is corrosion-resistant, it is known that metallic Ti can react with water to produce hydrogen; thus, the surface of the Ti head was oxidized prior to use. The flexible gold reaction cell allows on-line sampling of the aqueous fluid at almost constant temperature and pressure simply by adding a small amount of pressurized water to the space surrounding the reaction cell in the autoclave. In this way, fluid samples can be obtained from the reaction cell through a gold-lined sampling tube at any time during an ongoing experiment. All materials that would come into contact with the reaction fluid in the experiment were baked in a muffle furnace at 500 °C for 3 h prior to use to eliminate organic matter.

Fig. 1

Schematic of the hydrothermal reactor used in this study, modified after Yoshizaki et al. (2009)

The NaCl solution (approximately 6–7 mol/kg) was prepared from NaCl and pure water to simulate the Hadean seawater, which was potentially saltier than that of the present day owing to the absence of continental crust and associated salt deposits in the Hadean (Knauth 2005). To avoid contamination with organic matter, the NaCl was baked prior to use. The water/rock mass ratio was adjusted to four at the beginning of the experiment (approximately 60 g of solution and 15 g of rock powder) based on the results of Wetzel and Shock (2000) that the water/rock mass ratio in the high-temperature region in the subseafloor hydrothermal system is limited to values less than five. The overall background level of hydrogen concentration in hydrothermal fluid is lower than 0.002 mmol/kg, which was estimated from a 3-month experiment using pure silica powder and NaCl solution under the same temperature and pressure conditions.

Sampling and analysis

Fluid samples were obtained from the reaction cell through a gold-lined sampling tube several times during the ongoing experiment. For the analysis of H₂ concentration, approximately 0.5 ml of fluid was directly introduced into an Ar-purged, sealed vial without air contamination at room temperature. After equilibration between gas and liquid phases in the vial, the H₂ concentration in the gas phase was quantitatively analyzed by gas chromatography (GC) at JAMSTEC. The analytical reproducibility was better than 5 % (1σ). The solid materials of alteration products were dried in an oven immediately after the experiments and then preserved in a vacuum desiccator. The alteration products were analyzed by X-ray diffraction (XRD) at JAMSTEC and by a magnetometer (see below) after the experiment.

Magnetic measurements

For the starting solid material and alteration product of the experiments with olivine, Al₂O₃-5 %, and Al₂O₃-10 % komatiites, the magnetic hysteresis loops at room temperature were measured to estimate the amount of magnetite generated through the experiments using a MicroMag 2900 Alternating Gradient Magnetometer (AGM, Princeton Measurements Corporation) at Kyushu University with a maximum field of 500 mT. The strong-field thermomagnetic curve of the alteration product of the olivine experiment was measured between 50 °C and 780 °C using a MicroMag 3900 Vibrating Sample Magnetometer (VSM, Princeton Measurements Corporation) at the Tokyo Institute of Technology. The magnetic hysteresis loops at low temperatures (10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, and 65 K) were measured for the starting materials and alteration products of the komatiite experiments using an MPMS-XL5 Magnetic Property Measurement System (MPMS, Quantum Design) at the Center for Advanced Marine Core Research (CMCR), Kochi University. In the low-temperature measurements, the maximum field in the hysteresis loop measurement was set to be 3 T.

Results and discussion

The powdered olivine, Al₂O₃-5 %, and Al₂O₃-10 % komatiites were reacted with NaCl solution at 300 °C and 500 bar for 2112–2688 h to assess their H₂ generation potential in seafloor hydrothermal systems. In this section, we discuss the alteration of solid materials and its relation to the H₂ concentration in hydrothermal fluid, which yields implications for H₂-rich hydrothermal environments in the Hadean ocean.

