A NMR and molecular dynamics study of CO2-bearing basaltic melts and glasses

International audienceThe presence of volatile, especially carbon dioxide (CO2), in silicate liquids is considered as a key parameter to magmatic degassing and eruptive processes. Unfortunately, due to experimental difficulties, our current knowledge on the CO2 effect on silicate melt structure is weak and relies on the observation of ex-situ recovered CO2-bearing glasses.In the present work, we confront the results obtained from NMR spectroscopic observations of glass synthesised at pressure between 0.5 and 3.0 GPa and theoretical investigations from first-principles molecular dynamics (FPMD) simulations conducted at 5.0 and 8.0 GPa on high temperature melt for a simplified basaltic composition.The results obtained on the aluminosilicate framework (molar fraction of silicon species and Al average coordination number) suggest that both ex-situ and in-situ results compare adequately. The results are in agreement with our current knowledge on the change in aluminosilicate melt/glass structure with changing intensive conditions. Increasing pressure from 0.5 to 8.0 GPa induces 1) an increase in the average Al coordination number from 4.1 to almost 5.0 and 2) an increase in the degree of polymerisation with NBO/Si changing from 1.30 to 0.80.The presence of CO2 does not seem to induce a dramatic change on both the average Al coordination number and the NBO/Si. FPMD simulations performed with 0 and 20 wt.% CO2 at 8.0 GPa result in a change from 4.84 to 4.96 for the average Al coordination number and in a change from 0.87 to 0.80 for the NBO/Si value, respectively.On the contrary, there is a lack of consistency in between the CO2 speciation obtained from NMR spectroscopy and from FPMD simulations. Whereas the analysis of glasses does not reveal the presence of CO2mol species, the FPMD simulation results suggests the existence of a small proportion of CO2mol. Further work with in-situ experimental approach is therefore required to explain the observed lack of consistency between the CO2 speciation in glass and in high temperature melt with basaltic composition

C-O-H fluid solubility in Haplo-basalt under reducing conditions: An experimental study

International audienceWe conducted an experimental study to constrain the C-H-O solubility and speciation in hydrous silicate melts equilibrated under reduced fO2 conditions. Haplo-basaltic glasses in the NCMAS-C-O-H system were synthesised using IHPV at 1250 °C, 200-300 MPa with variable applied fH2 so as to vary fO2. Recovered rapidly quenched glasses were characterized using various spectroscopic methods: Micro-FTIR, Raman and 13C-MAS NMR. Glass CO2 content changes from 680 to 1320 ppm between ΔFMQ-2.6 and ΔFMQ + 2.6 independently of H2O content changing from 1.3 to 4.0 wt.%. Recent thermodynamic modelling of isobaric CO2-H2O solubility fails to reproduce our CO2-H2O solubility trend under reducing conditions. The lower CO2 solubility in the melt as compared to more oxidized conditions is directly correlated to the decrease of fCO2 within the fluid phase under reducing conditions. Carbonate groups (CO32-), OH- and H2Omol are the volatile species in the glasses. No evidence for CH4, carbides or organic compounds was observed. 13C MAS NMR analysis suggests that several carbonate units are coexisting in the glasses. {1H} 13C-CPMAS NMR suggests that all CO32- units are surrounded by OH groups. Those environments appear to slightly change with changing fO2 conditions suggesting different degree of hydrogenation in the vicinity of the carbonate groups. Our data show that the presence of a significant amount of dissolved does not increase the solubility of species such as CO or CH4. In other words, such species remain insoluble in basaltic melts, as established under dry conditions. Altogether, our CO2 solubility results show that a wet but reduced basalt will degass more C-species than if oxidized, owing to the lower prevailing fCO2 and insoluble character of CO. The presence of an important fraction of CO in the fluid phase will have a large impact on the primitive atmospheric compositions of Mars and the Earth

A model for the activity of silica along the carbonatite-kimberlite-mellilitite-basanite melt compositional joint

