Serpentinite Rocks Research Articles

Large salt accumulations as a consequence of hydrothermal processes associated with ‘Wilson cycles’: A review Part 1: Towards a new understanding

Abstract The formation of large salt deposits is observed especially in areas with a geological history of high tectonic activity. Over the last decade it has become a well-established fact that heavy brines form and solid salts precipitate, due to the thermodynamic and physico-chemical properties of seawater at high temperatures and pressures encountered within hydrothermal systems. This article reviews the modern theoretical and experimental research behind these findings, and also describes geological settings that most likely cause brine- and salt-forming hydrothermal processes to occur. This analysis has led to the identification of a set of specific conditions, properties, and processes (referred to as Conceptual elements) that are used to explain the often complex processes of brine behavior that leads to hydrothermal formation of solid salt. The main objective of this review is to present hydrothermal conditions known to occur during Wilson cycles: subduction, collision, and rifting, e.g., zones of repeated tectonic unrest, where brines (commonly derived from seawater) are concentrated into heavy brines and precipitate solid salts. The internal heat of the Earth and its interaction with deeply-circulating seawater in hydrothermal systems and also the immense recycling of crustal materials, including porous oceanic crust and serpentinite (hydrated) rocks via mantle processes may lead to the formation of salt accumulations. It is also acknowledged that such brines and solid salts may often be stored sub-surface for long time periods, extending from one Wilson cycle to another. Thus, on the basis of this analysis, it is cautiously suggested that large amounts of salts ‘hidden’ inside subduction zones may appear on the surface during subsequent rifting and oceanization phases. In Part 2 of this review, the Conceptual elements, which are described and discussed herein (Part 1) are applied to selected cases, including the Andean Mountains, the East African Rift, the Red Sea Rift, and other locations.

Characteristics of phosphorus fractions in the soils derived from sedimentary and serpentinite rocks in lowland tropical rain forests, Borneo

ABSTRACT Soil organic phosphorus (P) is an important P source for biota especially in P-limited forests. Organic P has various chemical formations which differ in bioavailability and these organic P can be degraded by phosphatase enzymes. Here, we report soil P fractions inferred from solution 31P-NMR spectroscopy and soil phosphatase activities of two tropical rain forests on contrasting parent materials; sedimentary and ultramafic igneous (serpentinite) rocks. Compared to the sedimentary soils and previous studies, P fractions of the serpentinite soils have distinctly high proportions of pyrophosphate and scyllo-inositol hexakisphosphate (scyllo-IP6). The accumulation of pyrophosphate and scyllo-IP6 may be related to strong sorptive capacity of iron oxides present in the serpentinite soils, which implies a consequent low P availability in the serpentinite soils. Mean value of soil phosphatase activities was higher in the serpentinite soils than in the sedimentary soils, suggesting that biota in these serpentinite forests depend more on soil organic P as a P source.

Experimental insight into redox transfer by iron- and sulfur-bearing serpentinite dehydration in subduction zones

Dehydration of antigorite in subduction zones releases a large amount of aqueous fluid and volatile elements, which can potentially oxidize the mantle wedge. The redox capacity of three synthetic serpentinites with variable Fetotal, Fe3+ and S− contents is investigated using XANES spectroscopy at both, Fe and S K-edges. Experiments are performed between 450 and 900 °C, at 2 GPa and fO2∼QFM-2; conditions similar to those encountered in subduction zones.Redox reactions in the synthetic serpentinites, which involve Fe and S can be summarized as follows: 1) the reduction of (S−)-pyrite into (S2−)-pyrrhotite (∼450 °C), with ∼4.4 mg/g of the sulfur degassed most likely as H2S, 2) the consumption of magnetite that reacts with antigorite to form Fe-rich olivine (<500 °C), 3) the reduction of (Fe3+)-antigorite into (Fe2+)-antigorite (∼580 °C), occurring about 100 °C below the temperature of antigorite breakdown, 4) the main (Fe2+)-antigorite breakdown that forms olivine and enstatite (∼675 °C), and 5) the decomposition of minor amounts of (Fe2+/3+)-clinochlore (∼800 °C). The bulk Fe3+/Fetotal ratio is found to decrease with run temperature from ∼0.82–0.97 depending on the hydrous starting material, down to 0.1–0.2 in the high-temperature anhydrous assemblages.The evolution of mineral modes and Fe3+/Fetotal with temperature in our synthetic samples shows similar trends to what has been reported in serpentinite rocks collected, for example, along a metamorphic transect in the western Alps. We show that a large amount of O2-equivalent – up to 10 mol/kg of rock – can be generated at temperature around 450 °C due to the presence of oxides and sulfides such as magnetite and pyrite. Owing to the poor capacity of aqueous fluid to transfer redox conditions, we surmise that this O2-equivalent is “consumed” at the scale of the lithospheric-mantle top which is partially serpentinized and therefore bear strong redox gradients.

