A synthesis and meta-analysis of the Fe chemistry of serpentinites and serpentine minerals Mayhew, Lisa E; Ellison, Eric T
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
02/2020, Letnik:
378, Številka:
2165
Journal Article
Recenzirano
Odprti dostop
The iron chemistry of serpentinites and serpentine group minerals is often invoked as a record of the setting and conditions of serpentinization because Fe behaviour is influenced by reaction ...conditions. Iron can be partitioned into a variety of secondary mineral phases and undergo variable extents of oxidation and/or reduction during serpentinization. This behaviour influences geophysical, geochemical and biological aspects of serpentinizing systems and, more broadly, earth systems. Iron chemistry of serpentinites and serpentines is frequently analysed and reported for single systems. Interpretations of the controls on, and the implications of, Fe behaviour drawn from a single system are often widely extrapolated. There is a wealth of serpentinite/serpentine chemical composition data available in the literature. Consequently, compilation of a database including potential predictors of Fe behaviour and measures of Fe chemistry enables systematic investigation of trends in Fe behaviour across a variety of systems and conditions. The database presented here contains approximately 2000 individual data points including both bulk rock and serpentine mineral geochemical data which are paired whenever possible. Measures of total Fe and Fe oxidation state, which are more limited, are compiled with characteristics of the systems from which they were sampled. Observations of trends in Fe chemistry in serpentinites and serpentines across the variety of geologic systems and parameters will aid in verifying and strengthening interpretations made on the basis of Fe chemistry. This article is part of a discussion meeting issue 'Serpentinite in the Earth system'.
IODP Expedition 357 used two seabed drills to core 17 shallow holes at 9 sites across Atlantis Massif ocean core complex (Mid-Atlantic Ridge 30°N). The goals of this expedition were to investigate ...serpentinization processes and microbial activity in the shallow subsurface of highly altered ultramafic and mafic sequences that have been uplifted to the seafloor along a major detachment fault zone. More than 57 m of core were recovered, with borehole penetration ranging from 1.3 to 16.4 meters below seafloor, and core recovery as high as 75% of total penetration in one borehole. The cores show highly heterogeneous rock types and alteration associated with changes in bulk rock chemistry that reflect multiple phases of magmatism, fluid-rock interaction and mass transfer within the detachment fault zone. Recovered ultramafic rocks are dominated by pervasively serpentinized harzburgite with intervals of serpentinized dunite and minor pyroxenite veins; gabbroic rocks occur as melt impregnations and veins. Dolerite intrusions and basaltic rocks represent the latest magmatic activity. The proportion of mafic rocks is volumetrically less than the amount of mafic rocks recovered previously by drilling the central dome of Atlantis Massif at IODP Site U1309. This suggests a different mode of melt accumulation in the mantle peridotites at the ridge-transform intersection and/or a tectonic transposition of rock types within a complex detachment fault zone. The cores revealed a high degree of serpentinization and metasomatic alteration dominated by talc-amphibole-chlorite overprinting. Metasomatism is most prevalent at contacts between ultramafic and mafic domains (gabbroic and/or doleritic intrusions) and points to channeled fluid flow and silica mobility during exhumation along the detachment fault. The presence of the mafic lenses within the serpentinites and their alteration to mechanically weak talc, serpentine and chlorite may also be critical in the development of the detachment fault zone and may aid in continued unroofing of the upper mantle peridotite/gabbro sequences.
New technologies were also developed for the seabed drills to enable biogeochemical and microbiological characterization of the environment. An in situ sensor package and water sampling system recorded real-time variations in dissolved methane, oxygen, pH, oxidation reduction potential (Eh), and temperature and during drilling and sampled bottom water after drilling. Systematic excursions in these parameters together with elevated hydrogen and methane concentrations in post-drilling fluids provide evidence for active serpentinization at all sites. In addition, chemical tracers were delivered into the drilling fluids for contamination testing, and a borehole plug system was successfully deployed at some sites for future fluid sampling. A major achievement of IODP Expedition 357 was to obtain microbiological samples along a west–east profile, which will provide a better understanding of how microbial communities evolve as ultramafic and mafic rocks are altered and emplaced on the seafloor. Strict sampling handling protocols allowed for very low limits of microbial cell detection, and our results show that the Atlantis Massif subsurface contains a relatively low density of microbial life.
