Article

En dépit de l’intérêt du CO2 pour les sciences planétaires, peu d’études ont été dédiées à ce jour au système H2O − CO2 aux pressions et températures représenta-tives de l’intérieur des corps planétaires, et plus particulièrement des lunes de glace du système solaire externe. Cette étude présente de nouveaux résultats expérimen-taux obtenus en cellules à enclumes de saphir et de diamant à des températures de 250 à 330 K et des pressions de 0 à 1.7 GPa, offrant de nouvelles contraintes sur la stabilité des hydrates de CO2 et sur le liquidus de la glace H2O VI à satura-tion en CO2. L’identification des phases et la caractérisation des équilibres au cours des expériences ont été assurées par spectroscopie Raman et suivi visuel. L’équi-libre entre le clathrate sI de CO2 et la phase liquide riche en H2O saturée en CO2

a été exploré sur toute la gamme de stabilité de l’hydrate, jusqu’à 0.7 − 0.8 GPa, complétant adéquatement les résultats disponibles à plus faible pression. Au-delà de cette pression, les présentes expériences ont confirmé l’existence jusqu’à 1 GPa d’un nouvel hydrate de CO2 récemment décrit dans la littérature. Enfin, ces expériences contraignent pour la première fois la courbe de fusion de la glace H2O VI à satura-tion en CO2, en l’absence des hydrates, entre 0.8 et 1.7 GPa. À partir d’un modèle décrivant le potentiel chimique de H2O à ces températures et pressions, ces derniers résultats permettent d’établir une première estimation de la solubilité du CO2 dans la phase fluide riche en H2O le long de la courbe de fusion observée. En s’appuyant sur les nouvelles données présentées et sur celles disponibles de précédents travaux, une description en pression, température et composition du système binaire a été construite. Cette étude présente également les signatures Raman des deux hydrates de CO2 et leur évolution en fonction de la pression et replace le comportement des deux hydrates dans le contexte plus général des systèmes binaires formant des phases clathrates.

Phase equilibria in the H

O–CO

system between 250–330 K

and 0–1.7 GPa: Stability of the CO

hydrates and H

O-ice VI

at CO

saturation

Olivier Bollengier^a,⇑, Mathieu Choukroun^b, Olivier Grasset^a, Erwan Le Menn^a, Guillaume Bellino^a,1, Yann Morizet^a, Lucile Bezacier^a,2, Adriana Oancea^a,

Ce´cile Taffin^a, Gabriel Tobie^a

a

Universite´ de Nantes, CNRS, Laboratoire de Plane´tologie et Ge´odynamique de Nantes, UMR 6112, 44322 Nantes Cedex 3, France

b

Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, United States Received 7 January 2013; accepted in revised form 2 June 2013; Available online 11 June 2013

Abstract

Although carbon dioxide is of interest in planetary science, few studies have been devoted so far to the H2O–CO2system at pressures and temperatures relevant to planetary interiors, especially of the icy moons of the giant planets. In this study, new sapphire and diamond anvil cell experiments were conducted in this binary system to constrain the stability of the CO2

hydrates and H2O-ice VI at CO2saturation in the 250–330 K and 0–1.7 GPa temperature and pressure ranges. Phases and equilibria were characterized by in situ Raman spectroscopy and optical monitoring. The equilibrium between the CO2sI clathrate hydrate and the H2O-rich liquid phase was constrained over the entire pressure range of stability of the hydrate, up to 0.7–0.8 GPa, with results in agreement with previous studies at lower pressures. Above this pressure and below 1 GPa, our experiments confirmed the existence of the new CO2high-pressure hydrate reported recently. Finally, the melting curve of the H2O-ice VI at CO2saturation in the absence of CO2hydrates was determined between 0.8 and 1.7 GPa. Using an available chemical potential model for H2O, a first assessment of the solubility of CO2along the H2O-ice VI melting curve is given. Consistent with these new results and previous studies of the H2O–CO2system, a P–T–X description of the binary sys-tem is proposed. The evolution with pressure of the Raman signatures of the two CO2hydrates is detailed, and their stability is discussed in light of other clathrate hydrate-forming systems.

