The unusual structural properties of the Fe-Mg serpentine minerals permit significant chemical variability, but the mechanisms and extent of elemental substitution have only recently come to light. New results show that greenalite forms solid solutions with the Fe(III) end-member hisingerite, cronstedtite, and Mg-serpentines, with the composition depending on formation conditions. Leveraging this new mineralogical context enables quantitative estimation of H2 production on Earth and Mars. Together, these advances indicate that greenalite solid solutions in ancient rocks produced and released H2 and thus contributed to planetary habitability. Examination of Martian rocks and analogous Earth materials shows greenalite-hisingerite minerals were responsible for H2 fluxes to the ancient Martian atmosphere and could be important contributors to planetary habitability throughout the Solar System.

The Samail Ophiolite in Oman, the largest exposed body of ultramafic rocks at the Earth’s surface, produces a continuous flux of hydrogen through low-temperature water/rock reactions. In turn, the scale of the subsurface microbial biosphere is sufficient to consume much of this hydrogen, except where H 2 is delivered to surface seeps via faults. By integrating data from recent investigations into the alteration history of the peridotites, groundwater dynamics, and the serpentinite-hosted microbial communities, we identify feasible subsurface conditions for a pilot demonstration of stimulated geological hydrogen production. A simple technoeconomic analysis shows that the stimulation methods to be used must increase the rate of net hydrogen production at least 10,000-fold compared to the estimated natural rate to economically produce hydrogen from engineered water/rock reactions in the peridotite formations. It may be possible to meet this challenge within the upper 1–2 km, given the projected availability of reactive Fe(II)-bearing phases and the lower drilling costs associated with shallower operations. Achieving ≥10,000-fold increases in the H 2 production rate will require a combination of stimuli. It will likely be necessary to increase the density of fracturing in the reaction volume by at least two orders of magnitude. Then, the H 2 -production rates must also be increased by another two orders of magnitude by increasing the water/rock ratio and modifying the chemistry of the injected fluids to optimize formation of Fe(III)-bearing secondary phases. These fluid modifications must be designed to simultaneously minimize microbial consumption of H 2 within the stimulation volume. In contrast, preserving the high potentials for biological H 2 consumption in the shallow groundwaters replete with oxidants such as nitrate, sulfate and dissolved inorganic carbon will reduce the potential for any inadvertent leaks of hydrogen to the atmosphere, where it acts as an indirect greenhouse gas.

Mass transfer processes between fluids and ultramafic rocks produce subsurface environments encompassing a wide range of redox conditions. A notable locality where an extensive range of redox conditions is observed in one location is Hole BA1B, a similar to 400 m borehole drilled by the Oman Drilling Project. A sulfur-enriched serpentinite zone, containing up to 0.6 wt% S, occurs between shallow oxidized serpentinites (<30 m) and deep partially serpentinized harzburgite (>150 m). All three alteration zones are predominantly composed of serpentine. However, microanalysis of samples from the sulfur-enriched zone shows that mesh textures after olivine are composed of serpentine, brucite, and tochilinite mixtures, yielding optically black thin-section samples that characterize this sulfidic zone. It is proposed that sulfur accumulates in this zone via a process similar to those found in supergene ore deposits. Reaction-path models show that at shallow conditions open to atmospheric input, sulfur is mobilized via oxidative weathering of serpentinized dunite and harzburgite. Sulfatebearing fluids percolate deeper and react with host rocks in a system closed to atmospheric input. As fluids become more reduced, dissolved sulfate is precipitated as sulfide minerals yielding rocks with similar to 0.4 wt% S, like those observed in Hole BA1B. Despite enrichment of S in the sulfidic zone in Hole BA1B, Ni and Co contents are uniform throughout all three layers in the borehole. This is consistent with model results which show that Ni (and, by analogy, Co) is less mobile than S, and can be hosted in serpentine and NiFe alloys in addition to sulfides. The sulfur enrichment process may occur abiotically. However, sulfide enrichment via microbial reduction of sulfate and other sulfur species can also facilitate the formation of the sulfidic zone. Bioenergetic calculations show that abundant energy is available for sulfur reducing microbes, consistent with previous work demonstrating the presence of active, sulfate-reducing microorganisms in Hole BA1B and other nearby boreholes. This suggests that the observed sulfur enrichment is an ongoing process. Overall, this work shows that variable redox conditions are attained as fluids percolate and react with serpentinized ultramafic rocks at variable extents of interaction between aquifer fluids, host ultramafic rocks, and the atmosphere.