Naturally occurring asbestos: potential for human exposure, San Severino Lucano (Basilicata, Southern Italy)

In recent years, environmental exposure to naturally occurring asbestos (NOA) has been shown to be a cause of several types of lung disease, which may occur when NOA outcrops are in proximity to residential areas. In this study, we present the results of our investigation of NOA hosted in outcrops of serpentinites in Basilicata region (Southern Italy), which could be potentially harmful to human health. For this purpose, fifteen serpentinite rock samples collected within and near the village of San Severino Lucano were characterized in detail by optical microscope (OM), X-ray powder diffraction (XRPD), thermal analysis (TG/DSC/DTG/DDSC), scanning and transmission electron microscopy combined with energy-dispersive spectrometry, analytical electron microscopy (SEM/EDS and TEM/AEM). Chrysotile and asbestos tremolite or asbestos actinolite were identified in all samples. The NOA in these outcrops could be subject to weathering processes and/or human activities, which generate airborne particles, thus exposing the neighbouring populations to a series of health risks. The size and morphology of NOA are also discussed, regarding the carcinogenic effect that these factors may have on human health. Due to the health risks associated with disturbing NOA-bearing rock, an asbestos exposure control plan must be instituted.

PETROLOGICAL, MINERALOGICAL AND GEOCHEMICAL DATA FROM THE EOHELLENIC OPHIOLITIC NAPPE IN THE ISLAND OF SKYROS, GREECE

The ophiolite of Skyros mainly consists of serpentinized harzburgites, gabbroic rocks, dolerites, tholeiitic basaltic lavas, rodingites, as well as ophicalcites. This ophiolitic sequence comprises of an ophiolitic mélange of pre-Upper Cretaceous age belonging to the Eohellenic nappe, has been affected by low to moderate metamorphic and metasomatic processes. Their mineral chemistry as well as their whole rock chemistry suggests that these ophiolitic rocks are associated with supra-subduction related processes. The presence of vesuvianite crystals in rodingites, as well as the occurrence of relict spinels within serpentinitic rocks, further confirms this assumption. Geochemical and petrological comparison between Skyros ophiolitic rocks and similar rocks of the Eohellenic nappe in East Thessaly and other N. Sporades islands, reveal that they all share many petrogenetical features. These eastcentral Greece Eohellenic ophiolites, and the ophiolitic rocks of the upper tectonic unit of the Attico-Cycladic Zone may possibly form an elongated ophiolitic zone associated to the hanging wall of the North Cycladic Detachment System

An Experimental Study of the Carbonation of Serpentinite and Partially Serpentinised Peridotites