•Seabed rock drills and real-time fluid monitoring for first time in ocean drilling•First time recovery of continuous sequences along oceanic detachment fault zone•Highly heterogeneous rock type and alteration in shallow detachment fault zone•High methane and hydrogen concentrations in Atlantis Massif shallow basement•Oceanic serpentinites potentially provide important niches for microbial life
Serpentinized peridotite is reacting with groundwater in the subsurface of the Samail ophiolite in Oman. Although these rocks are partially to completely serpentinized, they interact with a ...groundwater aquifer containing hyperalkaline fluids rich in H2 and CH4. Since the mechanisms by which H2 production may continue at low temperatures (<50°C) are not fully understood, core recovered during the Oman Drilling Project provides an excellent opportunity to study the mineralogy and Fe speciation in highly serpentinized harzburgite recording multiple stages of water/rock interaction. In Hole BA3A, early hydration of olivine and pyroxene, which likely occurred at temperatures of ∼100°C–200°C, produced serpentine, Fe‐rich brucite, awaruite, and little magnetite. Notably, Fe‐rich brucite is only preserved at >∼100 m depth in the core. Fe‐rich brucite is nearly absent within two near‐surface reaction zones where later stages of reaction are recorded, which include replacement of Fe‐rich brucite by Fe(III)‐bearing serpentine, and increases in the proportion of other Fe(III)‐bearing phases such as magnetite and hydroandradite. Thus, Fe‐rich brucite at depth represents substantial stored capacity for H2 production that can continue at low temperature, even after primary olivine and pyroxene are exhausted, thereby sustaining habitable conditions for microbial life.
Plain Language Summary
When ultramafic rocks from Earth's mantle come into contact with water, they undergo a set of hydration reactions leading to the formation of secondary minerals such as serpentine and brucite. Such “serpentinization reactions” also often involve the oxidation of iron, which can result in the production of hydrogen gas. Many models predict abundant hydrogen production at high temperatures >250°C, but the potential reactions that may produce hydrogen are more enigmatic at lower temperatures. Using rock cores drilled from the Samail ophiolite in Oman, we show that in highly serpentinized rocks in contact with hyperalkaline, low‐temperature, hydrogen‐rich fluids, multiple stages of reactions can be observed. By analyzing changes in mineralogy with depth, it is possible to identify reaction fronts where rocks that initially formed abundant Fe(II)‐bearing minerals such as brucite are now interacting with modern groundwaters at low‐temperature to form Fe(III)‐bearing minerals including magnetite, hydroandradite, and Fe(III)‐bearing serpentine. These reactions are likely responsible for the production of hydrogen gas that is able to support a rock‐hosted microbial community, and show that habitable conditions can be produced even during late, non‐hydrothermal stages of ultramafic rock alteration.
Key Points
Subsurface serpentinites from the Samail ophiolite contain distinct reaction fronts with implications for H2 production and habitability
Where Fe(II)‐rich brucite is present, it has the potential to fuel future H2 production
Fe(II,III)‐rich serpentine, hydroandradite, and magnetite all record past H2 production at low temperatures <200°C
Dissolved hydrogen is common in mafic and ultramafic aquifers; however, the water/rock reactions that give rise to hydrogen production at near-surface temperatures are enigmatic. Similarly, mineral ...hydration experiments have not yet unequivocally demonstrated whether H2 can be produced at low-temperatures at significant rates from reaction of aqueous fluids with basalts and peridotites for prolonged amounts of time. We conducted laboratory-based water/rock reactions between partially serpentinized Oman dunite and a simulated Oman rainwater (RW) media, as well as a simulated seawater (SW) media, to quantify H2 generation rates at 100°C. Throughout more than 9months of water/rock reaction, extensive hydrogen production and consumption were observed in RW and SW media. In the first 24h of reaction in anoxic fluids containing only dissolved N2 and CO2, the room-temperature pH in both RW and SW media increased from 6.5 to ∼9, and the average pH then remained relatively constant at pH 8.5 (±0.5 pH) for the duration of the experiments. We also measured some of the highest hydrogen concentrations observed in experimental low-temperature serpentinization reactions. The maximum measured H2 concentrations in SW media were 470nmol H2 per g mineral after ∼3months, while RW media H2 concentrations reached 280nmol/g H2 after ∼3months. After reaching micromolar dissolved H2(aq), the H2 concentrations notably declined, and CO2 was almost fully consumed. We measured the formation of formate (up to 98μM) and acetate (up to 91μM) associated with a drawdown of H2 and CO2 in the experiments. No CH4 or carbonate formation was observed. To identify reactions giving rise to low-temperature hydrogen production, the mineralogy and oxidation state of the Fe-bearing species in the dunite were extensively characterized before and after reaction using Raman spectroscopy, Quantitative Evaluation of Minerals by SCANing electron microscopy (QEMSCAN), powder X-ray diffraction (XRD), magnetic