Low-temperature Serpentinization Research Articles

Quaternary low-temperature serpentinization and carbonation in the New Caledonia ophiolite

The low-temperature alteration (< 150 °C) of ophiolites by infiltrated meteoric waters removes atmospheric CO2 through mineral carbonation and is assumed to generate H2 and possibly CH4 according to so-called serpentinization reactions. This overall alteration pattern is primarily constrained by the chemical composition of alkaline springs that are issued in several ophiolites worldwide. Here we report on the fingerprint, as veinlet mineralization, of the reactive percolation of such meteoric waters in the New Caledonia ophiolite (Massif du Sud). The mineralization which resulted from carbonation and serpentinization reactions, is young (< 2 Ma) and formed at a temperature of ca. 95 °C. It is mainly composed of lizardite, dolomite, magnetite ± pyroaurite. Thermochemical simulation of mineral–water equilibria shows that the percolating aqueous fluid was alkaline and H2 bearing. The δ13C of dolomite is exceptionally high, between 7.1 and up to 17.3‰, and is interpreted as evidence of low-temperature methanogenesis. Overall, the percolating fluid had a chemical composition similar to that of the waters issued today in the (hyper)alkaline springs of the Massif du Sud. The studied veinlets are thus interpreted as a sample of the plumbing system that fed an ancient Quaternary alkaline spring in the area.

Progressive carbonation and Ca-metasomatism of serpentinized ultramafic rocks: insights from natural occurrences and hydrothermal experiments

Hydration, carbonation, and related metasomatism of mantle peridotite play a significant role in the global geochemical cycle. In this study, we combined an analysis of carbonated serpentinite with hydrothermal experiments on carbonation and Ca-metasomatism for samples from the Manlay ophiolite, southern Mongolia to investigate that carbonation mechanism of the serpentinite body after serpentinization. Samples show that the serpentinite was either transected by calcite and dolomite veins or was completely replaced by carbonates (calcite with minor dolomite) and quartz, in which the original mesh texture of serpentinite was preserved. Carbonation occurred after low-temperature serpentinization (lizardite/chrysotile), suggesting that carbonation occurred at temperatures lower than 300 ˚C. Calcite in the serpentinite showed δ13 CVPDB values ranging from -8.83 to -5.11 ‰ and δ18 OVSMOW from + 20.1 to + 24.4 ‰, suggesting that CO2 in the fluids could be derived from the degradation of organic material or methanotrophic processes rather than the origin of seafloor limestone. Three batch-type experiments, i.e., single step experiments (1) Olivine + NaHCO3,aq + CaCl2,aq and (2) Chrysotile + NaHCO3,aq + wollastonite (Ca source), and two steps experiment (3) Olivine carbonation and Ca-metasomatism, were conducted at 275 °C and 5.7 MPa to constrain the mechanism of calcite replacement of serpentinite. We found that calcite precipitated from the solution directly in the first two experiments, but replacement of serpentinite by calcite was not observed. In contrast, the third experiment caused the initial carbonation to form magnesite and then changed to calcite by later alteration. The natural occurrences and experiments revealed the possibility that the carbonation of olivine followed by Ca-rich fluid infiltration produced calcite in the carbonated serpentinite. Such Ca-metasomatism of Mg carbonates could easily occur in the ultramafic bodies and significantly affect the global carbon cycle.

An empirical calibration of the serpentine-water oxygen isotope fractionation at T = 25–100 °C