Reduced (H2- and CH4-rich) and hyperalkaline fluids are products of subsurface reactions accompanying serpentinization of ultramafic rocks. H2 and CH4 produced during serpentinization can fuel microorgan-isms and support habitable subsurface environments. CH4 is also a potent greenhouse gas and can offset negative greenhouse emissions arising from active CO2 removal accompanying carbon mineralization in ultramafic rocks. However, the rate at which reduced volatiles are delivered to the surface and the rate of reactions that generate these volatiles at low-temperature conditions are poorly known. In this work, we measured H-2 and CH4 outgassing rates in several hyperalkaline spring sites in the Samail ophiolite, Oman. H-2 and CH4 outgassing in these sites are variable and range up to 70,000 and 7,000 mol yr(1), respectively. CH4 outgassing in spring sites are unlikely to offset negative carbon emissions estimated from active car-bon mineralization reactions in the Samail ophiolite. However, diffused CH4 outgassing from peridotite outcrops remain unconstrained. Compositional and isotopic constraints show that volatiles are likely derived from active serpentinization, fluid inclusion decrepitation, or a combination of both. Calculated active serpentinization rates of up to 8 x 10-14 sec(-1) account for measured outgassing rates and these are consistent with slow rates expected at low temperatures. In calculations of serpentinization rates, this work uses reaction-path models to account incorporation of both ferrous and ferric iron in the resulting alteration assemblages, which yields similar to 0.3 mol H-2 kg(-1) of ultramafic rock altered, lower than simulations based on iron oxidation to magnetite only. Contribution from decrepitation of H2- and CH(4-)bearing fluid inclusions is possible but would require much more mass of ultramafic rocks to account for observed out -gassing. Further studies can help quantify extents of each source on active outgassing in Oman. Overall, this work shows that low-temperature serpentinization on geologically short timescales can account for observed flux of reduced volatiles in hyperalkaline environments.(c) 2023 Elsevier Ltd. All rights reserved.

In subduction zones, carbon from the surface can be transported into Earth's interior. Fluids released during subduction can mobilize carbon bound in solid minerals in the subducting plate, and transport it into the overlying mantle. While there is increasing attention on the composition and redox state of carbon-bearing, subduction-mobilized fluids, the fate of fluids as they migrate through and react with overlying mantle rocks is less well characterized. Using thermodynamic modeling, we quantify the evolution of carbon-bearing aqueous fluids mobilized from subducting sedimentary rocks, as they follow several possible pathways through the overlying mantle. The extent of carbon mineralization in the overlying mantle depends on the source (siliciclastic vs carbonate rock) and composition of the fluids as well as the fluid path (isothermal, cooling, heating). A heating path towards the hot core of the mantle wedge does not favor sequestration of carbon into solid phases. Along cooling trajectories, most of the dissolved carbon (61 to 98 %) forms in solid minerals when carbon-rich, siliciclastic-equilibrated fluids react with mantle rocks. In contrast to fluids from siliciclastic sediments, less carbon is sequestered in the mantle when relatively carbon-poor, limestone-equilibrated fluids react with mantle rocks. The formation of quartz-rich (birbirite), quartz-magnesite (listvenite), talc-magnesite (soapstone), and serpentine-rich (serpentinite) parageneses is predicted at decreasing water-rock ratios. Soapstones are less favored to form along fractional crystallization paths and/or lower temperatures. Listvenite assemblages form from migrating fluids with dissolved carbon concentration > 20,000 ppm. Graphite is favored to form along closed reaction, cooling paths. Moreover, several cooling paths form reduced fluids rich in hydrogen (H2), methane (CH4), and organic species. Overall, this work provides quantitative models that can comprehensively inform studies on the movement of carbon in subduction zones and into the deep Earth.