In situ sequestration of CO2 in mantle peridotites has been proposed as a method to alleviate the amount of anthropogenic CO2 in the atmosphere. This study presents the results of eight-month long laboratory fluid-rock experiments on representative mantle rocks from the Oman-United Arab Emirates ophiolite to investigate this process. Small core samples (3 cm long) were reacted in wet supercritical CO2 and CO2-saturated brine at 100 bar and 70°C. The extent of carbonate formation, and hence the degree of carbon sequestration, varied greatly depending on rock type, with serpentinite (lizardite-dominated) exhibiting the highest capacity, manifested by the precipitation of magnesite MgCO3 and ferroan magnesite (Mg,Fe)CO3. The carbonate precipitation occurred predominantly on the surface of the core and subordinately within cross-cutting fractures. The extent of the CO2 reactions appeared to be principally controlled by the chemical and mineralogical composition of the rock, as well as the rock texture, with all these factors influencing the extent and rate of mineral dissolution and release of Mg and Fe for subsequent reaction with the CO2. It was calculated that ≈ 0.7 g of CO2 was captured by reacting ≈ 23 g of serpentinite, determined by the mass of magnesite formed. This equates to ≈ 30 kg CO2 per tonne of host rock, equivalent to ≈ 3% carbonation in half a year. However, recycling of carbonate present in veins within the original rock sample could mean that the overall amount is around 2%. The increased reactivity of serpentinite was associated with preferential dissolution of more reactive types of serpentine minerals and brucite, that were mainly present in the cross-cutting veins. The bulk of the serpentinite rock was little affected. This study, using relatively short term experiments, suggests that serpentinite might be a good host rock for CO2 sequestration, although long term experiments might prove that dunite and harzburgite could be an effective in an engineered system of CCSM. Wet scCO2 proved to be chemically aggressive than CO2-saturated brine and its ingress along fractures and grain boundaries resulted in greater host rock dissolution and subsequent carbonate precipitation.

Prediction of Unconfined Compressive Strength for Jointed Rocks Using Point Load Index Based on Joint Asperity Angle

This research paper is aimed to briefly highlight the correlation between unconfined compressive strength and point load index for jointed rocks based on joint asperity & orientation. In this observe, specimens were tested to obtain their unconfined compression strength and point load index for a different joint condition. The different joint conditions considered for this study were clean joint and joint filled condition. For both clean joint and joint filled specimens were prepared by various asperity angles of 30°, 45°, 60° and 90° with different orientation angles such as 0°, 30°, 45°, 60°, 90°. Plaster of Paris was used as model material to simulate weak rock mass in the field. By testing intact model specimens for unconfined compressive strength leads to revealing of optimum moisture content for further testing. The curing period for the model specimens is 3 days at room temperature. To simulate jointed rocks, various moulds of different orientation of joint with respect to major principal stress are prepared separately. The inner diameter of the mould is 50 mm and height is 100 mm. After casting, a rough joint was created by cutting the prepared sample using the cutter. The specimens are tested for both clean joint and joint filled condition to determine the favorable joint orientation and asperity angle. After curing, the specimens are tested for unconfined compressive strength and Point load index. The new multi-linear correlation for determining unconfined compressive strength with the help of point load index is developed and cross checked with equations formed for actual rock. On comparing both results it is found that the new equation can suitable for assessing the unconfined compressive strength of limestone and serpentinite rocks through point load index value.

CO2 mineral sequestration integrated with water-gas shift reaction

Mineralisation of CO2 using magnesium silicate rock offers a large carbon and storage (CCS) potential with documented advantages compared to underground storage of pure CO2. Work in Finland has resulted in what is referred to as “the ÅA route”, involving stepwise carbonation of serpentinite rock. Magnesium is extracted and converted into magnesium hydroxide (Mg(OH)2), which is carbonated in a pressurised fluidised bed (PFB) reactor at elevated pressure and temperature. The combined operation of a water-shift reaction and carbonation of Mg(OH)2 is addressed in this paper for (coal) gasification syngas and, in more detail, blast furnace top gas. Water produced during the carbonation step can drive the water-gas shift reaction. HSC and Aspen Plus are used for thermodynamic equilibrium product gas and solid products composition analysis. Optimal process conditions appear to be 400–450 °C, at a pressure of 40 bar or higher, for acceptable degrees of conversion. This is partly the result of the water-gas shift reaction equilibrium moving to the CO side at higher temperatures, besides increased calcination of Mg(OH)2 to much less reactive MgO. An energy requirement assessment for blast furnace top gas processing shows that power input requirements may be more than compensated for by waste heat.