Alteration products

The XRD analyses of the alteration products revealed that all the experiments generated a certain amount of serpentine during the hydrothermal reactions (Fig. 2), indicating that serpentinization occurred during these experiments. However, peaks of smectite were observed in the alteration products after the experiments with both Al₂O₃-5 % and Al₂O₃-10 % komatiites. The peaks of smectite in the alteration products were larger for Al₂O₃-10 % komatiite than for Al₂O₃-5 % komatiite; this suggests that smectite becomes more stable as the Al₂O₃ content of the starting material is increased. In addition, CaO originally contained in the starting material of both komatiites was converted to xonotlite.

Fig. 2

Results of the XRD analysis (CuKα) of the alteration products. XRD pattern of the alteration products from the experiments with (a) olivine, (b) Al₂O₃-5 % komatiite, and (c) Al₂O₃-10 % komatiite. Mineral abbreviations and ideal chemical formula: Ol = olivine ((Mg, Fe)₂SiO₄), Sme = smectite ((Na, Ca)_0.3(Al, Fe, Mg)_2–3(Si, Al)₄O₁₀(OH)₂ · nH₂O), Srp = serpentine ((Mg, Fe)₃Si₂O₅(OH)₄), Brc = brucite ((Mg, Fe)(OH)₂), and Xon = xonotlite (Ca₆Si₆O₁₇(OH)₂)

In general, H₂ generation during serpentinization is caused by the reduction of water as a result of oxidation of ferrous iron in fresh ultramafic rocks (e.g., Janecky and Seyfried 1986). This process can be written as

$$ 2{\left(\mathrm{F}\mathrm{e}\mathrm{O}\right)}_{\mathrm{rock}} + {\mathrm{H}}_2\mathrm{O}\ \to\ {\left({\mathrm{Fe}}_2{\mathrm{O}}_3\right)}_{\mathrm{rock}} + {\mathrm{H}}_2, $$

where (FeO)_rock refers to the ferrous oxide component in silicate minerals (and glass) while (Fe₂O₃)_rock indicates the ferric oxide component of minerals in a serpentinized rock (e.g., McCollom and Bach 2009). Because the ferric oxide typically precipitates as magnetite (ferric/ferrous oxide: Fe₃O₄), the amount of magnetite generated through reactions is strongly related to the H₂ concentration in hydrothermal fluid. In the XRD analyses of the alteration products, no clear peaks of magnetite were identified owing to its small quantities (below the detection limit). However, the magnetic measurements provided constraints on the amounts of effective magnetite generated in the experiments. Figure 3 shows the hysteresis loops of the starting material and alteration product of all experiments. From the hysteresis loops, the saturation magnetization (M _s) values were also calculated (Table 2). In the olivine experiment, the M _s values of the alteration product and starting material were estimated from the room-temperature hysteresis loops to be 3.69 Am²/kg and less than 0.01 Am²/kg, respectively (Fig. 3a, b, and Table 2). Furthermore, the strong-field thermomagnetic curve of the alteration product of the olivine experiment shows that the Curie temperature (T _C) was 538 °C (Fig. 4a), indicating the formation of nearly pure magnetite during the experiment. Using the well-known magnetite M _s value of 92 Am²/kg (Hunt et al. 1995), the alteration product of the olivine experiment is estimated to contain 4.0 wt% magnetite.

Fig. 3

Magnetic hysteresis loops of (a) the starting material and (b) the alteration product of the olivine experiment, (c) the starting material and (d) the alteration product of the Al₂O₃-5 % komatiite experiment, and (e) the starting material and (f) the alteration product of the Al₂O₃-10 % komatiite experiment. The hysteresis loops were corrected by subtracting diamagnetic/paramagnetic slopes. Note that the loops were measured at 10 K for both komatiite samples and at room temperature for the olivine samples

Table 2 Summary of the magnetic measurements

Fig. 4

Results of the Curie temperature measurements. a Strong-field thermomagnetic curve for the alteration product of the olivine experiment. b Temperature dependency of saturation magnetization of the starting material and alteration product from the Al₂O₃-5 % and Al₂O₃-10 % komatiite experiments, which were calculated from their hysteresis loops measured at various temperatures (e.g., Fig. 3)