A geochemical gradient established by mixing between reduced, hyperalkaline (pH > 11), H 2 ‐rich fluids generated through the process of serpentinization and surrounding surface water (pH ∼ 8) in the Samail Ophiolite of Oman provides an opportunity to characterize the geochemical and biological factors that influence the distribution of H 2 oxidizing chemotrophs, hydrogenotrophs. In this study, 16S rRNA gene amplicon sequencing was implemented to characterize hydrogenotrophs in sediments underlying surface expressed serpentinized fluids in Oman. Hydrogenotroph phylotype distribution was evaluated as functions of chemical energy supplies for their given metabolic redox reactions. Through this approach, it was discovered that hydrogenotrophic taxa are likely constrained to sediments with overlying fluids that have <∼60 μ m O 2 , including microorganisms of the genus, Hydrogenophaga . Sulfate reducers of the family, Thermodesulfovibrionaceae , likely require >∼10 μ m SO 4 −2 for survival. In sediments with fluids having >∼10 μ m SO 4 −2 , sulfate reducers likely outcompete microorganisms of the methanogen genus, Methanobacterium , for H 2 . Additionally, differences in distribution between Thermodesulfovibrionaceae and Methanobacterium may be driven by the availability of electron acceptors and the redox reaction that is most energy yielding in the fluid. Taken together, observations from the Oman geochemical gradient result in a hydrogenotroph niche model that can be used to evaluate global distribution patterns of hydrogenotrophs in continental serpentinized fluids. On a global scale, based on previous studies, Methanobacterium is constrained to fluids that have <∼10 μ m SO 4 −2 .

The Oman Drilling Project “Multi‐Borehole Observatory” (MBO) samples an area of active weathering of tectonically exposed peridotite. This article reviews the geology of the MBO region, summarizes recent research, and provides new data constraining ongoing alteration. Host rocks are partially to completely serpentinized, residual mantle harzburgites, and replacive. Dunites show evidence for “reactive fractionation,” in which cooling, crystallizing magmas reacted with older residues of melting. Harzburgites and dunites are 65%–100% hydrated. Ferric to total iron ratios vary from 50% to 90%. In Hole BA1B, alteration extent decreases with depth. Gradients in water and core composition are correlated. Serpentine veins are intergrown with, and cut, carbonate veins with measurable 14C. Ongoing hydration is accompanied by SiO2 addition. Sulfur enrichment in Hole BA1B may result from oxidative leaching of sulfur from the upper 30 m, coupled with sulfate reduction and sulfide precipitation at 30–150 m. Oxygen fugacity deep in Holes BA3A, NSHQ14, and BA2A is fixed by the reaction 2H2O = 2H2 + O2 combined with oxidation of ferrous iron in serpentine, brucite, and olivine. fO2 deep in Holes BA1A, BA1D, and BA4A is 3–4 log units above the H2O‐H2 limit, controlled by equilibria involving serpentine and brucite. Variations in alteration are correlated with texture, with reduced, low SiO2 assemblages in mesh cores recording very low water/rock ratios, juxtaposed with adjacent veins recording much higher ratios. The proportion of reduced mesh cores versus oxidized veins increases with depth, and the difference in fO2 recorded in cores and veins decreases with depth.

At present, molecular hydrogen (H 2 ) produced through Fe(II) oxidation during serpentinization of ultramafic rocks represents a small fraction of the global sink for O 2 due to limited exposures of ultramafic rocks. In contrast, ultramafic rocks such as komatiites were much more common in the Early Earth and H 2 production via serpentinization was a likely factor in maintaining an O 2 -free atmosphere throughout most of the Archean. Using thermodynamic simulations, this work quantifies the global O 2 consumption attributed to serpentinization during the past 3.5 billion years. Results show that H 2 generation is strongly dependent on rock compositions where serpentinization of more magnesian lithologies generated substantially higher amounts of H 2 . Consumption of >2 Tmole O 2 yr −1 via low-temperature serpentinization of Archean continents and seafloor is possible. This O 2 sink diminished greatly towards the end of the Archean as ultramafic rocks became less common and helped set the stage for the Great Oxidation Event.

