Explore the vast range of titles available.

Abstract Hydrogen is gaining attention as an energy source for its zero CO2 emission. Hydrogen may be manufactured by breaking down water using solar and wind energy. However, the high cost of manufactured hydrogen makes it uneconomical. Recently, earth scientists have discovered that hydrogen is generated naturally in Earth’s crust through a process called serpentinization, and may be extracted with a near-zero CO2 footprint. Serpentinization adds magnetite to the hydrogen source rocks and makes them more magnetic, providing the basis for understanding natural hydrogen systems using geophysics. We investigate the magnetic data over the Iowa section of the Midcontinent Rift and identify the signature of serpentinization in conjunction with gravity and electromagnetic data. Our research results show that a huge volume of igneous rock has undergone serpentinization in the Iowa section of Midcontinent Rift. Consequently, significant accumulation of natural hydrogen may be present under the Iowa farmlands.

The southern Mariana subduction zone, home to the Challenger Deep—the deepest known point on Earth—poses significant challenges for studying the hydration of the subducting plate due to its extreme depth. This study uses S‐wave seismic tomography and Vp/Vs ratios to investigate hydration and serpentinization at the Challenger Deep. We observe a low Vp and Vs layer in the upper mantle with Vp/Vs ratios exceeding 1.8, reaching up to 1.95 at the Moho. These high ratios indicate a strong serpentinized layer (>15 vol%) with significant changes in the mechanical properties of the serpentinized peridotite. Additionally, Vp/Vs ratios in the crust and uppermost mantle increase from the outer rise to the trench axis, demonstrating that bending‐related faulting and hydration intensify as the plate approaches the trench. Our results suggest extensive faulting, hydration, and mantle serpentinization at the Challenger Deep, making this region an extreme example of water cycling in subduction zones. Plain Language Summary The southern Mariana Trench, containing the deepest point on Earth's surface, is where the old Pacific Plate (∼125 Ma) is subducting beneath the Philippine plate. Understanding the processes of bending‐related faulting and hydration of the incoming subducting plate has been challenging due to the limitations of using only P‐wave velocity (Vp), which does not provide detailed lithological information. In this study, we identified valuable converted S‐wave arrivals from the incoming plate, allowing us to determine the S‐wave velocity (Vs) structure and calculate the Vp/Vs ratios. Our results reveal that the low Vp layer in the upper mantle is a strongly serpentinized layer. Compared to other subduction zones, the combination of lower Vp and Vs values with higher Vp/Vs ratios suggests more intense serpentinization within the incoming plate at the southern Mariana subduction zone. This study provides a clearer understanding of mantle hydration processes in extreme subduction environments and highlights how plate characteristics influence serpentinization intensity. Key Points S‐wave tomography and Vp/Vs ratios reveal extensive serpentinization and hydration in the subducting plate of the southern Mariana Trench Vp/Vs ratios in the crust and uppermost mantle increase toward the trench axis, indicating intensified hydration as the plate approaching Challenger Deep is an extreme example of water cycling in subduction zones

Fluids circulating through actively serpentinizing systems are often highly enriched in methane (CH₄). In many cases, the CH₄ in these fluids is thought to derive from abiotic reduction of inorganic carbon, but the conditions under which this process can occur in natural systems remain unclear. In recent years, several studies have reported abiotic formation of CH₄ during experimental serpentinization of olivine at temperatures at or below 200 °C. However, these results seem to contradict studies conducted at higher temperatures (300 °C to 400 °C), where substantial kinetic barriers to CH₄ synthesis have been observed. Here, the potential for abiotic formation of CH₄ from dissolved inorganic carbon during olivine serpentinization is reevaluated in a series of laboratory experiments conducted at 200 °C to 320 °C. A 13C-labeled inorganic carbon source was used to unambiguously determine the origin of CH₄ generated in the experiments. Consistent with previous high-temperature studies, the results indicate that abiotic formation of CH₄ from reduction of dissolved inorganic carbon during the experiments is extremely limited, with nearly all of the observed CH₄ derived from background sources. The results indicate that the potential for abiotic synthesis of CH₄ in low-temperature serpentinizing environments may be much more limited than some recent studies have suggested. However, more extensive production of CH₄ was observed in one experiment performed under conditions that allowed an H₂-rich vapor phase to form, suggesting that shallow serpentinization environments where a separate gas phase is present may be more favorable for abiotic synthesis of CH₄.