Ó2013 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

Carbon dioxide is one of the major compounds of the cold, volatile-rich outer Solar System. To this day, spectral analyses support its presence at the surface of almost every major satellite of the four giant planets (Dalton, 2010),

making it one of the most widespread compound on these bodies, second only to water. Even though its physical state and origin on these planetary surfaces are still unresolved (e.g. Dalton, 2010; Dalton et al., 2010; Oancea et al., 2012), its abundance in comets and nearby interstellar clouds (Bockele´e-Morvan et al., 2004; Dartois and Schmitt, 2009and references therein), and its detection in the plumes of Enceladus (Waite et al., 2009) and at the surface of Titan

(Niemann et al., 2010) suggest that this molecule may be a

major internal constituent of icy moons (Mousis and

Alibert, 2006; Tobie et al., 2012). Understanding the

evolution and present state of these bodies therefore re-quires knowledge of phase equilibria in CO2-bearing H2O-rich systems at the high pressures and low

0016-7037/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.gca.2013.06.006 ⇑Corresponding author. Tel.: +33 2 51 12 55 83.

E-mail address: olivier.bollengier@univ-nantes.fr

(O. Bollengier).

1Present address: Universite´ de Lille, CNRS, Unite´ Mate´riaux et Transformations, UMR 8207, 59655 Villeneuve d’Ascq, France.

2Present address: European Synchrotron Radiation Facility, 6 rue Jules Horowitz, 38000 Grenoble, France.

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temperatures relevant to their interior and, more specifi-cally, to their past or present internal oceans. The pressure at the base of the hydrosphere of the largest icy moons of the Solar System is expected to reach at least 0.9 GPa for Titan (Fortes, 2012; Tobie et al., 2012) and 1.3 GPa for Ganymede (Hussmann et al., 2007; Bland et al., 2009). From the surface of the moons to the base of their hydro-sphere, these pressures cover liquid-ice equilibria tempera-tures of 250 to 300 K for Titan and up to 320 K for Ganymede in the H2O unary system.

Of particular interest to this study is the stability of CO2 -bearing H2O solid compounds (i.e. CO2 hydrates). Like other gas–water systems, the H2O–CO2 binary is known to form specific hydrates called clathrate hydrates. Clath-rate hydClath-rates are non-stoichiometric crystalline compounds formed by a lattice of hydrogen-bonded H2O molecules encaging variable quantities of guest molecules. Because of their nature, clathrate hydrates exhibit distinct thermal and physical properties and are able to trap large amounts of guest molecules (e.g. the sI clathrate hydrate structure has an hydration number n = 5.75 corresponding to the number of H2O molecules per guest–hosting cavity in the structure, or a total guest content of 14.8 mol% if each cav-ity is filled with a single guest molecule) (Sloan and Koh,

2008; Choukroun et al., 2013). Between a few tens of

MPa and up to the GPa range, most clathrate hydrates are more stable than the H2O-ice polymorphs over wide ranges of composition relative to the solubility of the guest compound in water. Because of these properties, gas hy-drates are believed to have impacted the evolution and structure of icy moons over their entire history by affecting the trapping and exchange of volatiles from their high-pres-sure interiors to their surface, or, for Titan, to its atmo-sphere (Sohl et al., 2010; Choukroun and Sotin, 2012; Marion et al., 2012; Mun˜oz-Iglesias et al., 2012;

Choukroun et al., 2013). Understanding the behavior of

CO2hydrates over these large pressure ranges is thus essen-tial to understand the mechanisms of trapping and transfer of the volatile inside the hydrospheres of icy moons and the implications for the thermal and chemical evolution of these planetary bodies.