susceptibility, scanning electron microscopy (SEM), and Fe K-edge X-ray absorption near edge structure (XANES) spectroscopic techniques. The mineralogy of the solid starting material was dominated by olivine and serpentine with minor brucite, pyroxene and spinel. After reaction, additional serpentine and magnetite could be detected as reaction products, and pre-existing brucite was consumed. No changes were observed in the abundance or grain sizes of olivine or pyroxene. Thus, we propose that the destabilization of Fe(II)-bearing brucite and the subsequent oxidation of the aqueous Fe(II) to form magnetite and Fe(III)-rich serpentine give rise to H2 production at 100°C. This work demonstrates that dissolved hydrogen and low molecular weight organic acids can be produced by the reaction of labile Fe(II)-bearing minerals generated during a prior stage of water/rock reactions. In particular, progressive alteration of partially-serpentinized peridotites containing brucite may generate sufficient electron donors to fuel in-situ subsurface microbial activity.
The Mars Sample Return mission intends to retrieve a sealed collection of rocks, regolith, and atmosphere sampled from Jezero Crater, Mars, by the NASA Perseverance rover mission. For all ...life-related research, it is necessary to evaluate water availability in the samples and on Mars. Within the first Martian year, Perseverance has acquired an estimated total mass of 355 g of rocks and regolith, and 38 μmoles of Martian atmospheric gas. Using in-situ observations acquired by the Perseverance rover, we show that the present-day environmental conditions at Jezero allow for the hydration of sulfates, chlorides, and perchlorates and the occasional formation of frost as well as a diurnal atmospheric-surface water exchange of 0.5-10 g water per m
(assuming a well-mixed atmosphere). At night, when the temperature drops below 190 K, the surface water activity can exceed 0.5, the lowest limit for cell reproduction. During the day, when the temperature is above the cell replication limit of 245 K, water activity is less than 0.02. The environmental conditions at the surface of Jezero Crater, where these samples were acquired, are incompatible with the cell replication limits currently known on Earth.
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 × 101 to 2.9 × 107 cells/g. Low levels of biological sulfate reduction (2–1,000 fmol/cm3/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 low‐temperature 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 search for rock‐hosted life and any specific biosignatures it may produce on Earth and perhaps on other planetary bodies.
Key Points
Highly serpentinized subsurface rocks exhibit steep redox gradients and host microbial cell abundances that vary >6 orders of magnitude
Low rates of microbial sulfate reduction in rock cores are inferred to result in optical darkening and sulfide overprinting of the mineralogy
Widespread andradite garnet, abundant ferroan brucite, and rare carbonate are targets for future spatially resolved life‐detection efforts
Partially serpentinized peridotites in the Samail ophiolite in the Sultanate of Oman currently undergo low temperature alteration and hydration both at shallow levels, with water recently in contact ...with the atmosphere, and at depth, with anoxic, reducing fluids. However, it is unclear how changes in the distribution and oxidation state of Fe are driving the production of energy-rich gases such as hydrogen and methane detected in peridotite catchments. We track the Fe transformations in a suite of outcrop samples representing a subset of the spectrum of least to most altered end-members of the Oman peridotites. We use microscale mineralogical and geochemical analyses including QEMSCAN, Raman spectroscopy, synchrotron radiation X-ray fluorescence (XRF) mapping, and electron microprobe wavelength dispersive spectroscopy. The less-altered peridotites possess a diversity of Fe-bearing phases including relict primary minerals (e.g. olivine, pyroxene, chromite) and secondary phases (e.g. serpentine and brucite). Raman spectroscopy and electron microprobe data (Si/(Mg + Fe)) indicate that much of the serpentine is significantly intergrown with brucite on the sub-micron scale. These data also indicate that the Fe content of the brucite ranges from 10 to 20 wt% FeO. The mineral assemblage of the highly reacted rocks is less diverse, dominated by serpentine and carbonate while olivine and brucite are absent. Magnetite is relatively rare and mainly associated with chromite. Goethite and hematite, both Fe(III)-hydroxides, were also identified in the highly altered rocks. Whole rock chemical analyses reflect these mineralogical differences and show that Fe in the partially serpentinized samples is on average more reduced (∼0.40–0.55 Fe3+/FeTotal) than Fe in the highly reacted rocks (∼0.85–0.90 Fe3+/FeTotal). We propose that olivine, brucite, chromite and, perhaps, serpentine in the less-altered peridotites act as reactive phases during low temperature alteration of the Oman peridotite. The pervasive oxidation of Fe(II) in the less-altered peridotites to Fe(III) in the most-altered peridotites during water-rock reaction in the subsurface of the Samail ophiolite may produce H2 which will influence the development of microbial energy sources and habitats, and carbon cycling and sequestration within the (ultra)mafic ocean crust.
