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International audience; Interpreting the environmental conditions under which ancient microbialites formed relies upon comparisons with modern analogues. This is why we need a detailed reference framework relating the chemical and mineralogical compositions of modern microbialites to the physical and chemical parameters prevailing in the environments where they form. Here, we measured the chemical, including major and trace elements, and mineralogical composition of microbialites from ten Mexican lakes as well as the chemical composition of the surrounding waters. Saturation states of lakes with different mineral phases were systematically determined and correlations between solution and solid chemical analyses were assessed using multivariate analyses. A large diversity of microbialites was observed in terms of mineralogical composition, with occurrence of diverse carbonate phases such as (Mg-)calcite, monohydrocalcite, aragonite, hydromagnesite, and dolomite as well as authigenic Mg-silicate phases (kerolite and/or stevensite). All lakes harbouring microbialites were saturated or supersaturated with monohydrocalcite, suggesting that such a saturation state might be required for the onset of microbialite formation and that precursor soluble phases such as amorphous calcium carbonate and monohydrocalcite play a pivotal role in these lakes. Subsequently, monohydrocalcite transforms partly or completely to aragonite or Mg-calcite, depending on the lake (Mg/Ca)aq. Moreover, lakes harbouring hydromagnesite-containing microbialites were saturated with an amorphous magnesium carbonate phase, supporting again the involvement of precursor carbonate phases. Last, authigenic Mg-silicates formed by homogenous or heterogenous nucleation in lakes saturated or supersaturated with a phase reported in the literature as “amorphous sepiolite” and with a H4SiO4 concentration superior to 0.2 mM. A strong correlation between the alkalinity and the salinity of all the lakes was observed. The observed large variations of alkalinity between the lakes relate to varying concentration stages of an initial alkaline dilute water, due to a varying hydrochemical functioning. In all cases, the size of microbialites in the lakes correlated positively with salinity, (Mg/Ca)aq ratio and alkalinity. The trace element compositions of the microbialites also varied significantly between the lakes. Detrital contamination of the studied microbialites was the major factor affecting their rare earth elements (REE) + Y patterns. In particular, the microbialites highly affected by detrital contamination showed a high (REE + Y) content and flat (REE + Y) patterns. In contrast, some microbialites poorly affected by detrital contamination showed (REE + Y) patterns with features commonly reported for marine microbialites, such as a superchondritic Y/Ho ratio, enrichment in heavy REE and a negative Ce anomaly. This last observation questions the possibility to infer the marine versus lacustrine origin of a microbialite based on (REE + Y) patterns only. Overall, while microorganisms can impact nucleation processes and textural arrangements in microbialites, we observe that the hydrogeochemical evolution of lakes exerts a primary control over the onset of microbialite formation and the evolution of their chemical and mineralogical composition. Moreover, while changes of all these chemical and mineralogical features upon diagenesis and metamorphism will need to be assessed, the present study, together with recent meta-analyses of modern microbialites, broadens the set of modern references available for comparisons with geological archives.

Achromatium is a long known uncultured giant gammaproteobacterium forming intracellular CaCO3 that impacts C and S geochemical cycles functioning in some anoxic sediments and at oxic-anoxic boundaries. While intracellular CaCO3 granules have first been described as Ca oxalate then colloidal CaCO3 more than one century ago, they have often been referred to as crystalline solids and more specifically calcite over the last 25 years. Such a crystallographic distinction is important since the respective chemical reactivities of amorphous calcium carbonate (ACC) and calcite, hence their potential physiological role and conditions of formation, are significantly different. Here, we analyzed the intracellular CaCO3 granules of Achromatium cells from Lake Pavin using a combination of Raman microspectroscopy and scanning electron microscopy. Granules in intact Achromatium cells were unequivocally composed of ACC. Moreover, ACC spontaneously transformed into calcite when irradiated at high laser irradiance during Raman analyses. Few ACC granules also transformed spontaneously into calcite in lysed cells upon cell death and/or sample preparation. Overall, the present study supports the original claims that intracellular Ca-carbonates in Achromatium are amorphous and not crystalline. In that sense, Achromatium is similar to a diverse group of Cyanobacteria and a recently discovered magnetotactic alphaproteobacterium, which all form intracellular ACC. The implications for the physiology and ecology of Achromatium are discussed. Whether the mechanisms responsible for the preservation of such unstable compounds in these bacteria are similar to those involved in numerous ACC-forming eukaryotes remains to be discovered. Last, we recommend to future studies addressing the crystallinity of CaCO3 granules in Achromatium cells recovered from diverse environments all over the world to take care of the potential pitfalls evidenced by the present study.

International audience; The chemical and isotopic compositions of biogenic apatite are important geochemical markers, which can suffer modifications during fossilisation. Compared with modern ones, fossil apatites generally exhibit variations in carbonate content, enrichment in fluorine, incorporation of trace elements and an increase in crystallinity parameters. Detailed understanding of these transformations induced by fossilisation should help assess the preservation of geochemical records in apatites. In this contribution, we investigate the transformation of modern bone altered under controlled conditions. Modern bone samples were soaked in aqueous solutions of neutral to alkaline pH (9–10) for one and three weeks at various temperatures (20 and 70 °C) and experiments were duplicated in fluorine-free and in 10−2 M NaF solutions. Bone transformation was monitored through the modifications of chemical (F, Ca, P) and isotopic (δ13C, δ18Oc, δ18Op) composition as well as using vibrational (ATR-FTIR, Raman) and solid-state (1H, 13C, 19F) NMR spectroscopies. The observed modifications sustain a transformation mechanism through partial dissolution of biogenic apatite and precipitation of secondary apatite. This transformation occurs irrespective of the presence or absence of fluorine and leads to the formation of carbonate-bearing fluorapatite or carbonate-bearing hydroxylapatite, respectively. The fraction of secondary apatite seems to be limited to ~60%, suggesting that its formation has a protecting role against further dissolution of the primary apatite. The observation of clumped (CO32−, F−) defects in the structural B-site of some samples as well as perturbation of the isotopic compositions attest to carbonate incorporation in the secondary apatite. Although the incorporation of fluoride ions can serve as a probe revealing dissolution-recrystallization of bone, the present study also underlines the difficulty to systematically relate mineralogical transformations to an open-system behaviour in bioapatite.