This paper provides an overview of research on core from Oman Drilling Project Hole BT1B and the surrounding area, plus new data and calculations, constraining processes in the Tethyan subduction zone beneath the Samail ophiolite. The area is underlain by gently dipping, broadly folded layers of allochthonous Hawasina pelagic sediments, the metamorphic sole of the Samail ophiolite, and Banded Unit peridotites at the base of the Samail mantle section. Despite reactivation of some faults during uplift of the Jebel Akdar and Saih Hatat domes, the area preserves the tectonic “stratigraphy” of the Cretaceous subduction zone. Gently dipping listvenite bands, parallel to peridotite banding and to contacts between the peridotite and the metamorphic sole, replace peridotite at and near the basal thrust. Listvenites formed at less than 200°C and (poorly constrained) depths of 25–40 km by reaction with CO 2 ‐rich, aqueous fluids migrating from greater depths, derived from devolatilization of subducting sediments analogous to clastic sediments in the Hawasina Formation, at 400°–500°. Such processes could form important reservoirs for subducted CO 2 . Listvenite formation was accompanied by ductile deformation of serpentinites and listvenites—perhaps facilitated by fluid‐rock reaction—in a process that could lead to aseismic subduction in some regions. Addition of H 2 O and CO 2 to the mantle wedge, forming serpentinites and listvenites, caused large increases in the solid mass and volume of the rocks. This may have been accommodated by fractures formed as a result of volume changes, mainly at a serpentinization front.

Thermodynamic calculations provide valuable insights into the reactions that drive the profound fluid transformations during serpentinization, where surface fluids are transformed into some of the most reduced and alkaline fluids on Earth. However, environmental observations usually deviate from thermodynamic predictions, especially those occurring at low temperatures where equilibrium is slowly reached. In this work, we analyzed 138 low‐temperature (<40°C) fluids from the Samail ophiolite in Oman to test thermodynamic predictions with environmental observations. Four fluid types were identified through this work. (i) Type 1 circumneutral (pH 7–9) fluids result from fluid interactions with serpentinized rocks common in the shallow subsurface. (ii) Fluids with pH ranging from 9 to 11 and low Si concentrations are products of intermediate stages of serpentinization. (iii) Type 2 hyperalkaline (pH > 11) fluids approach equilibrium with diopside, and with serpentine and brucite actively forming during advanced stages of serpentinization. Lastly, (iv) most fluids sampled in this work deviate from predicted equilibrium compositions and depict various degrees of mixing between Type 1 and 2 fluids. Mixed fluids fall within the same pH range but have considerably higher dissolved Si than intermediate‐type fluids. Hyperalkaline fluids exhibit variable degrees of mixing despite maintaining pH > 11, implying strong buffering capacity of serpentinization‐generated fluids. Overall, this work demonstrates that predicted and measured compositions of serpentinization‐derived fluids can be reconciled using a combination of equilibrium and fluid‐transport simulations. This work substantiates these calculations as useful tools in exploring serpentinization reactions in continents and perhaps in other low‐temperature environments on Earth and beyond.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Carbon Capture and Storage (CCS) aims to gather and store atmospheric CO2, often in geologic reservoirs, to mitigate the increasing atmospheric CO2 concentrations that lead to climate change. While the majority of CCS projects to date focus on structurally trapping CO2 in gaseous form in porous sedimentary rocks, carbon mineralization approaches storage from a much more secure perspective by storing CO2 as a solid carbonate mineral phase. During the carbon mineralization process, interactions between the host rock and the fluids flowing through the rock’s permeable pathways exert a primary control on the evolution of permeability of the system. Precipitation of mineral phases within the fracture network can significantly reduce the permeability of the overall system (clogging), whereas mineral dissolution and volume positive mineral reactions (leading to cracking) can enhance permeability. The coupling between these competing processes dictates reservoir permeability and thus the long-term storage capacity and lifetime of CO2 storage reservoirs. Experimental studies are therefore vital to understand the chemo-mechanical controls on dissolution, precipitation, and carbonation-induced cracking, as well as to quantify their effect on the permeability of the system. In this study, we perform experiments using a new AutoLab triaxial deformation apparatus equipped with independently servo-controlled axial load, confining, and fluid pressures. Samples are prepared via cold press from Twin Sisters peridotite powdered to a mean particle size of 94 µm. Experimental conditions are set to reproduce shallow crust conditions at viable injection depths and are controlled at a confining pressure of 20 MPa and fluid pressures of 10 MPa. Experimental temperatures range from 20 to 150 °C. Pore fluids are mixed in a joint mixing vessel using deionized water and sodium bicarbonate forming a solution of 0.6 M concentration. The solution is then pressurized using CO2 (99.9% purity) to a pressure of 3.5 MPa serving both as a vehicle for CO2 transport and as pH buffer. Permeability, ultrasonic wave velocities, axial strain, pH and fluid composition are monitored during these flow-through experiments. Preliminary results relate progress of the mineral carbonation reaction through the sample with a systematic decrease in permeability and an associated increase in P wave velocity. The results of this experimental study will be used to constrain the most favourable conditions for CO2 storage in a solid form, which is fundamental to the upscaling of carbon mineralization as an innovative, efficient and safe method for CO2 storage.