Until recently, the only stable hydrate structure known to form in the H2O–CO2system was the cubic structure I clathrate hydrate (hereafter CO2-sI hydrate). The stability of the CO2-sI hydrate has been explored below the CO2

liquid-ice equilibrium near 0.5 GPa (Takenouchi and Kennedy, 1965; Ohgaki and Hamanaka, 1995; Nakano

et al., 1998) and, more recently, up to the H2O liquid-ice

VI equilibrium (Manakov et al., 2009). These last results suggest that the CO2-sI hydrate may be stable up to 0.7 GPa. The most recent high-pressure study in the H2O–CO2 system reported the existence of a transition from the CO2-sI hydrate to a new “high-pressure” hydrate (hereafter CO2-HP hydrate) below 280 K and up to 1 GPa

(Hirai et al., 2010). From these experiments, the pressure

range of the stability field of the CO2-HP hydrate has been well constrained, but its dissociation temperature remains unknown, as no CO2 hydrate was found at any pressure near 300 K (Manakov et al., 2009). Furthermore, the exact nature of this CO2-HP hydrate is still to be determined, as

X-ray diffraction patterns of the phase did not match those of known clathrate hydrate structures (a detailed review of these structures can be found in Loveday and Nelmes,

2008).

Equilibria in the H2O–CO2 system between the H2 O-rich liquid and solid phases (ices or hydrates) are affected by the quantity of dissolved compounds (here CO2) in the liquid phase. Consequently, estimating the solubility of CO2in H2O is essential to assess the stability of hydrates and H2O ices. Despite many experimental studies, the behavior of the H2O–CO2system remains largely unknown above a few hundred MPa at temperatures near the melting point of the H2O ices (extensive reviews of the available PVTx experimental studies reported in this system are avail-able from Diamond and Akinfiev, 2003; Spycher et al., 2003; Chapoy et al., 2004; Duan et al., 2006; Duan and Zhang 2006; Hu et al., 2007; Mao et al., 2010; Yan etal., 2011). Almost all available experiments reported below 400 K were carried out at pressures of at most 0.1 GPa, whereas higher pressure experiments were conducted at temperatures above 500 K, near or above the critical point of water (647 K). The only exceptions are the studies of

Takenouchi and Kennedy (1964)up to 0.16 GPa and down

to 383 K and of To¨dheide and Franck (1963) up to 0.35 GPa and down to 325 K. As a result, thermodynamic modeling of CO2solubility down to the melting point of the H2O ices or the dissociation of CO2hydrates has been developed only up to 0.2 GPa (e.g. Duan et al., 1992a, 1992b; Diamond and Akinfiev, 2003; Sun and Dubessy, 2010up to 0.1 GPa;Yan and Chen, 2010up to 0.15 GPa;

Duan and Sun, 2003up to 0.2 GPa), supporting solubility

values of about 5 mol% near 0.2 GPa and 300 K (Duan

and Sun, 2003). The consensus that the H2O–CO2binary

departs strongly from ideality upon a decrease of tempera-ture or an increase of pressure (e.g. Duan et al., 1992b; Joyce and Blencoe, 1994; Anovitz et al., 1998; Aranovich

and Newton, 1999; Blencoe et al., 1999) makes uncertain

the extrapolation of available data and models to these con-ditions. Although models basedon equations of state offer new qualitative insights above the GPa range (see the work

of dos Ramos et al., 2007), no experimental data are yet

available to assess their precision. In the GPa range,

Manakov et al. (2009)were the first to assess experimentally

the effect of CO2on the stability of the H2O-ice VI and VII at room temperature up to 2.6 GPa. Their proposed tem-perature depression of the H2O-ice VI melting curve (i.e. freezing point depression), up to 30 K near 1.25 GPa and 60 K near 2.2 GPa, would imply a dramatic increase of the solubility of CO2and/or a notable decrease of the activ-ity coefficient of H2O in the solution. In contrast, Qin

et al.(2010)observed that in a sample with a bulk CO2

con-tent of about 5 ± 1 mol%, CO2ice crystals were stable up to 1.26 GPa in the H2O-rich liquid phase at 293 K, meaning that the solubility of CO2above 1 GPa may in fact be lower than 5%.