International audience; A combination of adapted chlorite thermometry and high-spatial-resolution analytical techniques (TEM-EDX/FIB) shows that the low-grade metamorphic chlorites of the Glarus Alps, Central Alps (Switzerland), do not record the peak metamorphic conditions as commonly assumed in previous studies of this reference area for low-grade metamorphism. Chlorites have rather recorded several stages of the retrograde path, through an intracrystalline nanometric-scale compositional zoning. The consistency of the nanoscale zoning patterns observed both within sample and along the transect suggests that local equilibrium was achieved at this spatial scale and maintained during growth or re-equilibration. Applying recent thermobarometers, we highlight that chlorites recorded a distinct behaviour between the northern and southern part of the studied transect: the south of the Glarus area displays a regular P-T exhumation path, from 3.0 ± 0.2 kbar and 310 ± 20 °C for maximum P-T (estimated with conventional thermometers), to ~0.8 kbar and ~220 °C according to chlorite crystal-core analysis, and to ~0.3 kbar and ~190 °C according to chlorite crystal-rim analysis. On the contrary, the north of the Glarus area shows an apparent break in the exhumation, with P estimates from chlorite crystal rims (~1.3 kbar) higher than from chlorite crystal cores (~0.8 kbar). Even if the absolute pressure values are fraught with large uncertainty, their contrasting core/rim pattern is not model dependent. We try to correlate these new thermobarometric results with independent data to refine the exhumation scenario of the North-Helvetic flyschs, confirming differential uplift along the transect. This reappraisal of Glarus chlorite thermometry demonstrates that an adapted thermometry/analysis protocol opens new prospects for investigating the evolution of low-grade metamorphic terranes.

Graphitic carbon, with its diverse structures and unique properties, is everywhere at Earth's surface. Strategically located at the interface between the lithosphere, biosphere, hydrosphere, and atmosphere, graphitic carbon constitutes a major terrestrial carbon reservoir. Natural and synthetic graphitic carbon is also used in a broad range of applications, and graphitic carbon, so widely varied in its physical properties, has proven to be adaptable to many uses in society. Graphitic carbon has played an important role in human history (for example, coal mining) and is now a building block of nanotechnology, but this remarkable material is also an active player in geological processes.

International audience; Laser induced breakdown spectroscopy (LIBS), as performed by the ChemCam instrument, provides a new technique to measure hydrogen at the surface of Mars. Using a laboratory replica of the LIBS instrument onboard the Curiosity rover, different types of hydrated samples (basalts, calcium and magnesium sulfates, opals and apatites) covering a range of targets observed on Mars have been characterized and analyzed. A number of factors related to laser parameters, atmospheric conditions and differences in targets properties can affect the standoff LIBS signal, and in particular the hydrogen emission peak. Dedicated laboratory tests were run to identify a normalization of the hydrogen signal which could best compensate for these effects and enable the application of the laboratory calibration to Mars data. We check that the hydrogen signal increases linearly with water content; and normalization of the hydrogen emission peak using to oxygen and carbon emission peaks (related to the breakdown of atmospheric carbon dioxide) constitutes a robust approach. Moreover, the calibration curve obtained is relatively independent of the samples types

Hybrid rocks formed by fluid–rock interactions at high-pressure (HP) metamorphic conditions are active players in the recycling of volatiles in subduction zones. Such rocks include chlorite–talc–amphibole-rich (± carbonate) rocks formed by chemical and mechanical mixing of mafic, ultramafic and sedimentary protoliths. Recent discovery of widespread formation of lawsonite-rich hybrid rocks extends the composition range of these rocks and their significance for volatile transfer to the deep Earth. In this study we quantify the drastic water enrichment across a metasomatic aureole characterized by intense chemical resetting and massive lawsonite precipitation in Alpine Corsica (France). The chemical composition of the metasomatic product, which in many cases approaches the CASH system, favors (i) the precipitation of lawsonite and the unexpected reincorporation of free water at HP conditions, and (ii) the stability of lawsonite at higher temperature and at greater depth compared to the MORB + H 2 O system. We conclude that these hybrid rocks may contribute to transfer water to great depth in subduction, with implications for water cycling to the mantle.

