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1.7Gpa: Stability of the CO2 Hydrates And Available online at www.sciencedirect.com ScienceDirect Geochimica et Cosmochimica Acta 119 (2013) 322–339 www.elsevier.com/locate/gca Phase equilibria in the H2O–CO2 system between 250–330 K and 0–1.7 GPa: Stability of the CO2 hydrates and H2O-ice VI at CO2 saturation a, b a a Olivier Bollengier ⇑, Mathieu Choukroun , Olivier Grasset , Erwan Le Menn , 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–CO2 system 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 CO2 saturation 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 CO2 sI 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 CO2 high-pressure hydrate reported recently. Finally, the melting curve of the H2O-ice VI at CO2 saturation in the absence of CO2 hydrates was determined between 0.8 and 1.7 GPa. Using an available chemical potential model for H2O, a first assessment of the solubility of CO2 along the H2O-ice VI melting curve is given. Consistent with these new results and previous studies of the H2O–CO2 system, a P–T–X description of the binary sys- tem is proposed. The evolution with pressure of the Raman signatures of the two CO2 hydrates is detailed, and their stability is discussed in light of other clathrate hydrate-forming systems. Ó 2013 Elsevier Ltd. All rights reserved. 1. INTRODUCTION making it one of the most widespread compound on these bodies, second only to water. Even though its physical state Carbon dioxide is one of the major compounds of the and origin on these planetary surfaces are still unresolved cold, volatile-rich outer Solar System. To this day, spectral (e.g. Dalton, 2010; Dalton et al., 2010; Oancea et al., analyses support its presence at the surface of almost every 2012), its abundance in comets and nearby interstellar major satellite of the four giant planets (Dalton, 2010), clouds (Bockele´e-Morvan et al., 2004; Dartois and Schmitt, 2009 and references therein), and its detection in the plumes of Enceladus (Waite et al., 2009) and at the surface of Titan ⇑ Corresponding author. Tel.: +33 2 51 12 55 83. (Niemann et al., 2010) suggest that this molecule may be a E-mail address: [email protected] major internal constituent of icy moons (Mousis and (O. Bollengier). Alibert, 2006; Tobie et al., 2012). Understanding the 1 Present address: Universite´ de Lille, CNRS, Unite´ Mate´riaux et evolution and present state of these bodies therefore re- Transformations, UMR 8207, 59655 Villeneuve d’Ascq, France. 2 Present address: European Synchrotron Radiation Facility, 6 quires knowledge of phase equilibria in CO2-bearing rue Jules Horowitz, 38000 Grenoble, France. 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 O. Bollengier et al. / Geochimica et Cosmochimica Acta 119 (2013) 322–339 323 temperatures relevant to their interior and, more specifi- X-ray diffraction patterns of the phase did not match those cally, to their past or present internal oceans. The pressure of known clathrate hydrate structures (a detailed review of at the base of the hydrosphere of the largest icy moons of these structures can be found in Loveday and Nelmes, the Solar System is expected to reach at least 0.9 GPa for 2008). Titan (Fortes, 2012; Tobie et al., 2012) and 1.3 GPa for Equilibria in the H2O–CO2 system between the H2O- Ganymede (Hussmann et al., 2007; Bland et al., 2009). rich liquid and solid phases (ices or hydrates) are affected From the surface of the moons to the base of their hydro- by the quantity of dissolved compounds (here CO2) in the sphere, these pressures cover liquid-ice equilibria tempera- liquid phase. Consequently, estimating the solubility of tures of 250 to 300 K for Titan and up to 320 K for CO2 in H2O is essential to assess the stability of hydrates Ganymede in the H2O unary system. and H2O ices. Despite many experimental studies, the Of particular interest to this study is the stability of CO2- behavior of the H2O–CO2 system remains largely unknown bearing H2O solid compounds (i.e. CO2 hydrates). Like above a few hundred MPa at temperatures near the melting other