In both komatiite experiments, the M _s values of the starting material and alteration product were less than 0.05 Am²/kg at room temperature (Table 2), which indicates that these solid materials contain no or only minute quantities of ferromagnetic minerals with T _C higher than room temperature. In contrast, the M _s values calculated from the hysteresis loops measured at 10 K for these samples were greater than 0.80 Am²/kg (Fig. 3c–f, Table 2). These results revealed that the solid materials mainly contained a magnetic mineral with T _C lower than room temperature. The temperature dependences of M _s were also calculated from their hysteresis loops (Fig. 4b), which yielded T _C values of these materials of approximately 30 K. Although it is difficult to determine the composition of these magnetic minerals, the most probable candidate with a T _C value of approximately 30 K is a solid solution between magnetite and spinel phases such as (Fe₃O₄)_x(MgAl₂O₄)_1−x (Harrison and Putnis 1996). The starting materials showed a certain level of M _s value (Table 2), which is consistent with the starting materials originally containing spinel. In addition, the normalized M _s of the starting material of the Al₂O₃-5 % komatiite experiment is slightly higher than that of the alteration product at temperatures above 30 K (Fig. 4b), indicating that a tiny amount of magnetite is probably contained in the starting material in addition to the magnetite–spinel solid solution. On the other hand, the M _s values of the alteration products are clearly higher than those of the starting materials in both experiments. More importantly, the increment of the M _s value in the Al₂O₃-5 % komatiite experiment is higher than that in the Al₂O₃-10 % komatiite experiment (Table 2). This trend strongly suggests that the total amount of effective magnetite component in the magnetite–spinel solid solution that was newly generated during serpentinization is greater in the Al₂O₃-5 % komatiite experiment than in the Al₂O₃-10 % komatiite experiment.

Even considering the results of all experiments, the increment of the M _s value decreases with increasing Al₂O₃ content in the starting material, which is likely derived from the decrease in the amount of effective magnetite generated with increasing Al₂O₃ content in the starting material. It is therefore suggested that the increase in Al₂O₃ content in ultramafic rock elevates the amount of smectite and reduces the amount of magnetite in the alteration minerals. The formation of smectite presumably inhibited magnetite formation because the FeO originally contained in the starting material was incorporated directly into smectite.

Effect of Al₂O₃ content in rocks on H₂ concentration in hydrothermal fluid

The three experiments revealed that the final, steady-state H₂ concentrations of fluids were approximately 60 mmol/kg (with olivine), 20 mmol/kg (with Al₂O₃-5 % komatiite), and 0.05 mmol/kg (with Al₂O₃-10 % komatiite) (Figs. 5 and 6; Table 3). The H₂ concentration in the olivine experiment is significantly higher than that obtained in a previous experiment with olivine at 400 °C (Allen and Seyfried 2003) and is rather comparable with that generated by an experiment simulating serpentinization of lherzolite at 200 °C (Seyfried et al. 2007). This is consistent with the thermodynamic analysis of serpentinization; olivine becomes stable above approximately 315–390 °C, which limits magnetite formation and H₂ generation in hydrothermal fluid (McCollom and Bach 2009). The steady-state H₂ concentration in the Al₂O₃-5 % komatiite experiment is clearly higher than that obtained from the previous ongoing experiment using Al₂O₃-5 % komatiite and pure water (2.4 mmol/kg) (Yoshizaki et al. 2009). This may be due to the difference of initial solution because NaCl solution is generally much more reactive with rocks than pure water. More importantly, the experiments in this study showed that the steady-state H₂ concentration in fluid increases as the Al₂O₃ content in the starting material is reduced (Fig. 7). This pattern is also consistent with the amount of generated magnetite, which increases with decreasing Al₂O₃ content in the starting material. Therefore, it is suggested that the Al₂O₃ content in the starting material strongly affects the generated H₂ concentration in fluid because the Al₂O₃ level controls the formation of smectite, which potentially inhibits magnetite formation.