International audience; Estimation of Fe3+/ΣFe ratios in materials at the submicrometre scale has been a long-standing challenge in the Earth and environmental sciences because of the usefulness of this ratio in estimating redox conditions as well as for geothermometry. To date, few quantitative methods with submicrometric resolution have been developed for this purpose, and most of them have used electron energy-loss spectroscopy carried out in the ultra-high vacuum environment of a transmission electron microscope (TEM). Scanning transmission X-ray microscopy (STXM) is a relatively new technique complementary to TEM and is increasingly being used in the Earth sciences. Here, we detail an analytical procedure to quantify the Fe3+/ΣFe ratio in silicates using Fe L2,3-edge X-ray absorption near edge structure (XANES) spectra obtained by STXM, and we discuss its advantages and limitations. Two different methods for retrieving Fe3+/ΣFe ratios from XANES spectra are calibrated using reference samples with known Fe3+ content by independent approaches. The first method uses the intensity ratio of the two major peaks at the L3-edge. This method allows mapping of Fe3+/ΣFe ratios at a spatial scale better than 50 nm by the acquisition of 5 images only. The second method employs a 2-eV-wide integration window centred on the L2 maximum for Fe3+, which is compared to the total integral intensity of the Fe L2-edge. These two approaches are applied to metapelites from the Glarus massif (Switzerland), containing micrometre-sized chlorite and illite grains and prepared as ultrathin foils by focused ion beam milling. Nanometre-scale mapping of iron redox in these samples is presented and shows evidence of compositional zonation. The existence of such zonation has crucial implications for geothermometry and illustrates the importance of being able to measure Fe3+/ΣFe ratios at the submicrometre scale in geological samples.

International audience; Carbonate-bearing lithologies are the main carbon carrier into subduction zones. Their evolution during metamorphism largely controls the fate of carbon, regulating its fluxes between shallow and deep reservoirs. Recent estimates predict that almost all subducted carbon is transferred into the crust and lithospheric mantle during subduction metamorphism via decarbonation and dissolution reactions at high-pressure conditions. Here we report the occurrence of eclogite-facies marbles associated with metasomatic systems in Alpine Corsica (France). The occurrence of these marbles along major fluid-conduits as well as textural, geochemical and isotopic data indicating fluid–mineral reactions are compelling evidence for the precipitation of these carbonate-rich assemblages from carbonic fluids during metamorphism. The discovery of metasomatic marbles brings new insights into the fate of carbonic fluids formed in subducting slabs. We infer that rock carbonation can occur at high-pressure conditions by either vein-injection or chemical replacement mechanisms. This indicates that carbonic fluids produced by decarbonation reactions and carbonate dissolution may not be directly transferred to the mantle wedge, but can interact with slab and mantle-forming rocks. Rock-carbonation by fluid–rock interactions may have an important impact on the residence time of carbon and oxygen in subduction zones and lithospheric mantle reservoirs as well as carbonate isotopic signatures in subduction zones. Furthermore, carbonation may modulate the emission of CO2 at volcanic arcs over geological time scales.

The insoluble organic matter (IOM) of an unequilibrated enstatite chondrite Sahara (SAH) 97096 has been investigated using a battery of analytical techniques. As the enstatite chondrites are thought to have formed in a reduced environment at higher temperatures than carbonaceous chondrites, they constitute an interesting comparative material to test the heterogeneities of the IOM in the solar system and to constrain the processes that could affect IOM during solar system evolution. The SAH 97096 IOM is found in situ: as submicrometer grains in the network of fine-grained matrix occurring mostly around chondrules and as inclusions in metallic nodules, where the carbonaceous matter appears to be more graphitized. IOM in these two settings has very similar $\delta^{15}N$ and $\delta^{13}C$; this supports the idea that graphitized inclusions in metal could be formed by metal catalytic graphitization of matrix IOM. A detailed comparison between the IOM extracted from a fresh part and a terrestrially weathered part of SAH 97096 shows the similarity between both IOM samples in spite of the high degree of mineral alteration in the latter. The isolated IOM exhibits a heterogeneous polyaromatic macromolecular structure, sometimes highly graphitized, without any detectable free radicals and deuterium-heterogeneity and having mean H- and N-isotopic compositions in the range of values observed for carbonaceous chondrites. It contains some submicrometer-sized areas highly enriched in $^{15}N$ ($\delta^{15}N$ up to 1600 permil). These observations reinforce the idea that the IOM found in carbonaceous chondrites is a common component widespread in the solar system. Most of the features of SAH 97096 IOM could be explained by the thermal modification of this main component.

International audience; Abstract Pyritized plant tissues with well-preserved morphology were studied in rocks from Vanoise (western Alps, France) that experienced high-pressure, low-temperature metamorphic conditions in the blueschist facies during the Alpine orogeny. Organic and inorganic phases composing these fossils were characterized down to the nanometer scale by Raman microspectroscopy, scanning transmission X-ray microscopy and transmission electron microscopy. The graphitic but disordered organic matter composing these fossils is chemically and structurally homogeneous and mostly contains aromatic functional groups. Its original chemistry remains undefined likely because it was significantly transformed by diagenetic processes and/or thermal degradation during metamorphism. Various mineral phases are closely associated with this organic matter, including sulphides such as pyrite and pyrrhotite, carbonates such as ankerite and calcite, and iron oxides. A tentative time sequence of formation of these diverse mineral phases relative to organic matter decay is proposed. The absence of traces of organic matter sulphurization, the pervasive pyritization of the vascular tissues and the presence of ankerite suggest that the depositional/diagenetic environment of these metasediments was likely rich in reactive iron. Fe-sulphides and ankerite likely precipitated early and might have promoted the preservation of the fossilized biological soft tissues by providing mechanical resistance to compaction during diagenesis and subsequent metamorphism. In contrast, iron oxides which form rims of 100-nm in thickness at the interface between organic matter and Fe-sulphides may result from metamorphic processes. This study illustrates that it may be possible in some instances to deconvolve metamorphic from diagenetic imprints and opens new avenues to better constrain processes that may allow the preservation of organic fossils during diagenesis and metamorphism.