The aim of this work is to bring further constraints on the phase equilibria in the H2O–CO2binary system up to 1.7 GPa, in the temperature range relevant to the stability of the CO2hydrates and H2O ices. In this paper, we report on three consistent, new sets of experiments covering (1) the

of stability of the CO2-HP hydrate up to its pressure of dis-sociation into CO2and H2O ices, and (3) the stability of the H2O-ice VI at CO2saturation. Using these results and pre-viously reported data, a P–T–X description of phase equi-libria in the binary system is proposed. We also report the first set of Raman spectra of the CO2-HP hydrate and the effect of pressure on the Raman signatures of the two CO2hydrates. Finally, we discuss the implications of our results on CO2 hydrates, particularly the high-pressure phase, for the structural evolution of clathrate hydrates un-der pressure.

2. EXPERIMENTAL 2.1. Apparatus

The full range of pressures up to 1.7 GPa was explored through eight experiments performed with a MDAC-THP Betsa diamond anvil cell (DAC) coupled to a CBN 8-30 Heto chiller. The cell was fitted with 2.5 mm thick Ia-type diamonds with 500 lm culets. For each experiment, a 200-lm wide sample chamber was formed by drilling a hole in a 80-lm thick pre-indented 301L stainless steel gasket. The temperature was measured close to the sample with a K-type thermocouple, with an estimated accuracy of ±1 K. The pressure was measured in situ using the fluores-cence of ruby balls (Grasset, 2001) with a pressure accuracy estimated to ±50 MPa. The applicability of the pressure law to the pressures and temperatures explored in this study was checked by monitoring the melting temperature of the H2O-ice VI of pure H2O samples in two separate DAC experiments.

A single experiment limited to a pressure of 1 GPa was conducted in a sapphire anvil cell (SAC) set in a liquid nitrogen-cooled cryostat. This experimental apparatus was described and used in several previous studies (Grasset,

2001; Grasset et al., 2005; Choukroun et al., 2007). This

experiment was carried out with 3.5-mm thick sapphire an-vils having 1.5 mm culets. A 0.6-mm wide sample chamber was formed by drilling a hole in a 140-lm thick pre-in-dented copper gasket. The temperature was measured with a single Pt100 thermometer, with an estimated accuracy of ±0.5 K. The pressure was measured using the same method than for the DAC experiments.

The DAC apparatus was used to investigate the transi-tion between the CO2-sI and the CO2-HP hydrates, and to constrain the H2O-ice VI melting curve at CO2 satura-tion. The SAC experiment was used to monitor the dissoci-ation curve of the CO2-sI hydrate and the transition between the two hydrates. To assess the consistency be-tween the DAC and SAC datasets, a few points on the dis-sociation curve of the CO2-sI hydrate were acquired during the DAC experiments, and the H2O-ice VI melting curve at CO2saturation was observed below 1 GPa during the SAC experiment.

These two optical cell systems were coupled to a Horiba Jobin–Yvon LabRAM 300 Raman spectrometer and microscope for confocal analyses. Samples were excited

lines/mm grating, spectral spacing (and spectral width) on our spectra ranges from 1.1 cm 1(4.5 cm 1) at low wave-numbers (near 300 cm ¹) to 0.75 cm ¹(2.5 cm ¹) at high wavenumbers (near 3000 cm 1).