Microbial CO2-driven methanogenesis, the biologically mediated conversion of hydrogen and carbon dioxide to methane, is potentially one of the earliest metabolic strategies to appear on Earth and ...likely originated in ecosystems sustained by water/rock interaction. The potential for methanogens to influence secondary mineralization pathways during water–rock reactions is not well known. Yet geochemical signatures of the presence and activity of life in rock-hosted environments are of great interest in the search for life and signs of life preserved on Earth and other planets. We designed a laboratory experiment to evaluate microbial growth and secondary mineralization pathways in the presence and absence of microbial methanogenesis. Methanothermobacter thermoflexus, a moderately thermophilic methanogen was supported by the continuous production of H2 from water–basalt–Fe0 reactions at 55°C for over one year. As H2 was converted to CH4 in culture experiments, the H2 concentration was maintained at 0.1–0.5μM, orders of magnitude lower than the concentration in comparable abiotic experiments. The pH in active cultures was lower than in abiotic experiments. The concentration of CO2(g), and thus CO2(aq), was higher in the active cultures vs. abiotic and inactive culture experiments. These differences in chemistry between the two systems led to the prediction, from geochemical equilibrium models, of distinct Fe-bearing secondary phases stable in the active cultures (carbonate) vs. abiotic and inactive culture experiments (phyllosilicate). The application of synchrotron X-ray absorption spectroscopic techniques enabled the detection of rare and microscale Fe-bearing solid-phase reaction products. A phyllosilicate dominated the Fe-bearing secondary mineralogy of the abiotic and inactive culture experiments. In contrast, the phyllosilicate was not formed in the active cultures. However, a carbonate phase was not robustly detected. Surprisingly, an Fe-bearing pyroxene was detected in some cultures, and formed through an unknown pathway. Thus distinct secondary mineralization occurred in the presence vs. absence of microbial methanogenesis, whether it was directly or indirectly caused by in-situ biological activity.
•distinct differences in gaseous, aqueous and solid phases of water-basalt-Fe0 reactions in abiotic vs. culture experiments•lower pH conditions (~0.5 pH unit) maintained in active cultures versus abiotic and inactive culture experiments•thermodynamically predicted mineral phase absent from experiments with active methanogenesis•detection of distinct mineral assemblages representative of active cultures vs. abiotic systems
Abstract
Herein, we present 2 cases referred to the North Carolina Office of the Chief Medical Examiner (NC OCME) in which ethanol results reported by different hospital laboratories, using alcohol ...dehydrogenase (ADH)–based assays, were positive, whereas results of headspace gas chromatography testing performed in the NC OCME laboratory were negative. Literature reports suggest that false-positive ethanol measurements from ADH-based assays can occur when a combination of elevated lactate and lactate dehydrogenase (LD) are present in the specimen. The results were reported in perimortem specimens collected from 2 children with unrelated medical conditions. The cases and associated clinical parameters are considered based on the lactate/LD explanation for the false-positive results, to facilitate the recognition of circumstances that can produce erroneous serum ethanol results.
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