Estimation of serpentinite rock mass strength of Placetas - Cuba underground gold mine deposit

This study estimated the strength of the serpentinite rock mass of the underground gold mine “Oro Descanso” Placetas, Cuba. The rock mass was classified into its lithological group of massive, sheared serpentinite rocks and gabbros. The geo-technical information from the well log data obtained during drilling process (geological logs). The structural analysis was carried out through field observation and quantified by Geological Strength Index (GSI) of average values for massive serpentinite 60, sheared serpentinite 38 and gabbros 78. The generalized Hoek-Brown criterion with software programme, Rocklab 1.0, 2004 version was employed for the analysis and the determination of the rock mass local compressive strength (massive serpentinite = 1.733Mpa; sheared serpentinite = 0.464Mpa; gabbros = 10.354Mpa) and the global strength (massive serpentinite = 6.561Mpa, sheared serpentinite = 5.657Mpa and gabbros = 22.547Mpa). These estimated values characterize brittle type of failure mode and thus supports are recommended.

Serpentinization-Influenced Groundwater Harbors Extremely Low Diversity Microbial Communities Adapted to High pH.

Serpentinization is a widespread geochemical process associated with aqueous alteration of ultramafic rocks that produces abundant reductants (H2 and CH4) for life to exploit, but also potentially challenging conditions, including high pH, limited availability of terminal electron acceptors, and low concentrations of inorganic carbon. As a consequence, past studies of serpentinites have reported low cellular abundances and limited microbial diversity. Establishment of the Coast Range Ophiolite Microbial Observatory (California, U.S.A.) allowed a comparison of microbial communities and physicochemical parameters directly within serpentinization-influenced subsurface aquifers. Samples collected from seven wells were subjected to a range of analyses, including solute and gas chemistry, microbial diversity by 16S rRNA gene sequencing, and metabolic potential by shotgun metagenomics, in an attempt to elucidate what factors drive microbial activities in serpentinite habitats. This study describes the first comprehensive interdisciplinary analysis of microbial communities in hyperalkaline groundwater directly accessed by boreholes into serpentinite rocks. Several environmental factors, including pH, methane, and carbon monoxide, were strongly associated with the predominant subsurface microbial communities. A single operational taxonomic unit (OTU) of Betaproteobacteria and a few OTUs of Clostridia were the almost exclusive inhabitants of fluids exhibiting the most serpentinized character. Metagenomes from these extreme samples contained abundant sequences encoding proteins associated with hydrogen metabolism, carbon monoxide oxidation, carbon fixation, and acetogenesis. Metabolic pathways encoded by Clostridia and Betaproteobacteria, in particular, are likely to play important roles in the ecosystems of serpentinizing groundwater. These data provide a basis for further biogeochemical studies of key processes in serpentinite subsurface environments.

Metagenomic identification of active methanogens and methanotrophs in serpentinite springs of the Voltri Massif, Italy.

The production of hydrogen and methane by geochemical reactions associated with the serpentinization of ultramafic rocks can potentially support subsurface microbial ecosystems independent of the photosynthetic biosphere. Methanogenic and methanotrophic microorganisms are abundant in marine hydrothermal systems heavily influenced by serpentinization, but evidence for methane-cycling archaea and bacteria in continental serpentinite springs has been limited. This report provides metagenomic and experimental evidence for active methanogenesis and methanotrophy by microbial communities in serpentinite springs of the Voltri Massif, Italy. Methanogens belonging to family Methanobacteriaceae and methanotrophic bacteria belonging to family Methylococcaceae were heavily enriched in three ultrabasic springs (pH 12). Metagenomic data also suggest the potential for hydrogen oxidation, hydrogen production, carbon fixation, fermentation, and organic acid metabolism in the ultrabasic springs. The predicted metabolic capabilities are consistent with an active subsurface ecosystem supported by energy and carbon liberated by geochemical reactions within the serpentinite rocks of the Voltri Massif.

Influence of physicochemical properties of Brazilian serpentinites on the leaching process for indirect CO2 mineral carbonation