The ability to precisely constrain the physical–chemical conditions of serpentinization, such as temperature and fluid source, is limited by the accuracy of calibrations for oxygen isotope fractionation between serpentine and water – i.e., 1000 lnα(serpentine-water) – which disagree by up to 20‰ when extrapolated to T < 200 °C. In this study, we present a new empirical calibration of 1000 lnα(serpentine-water) aiming to improve applications of oxygen isotope geochemistry to very low-T serpentinization (T < 100 °C). We used the high-spatial resolution capabilities of Secondary Ion Mass Spectrometry (SIMS) to analyze oxygen isotope ratios in mineral pairs of calcite + serpentine, quartz + serpentine and talc + serpentine co-crystallized at scales ≤ 50 μm in six serpentinite samples from the Samail ophiolite (Oman). SIMS analysis shows that the mineral pairs are relatively homogenous in oxygen isotope ratios with variability in δ18O values ≤2‰ (2 s). Clumped isotope thermometry and petrological constraints indicate crystallization temperatures from 25 to 100 °C for the investigated samples. These independent constraints on temperature allowed us to derive 1000 lnα(serpentine-water) by combining Δ18O(quartz-serpentine), Δ18O(talc-serpentine) and Δ18O(calcite-serpentine) measured by SIMS in the investigated samples with Δ18O(quartz-water), Δ18O(talc-water) and Δ18O(calcite-water) calibrations available in the literature. Our empirical calibration of 1000 lnα(serpentine-water) = 1.04 ± 0.20 (2SE) × 106/T2 (T in K), from T = 25 to 100 °C, is within uncertainty of former high-temperature empirical calibrations extrapolated to T < 100 °C and matches experimental calibrations when extrapolated to T = 250 °C. The new serpentine-water calibration enables more accurate reconstructions of fluid-rock interactions occurring during low-temperature serpentinization processes in various tectonic settings.

H2 and CH4 outgassing rates in the Samail ophiolite, Oman: Implications for low-temperature, continental serpentinization rates

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 microorganisms 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 H2 and CH4 outgassing rates in several hyperalkaline spring sites in the Samail ophiolite, Oman. H2 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 carbon 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 × 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 ∼0.3 mol H2 kg−1 of ultramafic rock altered, lower than simulations based on iron oxidation to magnetite only. Contribution from decrepitation of H2- and CH4-bearing fluid inclusions is possible but would require much more mass of ultramafic rocks to account for observed outgassing. 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.

Platinum Group Elements (PGE) Geochemistry and Mineralogy of Low Economic Potential (Rh-Pt-Pd)-Rich Chromitites from Ophiolite Complexes

This contribution provides an overview of platinum group elements (PGE) distribution and mineralogy in ophiolitic chromitites, which are unusually enriched in the low melting-point Rh, Pt and Pd (PPGE) compared with most chromite deposits associated with ophiolites, which are dominated by the refractory Os, Ir and Ru (IPGE). The PPGE-rich chromitites examined in this paper have a PPGE/IPGE ratio equal to or higher than 1 and represent about 7% of the ophiolitic chromitite population. These chromitites occur in the mantle unit, in the mantle-transition zone (MTZ), as well as in the supra-Moho cumulate sequence of ophiolite complexes. The age of their host ophiolites varies from Proterozoic to Eocene and, based on their composition, the chromitites can be classified into Cr-rich and Al-rich categories. Mineralogical assemblages observed in this investigation suggest that the PPGE enrichment was achieved in the magmatic stage thanks to the formation of an immiscible sulfide liquid segregating during or immediately after chromite precipitation. The sulfide liquid collected the available chalcophile PPGE that precipitated as specific phases together with Ni-Cu-Fe sulfides in the host chromitite and the silicate matrix. After their magmatic precipitation, the PPGM and associated sulfides were altered during low-temperature serpentinization and hydrothermal processes. Therefore, the original high-temperature assemblage underwent desulfurization, generating awaruite and alloys characterized by variable Pt-Pd-Rh-Cu-Ni-Fe assemblages. The occurrence of secondary PPGM containing Sb, As, Bi, Te, Sn, Hg, Pb and Au suggests that these elements might have been originally present in the differentiating magmatic sulfide liquid or, alternatively, they were introduced by an external source transported by hydrothermal and hydrous fluids during the low-temperature evolution of the host ophiolite. Although the PGE content may be as high as 81,867 ppb, as was found in one sample from Shetland chromite deposits, the ophiolitic chromitites are not presently considered as a potential resource because of the following circumstances: (1) enrichment of PPGE in podiform chromitites is a local event that occurs randomly in ophiolite sequences, (2) ore deposits are small and characterized by uneven distribution and high discontinuity, (3) physical characters of the mineralization only allow poor recovery of the precious metals mainly due to the minute grain size, and (4) for these reasons, the PPGE reserves in ophiolitic chromitites cannot compete, at the moment, with those in chromite deposits of the Bushveld type that will supply world demands for centuries using current mining techniques.