The occurrence and preservation of aragonite in eclogite-facies rocks of north-eastern Corsica is linked to an uncommon microtexture. Aragonite exclusively occurs as oriented fibres in garnet crystals of a graphitic, more or less siliceous marble that immediately overlies a serpentinite body of the meta-ophiolitic unit. The arrangement of the fibres is grossly radial, but more clearly sectoral in subhedral garnet, the fibres growing perpendicular to the garnet/matrix interface. Raman mapping reveals that the carbonate is calcite in the matrix and in poikiloblastic garnet cores, and that the fibres in the garnet mantle are aragonite alone in the case of a carbonate matrix, and both aragonite and quartz (in distinct fibres) in a quartz–carbonate matrix. These features are interpreted as prograde intergrowths, the result of garnet nucleation and growth in a calcite and then aragonite matrix (±quartz). Upon further heating and/or decompression, the aragonite matrix transformed back to calcite while the carbonates included in garnet retained their original structure, in spite of the relatively high temperature attained ( ca. 500 °C). These aragonite relics are one more example of the preservation of a high-pressure polymorph through mechanical shielding of inclusions in a rigid host. The aragonite–garnet intergrowths are similar to quartz–garnet intergrowths described in amphibolite-facies graphitic schists. They are evidence that oriented inclusions in garnet are not necessarily precipitates (‘exsolutions’). Unlike precipitates, their orientation is controlled more by the shape of the garnet growth front than by symmetry constraints.

International audience; Reconstructing the original biogeochemistry of organic microfossils requires quantifying the extent of the chemical transformations they experienced during burial and maturation processes. In the present study, fossilization experiments have been performed using modern sporopollenin chosen as an analogue for the resistant biocompounds possibly constituting the wall of many organic microfossils. Sporopollenin powder has been processed thermally under argon atmosphere at different temperatures (up to 1000 °C) for varying durations (up to 900 min). Solid residues of each experiment have been characterized using infrared, Raman and synchrotron-based XANES spectroscopies. Results indicate that significant defunctionalisation and aromatization affect the molecular structure of sporopollenin with increasing temperature. Two distinct stages of evolution with temperature are observed: in a first stage, sporopollenin experiences dehydrogenation and deoxygenation simultaneously (below 500 °C); in a second stage (above 500 °C) an increasing concentration in aromatic groups and a lateral growth of aromatic layers are observed. With increasing heating duration (up to 900 min) at a constant temperature (360 °C), oxygen is progressively lost and conjugated carbon–carbon chains or domains grow progressively, following a log-linear kinetic behavior. Based on the comparison with natural spores fossilized within metasediments which experienced intense metamorphism, we show that the present experimental simulations may not perfectly mimic natural diagenesis and metamorphism. Yet, performing such laboratory experiments provides key insights on the processes transforming biogenic molecules into molecular fossils.

International audience; The Oisans Massif, located in the external zones of the western Alps, experienced significant shortening during the Alpine collision. While a series of major top-to-the west shear zones was recently described, the general low grade of the metamorphism has not attracted much petrological and geochronological studies. This paper provides combined temperature and age constraints on the evolution of the Oisans Massif. Temperature was estimated with the Raman Spectrometry of Carbonaceous Material (RSCM) method and chlorite geothermometry. Maximum temperature reached by the Mesozoic cover (Tmax) from Grenoble to the Galibier pass (E-W) and from Saint Jean de Maurienne to Embrun (N-S) yielded almost constant ca. 330 °C temperatures all through the Massif. Temperatures however strongly decrease either westward towards the top of the Vercors sedimentary sequence or eastward towards the Penninic Frontal Thrust. Age constraints were retrieved using 40Ar/39Ar in situ analyses performed on variously strained samples from Alpine shear zones. Over strain gradients, incipient Alpine recrystallizations progressively develop at the expense of former Variscan parageneses. A combined textural and EPMA approach permitted to identify newly formed chlorite and phengite that unequivocally grew in response to deformation or to low grade metamorphism. Chlorites recorded temperatures from ca. 350–150 °C during the activity of shear zones. In parallel, 40Ar/39Ar in situ experiments enabled dating deformation using both synkinematic phengites crystallized below the closure temperature of white-micas and former Variscan muscovite whose isotopic system have been, at least partially re-opened. Activity of top-to-the-west shear zones responsible for the shortening and thickening of the Oisans massif thus occurred between 34 and 33 and 25 Ma. Integrating these new age-constraints, Tmax estimates, and published geological data on the Oisans Massif and neighbouring areas allow proposing a new shortening scenario for the external zones. From ca. 34–33 Ma, the Oisans Massif was buried as a rigid block below more internal units and reached a temperature of ca. 330 °C. Shearing progressively localized along a series of top-to-the-W shear zones in the basement until 25 Ma. Deformation then localized along a major thrust at the base of the ECM massif and propagated along the basement-cover interface below the Vercors massif after 16 Ma.