International audienceCarbon dioxide and water, being present in the Earth’s mantle at concentration levels of tens to hundreds of ppm, greatly lower the peridotite solidus temperature and drastically modify the composition of produced melts. The presence of CO2 produces silica-poor, carbonate-rich liquids at the onset of melting, and these liquids shift toward silica rich compositions as the degree of melting increases. Numerous geochemical observations and experimental studies have revealed the complexity of the transition between carbonate-rich and silicate-rich melts. It is characterized by a strongly non-linear evolution and, under specific conditions, by immiscibility. To better constrain this transition, we have used the thermodynamic activity of silica as a probe of the mixing properties between molten carbonate and molten silicate. The activity of silica (image) was calculated for a large number of experimental liquids from two equilibria: olivine-orthopyroxene-melt and immiscible silicate-rich melt-carbonate-rich melt (491 data points ranging from 1 to 14 GPa and 1090 to 1800°C). We modeled image during incipient melting of the peridotite in presence of CO2 with a generalized Margules function. Our model well reproduces the silica activity–composition relationships of the experimental database, and can be used to predict the silica content of the melts coexisting with olivine and orthopyroxene. We show that water content and Ca/Mg ratio in the melts have an important influence on the image. In contrast to a recent empirical model (Dasgupta et al., 2013), the analysis of the experimental database reveals that the transition from carbonate to silicate melt with decreasing depth should occur abruptly in oceanic mantle. Our model predict that carbonatitic melts with ~ 5 wt.% SiO2 can be stabilized from ~ 150 km depth, at the onset of incipient melting by “redox melting”, up to ~ 75 km, above which the liquid evolves abruptly to a carbonated silicate composition (> ~ 25 wt.% SiO2). In the cratonic mantle lithosphere, our model predicts that carbonatitic melts are prevailing up to shallow depth, and conflicts the recent model (Russell et al., 2012) of CO2-saturation triggered by orthopyroxene assimilation during kimberlite ascent

CO 2 Solubility in Kimberlite melts CO 2 Solubility in Kimberlite melts

International audienceCarbon dioxide is the most abundant volatile in kimberlite melts and its solubility exerts a prime influence on the melt structure, buoyancy, transport rate and hence eruption dynamics. The actual primary composition of kimberlite magma is the matter of some debate but the solubility of CO2 in kimberlitic melts is also poorly constrained due to difficulties in quenching these compositions to a glass that retains the equilibrium CO2 content. In this study we used a range of synthetic, melt compositions with broadly kimberlitic to carbonatitic characteristics which can, under certain conditions, be quenched fast enough to produce a glass. These materials are used to determine the CO2 solubility as a function of chemical composition and pressure (0.05-1.5 GPa). Our results suggest that the solubility of CO2 decreases steadily with increasing amount of network forming cations from ~ 30 wt% CO2 at 12 wt% SiO2 down to ~ 3 wt% CO2 at 40 wt% SiO2. For low silica melts, CO2 solubility correlates non-linearly with pressure showing a sudden increase from 0.1 to 100 MPa and a smooth increase for pressure > 100 MPa. This peculiar pressure-solubility relationship in low silica melts implies that CO2 degassing must mostly occur within the last 3 km of ascent to the surface having potential links with the highly explosive nature of kimberlite magmas and some of the geo-morphological features of their root zone. We present an empirical CO2 solubility model covering a large range of melt composition from 11 to 55 wt% SiO2 spanning the transition from carbonatitic to kimberlitic at pressures from 1500 to 50 MPa

The effect of sulfur on the glass transition temperature in anorthite-diopside eutectic glasses

International audienceThe effect of sulfur dissolved in anorthite-diopside eutectic (AD) glasses on the glasstransition temperature (Tg) has been investigated via Differential Scanning Calorimetricmeasurements (DSC) and Thermogravimetric Analysis (TGA) under moderately reducing tooxidizing conditions.In a series of AD glasses, we have measured the change in Tg as a function of S contentpresent as SO42- (HS- is also identified to a lesser extent) and H2O content. The AD glassesinvestigated have S contents ranging from 0 to 7519 ppm and H2O contents ranging from 0 to5.3 wt.%. In agreement with previous studies, increasing H2O content induces a strongexponential decrease in Tg: volatile free AD glass has a Tg at 758±13C and AD glass with5.18±0.48 wt.% H2O has a Tg at 450±11C. The change in Tg as a function of H2O is wellreproducedwith a third-order polynomial function and has been used to constrain Tg at anyH2O content. The effect of S on Tg is almost inexistent or towards a decrease in Tg withincreasing S content. For instance, at ~2.4 wt.% H2O, the addition of S induces a change inTg from 585±10°C with 0 ppm S to 523±3C with 2365±138 ppm S; a further increase in Sup to 7239±90 ppm S does not induce a dramatic change in Tg measured at 529±2C.The limited effect of S on the glass transition temperature contrasts with recent spectroscopicmeasurements suggesting that S dissolution as SO42- groups provokes an increase in thepolymerization degree. We propose an alternative view which reconciles the spectroscopicevidence with the Tg measurements. The dissolution of S as SO42- does not induce theformation of Si-O-Si molecular bonding through consumption of available non-bridgingoxygens (NBO) but instead we suggest that Si-O-S molecular bonds are formed which are not detectable by DSC measurements but mimic the increase in glass polymerization. Therefore, spectroscopic measurements must be used with caution in order to extract melt physicalproperties

A composition-independent quantitative determination of the water content in silicate glasses and silicate melt inclusions by confocal Raman spectroscopy