Fig. 5

Changes in H₂ concentration during the experiments with olivine, Al₂O₃-5 % and Al₂O₃-10 % komatiites. The arrow indicates the maximum background H₂ concentration after c. 2000 h

Fig. 6

Comparison of H₂ concentrations in hydrothermal fluid from experiments and modern natural hydrothermal vent systems. Data sources: Al₂O₃-5 % komatiite experiment with pure water at 300 °C and 500 bar (Yoshizaki et al. 2009), lherzolite experiment at 200 °C and 500 bar (Seyfried et al. 2007), four experiments with Ol, Opx, and/or Cpx at 400 °C and 500 bar (Allen and Seyfried 2003), EPR 9°50′N (Lilley et al. 2003; Von Damm and Lilley 2004), EPR 13°N (Von Damm 1995), EPR 17–19°S (Charlou et al. 1996b), EPR 21°S (Lilley et al. 1983), Lucky Strike and Menez Gwen (Charlou et al. 2000), MARK-1/2 (Charlou et al. 2002), TAG (Charlou et al. 1996a), Logatchev and Rainbow (Charlou et al. 2002), Kairei (Gallant and Von Damm 2006; Kumagai et al. 2008), and Lost City (Proskurowski et al. 2006). The H₂ concentrations of seafloor hydrothermal fluids were mainly obtained from the compilation of Nakamura and Takai (2014). Mineral abbreviations: Ol = olivine, Opx = orthopyroxene, Cpx = clinopyroxene

Table 3 Changes in H₂ concentration and pH of fluid samples through time

Fig. 7

Relationships between fluid H₂ concentration during serpentinization and Al₂O₃ content in the starting solid material. For comparison, the results of the lherzolite experiment at 200 °C (Seyfried et al. 2007) and the experiments with Ol, Opx, and/or Cpx at 400 °C (Allen and Seyfried 2003) are also shown

Comparison with known H₂ concentrations in modern seafloor hydrothermal vent fluids shows that the concentration in the Al₂O₃-10 % komatiite experiment is comparable to those observed in typical MORB-hosted systems (generally 0.1–1 mmol/kg) (Fig. 6). In contrast, natural peridotite-associated high-temperature hydrothermal fluids have higher H₂ concentrations of up to 16 mmol/kg (Charlou et al. 2002; Gallant and Von Damm 2006; Kumagai et al. 2008), which was successfully reconstructed in experiments using orthopyroxene ± clinopyroxene ± olivine at 400 °C (Allen and Seyfried 2003). Thus, the Al₂O₃-5 % komatiite has the potential to generate an H₂ concentration similar to or higher than those observed in modern peridotite-associated hydrothermal fluids.

Potential role of komatiite serpentinization in the origin of life and early ecosystems in the Hadean ocean

Theoretical predictions for the thermodynamic state of prebiotic chemical evolution have highlighted that the H₂-rich hydrothermal environment is energetically advantageous for synthesis and preservation of organic molecules such as amino acids and fatty acids (Amend and McCollom 2009; Amend et al. 2011). Furthermore, it has been hypothesized that the most plausible energy metabolisms to support the emergence and early evolution of life may have been H₂-driven hydrogenotrophic methanogenesis/acetogenesis and/or methanotrophic acetogenesis (Russell et al. 1989; Takai et al. 2006; Martin et al. 2008; Russell et al. 2010, 2014). In this section, we discuss the possible geological settings that may have generated H₂-rich hydrothermal systems in the Hadean ocean.