A coal and two reference synthetic-cokes, a lamellar graphitizing anthracene-based coke (AC) and a microporous non- graphitizing saccharose-based coke (SC), were held at pressures up to 8 GPa and temperatures up to 1473 K under dry and hydrous conditions. Their subsequent structural and microtextural modifications were characterized by high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy (microspectroscopy and area-mode spectroscopy). No significant transformation was observed in products held up to 95 hours at temperatures below 1273 K. At 1273 K and above, for constant run duration, the higher the pressure, the better is the carbonaceous material organization. The effect of pressure on the graphitization mechanisms is most obvious in the case of the non-graphitizing SC for which the main role of pressure is to transform the microporous microtexture towards a lamellar one. Graphitization is initiated in pore walls as a result of pore growth and thus appears as a local phenomenon. Triperiodic graphite was detected by HRTEM in all SC samples synthesized at 2 GPa and above (1273 K, 95 hours). In the case of the AC material, microtexture remains lamellar and the graphitization results in the reorientation and the in-plane growth of the aromatic layers and rather appears as a bulk phenomenon. The evolution of the coal is intermediate between the two synthetic cokes, as this precursor was microtexturally heterogeneous, composed of an intermediate microtexture between lamellar and microporous ones. Graphitization under pressure appears to be a progressive and continuous process that proceeds heterogeneously through the carbonaceous matrix. The products recovered from high-pressure experiments are structurally and microtexturally heterogeneous, and this heterogeneity raises important problems with respect to the characterization scale and transformation rate in the experimental products. Indeed, in comparison to graphitization temperature in natural samples, it is expected that longer run duration would have led to triperiodic graphite under most experimental conditions achieved here. However, our experimental results show that pressure mainly speeds up the graphitization process, or even makes it possible in the case of the SC, by inducing microtextural and subsequent structural transformations. In all cases, whatever the pressure, temperature defines the highest structural state.

Metasedimentary rocks generally contain carbonaceous material (CM) deriving from the evolution of organic matter originally present in the host sedimentary rock. During metamorphic processes, this organic matter is progressively transformed into graphite s.s. and the degree of organisation of CM is known as a reliable indicator of metamorphic grade. In this study, the degree of organisation of CM was systematically characterised by Raman microspectroscopy across several Mesozoic and Cenozoic reference metamorphic belts. This degree of organisation, including within-sample heterogeneity, was quantified by the relative area of the defect band (R2 ratio). The results from the Schistes Lustres (Western Alps) and Sanbagawa (Japan) cross-sections show that (1) even through simple visual inspection, changes in the CM Raman spectrum appear sensitive to variations of metamorphic grade, (2) there is an excellent agreement between the R2 values calculated for the two sections when considering samples with an equivalent metamorphic grade, and (3) the evolution of the R2 ratio with metamorphic grade is controlled by temperature (T). Along the Tinos cross-section (Greece), which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks, the R2 ratio is nearly constant. Consequently, the degree of organisation of CM is not affected by the retrogression and records peak metamorphic conditions. More generally, analysis of 54 samples representative of high-temperature, low-pressure to high-pressure, low-temperature metamorphic gradients shows that there is a linear correlation between the R2 ratio and the peak temperature [T(°C) = −445 R2 + 641], whatever the metamorphic gradient and, probably, the organic precursor. The Raman spectrum of CM can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism. Temperature can be estimated to ± 50 °C in the range 330–650 °C. A few technical indications are given for optimal application.

International audience; There is growing evidence that microbial populations associated with rocks may be involved in weathering processes; in particular they may play a role in the degradation of fossil organic carbon widely present in sedimentary rocks. In this study, we investigate the microbial diversity associated with two organic-rich rocks exhibiting contrasted organic chemical structure: a typical type I aliphatic carbon-rich kerogen (Autun shale) and a coal rich in aromatic carbon (Graissessac coal). In both rocks, the mineralogy and the geochemistry of the organic matter were investigated. Moreover, the microbial diversity associated with these rocks was characterized using bacterial 165 rRNA gene analysis. In the Autun shale, one dominant species is observed (Ralstonia) whereas a larger microbial diversity is retrieved from the Graissessac coal. Altogether, we conclude that potential hydrocarbonoclastic microorganisms detected by the presence of nag-like genes potentially encoding dioxygenase may be associated with a possible biodegradation of fossil organic carbon in the two samples. We conclude that under favorable conditions, aerobic biodegradation could occur and oxidize a minor fraction of these two different kerogens using a similar enzymatic process. (C) 2014 Elsevier B.V. All rights reserved.

At geological time scales, the role of continental erosion in the organic carbon (OC) cycle is determined by the balance between recent OC burial and petrogenic OC oxidation. Evaluating its net effect on the concentration of carbon dioxide and dioxygen in the atmosphere requires the fate of petrogenic OC to be assessed. Here, we report a multiscale (nanometer to micrometer) structural characterization of petrogenic OC in the Himalayan system. We show that graphitic carbon is preserved and buried in marine sediments, while the less graphitized forms are oxidized during fluvial transport. Radiocarbon dating indicates that 30 to 50% of the carbon initially present in the Himalayan rocks is conserved during the erosion cycle. Graphitization during metamorphism thus stabilizes carbon in the crust over geological time scales.