pH-swing mineral carbonation is kinetically favorable and requires a short reaction time. It must also obtain a high extraction rate for reactive elements in the leaching process. The main purpose of this study is to investigate the behavior of different serpentinite rocks in the leaching processes; the reactivity of Brazilian serpentinite rocks (such as: S-GO and S-MG) is analyzed based on physicochemical properties in order to understand their relationship to leaching efficiency. Surface area-to-volume ratio (SBET/Vp) and metals-to-silicon ratio (Σ(Mg, Ca)/Si) were used to measure reactivity. Leaching was carried out to determine Mg and Fe extraction. Reaction conditions for both serpentinite rocks were: 355–250μm particle size, 4M HCl concentration, 100°C, and 2h of reaction time. Characterization results show that both serpentinite rocks (S-GO and S-MG) have high magnesium (Mg) content. SBET/Vp was 36 for S-GO and 29 for S-MG, while Σ(Mg, Ca)/Si was 2.64 for S-GO and 1.20 for S-MG. These results suggest that S-GO is approximately 50% more reactive than S-MG, and that S-MG is limited by low accessible surface (SBET/Vp) and the high mineralogical complexity (Σ(Mg, Ca)/Si). Leaching results confirmed the reactivity; Mg and Fe extraction from S-GO was 94±1%. However, results for S-MG were 34% for Mg and 60% for Fe. In order to increase the reactivity of S-MG, particle size was reduced to 75–63μm. Even though S-MG was mechanically activated, Mg and Fe extraction has not increased significantly.

Conditions of stichtite (Mg6Cr2(OH)16[CO3]·4H2O) formation and its geochemical and isotope record of early phanerozoic serpentinizing environments

Stichtite is a magnesium-chromium hydroxycarbonate mineral found in association with early Phanerozoic chromite-rich serpentinite rocks of Tasmania, Australia and Tehuitzingo, Mexico. Elemental analysis of stichtite shows a range of compositions within the Cr-Fe-Al hydrotalcite group, with compositional trends associated with each discrete serpentinite host body. Elemental and textural analysis of stichtite-associated chromite indicates that stichtite forms in fore-arc setting rocks through interaction of chromite and methane-rich serpentinizing fluids. The degree to which chromite is replaced by stichtite is inferred to correlate with the length of time that the host rocks spent within the “stichtite window.” Carbon stable-isotope analyses of stichtite suggest carbon sourcing from marine kerogen with a minor marine carbonate component in some samples. The carbon and hydrogen stable isotope profile of stichtite ranges from the field of methane from active serpentinizing zones, to organic thermogenic methane. The association of the stichtite 2H polytype (nee barbertonite) with aragonite±antigorite suggests this is a higher pressure/temperature polytype of stichtite. Reaction completion textures, isotopic values, and qualitative mineral thermobarometric indicators indicate that stichtite forms during serpentinization of fore-arc setting rocks in a methane/H2-rich environment within fluid conduits, ranging from low temperatures and pressures near the surface, to depths where pressure is up to 0.8–1.2GPa and temperature is up to ∼300°C. These unique chemical, isotopic, and textural properties of stichtite from distinct serpentinite bodies likely record the duration of serpentinization at specific thermobarometric conditions, and provide a window into the conditions associated with a potentially habitable environment on early Earth and other bodies of the solar system.

Leaching optimization of mining wastes with lizardite and brucite contents for use in indirect mineral carbonation through the pH swing method

This study investigated the leaching process in order to maximize Mg and Fe extraction and to produce amorphous silica (SiO2) with high purity. For this, a mining waste identified as S-GO was employed; which is a serpentinite rock with high lizardite 1T and native brucite contents. A Taguchi Experiment Design was used in order to assess the parameters that influence the leaching process such as: granulometry, hydrochloric acid concentration (HCl), leaching temperature, and mass/volume ratio. Furthermore, thermogravimetric analysis (TGA) was done to understand the interrelation between the mineral structure and leaching performance. Results show that lizardite 1T-bearing serpentinite presents a low content of tetrahedral Al3+ and high octahedral Fe3+ contents on S-GO. Native brucite delayed the formation of a hydrated silica layer and improved dissolution of serpentines. For this, Mg and Fe extractions are efficient, reaching 88 ± 2% of Mg and Fe extracted during the first 30 min of reaction, under mild process conditions: stoichiometric mass/volume ratio, 1M HCl concentration, pressure of 1 bar, temperature of 100 °C, and 300 μm particle size. On the other hand, an excess of acid improves Mg and Fe extraction by only 10 ± 5% for S-GO. Such characteristics reduce energetic penalties and costs involved on indirect mineral carbonation processes by the pH swing method.