Trace and rare earth element distribution in hyperalkaline serpentinite-hosted spring waters and associated authigenic carbonates from the Ronda peridotite

The Ronda peridotite in southern Spain is subject to low-temperature serpentinization by circulating groundwaters that produces hyperalkaline fluids of high pH and low dissolved inorganic carbon content (DIC). When these waters resurface as hyperalkaline springs they take up atmospheric carbon dioxide (CO2), which triggers the precipitation of travertine carbonates. Spring waters at Baños del Duque, Fuente Romana, and Fuente Amargosa show typical chemical features of hyperalkaline springs such as high pH and low DIC, yet exhibit an unusual distribution of rare earth elements (REEs) enriched in lanthanum (La) and cerium (Ce). In order to constrain changes in spring water chemistry and carbonate precipitation during atmospheric CO2 uptake, a reaction path model was set up to illustrate that the distribution of REE species in the high pH and low DIC water is strongly dependent on REE complexation by hydroxide and carbonate species. Reaction path modeling results show that during spring fluid emission and CO2 uptake, REE speciation in the fluids shifts from a dominance of hydroxide to carbonate complexes. The complexation of rare earth elements in the high pH, low DIC fluids strongly depends on the DIC content of the respective fluid. The REE distribution of travertine deposits associated with the hyperalkaline springs deviates from that in the fluids, suggesting that travertines precipitated from fluids different to those currently emitted from the springs.

Low-temperature, diagenetic serpentinization of peridotite clasts in lower Miocene marine conglomerates, Torino Hill, NW Italy

Peridotite clasts in a marine Miocene conglomerate from the Torino Hill (NW Italy) are characterised by an isopachous serpentinized external rim (4–12 mm thick) made up of lizardite, surrounding a non-serpentinized peridotite core. This peculiar structure indicates a diagenetic, post-depositional serpentinization of the peridotite clasts. The conglomerate is cemented by a rim of lizardite, followed by Mg-calcite and Ca-dolomite spar. The burial history of the host sedimentary succession indicates that serpentinization and precipitation of lizardite cements probably occurred at very low temperature (max. 50–60 °C). This is the first reported case of diagenetic, very low temperature serpentinization affecting ultramafite clasts in sedimentary successions.

Transformation of low-molecular-weight organic acids by microbial endoliths in subsurface mafic and ultramafic igneous rock.

A growing body of work indicates that continental subsurface rocks host a substantial portion of the Earth's biosphere. However, the activities of microbial cells inhabiting pore spaces and microfractures in subsurface rocks remain underexplored. Here, we develop and optimize microcosm assays to detect organic acid transformation activities of cells residing in mafic to ultramafic igneous rocks. Application of this assay to gabbro core from the Stillwater Mine, Montana, USA, revealed maximal methane production from acetate at temperatures approximating that of the mine. Controls show that these activities are not due to contamination introduced during drilling, exhumation, or laboratory processing of the core. The assay was then applied to rocks cored from the Samail Ophiolite, Oman, which is undergoing low-temperature serpentinization. Production of (i) carbon dioxide from acetate and formate and (ii) methane from formate were detected in a dunite/harzburgite rock core interfacing pH9.6 waters, and estimates of microbial activities were up to three orders of magnitude higher in the rock core pore space than in corresponding waters. The detection of endolithic microbial activities in igneous rocks has implications for life detection on other planetary bodies where similar rock types prevail, such as Mars, Europa and Enceladus.

Multi-stage serpentinization of ultramafic rocks in the Manlay Ophiolite, southern Mongolia