A new approach was developed to measure the water content of silicate glasses using Raman spectroscopy, which is independent of the glass matrix composition and structure. Contrary to previous studies, the compositional range of our studied silicate glasses was not restricted to rhyolites, but included andesitic, basaltic and phonolitic glasses. We used 21 glasses with known water contents for calibration. To reduce the uncertainties caused by the baseline removal and correct for the influence of the glass composition on the spectra, we developed the following strategy: (1) application of a frequency-dependent intensity correction of the Raman spectra; (2) normalization of the water peak using the broad T-O and T-O-T vibration band at 850-1250cm−1 wavenumbers (instead of the low wavenumber T-O-T broad band, which appeared to be highly sensitive to the FeO content and the degree of polymerization of the melt); (3) normalization of the integrated Si-O band area by the total number of tetrahedral cations and the position of the band maximum. The calibration line shows a ±0.4wt% uncertainty at one relative standard deviation in the range of 0.8-9.5wt% water and a wide range of natural melt compositions. This method provides a simple, quick, broadly available and cost-effective way for a quantitative determination of the water content of silicate glasses. Application to silicate melt inclusions yielded data in good agreement with SIMS dat

The molecular structure of melts along the carbonatite–kimberlite–basalt compositional joint: CO2and polymerisation

International audienceTransitional melts, intermediate in composition between silicate and carbonate melts, form by low degree partial melting of mantle peridotite and might be the most abundant type of melt in the asthenosphere. Their role in the transport of volatile elements and in metasomatic processes at the planetary scale might be significant yet they have remained largely unstudied. Their molecular structure has remained elusive in part because these melts are difficult to quench to glass. Here we use FTIR, Raman, 13C and 29Si NMR spectroscopy together with First Principle Molecular Dynamic (FPMD) simulations to investigate the molecular structure of transitional melts and in particular to assess the effect of CO2on their structure. We found that carbon in these glasses forms free ionic carbonate groups attracting cations away from their usual ‘depolymerising’ role in breaking up the covalent silicate network. Solution of CO2in these melts strongly modifies their structure resulting in a significant polymerisation of the aluminosilicate network with a decrease in NBO/Si of about 0.2 for every 5 mol% CO2dissolved.This polymerisation effect is expected to influence the physical and transport properties of transitional melts. An increase in viscosity is expected with increasing CO2content, potentially leading to melt ponding at certain levels in the mantle such as at the lithosphere–asthenosphere boundary. Conversely an ascending and degassing transitional melt such as a kimberlite would become increasingly fluid during ascent hence potentially accelerate. Carbon-rich transitional melts are effectively composed of two sub-networks:a carbonate and a silicate one leading to peculiar physical and transport properties

The SuperCam Instrument Suite on the Mars 2020 Rover: Science Objectives and Mast-Unit Description

On the NASA 2020 rover mission to Jezero crater, the remote determination of the texture, mineralogy and chemistry of rocks is essential to quickly and thoroughly characterize an area and to optimize the selection of samples for return to Earth. As part of the Perseverance payload, SuperCam is a suite of five techniques that provide critical and complementary observations via Laser-Induced Breakdown Spectroscopy (LIBS), Time-Resolved Raman and Luminescence (TRR/L), visible and near-infrared spectroscopy (VISIR), high-resolution color imaging (RMI), and acoustic recording (MIC). SuperCam operates at remote distances, primarily 2-7 m, while providing data at sub-mm to mm scales. We report on SuperCam's science objectives in the context of the Mars 2020 mission goals and ways the different techniques can address these questions. The instrument is made up of three separate subsystems: the Mast Unit is designed and built in France; the Body Unit is provided by the United States; the calibration target holder is contributed by Spain, and the targets themselves by the entire science team. This publication focuses on the design, development, and tests of the Mast Unit; companion papers describe the other units. The goal of this work is to provide an understanding of the technical choices made, the constraints that were imposed, and ultimately the validated performance of the flight model as it leaves Earth, and it will serve as the foundation for Mars operations and future processing of the data.In France was provided by the Centre National d'Etudes Spatiales (CNES). Human resources were provided in part by the Centre National de la Recherche Scientifique (CNRS) and universities. Funding was provided in the US by NASA's Mars Exploration Program. Some funding of data analyses at Los Alamos National Laboratory (LANL) was provided by laboratory-directed research and development funds

Raman quantification factor calibration for CO-CO2 gas mixture in synthetic fluid inclusions: Application to oxygen fugacity calculation in magmatic systems.