The hydrous alteration of ultramafic rocks, known as serpentinization, generates fluids that can fuel microbial communities and enable the synthesis of simple organic compounds. Serpentinization reactions can proceed even at the ambient, low-temperature conditions present in continental aquifers raising questions about the limits of life deep in the Earth's subsurface. Through thermodynamic calculations, we investigate various reactions that facilitate the transformation of oxic, slightly acidic rainwater into reduced, hyperalkaline fluids during low-temperature serpentinization. We explore a suite of factors (variabilities in temperature, host-rock compositions, fluid salinity, and the buffering capacity of various serpentinization-relevant minerals) that offer broad insights into the chemical environments formed through low-temperature serpentinization. Results of calculations show that alteration of olivine-rich lithologies will lead to fluids constrained by the chrysotile-brucite-diopside equilibrium assemblage, close in pH to those measured from the most alkaline fluids hosted in ultramafic rocks. Variabilities in the compositions of fluids hosted by continental serpentinizing systems can be attributed to a shift from being in equilibrium with diopside to calcite, among other reactions. Results of calculations also show that it would be difficult to distinguish fluids reacting with either fresh or altered ultramafic rocks based solely on their pH, and total dissolved Ca, Mg and Si content. Our models also account for Fe incorporation into solid solutions of serpentine and brucite and show that the global H-2 flux from continental serpentinization could be considerably lower than estimates based on iron oxidation to magnetite only. Lastly, we present the energetic landscape available to subsurface microorganisms by focusing on two microbial process using H-2: methanogenesis and hydrogen oxidation. Limited but available energy (0.2-1.7 calories/kg fluid) can be exploited by methanogens, permitting the possibility of deep communities in serpentinizing aquifers. More energy is available for methanogenesis (0.2-6 calories/kg fluid) and hydrogen oxidation (0-17 calories/kg fluid) when upwelling, deep-seated, serpentinization-generated fluids mix with shallow groundwater. Ultimately, predictions set forth in this study provide a framework for testing ideas that can explain the compositions of fluids and microbial communities sampled at ultramafic environments here on Earth and perhaps in the near future, on ocean worlds in our solar system.

809 deep IODP Hole U1473A at Atlantis Bank, SWIR, is 2.2 km from 1,508-mHole 735B and 1.4 from 158-m Hole 1105A. With mapping, it provides the first 3-D view of the upper levels of a 660-km(2) lower crustal batholith. It is laterally and vertically zoned, representing a complex interplay of cyclic intrusion, and ongoing deformation, with kilometer-scale upward and lateral migration of interstial melt. Transform wall dives over the gabbro-peridotite contact found only evolved gabbro intruded directly into the mantle near the transform. There was no high-level melt lens, rather the gabbros crystallized at depth, and then emplaced into the zone of diking by diapiric rise of a crystal mush followed by crystal-plastic deformation and faulting. The residues to mass balance the crust to a parent melt composition lie at depth below the center of the massif-likely near the crust-mantle boundary. Thus, basalts erupted to the seafloor from >1,550 mbsf. By contrast, the Mid-Atlantic Ridge lower crust drilled at 23 degrees N and at Atlantis Massif experienced little high-temperature deformation and limited late-stage melt transport. They contain primitive cumulates and represent direct intrusion, storage, and crystallization of parental MORB in thinner crust below the dike-gabbro transition. The strong asymmetric spreading of the SWIR to the south was due to fault capture, with the northern rift valley wall faults cutoff by a detachment fault that extended across most of the zone of intrusion. This caused rapid migration of the plate boundary to the north, while the large majority of the lower crust to spread south unroofing Atlantis Bank and uplifting it into the rift mountains.