Serpentinization of ultramafic rocks in ophiolites is key to understanding the global cycle of elements and changes in the physical properties of lithospheric mantle. Mongolia, a central part of the Central Asian Orogenic Belt (CAOB), contains numerous ophiolite complexes, but the metamorphism of ultramafic rocks in these ophiolites has been little studied. Here we present the results of our study of the serpentinization of an ultramafic body in the Manlay Ophiolite, southern Mongolia. The ultramafic rocks were completely serpentinized, and no relics of olivine or orthopyroxene were found. The composition of Cr-spinels [Mg# = Mg/(Mg + Fe2+) = 0.54 and Cr# = Cr/(Cr + Al) = 0.56] and the bulk rock chemistry (Mg/Si = 1.21–1.24 and Al/Si &lt; 0.018) of the serpentinites indicate their origin from a fore-arc setting. Lizardite occurs in the cores and rims of mesh texture (Mg# = 0.97) and chrysotile is found in various occurrences, including in bastite (Mg# = 0.95), mesh cores (Mg# = 0.92), mesh rims (Mg# = 0.96), and later-stage large veins (Mg# = 0.94). The presence of lizardite and chrysotile and the absence of antigorite suggests low-temperature serpentinization (&lt;300 °C). The lack of brucite in the serpentinites implies infiltration of the ultramafic rocks of the Manlay Ophiolite by Si-rich fluids. Based on microtextures and mineral chemistry, the serpentinization of the ultramafic rocks in the Manlay Ophiolite took place in three stages: (1) replacement of olivine by lizardite, (2) chrysotile formation (bastite) after orthopyroxene and as a replacement of relics of olivine, and (3) the development of veins of chrysotile that cut across all previous textures. The complex texture of the serpentinites in the Manlay Ophiolite indicates multiple stages of fluid infiltration into the ultramafic parts of these ophiolites in southern Mongolia and the CAOB.

Decreasing extents of Archean serpentinization contributed to the rise of an oxidized atmosphere

At present, molecular hydrogen (H2) produced through Fe(II) oxidation during serpentinization of ultramafic rocks represents a small fraction of the global sink for O2 due to limited exposures of ultramafic rocks. In contrast, ultramafic rocks such as komatiites were much more common in the Early Earth and H2 production via serpentinization was a likely factor in maintaining an O2-free atmosphere throughout most of the Archean. Using thermodynamic simulations, this work quantifies the global O2 consumption attributed to serpentinization during the past 3.5 billion years. Results show that H2 generation is strongly dependent on rock compositions where serpentinization of more magnesian lithologies generated substantially higher amounts of H2. Consumption of >2 Tmole O2 yr−1 via low-temperature serpentinization of Archean continents and seafloor is possible. This O2 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.

Extending the dataset of fluid geochemistry of the Menez Gwen, Lucky Strike, Rainbow, TAG and Snake Pit hydrothermal vent fields: Investigation of temporal stability and organic contribution

The Menez Gwen, Lucky Strike, Rainbow, TAG (Transatlantic Geotraverse) and Snake Pit hydrothermal vent fields on the Mid-Atlantic Ridge were revisited and resampled for geochemical investigation during the BIOBAZ 2013 and BICOSE 2014 research cruises. Geochemical analysis of the major and minor elements of the hydrothermal fluid and concentrations of gases extends and complements the existing dataset. Our results are consistent with values previously reported and fall within the range of the analytical error. This indicates that the hydrothermal vent field system has remained relatively stable over the last few decades. However, some differences were observed and (i) suggested a recent eruption at Menez Gwen, (ii) supported the occurrence of low-temperature serpentinisation in this same site, (iii) supported a change in the reaction zone or axial magma chamber (AMC) depth at Lucky Strike, (iv) an increase of the temperature at depth at Snake Pit and (v) supported the hypothesis of large seawater entrainment through the TAG hydrothermal mound. Besides, it is possible that small temporal and spatial scale processes may control a significant part of the geochemistry, owing to the fact that some variations in the data could not be interpreted. However, our investigation of the organic geochemistry represents a pioneering addition to research for Menez Gwen, Snake Pit and TAG and a much more comprehensive study for Lucky Strike and Rainbow. Concentrations for a wide variety of semi volatile organic compounds (SVOCs) were obtained for the first time at all sites. Our results showed that a great part of the total organic carbon (TOC) could not be allocated by the total SVOCs studied here, suggesting that other processes/sources of organic carbon remain to be identified. The TAG organic geochemistry seemed entirely based on thermogenic processes whereas mixed processes may occur at the other vent field. The presence of n-alkanes suggested the contribution of a low-temperature fluid at all sites. An additional high-temperature organic matter degradation component was likely present at Menez Gwen and Lucky Strike. Our results also indicated that both abiogenic and biogenic processes produced organic compounds. Therefore, we suggest that a portion of the fatty acids at Menez Gwen and polyaromatic hydrocarbons (PAHs) at Rainbow may be derived from abiogenic processes, whereas biogenic processes could be responsible for the presence of n-fatty acids (n-FAs) at Lucky Strike and Rainbow. Moreover, organic geochemistry data appeared to be helpful in understanding some inorganic processes.

