Theoretical predictions vs environmental observations on serpentinization fluids: Lessons from the Samail ophiolite in Oman

Leong, James Andrew; Howells, Alta; Robinson, Kirtland J.; Cox, Alysia; Debes, Randall; Fecteau, Kristopher M.; Prapaipong, Panjai; Shock, Everett L.

doi:10.1002/essoar.10504642.1

Cited by 10 publications

(40 citation statements)

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“…A likely interpretation of the data on sulfide concentration in the core, the "black serpentinite" zone in the core, and the presence of stinky black goo possibly containing tochilinite, is that there is ongoing sulfur reduction and sulfide precipitation in this depth range, fed by sulfate-bearing fluids descending from shallower depths, where fluids leach of sulfur from a shallow, oxidative weathering horizon. Consistent with this hypothesis, at peridotite hosted alkaline springs in Oman, Type II fluids (derived from deeper systems) have orders of magnitude lower dissolved sulfate than Type I fluids which are derived from shallower systems Leong, Howells, et al, 2021;Paukert Vankeuren et al, 2012).…”

Section: Sulfur-rich "Black Serpentinite" Zone From 30 To 150 M Depth In Hole Ba1bmentioning

confidence: 87%

“…As for BA1B, BA3A, and BA4A were also logged just once, within a few months of drilling. In BA3A, a pH log 2 yr after drilling revealed a small increase in pH throughout the hole, to ∼11, suggestive of buffering involving Ca 2+ and OH − (Barnes & O'Neil, 1969;Bruni et al, 2002;Leong, Howells, et al, 2021;Leong & Shock, 2020;Neal & Stanger, 1984;Paukert Vankeuren et al, 2012). BA3A is 5 m from water monitoring well NSHQ14, drilled in 2004.…”

Section: Borehole Water Propertiesmentioning

confidence: 97%

“…The first step simulated reactions close to the surface while the second step modeled reactions in deeper aquifers. In contrast to Paukert Vankeuren et al ( 2012) and Leong, Howells, et al (2021), first step simulations in this work were allowed to run to higher reaction progress (i.e.., toward lower W/R ratio, down to ∼10) until modeled dissolved sulfate concentrations approximate field constraints before proceeding to the second step. Results of calculations are shown in Figure 29, for the first and second steps, respectively, and in Table S4 in Supporting Information S1.…”

Section: Thermodynamic Modeling Of Water/rock Reactionmentioning

confidence: 99%

“…In this work, calculations incorporating variable SiO 2 activities were used, ranging from a(SiO 2 ) dictated by Mg-serpentine (chrysotile) and Mg-brucite to higher a(SiO 2 ) dictated by Mg-serpentine and talc. These, encompass the range of a(SiO 2 ) in pH > 9 fluids from the Samail ophiolite (Leong, Howells, et al, 2021).…”

Section: Thermodynamic Modeling Of Water/rock Reactionmentioning

confidence: 99%

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Initial Results From the Oman Drilling Project Multi‐Borehole Observatory: Petrogenesis and Ongoing Alteration of Mantle Peridotite in the Weathering Horizon

et al. 2021

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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.

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Section: Sulfur-rich "Black Serpentinite" Zone From 30 To 150 M Depth In Hole Ba1bmentioning

confidence: 87%

Section: Borehole Water Propertiesmentioning

confidence: 97%

Section: Thermodynamic Modeling Of Water/rock Reactionmentioning

confidence: 99%

Section: Thermodynamic Modeling Of Water/rock Reactionmentioning

confidence: 99%

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Initial Results From the Oman Drilling Project Multi‐Borehole Observatory: Petrogenesis and Ongoing Alteration of Mantle Peridotite in the Weathering Horizon

et al. 2021

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“…As discussed by Leong et al. (2021) and Nothaft, Templeton, Boyd, et al. (2021), mixing between a moderately alkaline, oxidizing fluid and a hyperalkaline, highly reduced end‐member fluid close to equilibrium with serpentine, brucite and calcite (or diopside or hydroandradite instead of calcite) is likely occurring in the BA holes.…”