International audienceWith a combined approach using Solid State 13C-MAS NMR and Laser Raman Microspectroscopy, we investigated the CO2-CO gas composition (X(CO2)) in fluid inclusions synthesised under high pressure (200-300 MPa), high temperature (1225-1250 °C) and reducing conditions (17 < P(H2) < 62 bars). Fluid inclusions are entrapped in a volatile-bearing basaltic glass which was characterized by FTIR for determining the water solubility (H2Om). 13C-MAS NMR is used as a standard analysis for determining the X(CO2). The Raman quantification factors between 13CO2 and 13CO is determined from peak area (F-factor), peak height (G-factor) and according to the Placzek's polarizability theory. The calibration is derived for both CO2 Fermi diad resonances: 2n2 and n1. We obtain similar values for the main CO2 resonance (2n2) with 1.956 and 1.809 for F and G respectively. Results are consistent with the fact that peak height and area will measure the same quantity. For n1, multiple calibration trends are observed. The different trends are explained by the different 13C/12C ratio observed in between the samples. However, we suggest that such resonance is not suitable for determining the fluid inclusion compositions. We extended the 13C results for calibrating the F- and G-factors for 12CO2-12CO gas mixture in the fluid inclusions and for the main CO2 resonance. For 12CO2-12CO mixture, F and G values are 1.856 and 1.756 which is in the same order as the derived values for 13C species. Thus, we propose that no significant 13C/12C fractionation occurs in the fluid phase and both isotopes will behave in a similar way. Using the derived calibration for 12C and 13C species, the X(CO2) in the fluid phase was recalculated. Results are similar for both isotopes witnessing the similar behaviour of 12C and 13C fluid species during the experiments. The log f(O2) experienced by each sample has been calculated through a thermodynamic approach using 2 independent methods. The log f(O2) calculated from the H2Om in the glass and the X(CO2) in the fluid phase are in good agreement. Large discrepancy is observed for low H2Om content which gives lower log f(O2) value than expected from experimental conditions. Large uncertainties on the H2Om measurements will induce a very approximate value for the log f(O2). This method may not be accurate enough at low H2Om and using the X(CO2) in the fluid phase would therefore provide a better estimate of the log f(O2)

A multi-component model for partial melting in presence of CO2 and other volatiles in the mantle.

International audienceThe link between volatiles and mantle melting has so far been illuminated by experiments revealing punctually, at a given P-T condition and under a specific chemical system, properties such as solubility laws, redox equiblibra, and phase equilibria. The aim we are pursuing here is to establish a multi-component model describing the Gibbs free energy of melt produced by mantle melting in presence of CO2-H2O: Carbonatite-carbonated melt and basalts. The generated low melt fractions are often dominated by carbonate-rich compositions, whereas with increasing temperature, the melts evolve towards basaltic compositions. However, the transition between carbonate-dominated and silicate-dominated melts is complex and poorly constrained: it is characterized by a continuous evolution between a carbonated melt and a silicated melt, or show, under specific conditions, immiscibility between these two types of liquids. Several studies emphasize the role of alkalis in the immiscibility between a carbonate-dominated melt and a silicate-dominated melt. Consequently, we performed experiments in simplified systems to better understand the influence of each (K and Na) on this immiscibility. In addition, specific experiments on more complex compositions have been performed, in order to give first insights on the role of various volatiles present in the melts: water, chlorine. From a thermodynamic point of view, the carbonate-silicate transition is defined by the activity of the component SiO2 in the liquid and is calculated from experimental data (2-10 GPa, 1100-1600° C) using crystal-liquid and liquid-liquid equilibria. This silicate-carbonate immiscibility constitutes a powerful tool defining the mixing properties of the liquid. The miscibility gap defines equilibrium melts with different compositions, but the melt components are characterized by similar activities. This can be inverted to derive activity-composition relationships that are strictly independent of standard state properties. We will present a parameterization of the mixing properties allowing the complex activity-composition relationships for multi-component carbonated melts to be accounted for. Graphite-liquid and fluid-liquid data allow, for the first time, to constrain the standard state properties of CO2dissolved in liquid, and its activity. Activity-composition relationships for CO2 are strongly non-ideal in carbonated melts, but the presence of water apparently tends to minimize this non-ideality. We suggest that water may have a role on the redox stability of C relative to CO32-, and consequently on the distribution of graphite/diamond vs. carbonate species and on the onset of melting in C-O-H-bearing mantle. We propose several applications allowing the composition of incipient melts to be calculated as a function of depth underneath Mid-Ocean-Ridges and underneath Hot-Spots. In the oceanic mantle, the top of the Lithosphere-Asthenosphere boundary is identified by seismic data as a discontinuity at an average depth of 65 km. This observation correlates with the onset of peridotite melting in presence of both H2O and CO2. Therefore, partial melting must occur at 65 km, implying production of H2O-rich carbonatitic melts as shown by our present model, and which are to the origin of the weakening. This thermodynamic study, supported by experimental investigation, constitutes an essential step in modeling the distribution and fate of volatiles, especially carbon, in the Earth's mantle