gas–water systems, the H2O–CO2 binary is known point of the H2O ices (extensive reviews of the available to form specific hydrates called clathrate hydrates. Clath- PVTx experimental studies reported in this system are avail- rate hydrates are non-stoichiometric crystalline compounds able from Diamond and Akinfiev, 2003; Spycher et al., formed by a lattice of hydrogen-bonded H2O molecules 2003; Chapoy et al., 2004; Duan et al., 2006; Duan and encaging variable quantities of guest molecules. Because Zhang 2006; Hu et al., 2007; Mao et al., 2010; Yan etal., of their nature, clathrate hydrates exhibit distinct thermal 2011). Almost all available experiments reported below and physical properties and are able to trap large amounts 400 K were carried out at pressures of at most 0.1 GPa, of guest molecules (e.g. the sI clathrate hydrate structure whereas higher pressure experiments were conducted at has an hydration number n = 5.75 corresponding to the temperatures above 500 K, near or above the critical point number of H2O molecules per guest–hosting cavity in the of water (647 K). The only exceptions are the studies of structure, or a total guest content of 14.8 mol% if each cav- Takenouchi and Kennedy (1964) up to 0.16 GPa and down ity is filled with a single guest molecule) (Sloan and Koh, to 383 K and of To¨dheide and Franck (1963) up to 2008; Choukroun et al., 2013). Between a few tens of 0.35 GPa and down to 325 K. As a result, thermodynamic MPa and up to the GPa range, most clathrate hydrates modeling of CO2 solubility down to the melting point of are more stable than the H2O-ice polymorphs over wide the H2O ices or the dissociation of CO2 hydrates has been ranges of composition relative to the solubility of the guest developed only up to 0.2 GPa (e.g. Duan et al., 1992a, compound in water. Because of these properties, gas hy- 1992b; Diamond and Akinfiev, 2003; Sun and Dubessy, drates are believed to have impacted the evolution and 2010 up to 0.1 GPa; Yan and Chen, 2010 up to 0.15 GPa; structure of icy moons over their entire history by affecting Duan and Sun, 2003 up to 0.2 GPa), supporting solubility the trapping and exchange of volatiles from their high-pres- values of about 5 mol% near 0.2 GPa and 300 K (Duan sure interiors to their surface, or, for Titan, to its atmo- and Sun, 2003). The consensus that the H2O–CO2 binary sphere (Sohl et al., 2010; Choukroun and Sotin, 2012; departs strongly from ideality upon a decrease of tempera- Marion et al., 2012; Mun˜oz-Iglesias et al., 2012; ture or an increase of pressure (e.g. Duan et al., 1992b; Choukroun et al., 2013). Understanding the behavior of Joyce and Blencoe, 1994; Anovitz et al., 1998; Aranovich CO2 hydrates over these large pressure ranges is thus essen- and Newton, 1999; Blencoe et al., 1999) makes uncertain tial to understand the mechanisms of trapping and transfer the extrapolation of available data and models to these con- of the volatile inside the hydrospheres of icy moons and the ditions. Although models basedon equations of state offer implications for the thermal and chemical evolution of new qualitative insights above the GPa range (see the work these planetary bodies. of dos Ramos et al., 2007), no experimental data are yet Until recently, the only stable hydrate structure known available to assess their precision. In the GPa range, to form in the H2O–CO2 system was the cubic structure I Manakov et al. (2009) were the first to assess experimentally clathrate hydrate (hereafter CO2-sI hydrate). The stability the effect of CO2 on the stability of the H2O-ice VI and VII of the CO2-sI hydrate has been explored below the CO2 at room temperature up to 2.6 GPa. Their proposed tem- liquid-ice equilibrium near 0.5 GPa (Takenouchi and perature depression of the H2O-ice VI melting curve (i.e. Kennedy, 1965; Ohgaki and Hamanaka, 1995; Nakano freezing point depression), up to 30 K near 1.25 GPa and et al., 1998) and, more recently, up to the H2O liquid-ice 60 K near 2.2 GPa, would imply a dramatic increase of VI equilibrium (Manakov et al., 2009).
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