An Overlooked Natural Hydrogen Evolution Pathway: Ni2+ Boosting H2 O Reduction by Fe(OH)2 Oxidation during Low-Temperature Serpentinization.

Natural hydrogen (H2 ) has gained considerable attentions as a renewable energy resource to mitigate the globally increasing environmental concerns. Low-temperature serpentinization (<200 °C) as a typical water-rock reaction is a major source of the natural H2 . However, the reaction mechanism and the controlling step to product H2 remained unclear, which hinders the further utilization of natural H2 . Herein, we demonstrated that the H2 production rate could be determined by the Fe(OH)2 oxidation during low-temperature serpentinization. Moreover, the co-existence of Ni2+ could largely enhance the H2 production kinetics. With the addition of only 1 % Ni2+ , the H2 production rate was remarkably enhanced by about two orders of magnitude at 90 °C. D2 O isotopic experiment and theoretical calculations revealed that the enhanced H2 production kinetics could be attributed to the catalytic role of Ni2+ to promote the reduction of H2 O.

Molecular Evidence for an Active Microbial Methane Cycle in Subsurface Serpentinite-Hosted Groundwaters in the Samail Ophiolite, Oman.

Serpentinization can generate highly reduced fluids replete with hydrogen (H2) and methane (CH4), potent reductants capable of driving microbial methanogenesis and methanotrophy, respectively. However, CH4 in serpentinized waters is thought to be primarily abiogenic, raising key questions about the relative importance of methanogens and methanotrophs in the production and consumption of CH4 in these systems. Herein, we apply molecular approaches to examine the functional capability and activity of microbial CH4 cycling in serpentinization-impacted subsurface waters intersecting multiple rock and water types within the Samail Ophiolite of Oman. Abundant 16S rRNA genes and transcripts affiliated with the methanogenic genus Methanobacterium were recovered from the most alkaline (pH, >10), H2- and CH4-rich subsurface waters. Additionally, 16S rRNA genes and transcripts associated with the aerobic methanotrophic genus Methylococcus were detected in wells that spanned varied fluid geochemistry. Metagenomic sequencing yielded genes encoding homologs of proteins involved in the hydrogenotrophic pathway of microbial CH4 production and in microbial CH4 oxidation. Transcripts of several key genes encoding methanogenesis/methanotrophy enzymes were identified, predominantly in communities from the most hyperalkaline waters. These results indicate active methanogenic and methanotrophic populations in waters with hyperalkaline pH in the Samail Ophiolite, thereby supporting a role for biological CH4 cycling in aquifers that undergo low-temperature serpentinization.IMPORTANCE Serpentinization of ultramafic rock can generate conditions favorable for microbial methane (CH4) cycling, including the abiotic production of hydrogen (H2) and possibly CH4 Systems of low-temperature serpentinization are geobiological targets due to their potential to harbor microbial life and ubiquity throughout Earth's history. Biomass in fracture waters collected from the Samail Ophiolite of Oman, a system undergoing modern serpentinization, yielded DNA and RNA signatures indicative of active microbial methanogenesis and methanotrophy. Intriguingly, transcripts for proteins involved in methanogenesis were most abundant in the most highly reacted waters that have hyperalkaline pH and elevated concentrations of H2 and CH4 These findings suggest active biological methane cycling in serpentinite-hosted aquifers, even under extreme conditions of high pH and carbon limitation. These observations underscore the potential for microbial activity to influence the isotopic composition of CH4 in these systems, which is information that could help in identifying biosignatures of microbial activity on other planets.