Considering the relationship between the decreasing mantle temperature during the history of Earth and the chemical composition/production process of komatiites, the different H₂ generation potentials of ADK and AUK imply that komatiite-hosted hydrothermal fluid evolved from H₂-enriched to H₂-depleted through geologic time. Previously, high-pressure experiments on the generation of komatiite melts revealed that ADK melt is generated under high-temperature and high-pressure (>7 GPa) conditions, leaving majoritic garnet as a liquidus phase of mantle, whereas AUK may form owing to high degrees of mantle melting at shallower depths where olivine is a liquidus phase (e.g., Herzberg 1992; Arndt et al. 2008). Therefore, it is believed that the formation of ADK requires a higher mantle plume temperature than for the generation of AUK. This indicates that ADK was produced more abundantly in the oceanic-island/plateau volcanism of the earlier Earth with the hotter mantle than AUK, which is consistent with the geological records of ~3.5 Ga Barberton-type (Al-depleted) and c. 2.7 Ga Munro-type (Al-undepleted) komatiites (Nesbitt et al. 1979; Herzberg 1992). As the Al₂O₃-5 % (Al-depleted) komatiite has a greater potential to enrich H₂ in hydrothermal fluid than Al₂O₃-10 % (Al-undepleted) komatiite (Figs. 6 and 7), the H₂-rich hydrothermal environments hosted by ADK were present abundantly and ubiquitously on the ocean floor prior to the early Archean.

In contrast, the exposure of mantle peridotite at the ocean floor was probably rare in the Archean (Takai et al. 2006). Modern abyssal peridotite is frequently observed near slow-spreading ridges without sufficient magmatic supply because large-scale normal faults (e.g., detachment faults) penetrate deeply, and the mantle peridotite is dragged up to the seafloor (Escartín et al. 2003). However, it has been revealed that the Archean oceanic crust was much thicker than the modern equivalent owing to the higher potential mantle temperature at that time (Moores 2002; Ohta et al. 1996; Komiya 2004; Shibuya et al. 2007, 2012). The thicker ocean crust would have limited the development of deep normal faults reaching to the mantle and exposure of mantle peridotite, which has led to the hypothesis that the early Archean H₂-rich hydrothermal environment was mainly driven by komatiite volcanism instead of much less abundant fault-related peridotite (Takai et al. 2006). This hypothesis is further substantiated by the results obtained in this study and can be applied to the Hadean. The serpentinization of ADK may have generated abundant H₂-rich (ca. 20 mmol/kg) hydrothermal fluid and proximal H₂-rich hydrothermal fluid–seawater mixing zones in the Hadean ocean, which could potentially have served as nurseries for prebiotic chemical evolution and the subsequent emergence and early evolution of life on Earth. Furthermore, considering that the oceanic crust was probably sufficiently thick throughout the Hadean and Archean to limit the exposure of peridotite on the seafloor and that ADK disappeared after the early Archean, the Hadean era was more favorable for the emergence of life than the Archean era in terms of H₂-rich hydrothermal environments.

Conclusions

The H₂ generation potential of ADK and AUK during serpentinization was estimated by the experimental hydrothermal reactions of komatiites at 300 °C and 500 bar. The experiments revealed that the H₂ concentration (ca. 20 mmol/kg) in hydrothermal fluid generated from the serpentinization of ADK is comparable to that of the most H₂-rich hydrothermal systems in modern oceans. Furthermore, considering the limited exposure of mantle peridotite owing to the thick lid of oceanic crusts in the Hadean ocean floor, ADK was likely the most ubiquitous seafloor ultramafic rock that could host H₂-rich hydrothermal system. Such H₂-rich hydrothermal systems would have provided a favorable environment (redox, electrochemical, and thermal gradients in the seawater/hydrothermal fluid mixing zone) for the acquisition of bioavailable free energy and for prebiotic chemical evolution in the Hadean. Further experiments and thermodynamic calculations to simulate the mixing between Hadean seawater and H₂-rich hydrothermal fluid should be carried out to test this hypothesis.