Extraction of magnesium from four Finnish magnesium silicate rocks for CO2 mineralisation - Part 1: Thermal solid/solid extraction

The only option for carbon capture and storage (CCS) in Finland is mineral carbonation, which has been extensively researched at Åbo Akademi University (ÅA). Finding suitable minerals for mineral carbonation in different regions of Finland will keep transportation of either CO2 or the actual mineral at a lowest possible level. Four different rocks from different regions in Finland have been compared with respect to composition and possibility to extract magnesium. This paper presents experiments applying thermal solid/solid extraction. It was found that a mixture of ammonium sulphate (AS) and ammonium bisulphate (ABS) could extract a significant part of magnesium compared to tests using only AS or ABS. Serpentinite rock is also the best option for magnesium extraction while diopside and magnesiohornblende are much less reactive. Continuous mixing during the experiment in a rotary tube and adding some water to the solid/solid mixture gives better extraction results. The binding capacities of CO2 in the serpetinites with respect to the magnesium extractions achieved in this study are 240kg CO2/ton rock (Serp-A) and 207kg CO2/ton rock (Serp-B).

Pinguicula ramosa Miyoshi ' a botanical review

Pinguicula ramosa Miyoshi (Lentibulariaceae), endemic to Japan, is one of the most unique Pinguicula species often having a branched pedicel. Ecologically it occurs on cliffs formed by tuffs, porous volcanic rocks, which is also unusual as many Pinguicula species can be more commonly found on calcareous soils or serpentinite rocks. However, little attention has been previously given to this tiny endangered species with very few references in the literature outside of Japan, or even in there. To reveal the species, observations of the plants in situ, herbarium specimen examinations, and review of the literature have been performed. In addition, comparisons with morphologically and phylogenetically closely related species, P. variegata Turcz. and P. villosa L., will be summarized. This work is a botanical review overall P. ramosa discussing its morphology, distribution, taxonomy, ecology, history, conservation, and other scientific matters.

Naturally occurring asbestos (NOA) in rock and soil and relation with human activities: the monitoring example of selected sites in Calabria (southern Italy)

The major aim of our research is to assess the occurrence, within the serpentinites and serpentinite-derived soils located in the area of Sila Piccola (northern Calabria), of asbestiform minerals that could be potentially harmful for human health. At this purpose, nine serpentinite rock samples and three agricultural soils have been collected nearby two inhabited centres (Gimigliano and Conflenti towns) and characterized in detail by means of Polarized Light Microscopy (PLM), X-Ray Powder Diffractometry (XRPD), Scanning Electron Microscopy combined with Energy-Dispersive Spectrometry (SEM/EDS). Further characterization by Differential Scanning Calorimetry (DSC), Thermogravimetric (TG) and by μ-Raman spectroscopy (μ-R) have allowed the discrimination among the three serpentine polytypes. Serpentinites from Gimigliano mainly consist of lizardite, antigorite while chrysotile is the dominant asbestos phase in the samples from Conflenti. Moreover in both investigated sample groups asbestos tremolite- actinolite was also detected. Furthermore, high amounts of chrysotile and asbestos tremolite-actinolite were found in agricultural soils, suggesting that human activities can disturb and provoke the release of inhalable asbestos in the atmosphere, triggering thus mechanisms of hazardous exposition for population.

The significance of the serpentinite characteristics in mineral carbonation by “the ÅA Route”

Serpentinite rocks are considered to be a suitable raw material for Carbon Capture and Mineralization (CCM), which has been recognized as a potential method for the management of the industrial CO2-emissions. Different serpentine minerals, that form serpentinite rocks, feature divergent characteristics that influence the eventual suitability of a serpentinite rock for CCM. The present study focuses on these characteristics, sub-divides them and explains how each characteristic influences the suitability. The carbonation method employed in this study is developed at Åbo Akademi University. The good, yet sometimes inconsistent results gained with this method, gave grounds to a closer mineralogical study. Samples in this study consist of an antigorite dominating serpentinite, a lizardite dominating serpentinite, an antigorite seam, a lizardite rock and a serpentinite rock with high water content (15.15%). A serpentinite tailing is used as a base sample. Mineralogical studies were executed with an optical microscope, Scanning Electron Microscope with Energy Dispersive X-Ray Spectrometer (SEM-EDX) and X-ray Diffraction (XRD).The results state that the characteristics that influence the suitability of a serpentinite rock for mineral carbonation are the mineralogical structure, parent rock and later transformation of the rock through metamorphosis or other type of alteration. The features that positively influence the suitability are more common in lizardite than in antigorite.As serpentinite rocks are widely available, additional knowledge on their behavior based on the mineralogical characteristics, and understanding these variables, enables improvement of the method and widens its range of application. The results can also be used as guidelines to other rocks that are found to be possible raw materials for the carbonation method. The results do not apply to carbonation processes only, but also other purposes, for example among industrial exploitations of quarried materials and their further utilization as profitable goods.