Low-temperature serpentinization reaction process and its biogeochemical significance

Low-temperature serpentinization reaction process and its biogeochemical significance

Thermo-Structural Evolution of the Val Malenco (Italy) Peridotite: A Petrological, Geochemical and Microstructural Study

The Val Malenco peridotite massif is one of the largest exposed ultramafic massifs in Alpine orogen. To better constrain its tectonic history, we have performed a comprehensive petro-structural and geochemical study. Our results show that the Val Malenco serpentinized peridotite recorded both pre-Alpine extension and Alpine convergence events. The pre-Alpine extension is recorded by microstructural and geochemical features preserved in clinopyroxene and olivine porphyroblasts, including partial melting and refertilisation, high-temperature (900–1000 °C) deformation and a cooling, and fluid-rock reaction. The following Alpine convergence in a supra-subduction zone setting is documented by subduction-related prograde metamorphism features preserved in the coarse-grained antigorite and olivine grains in the less-strained olivine-rich layers, and later low-temperature (&lt;350 °C) serpentinization in the fine-grained antigorite in the more strained antigorite-rich layers. The strain shadow structure in the more strained antigorite-rich layer composed of dissolving clinopyroxene porphyroblast and the precipitated oriented diopside and olivine suggest dissolution and precipitation creep, while the consistency between the strain shadow structure and alternating less- and more-strained serpentinized domains highlights the increasing role of strain localization induced by the dissolution-precipitation creep with decreasing temperature during exhumation in Alpine convergence events.

Origin of Indus ophiolite-hosted ophicarbonate veins: Isotopic evidence of mixing between seawater and continental crust-derived fluid during Neo-Tethys closure

Ophicarbonates are found in ophiolite-associated peridotites and bear signature of fluid-rock interaction either on ocean floor or during ophiolite emplacement. The occurrence of ophicarbonate veins hosted by ultramafic rocks of the Indus Ophiolite, Nidar Valley, Ladakh Himalaya, is interpreted within the context of serpentinization of the allochthonous Cretaceous upper mantle section during uplift of the Himalaya orogen. Mixed dolomite-calcite obtained from two veins show carbon isotope ratios consistent with an origin from bicarbonate-dominated fluid and inconsistent with incorporation of atmospheric carbon dioxide in subaerial contact with high pH waters of serpentinization. Strontium isotope ratio of 0.711452–0.712041 are consistent with serpentinization subsequently to emplacement of the ophiolite and by interaction with Cretaceous seawater mixed with continental crust-derived fluid during Neo-Tethys closure. The predominance of lizardite and chrysotile in ophicarbonate veins as documented by Raman spectra also supports a low temperature serpentinization during ophiolite emplacement. The presence of hydrocarbon peaks in fluid inclusions in serpentinized olivine suggests that some of the dissolved inorganic carbon reacted with hydrogen released through oxidation of iron. The ophicarbonates and serpentines in peridotite of Indus ophiolite bear signature of a paleo-environment for Fischer-Tropsch reactions during the last phase of Neo-Tethys closure.

A seawater throttle on H2 production in Precambrian serpentinizing systems

Since the initial discovery of low-temperature alkaline hydrothermal vents off the Mid-Atlantic Ridge axis nearly 20 y ago, the observation that serpentinizing systems produce abundant H2 has strongly influenced models of atmospheric evolution and geological scenarios for the origin of life. Nevertheless, the principal mechanisms that generate H2 in these systems, and how secular changes in seawater composition may have modified serpentinization-driven H2 fluxes, remain poorly constrained. Here, we demonstrate that the dominant mechanism for H2 production during low-temperature serpentinization is directly related to a Si deficiency in the serpentine structure, which itself is caused by low SiO2(aq) concentrations in serpentinizing fluids derived from modern seawater. Geochemical calculations explicitly incorporating this mechanism illustrate that H2 production is directly proportional to both the SiO2(aq) concentration and temperature of serpentinization. These results imply that, before the emergence of silica-secreting organisms, elevated SiO2(aq) concentrations in Precambrian seawater would have generated serpentinites that produced up to two orders of magnitude less H2 than their modern counterparts, consistent with Fe-oxidation states measured on ancient igneous rocks. A mechanistic link between the marine Si cycle and off-axis H2 production requires a reevaluation of the processes that supplied H2 to prebiotic and early microbial systems, as well as those that balanced ocean-atmosphere redox through time.