A series of three laboratory experiments were conducted to investigate how pH affects reaction pathways and rates during serpentinization. Two experiments were conducted under strongly alkaline conditions using olivine as reactant at 200 and 230°C, and the results were compared with previous studies performed using the same reactants and methods at more neutral pH. For both experiments, higher pH resulted in more rapid serpentinization of the olivine and generation of larger amounts of H 2 for comparable reaction times. Proportionally greater amounts of Fe were partitioned into brucite and chrysotile and less into magnetite in the experiments conducted at higher pH. In a third experiment, alkaline fluids were injected into an ongoing experiment containing olivine and orthopyroxene to raise the pH from circumneutral to strongly alkaline conditions. Increasing the pH of the olivine-orthopyroxene experiment resulted in an immediate and steep increase in H 2 production, and led to far more extensive reaction of the primary minerals compared to a similar experiment conducted under more neutral conditions. The results suggest that the development of strongly alkaline conditions in actively serpentinizing systems promotes increased rates of reaction and H 2 production, enhancing the flux of H 2 available to support biological activity in these environments. This article is part of a discussion meeting issue ‘Serpentinite in the Earth System’.

We examined the mineralogical, chemical and isotopic compositions of secondary fluid inclusions in olivine-rich rocks from two active serpentinization systems: the Von Damm hydrothermal field (Mid-Cayman Rise) and the Zambales ophiolite (Philippines). Peridotite, troctolite and gabbroic rocks in these systems contain abundant CH 4 -rich secondary inclusions in olivine, with less abundant inclusions in plagioclase and clinopyroxene. Olivine-hosted secondary inclusions are chiefly composed of CH 4 and minor H 2 , in addition to secondary minerals including serpentine, brucite, magnetite and carbonates. Secondary inclusions in plagioclase are dominated by CH 4 with variable amounts of H 2 and H 2 O, while those in clinopyroxene contain only CH 4 . We determined hydrocarbon abundances and stable carbon isotope compositions by crushing whole rocks and analysing the released volatiles using isotope ratio monitoring—gas chromatography mass spectrometry. Bulk rock gas analyses yielded appreciable quantities of CH 4 and C 2 H 6 in samples from Cayman (4–313 nmol g −1 CH 4 and 0.02–0.99 nmol g −1 C 2 H 6 ), with lesser amounts in samples from Zambales (2–37 nmol g −1 CH 4 and 0.004–0.082 nmol g −1 C 2 H 6 ). Mafic and ultramafic rocks at Cayman exhibit δ 13 C CH 4 values of −16.7‰ to −4.4‰ and δ 13 C C 2 H 6 values of −20.3‰ to +0.7‰. Ultramafic rocks from Zambales exhibit δ 13 C CH 4 values of −12.4‰ to −2.8‰ and δ 13 C C 2 H 6 values of −1.2‰ to −0.9‰. Similarities in the carbon isotopic compositions of CH 4 and C 2 H 6 in plutonic rocks, Von Damm hydrothermal fluids, and Zambales gas seeps suggest that leaching of fluid inclusions may provide a significant contribution of abiotic hydrocarbons to deep-sea vent fluids and ophiolite-hosted gas seeps. Isotopic compositions of CH 4 and C 2 H 6 from a variety of hydrothermal fields hosted in olivine-rich rocks that are similar to those in Von Damm vent fluids further support the idea that a significant portion of abiotic hydrocarbons in ultramafic-influenced vent fluids is derived from fluid inclusions. This article is part of a discussion meeting issue ‘Serpentinite in the Earth system’.

Hydrogen from serpentinization is a source of chemical energy for some life forms on Earth. It is a potential fuel for life in the subsurface of Mars and in the icy ocean worlds in the outer solar system. Serpentinization is also implicated in life’s origin. Planetary exploration offers a way to investigate such theories by characterizing and ultimately searching for life in geochemical settings that no longer exist on Earth. At present, much of the current context of serpentinization on other worlds relies on inference from modelling and studies on Earth. While there is evidence from orbital spectral imaging and martian meteorites that serpentinization has occurred on Mars, the extent and duration of that activity has not been constrained. Similarly, ongoing serpentinization might explain hydrogen found in the ocean of Saturn’s tiny moon Enceladus, but this raises questions about how long such activity has persisted. Titan’s hydrocarbon-rich atmosphere may derive from ancient or present-day serpentinization at the bottom of its ocean. In Europa, volcanism or serpentinization may provide hydrogen as a redox couple to oxygen generated at the moon’s surface. We assess the potential extent of serpentinization in the solar system’s wet and rocky worlds, assuming that microfracturing from thermal expansion anisotropy sets an upper limit on the percolation depth of surface water into the rocky interiors. In this bulk geophysical model, planetary cooling from radiogenic decay implies the infiltration of water to greater depths through time, continuing to the present. The serpentinization of this newly exposed rock is assessed as a significant source of global hydrogen. Comparing the computed hydrogen and surface-generated oxygen delivered to Europa’s ocean reveals redox fluxes similar to Earth’s. Planned robotic exploration missions to other worlds can aid in understanding the planetary context of serpentinization, testing the predictions herein. This article is part of a discussion meeting issue ‘Serpentinite in the Earth System’.