Modern water/rock reactions in Oman hyperalkaline peridotite aquifers and implications for microbial habitability

The Samail ophiolite in Oman is undergoing modern hydration and carbonation of peridotite and may host a deep subsurface biosphere. Previous investigations of hyperalkaline fluids in Oman have focused on fluids released at surface seeps, which quickly lose their reducing character and precipitate carbonates upon contact with the O2/CO2-rich atmosphere. In this work, geochemical analysis of rocks and fluids from the subsurface provides new insights into the operative reactions in serpentinizing aquifers. Serpentinite rock and hyperalkaline fluids (pH>10), which exhibit millimolar concentrations of Ca2+, H2 and CH4, as well as variable sulfate and nitrate, were accessed from wells situated in mantle peridotite near Ibra and studied to investigate their aqueous geochemistry, gas concentrations, isotopic signatures, mineralogy, Fe speciation and microbial community composition.The bulk mineralogy of drill cuttings is dominated by olivine, pyroxene, brucite, serpentine and magnetite. At depth, Fe-bearing brucite is commonly intermixed with serpentine, whereas near the surface, olivine and brucite are lost and increased magnetite and serpentine is detected. Micro-Raman spectroscopy reveals at least two distinct generations of serpentine present in drill cuttings recovered from several depths from two wells. Fe K-edge X-ray absorption near-edge spectroscopy (XANES) analysis of the lizardite shows a strong tetrahedral Fe coordination, suggesting a mixture of both Fe(II) and Fe(III) in the serpentine. Magnetite veins are also closely associated with this second generation serpentine, and 2–10μm magnetite grains overprint all minerals in the drill cuttings. Thus we propose that the dissolved H2 that accumulates in the subsurface hyperalkaline fluids was evolved through low temperature oxidation and hydration of relict olivine, as well as destabilization of pre-existing brucite present in the partially serpentinized dunites and harzburgites. In particular, we hypothesize that Fe-bearing brucite is currently reacting with dissolved silica in the aquifer fluids to generate late-stage magnetite, additional serpentine and dissolved H2.Dissolved CH4 in the fluids exhibits the most isotopically heavy carbon in CH4 reported in the literature thus far. The CH4 may have formed through abiotic reduction of dissolved CO2 or through biogenic pathways under extreme carbon limitation. The methane isotopic composition may have also been modified by significant methane oxidation. 16S rRNA sequencing of DNA recovered from filtered hyperalkaline well fluids reveals an abundance of Meiothermus, Thermodesulfovibrionaceae (sulfate-reducers) and Clostridia (fermenters). The fluids also contain candidate phyla OP1 and OD1, as well as Methanobacterium (methanogen) and Methylococcus sp. (methanotroph). The composition of these microbial communities suggests that low-temperature hydrogen and methane generation, coupled with the presence of electron acceptors such as nitrate and sulfate, sustains subsurface microbial life within the Oman ophiolite.

Occurrence and characterization of asbestos minerals in the area of San Severino Lucano (Basilicata, Southern Italy)

Serpentinites are metamorphic rocks used for building and ornamental purposes since ancient times but they can contain asbestos minerals, thus currently their use is limited or banned in several countries. In this study we have investigated the possible presence of asbestos contained in the serpentinitic rocks outcropping near San Severino Lucano (Basilicata, Southern-Italy). Serpentinites have been characterized in detail combining different analytical techniques in order to detect asbestos minerals that maybe potentially harmful for human health.