Issues of structural control of demantoid mineralization at the Poldnevskoye deposit (Middle Urals)

Relevance of the work is due to the need to establish structural control of the mineralization of jewelry demantoid in ultrabasites of the Korkodinsky massif for forecasting, prospecting and exploration of its deposits. Purpose of the work: identification the criteria for structural control at the Poldnevsky demantoid field. Research methodology: analysis of literature data on research topics, studying the geology of the field and the nature of the distribution of mineralization, the conditions of occurrence of mineralized zones, the features of fault tectonics in the quarry of the Poldnevskoye field, the study of thin sections and thin sections by optical methods Results. A geotectonic model for the formation of the Korkodinsky ultrabasite massif represented by a crust-mantle mixture from the depth of 50–60 km is proposed. In the process of rock hoisting, decompression, metamorphism and hydrothermal-metasomatic transformations, decompression fracturing were experienced. The earliest fractures were made by hydrothermal-metasomatic clinopyroxenite, which then also underwent decompression fracturing. Ultrabasic rocks and clinopyroxenites underwent local antigoritization. Later decompression fractures are made by antigorite, clinochrysotile, carbonate, sometimes magnetite and demantoid. All of them were formed under conditions of continuous decrease in allround pressure and synchronous opening of decompression fractures. The largest fractures are oriented subparallel to the wings of the enclosing fault and have a steep fall. The fractures are sinuous and can be arranged en echelon. Mineral-forming fluids were released from the massif as a result of decompression and mixed with meteoric waters penetrating along tensile cracks in the roof of the massif. Late low-temperature serpentinization of the massif is accompanied by the manifestation of “serpentinite tectonics”, which can destroy bunches with demantoid mineralization when superimposition. The demantoid mineralization in weakly serpentinized dunites is preserved from destruction by “serpentinite tectonics”. Conclusion. Since demantoid mineralization is confined to decompression fractures, the possibilities of structural control are very limited. The largest fractures are oriented subconcordantly to the sides of the fault enclosing the massif. This pattern of the distribution of demantoid mineralization in ultrabasites is favorable for the formation of deluvial-alluvial placers, but is unfavorable for development in bedrock depth.

Thermodynamic constraints on the geochemistry of low-temperature, continental, serpentinization-generated fluids

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 Earth9s 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 H₂ 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 H₂: 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.

СЕРПЕНТИНИЗАЦИЯ МАНТИЙНЫХ ПЕРИДОТИТОВ КАК ОСНОВНОЙ ИСТОЧНИК ГЛУБИННЫХ УГЛЕВОДОРОДОВ ЗАПАДНО-СИБИРСКОГО НЕФТЕГАЗОНОСНОГО БАССЕЙНА

The authors have proposed a scheme for the formation of the hydrocarbon capacity of the West Siberian oil and gas basin in a paleosubduction geodynamic setting and continue developing the idea concerning the abiogenic deep genesis of hydrocarbons due to the low-temperature serpentinization of mantle peridotites during spreading in the Ordovician back-arc basin, which occupied at that time within the area of the future West Siberian basin, and the high-temperature hydration of peridotites in the mantle wedges above the subduction zones during the Selurian and Devonian. Another source of abyssal hydrocarbons is the serpentinization of mantle peridotites fragments in water saturated basement. A major role is being established in the formation of hydrocarbon deposits in the sedimentary cover of Western Siberia in the vertical migration of deep hydrocarbon fluids through degassing pipes, including anomalous annular zones, through deep thrusts, which are active gas-oil supply channels and along a large zone of Neogene-Quaternary low-amplitude horizontal shifts in the basement thickness. Deep-seated faults that comprise triassic rift system of West Siberian play a major role in water removal from deep sources. The feeding with new portions of oil and gas in hydrocarbon fields of West Siberian basin continues in present time, which provides renewability of regional hydrocarbons. The total volume of methane generated in the serpentinization zones of mantle peridotites is formed from two sources — abiotic (reaction of hydrogen with carbon dioxide dissolved in seawater) and biotic (methanogens). The origin of hydrocarbons is considered in the context of a new polygenetic scheme of oil and gas formation in the process of «abiogenic-biogenic» synthesis.