The conditions of methane (CH₄) formation in olivine-hosted secondary fluid inclusions and their prevalence in peridotite and gabbroic rocks from a wide range of geological settings were assessed using confocal Raman spectroscopy, optical and scanning electron microscopy, electron microprobe analysis, and thermodynamic modeling. Detailed examination of 160 samples from ultraslow- to fast-spreading midocean ridges, subduction zones, and ophiolites revealed that hydrogen (H₂) and CH₄ formation linked to serpentinization within olivine-hosted secondary fluid inclusions is a widespread process. Fluid inclusion contents are dominated by serpentine, brucite, and magnetite, as well as CH4(g) and H2(g) in varying proportions, consistent with serpentinization under strongly reducing, closed-system conditions. Thermodynamic constraints indicate that aqueous fluids entering the upper mantle or lower oceanic crust are trapped in olivine as secondary fluid inclusions at temperatures higher than ∼400 °C. When temperatures decrease below ∼340 °C, serpentinization of olivine lining the walls of the fluid inclusions leads to a near-quantitative consumption of trapped liquid H₂O. The generation of molecular H₂ through precipitation of Fe(III)-rich daughter minerals results in conditions that are conducive to the reduction of inorganic carbon and the formation of CH₄. Once formed, CH4(g) and H2(g) can be stored over geological timescales until extracted by dissolution or fracturing of the olivine host. Fluid inclusions represent a widespread and significant source of abiotic CH₄ and H₂ in submarine and subaerial vent systems on Earth, and possibly elsewhere in the solar system.

Mantle peridotite in Wadi Fins in eastern Oman exhibits three concentric alteration zones with oxide and sulfide mineralogy recording gradients in f O 2 and f S 2 (fugacity) of more than 20 orders of magnitude over 15–20 cm. The black cores of samples (approx. 5 cm in diameter) exhibit incomplete, nearly isochemical serpentinization, with relict primary mantle minerals (olivine, orthopyroxene and clinopyroxene) along with sulfide assemblages (pentlandite/heazlewoodite/bornite) recording low f O 2 and moderate f S 2 . In addition to the black cores, two alteration zones are evident from their colouration in outcrop and hand samples: green and red. These zones exhibit non-isochemical alteration characterized by intergrowths of stevensite/lizardite. All three reaction zones are cut by calcite ± serpentine veins, which are most abundant in the outer, red zones, sometimes are flanked by narrow red and/or green zones where they cut the black zones, and thus may be approximately coeval with all three alteration zones. The alteration zones record progressively higher f O 2 recorded by Ni-rich sulfides and iron oxides/hydroxides. These alteration zones lost 20–30% of their initial magnesium content, together with mobilization of iron over short distances from inner green zones into outer red zones, where iron is reprecipitated in goethite intermixed with silicates due to higher f O 2 . Thermodynamic modelling at 60°C and 50 MPa (estimated alteration conditions) reproduces sulfide assemblages, f O 2 changes and Mg and Fe mobility. This article is part of a discussion meeting issue ‘Serpentinite in the Earth system’.

Hydrogen is gaining attention as an energy source for its zero CO2 emission. Hydrogen may be manufactured by breaking down water using solar and wind energy. However, the high cost of manufactured hydrogen makes it uneconomical. Recently, earth scientists have discovered that hydrogen is generated naturally in Earth’s crust through a process called serpentinization, and may be extracted with a near-zero CO2 footprint. Serpentinization adds magnetite to the hydrogen source rocks and makes them more magnetic, providing the basis for understanding natural hydrogen systems using geophysics. We investigate the magnetic data over the Iowa section of the Midcontinent Rift and identify the signature of serpentinization in conjunction with gravity and electromagnetic data. Our research results show that a huge volume of igneous rock has undergone serpentinization in the Iowa section of Midcontinent Rift. Consequently, significant accumulation of natural hydrogen may be present under the Iowa farmlands.

The Lost City hydrothermal field is a dramatic example of the biological potential of serpentinization. Microbial life is prevalent throughout the Lost City chimneys, powered by the hydrogen gas and organic molecules produced by serpentinization and its associated geochemical reactions. Microbial life in the serpentinite subsurface below the Lost City chimneys, however, is unlikely to be as dense or active. The marine serpentinite subsurface poses serious challenges for microbial activity, including low porosities, the combination of stressors of elevated temperature, high pH and a lack of bioavailable ∑CO 2 . A better understanding of the biological opportunities and challenges in serpentinizing systems would provide important insights into the total habitable volume of Earth's crust and for the potential of the origin and persistence of life in Earth's subsurface environments. Furthermore, the limitations to life in serpentinizing subsurface environments on Earth have significant implications for the habitability of subsurface environments on ocean worlds such as Europa and Enceladus. Here, we review the requirements and limitations of life in serpentinizing systems, informed by our research at the Lost City and the underwater mountain on which it resides, the Atlantis Massif. This article is part of a discussion meeting issue ‘Serpentinite in the Earth System’.