International audience; Carbon is transported from Earth's surface into its interior at subduction zones. Carbonates in sediments overlying hydrothermally altered rocks (including serpentinites) within the subducted slab are the main carriers of this carbon1. Part of the carbon is recycled back to the surface by volcanism, but some is transferred to the deep Earth1, 2. Redox transformations during shallow subduction control the transfer and long-term fate of carbon, but are poorly explored1, 3. Here we use carbon stable isotopes and Raman spectroscopy to analyse the reduction of carbonate in an exhumed serpentinite-sediment contact in Alpine Corsica, France. We find that highly crystalline graphite was formed during subduction metamorphism and was concentrated in the sediment, within a reaction zone in direct contact with the serpentinite. The graphite in this reaction zone has a carbon isotopic signature (δ13C) of up to 0.8±0.1‰, similar to that of the original calcite that composed the sediments, and is texturally associated with the calcium-bearing mineral wollastonite that is also formed in the process. We use mass-balance calculations to show that about 9% of the total carbonaceous matter in the sedimentary unit results from complete calcite reduction in the reaction zone. We conclude that graphite formation, under reducing and low-temperature conditions, provides a mechanism to retain carbon in a subducting slab, aiding transport of carbon into the deeper Earth.

International audience; Phyllosilicates are among the most important stable minerals within the Earth’s crust. Their use as geo-thermometer bears great potential for application to the thermal history of rocks within the stability range of layered silicates and was tested here.A high-resolution analytical technique combining focused ion beam (FIB) milling and analytical electron microscopy (AEM) analysis has been applied to a series of sandstone core samples from the Gulf Coast (Texas, U.S.A.). The nanoscale compositional variations of K-deficient mica and chlorite flakes show that rim compositions are the most likely to approach equilibrium compositions, whereas core compositions may be relict, especially for illite-like phases. These rim analyses were used to test existing empirical or thermodynamically formulated thermo(baro)meters against maximum temperatures, which are perfectly constrained for the selected samples as they were measured in situ during drilling (100–230 °C and 300–1200 bars). The results show that most of the empirical models overestimate the temperature, while thermodynamic models yields reasonable estimates for diagenetic to anchizonal conditions, especially if the Fe3+ content is taken into account. This study clearly shows that phyllosilicates thermometry is reliable when combined with an analytical technique giving access to the fine-scale compositional variations that may represent local equilibration, whereas using micrometric compositional analysis precludes trustworthy application of such thermometers.

International audience; Multiple pieces of geologic evidence suggest that interfaces between contrasted lithologies exert a strong control on the fate of volatiles in subduction zones. Here we present results from a contact between serpentinites and sediments, located in Corsica and metamorphosed in the blueschist facies during the alpine orogeny. It was shown previously that carbonates in the sediments have been reduced to graphitic carbonaceous material within a 5-10-cm-thick reaction zone at the contact with serpentinites. In an effort to investigate the mechanisms governing this unusual process, bulk rock geochemical analyses incorporating a statistical analysis of compositional data are presented. Observations show that the fate of C was decoupled from that of other elements such as O, H, and large-ion lithophile elements--e.g. K, Sr, Ba...,As--that were extensively leached from the reaction zone. Notably, Na is strongly enriched in the reaction zone and structurally linked to pectolite. Reducing conditions, manifested by the depletion of O in the reaction zone compared to the bulk metasediment, were likely maintained by the presence of Fe(II) in the serpentinite. Moreover, thermodynamic calculations show that the low solubility of carbon in COH fluids at high-pressure and low-temperature conditions was the main driver for graphite precipitation synchronously with carbonate destabilization. This may have been kinetically favored by the presence of already existing graphitized carbonaceous material and phengite in the metasediment. Limited lateral flow might have contributed as well to the geochemical and petrological patterns observed in these rocks.

International audience; The evolution of chlorite composition with temperature (and pressure) serves as basis to a number of chlorite chemical thermometers, for which the oxidation state of iron has been recognised as a recurrent issue, especially at low temperature (T). A new chlorite geothermometer that does not require prior Fe3+ knowledge is formulated, calibrated on 161 analyses with well-constrained T data covering a wide range of geological contexts and tested here for low-T chlorites (T < 350 °C and pressures below 4 kbar). The new solid-solution model used involves six end-member components (the Mg and Fe end-members of ‘Al-free chlorite S’, sudoite and amesite) and so accounts for all low-T chlorite compositions; ideal mixing on site is assumed, with an ordered cationic distribution in tetrahedral and octahedral sites. Applied to chlorite analyses from three distinct low-T environments for which independent T data are available (Gulf Coast, Texas; Saint Martin, Lesser Antilles; Toyoha, Hokkaido), the new pure-Fe2+ thermometer performs at least as well as the recent models, which require an estimate of Fe3+ content. This relief from the ferric iron issue, combined with the simple formulation of the semi-empirical approach, makes the present thermometer a very practical tool, well suited for, for example, the handling of large analytical datasets—provided it is used in the calibration range (T < 350 °C, P < 4 kbar).

International audience; Vibrational properties of diaspore, a-AlOOH, have been re-investigated using room-temperature singlecrystal Raman spectroscopy and low-temperature powder infrared (IR) transmission spectroscopy. First-principles harmonic calculations based on density functional theory provide a convincing assignment of the major Raman peaks and infrared absorption bands. The large width of the Raman band related to OH stretching modes is ascribed to mode-mode anharmonic coupling due to medium-strength H-bonding. Additional broadening in the powder IR spectrum arises from depolarization effects in powder particles. The temperature dependence of the IR spectrum provides a further insight into the anharmonic properties of diaspore. Based on their frequency and temperature behavior, narrow absorption features at *2,000 cm-1 and anti-resonance at *2,966 cm-1 in the IR spectrum are interpreted as overtones of fundamental bending bands.