Section: Discussionmentioning

confidence: 91%

Accessing the Subsurface Biosphere Within Rocks Undergoing Active Low‐Temperature Serpentinization in the Samail Ophiolite (Oman Drilling Project)

et al. 2021

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The Oman Drilling Project established an "Active Alteration" multi-borehole observatory in peridotites undergoing low-temperature serpentinization in the Samail Ophiolite. The highly serpentinized rocks are in contact with strongly reducing fluids. Distinct hydrological regimes, governed by differences in rock porosity and fracture density, give rise to steep redox (Eh +200 to −750 mV) and pH (pH range 8.5-11.2) gradients within the 300-400 m deep boreholes. The serpentinites and fluids host an active subsurface ecosystem. Microbial cell abundances in serpentinite vary at least six orders of magnitude, from ≤3.5 × 10 1 to 2.9 × 10 7 cells/g. Low levels of biological sulfate reduction (2-1,000 fmol/ cm 3 /day) can be detected in rock cores, particularly in rocks in contact with reduced groundwaters with pH < 10.5. Thermodesulfovibrio is the predominant sulfate reducer identified via metagenomic sequencing of adjacent groundwater communities. We infer that transport and reaction of microbially generated sulfide with the serpentine and brucite assemblages gives rise to optical darkening and sulfide overprinting, including the formation of tochilinite-vallerite group minerals, potentially serving as an indicator that this system is inhabited by microbial life. Olivine mesh-cores replaced with ferroan brucite and minor awaruite, abundant veins containing hydroandradite garnet and polyhedral serpentine, and late-stage carbonate veins are suggested as targets for future spatially resolved life-detection investigations. The high-quality whole-round core samples that have been preserved can be further probed to define how life distributes itself and functions within a system where chemical disequilibria are sustained by lowtemperature water/rock interaction, and how biosignatures of in situ microbial activity are generated.Plain Language Summary Ultramafic rocks undergoing water/rock interaction, and storing fluids that are far from chemical equilibrium, may be one of the most common habitats in our solar system. Through the Oman Drilling Project we collected >1 km of intact serpentinite in contact with groundwaters. These cores capture parts of the rock-hosted biosphere and show how cells are distributed within serpentinites that vary in their mineralogical, physical and chemical properties. The cores are also biologically active, enabling us to detect specific metabolisms, such as when microorganisms combine hydrogen as reductant and sulfate as an oxidant to fuel their metabolism. Although the distribution of microbial cells in the rock cores is very heterogeneous, there are many intervals where the abundance of cells constitutes robust biomass. In the deeper cores, slow, albeit detectable, microbial sulfate reduction proceeds. We suggest that this pervasive biological activity releases byproducts such as sulfide that can react with the serpentinite and change the optical and chemical properties of the rocks. The feedbacks between the rock alteration and microbial activity produce markers that enable us to focus our sea...

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Hydrothermal Experiments with Protonated Benzylamines Provide Predictions of Temperature-Dependent Deamination Rates for Geochemical Modeling

Robinson

Gould

Hartnett

et al. 2021

ACS Earth Space Chem.

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In fluids of sufficient temperature and residence time, certain organic species are observed to equilibrate, and their abundances become diagnostic of reaction conditions, including temperature, redox state, and pH. Organic species released from remote geologic and planetary settings can therefore serve as geochemical tracers for environments that are difficult to observe directly. Here, we provide a framework for selecting organic compounds as geochemical tracers based on kinetic modeling. We characterized temperature-dependent rates of deamination substitution reactions for aqueous protonated benzylamines, i.e., benzylaminiums, to form benzyl alcohols and ammonium. Hydrothermal experiments were conducted at 200–300 °C at liquid–vapor water saturation pressures with ring-substituted benzylaminiums expected to have comparable deamination rates to environmentally abundant aminiums, e.g., amino acids. We compared rates extrapolated from experiments to idealized natural systems, taking into account fluid temperatures and residence times. Our results indicate that reversible deamination/hydration reactions may equilibrate over geologic time scales across diverse environments, including those approaching freezing temperatures for the most reactive benzylaminium. Therefore, similar reaction constituents may be useful targets for the exploration of potentially habitable subsurface environments, such as icy ocean worlds of the solar system. Our investigation supports previous findings that aqueous deamination of benzylaminiums operates via two simultaneous substitution mechanisms, SN1 and SN2. We find that for certain benzylaminiums, rates of each mechanism should be modeled individually to improve extrapolation across temperatures. Extrapolations of observed (i.e., bulk) deamination kinetics to near-ambient temperatures (∼50 °C) without mechanistic considerations can produce discrepancies in reaction half-lives on the order of a billion years.

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Theoretical predictions vs environmental observations on serpentinization fluids: Lessons from the Samail ophiolite in Oman

Cited by 10 publications

References 0 publications

Initial Results From the Oman Drilling Project Multi‐Borehole Observatory: Petrogenesis and Ongoing Alteration of Mantle Peridotite in the Weathering Horizon

Initial Results From the Oman Drilling Project Multi‐Borehole Observatory: Petrogenesis and Ongoing Alteration of Mantle Peridotite in the Weathering Horizon

Accessing the Subsurface Biosphere Within Rocks Undergoing Active Low‐Temperature Serpentinization in the Samail Ophiolite (Oman Drilling Project)

Hydrothermal Experiments with Protonated Benzylamines Provide Predictions of Temperature-Dependent Deamination Rates for Geochemical Modeling

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