2.2. Material

For the SAC and most DAC experiments, the anvil cells were loaded directly with CO2-sI clathrate hydrate. The hy-drate was synthesized as follows. (1) A 50 mL autoclave was filled with 10 mL of liquid H2O (LiChrosolvÒ

chro-matographic grade water, Merck Chemicals) and pressur-ized at room temperature with 50 bar of CO2 (purity P99.995%, N45 grade, Air Liquide). During the synthesis, mixing in the autoclave was ensured using a magnetic stir-rer. (2) After loading, the sample inside the autoclave was cooled to 263 K over a few hours, and (3) then heated to approximately 280 K over 1 h. This cycle ensures that only the CO2hydrate (not H2O ice) was present in the autoclave. (4) After this cycle, the autoclave was cooled to 255 K in a cold chamber until opening. Thesynthesis was carried out over a one to two-day period at the end at which the pres-sure in the autoclave was usually 18 bar. (5) Finally, the CO2hydrate was sampled and crushed into a fine powder and loaded in the anvil cell. Once closed, the cell was cooled from 255 K to about 100 K above liquid nitrogen to pre-serve the sample during the few minutes necessary to begin the experiment. To estimate the composition of these load-ings, six test samples were synthesized following the same procedure. Samples of a few grams were slowly heated from 255 to 293 K and dissociated on a Mettler Toledo AB 135-S balance (precision 10 5g) to monitor the loss of CO2(e.g.

Circone et al., 2003). From these experiments, we estimated

the compositions of our samples at the time of loading to be of 8 ± 1.5 mol%.

For three of the DAC experiments, samples were loaded as mixtures of H2O and CO2ices. H2O ice was formed from deposition of liquid H2O on a cold plate and crushed to a fine powder. CO2ice was obtained by vapor deposition as CO2gas was directed on the H2O ice powder kept around 100 K above liquid nitrogen. The bulk composition of these samples was asserted during the experiments by comparison with the first set of samples of phase abundances at similar P/T conditions. Only samples with compositions similar to the first group were kept. No difference was observed be-tween the results obtained from the two groups of samples. 2.3. Phase determination and sample monitoring

Phase determination during the experiments was achieved by Raman spectroscopy. Raman analyses were fo-cused on the three spectral regions bearing characteristic H2O and CO2features, at low wavenumbers (0–500 cm ¹, H2O and CO2external modes or “O–O” modes for H2O), intermediate (1200–1500 cm 1, CO2 internal modes with the m1and 2m2Fermi resonance bands) and high wave-numbers (2800–3800 cm 1, H2O internal or “O–H” modes). Over our explored pressure and temperature ranges,

Raman spectra are available in the literature for the CO2-ice I (Hanson and Jones, 1981), the only CO2-ice phase we observed (Iota and Yoo, 2001), for H2O-ice polymorphs (Mincˇeva-Sˇukarova et al., 1984, 1987; Grasset

et al., 2005), and for the CO2-sI hydrate (Nakano et al.,

1998; Ikeda et al., 1998).

No Raman spectrum of the CO2-HP hydrate had been reported yet. InFig. 1, the Raman spectrum of the CO2 -HP hydrate, recorded in this study, is compared to those of the CO2-sI hydrate and CO2-ice I at similar pressure and temperature conditions. As illustrated in Fig. 1, the two CO2hydrates exhibit Raman signatures significantly different from that of CO2-ice I, allowing the distinction of the hydrates with mixtures of H2O and CO2ices at any explored pressure and temperature.Fig. 1also illustrates that even though the two CO2hydrate signatures are simi-lar, they present distinct band positions and shapes at the same pressure and temperature. Observation of the spectra and monitoring of their evolution with pressure and at phase transitions allowed the identification of the two hy-drates in the course of the experiments. An analysis of the spectra of the two hydrate phases and implications about their structure are presented in Section4.2.

During the experiments, the evolution of the samples was also monitored by direct optical observation. Pictures ac-quired during SAC and DAC experiments in the three pres-sure–temperature ranges detailed in this study are presented

inFig. 2. At low pressures, in the stability field of the CO2-sI

hydrate (Fig. 2a), the pressure and temperature of dissocia-tion were recorded at the time the last hydrate crystal melted. At intermediate pressures, where the phase transition be-tween the two CO2hydrates occurs (Fig. 2b), the stability