In contrast to siliceous sponge spicules, the biomineralization in calcareous sponges is poorly understood. In particular, the existence of a differentiated central core in calcareous spicules is still controversial. Here we combine high-spatial resolution analyses, including NanoSIMS, Raman, SXM, AFM, SEM and TEM to investigate the composition, mineralogy and ultrastructure of the giant tetractines of Leuconia johnstoni Carter, 1871 (Baeriidae, Calcaronea) and the organization of surrounding cells. A compositionally distinct core is present in these spicule types. The core measures 3.5-10 mu m in diameter and is significantly depleted in Mg and lightly enriched in S compared with the adjacent outer layer in the spicule. Measured Mg/Ca ratios in the core range from 70 to 90 mmol/mol compared to 125-130 mmol/mol in the adjacent calcite envelope. However, this heterogeneous distribution of Mg and S is not reflected in the mineralogy and the microstructure. Raman spectroscopy demonstrates a purely calcitic mineralogy. SEM examination of slightly etched spicules indicates an ultrastructure organized hierarchically in a concentric pattern, with layers less than 250 nm in width inside layers averaging 535 +/- 260 nm. No change in structural pattern corresponds to the Mg/Ca variation observed. AFM and TEM observations show a nano-granular organization of the spicules with a network of intraspicular organic material intercalated between nanograins 60-130 nm in diameter. Observations of sclerocyte cells in the process of spiculogenesis suggest that the compositionally distinct core is produced by a sub-apical sclerocyte "founder cell" that controls axial growth, while the envelope is secreted by lateral sclerocytes "thickener cells", which control radial growth. (C) 2010 Elsevier Inc. All rights reserved.

The two long-term sources of atmospheric carbon are CO2 degassing from metamorphic and volcanic activity, and oxidation of organic carbon (OC) contained in sedimentary rocks, or petrogenic organic carbon (OCpetro). The latter fl ux is still poorly constrained. In this study, we report particulate organic carbon content and 14C activity measurements in Amazon River sediments, which allow for estimates of the OCpetro content of these sediments. A large decrease of OCpetro content in riverine sediments is observed from the outlet of the Andes to the mouth of the large tributaries. This loss reveals oxidation of OCpetro during transfer of sediments in the fl oodplain, and results in an escape of ~0.25 Mt C/yr to the atmosphere, which is on the same order of magnitude as the CO2 consumption by silicate weathering in the same area. Raman microspectroscopy investigations show that graphite is the most stable phase with respect to this oxidation process. These results emphasize the signifi cance of OCpetro oxidation in large river fl oodplains in the global carbon cycle.

International audience; The spatial distribution of CaCO3 polymorphs formed during the experimental carbonation of water saturated Portlandcement cores (30 mm in diameter), with supercritical CO2 at 90 C and 30 MPa, has been investigated using Raman microspectrometry on polished sample sections and X-ray microdiffraction. The three calcium carbonate polymorphs (calcite, aragonite and vaterite) were clearly distinguished using both techniques and their distribution along the main CO2 diffusion direction could be mapped at the millimetre scale using a dynamic line-scanning Raman mapping tool. The calcium carbonate 2D distribution clearly shows that vaterite, the least stable of the three CaCO3 polymorphs, is mostly located in a 500 mm wide ring ahead of the carbonation zone. This feature indicates that vaterite is the first CaCO3 polymorph to crystallize within the cement sample in the course of the carbonation process. The presence of a vaterite front indicates that local mineral–solution equilibration can be slower than species transport, even above ambient conditions, and that kinetics cannot be ignored in the cement carbonation process. By using calcite and vaterite precipitation kinetic data from the literature and assuming water–mineral kinetics based on the Transition State Theory, the vaterite front inferred from Raman mapping is reproduced with a purely diffusive 1D transport code.

International audience; Abstract Graphitizing anthracene-based cokes and non-graphitizing saccharose-based chars were processed at temperatures from 450 °C to 2900 °C at ambient pressure. This offers a whole set of samples that greatly differ in structure. Here, their structural evolution was monitored by combining XRD and visible (green) Raman spectroscopy as well as, for the first time, near-infrared Raman and synchrotron-based C-XANES spectroscopies. These different techniques provide complementary information especially regarding the spatial resolution they achieved. However, despite its importance, the quantitative comparison between the structural parameters extracted from these techniques is difficult. Based on a new signal deconvolution procedure to extract quantitative structural information from C-XANES data and the achievement of a new dataset on a complete series of graphitization, the reliability and the precision of the information which can be retrieved from each technique are discussed. C-XANES spectroscopy appears to provide reliable proxies for the extent of aromatic layers of graphitic compounds and an empirical calibration is proposed.

In phosphate-rich environments, vivianite (Fe(II)(3)(PO(4))(2), 8H(2)O) is an important sink for dissolved Fe(II) and is considered as a very stable mineral due to its low solubility at neutral pH. In the present study, we report the mineralogical transformation of vivianite in cultures of the nitrate-reducing iron-oxidizing bacterial strain BoFeN1 in the presence of dissolved Fe(II). Vivianite was first transformed into a greenish phase consisting mostly of an amorphous mixed valence Fe-phosphate. This precipitate became progressively orange and the final product of iron oxidation consisted of an amorphous Fe(III)-phosphate. The sub-micrometer analysis by scanning transmission X-ray microscopy of the iron redox state in samples collected at different stages of the culture indicated that iron was progressively oxidized at the contact of the bacteria and at a distance from the cells in extracellular minerals. Iron oxidation in the extracellular minerals was delayed by a few days compared with cell-associated Fe-minerals. This led to strong differences of Fe redox in between these two types of minerals and finally to local heterogeneities of redox within the sample. In the absence of dissolved Fe(II), vivianite was not significantly transformed by BoFeN1. Whereas Fe(II) oxidation at the cell contact is most probably directly catalyzed by the bacteria, vivianite transformation at a distance from the cells might result from oxidation by nitrite. In addition, processes leading to the export of Fe(III) from bacterial oxidation sites to extracellular minerals are discussed including some involving colloids observed by cryo-transmission electron microscopy in the culture medium.

Graphite in the Borrowdale (Cumbria, UK) deposit occurs as large masses within mineralized pipe-like bodies, in late graphite–chlorite veins, and disseminated through the volcanic host rocks. This occurrence shows the greatest variety of crystalline graphite morphologies recognized to date from a single deposit. These morphologies described herein include flakes, cryptocrystalline and spherulitic aggregates, and dish-like forms. Colloform textures, displayed by many of the cryptocrystalline aggregates, are reported here for the first time from any graphite deposit worldwide. Textural relationships indicate that spherulitic aggregates and colloform graphite formed earlier than flaky crystals. This sequence of crystallization is in agreement with the precipitation of graphite from fluids with progressively decreasing supersaturation. The structural characterization carried out by means of Raman spectroscopy shows that, with the exception of colloform graphite around silicate grains and pyrite within the host rocks, all graphite morphologies display very high crystallinity. The microscale SIMS study reveals light stable carbon isotope ratios for graphite (δ13C = -34.5 to -30.2%), which are compatible with the assimilation of carbon-bearing metapelites in the Borrowdale Volcanic Group magmas. Within the main mineralized breccia pipelike bodies, the isotopic signatures (with cryptocrystalline graphite being lighter than flaky graphite) are consistent with the composition and evolution of the mineralizing fluids inferred from fluid inclusion data which indicate a progressive loss of CO2. Late graphite–chlorite veins contain isotopically heavier spherulitic graphite than flaky graphite. This agrees with CH4-enriched fluids at this stage of the mineralizing event, resulting in the successive precipitation of isotopically heavier graphite morphologies. The isotopic variations of the different graphite morphologies can be attributed therefore, to changes in the speciation of carbon in the fluids coupled with concomitant changes in the XH2O during precipitation of graphite and associated hydrous minerals (mainly epidote and chlorite).

Continental erosion controls atmospheric carbon dioxide levels on geological timescales through silicate weathering, riverine transport and subsequent burial of organic carbon in oceanic sediments. The efficiency of organic carbon deposition in sedimentary basins is however limited by the organic carbon load capacity of the sediments and organic carbon oxidation in continental margins. At the global scale, previous studies have suggested that about 70 per cent of riverine organic carbon is returned to the atmosphere, such as in the Amazon basin. Here we present a comprehensive organic carbon budget for the Himalayan erosional system, including source rocks, river sediments and marine sediments buried in the Bengal fan. We show that organic carbon export is controlled by sediment properties, and that oxidative loss is negligible during transport and deposition to the ocean. Our results indicate that 70 to 85 per cent of the organic carbon is recent organic matter captured during transport, which serves as a net sink for atmospheric carbon dioxide. The amount of organic carbon deposited in the Bengal basin represents about 10 to 20 per cent of the total terrestrial organic carbon buried in oceanic sediments. High erosion rates in the Himalayas generate high sedimentation rates and low oxygen availability in the Bay of Bengal that sustain the observed extreme organic carbon burial efficiency. Active orogenic systems generate enhanced physical erosion and the resulting organic carbon burial buffers atmospheric carbon dioxide levels, thereby exerting a negative feedback on climate over geological timescales.

International audience; It is commonly believed that high-grade metamorphism erases all traces of fossils and, more generally, of biogenicity in rocks. To address this issue, we report in this study X-ray spectromicroscopy, Raman microspectroscopy, and electron microscopy observations at the nm-scale of fossil lycophytes megaspores found in Triassic metasedimentary rocks from the French Alps. These rocks have undergone high-pressure metamorphic conditions (not, vert, similar 14 kbar, not, vert, similar 360 °C) during the Alpine orogeny corresponding to a burial of not, vert, similar 35 km during subduction. Despite such metamorphism, the fossil megaspores are remarkably well-preserved texturally. An unusual mineralogical zonation consisting of magnetite, ankerite and calcite is superposed on the structures of the spore walls. In parallel, chemical heterogeneities of the organic matter (OM) composing the spore walls were detected. We discuss different interpretations for our observations, in particular a scenario in which these features are remnants of the original structural heterogeneities in the spores, and provide insights on the fossilization processes under high-grade metamorphic conditions. Whatever the processes leading to such heterogeneities, our observations oppose the usual assumption that high-grade metamorphism erases all indications of biogenicity in rocks and open new avenues for investigation at the nm-scale of the evolution of organic matter during diagenesis and metamorphism.

The applicability of Raman spectroscopy to characterize disordered and heterogeneous carbonaceous materials (CM) is discussed, by considering both natural and synthetic CM. First, different analytical mismatches during the measurement are discussed and technical indications are provided in order to eliminate them. Second, the accuracy and relevance of the different parameters obtained by the decomposition of spectra by conventional fitting procedure, is reviewed. Lastly, a new Raman technique (Raman area mode microspectroscopy) giving an homogeneous repartition of power within a large laser beam is presented, this technique being powerful to study strongly heterogeneous CM and/or photosensitive samples.