o nacl liquidi to 25 mass% - virginia tech geosciences ... 16 gpa for a 5 mass% nacl...

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Experimental determination of H2O–NaCl liquidi to 25 mass%NaCl and 1.4 GPa: Application to the Jovian satellite Europa

P. Valenti a, R.J. Bodnar b,⇑, C. Schmidt a

a Deutsches GeoForschungsZentrum (GFZ), Section 3.3, Telegrafenberg, 14473 Potsdam, Germanyb Fluids Research Laboratory, Department of Geosciences, Virginia Tech, Blacksburg, VA 24061-0420, USA

Received 5 July 2011; accepted in revised form 5 June 2012

Abstract

Liquidi in the system H2O + NaCl were determined experimentally for bulk compositions of 5, 10, 15, and 25 mass% NaClat pressures to �1.4 GPa using a high-pressure optical cell or a hydrothermal diamond-anvil cell combined with Raman spec-troscopy. The dP/dT slope of the ice I liquidus becomes steeper with increasing salinity, i.e., it decreases from ��9 MPa/�Cfor pure H2O, to ��11 MPa/�C for 5 and 10 mass% NaCl, and to ��15 MPa/�C for 15 mass% NaCl. The liquidi for othersolid phases at higher pressures always display a positive dP/dT slope. The liquidi of 5 mass% NaCl have dP/dT slopes similarto those of the corresponding ice melting curves of pure H2O, but the ice stability fields shift towards higher pressures, e.g., theice I + ice III + liquid triple point was observed at �300 MPa and �27.7 �C (that of pure water is at 209.9 MPa and�21.985 �C). At 15 mass% NaCl, a hydrohalite liquidus replaces the ice III and ice V liquidi, and also decreases the rangein pressure of ice I stability. Approximate P–T locations of triple points for 15 mass% NaCl are �1.07 GPa and 1.7 �C forhydrohalite + ice VI + liquid, and �25 MPa and �11.7 �C for ice I + hydrohalite + liquid (for comparison, the ice I + hydro-halite + L + V quadruple invariant point is at �21.2 �C, �0.0001 MPa, 23.15 mass% NaCl). The stability field of ice VI shiftscontinuously towards higher pressure with addition of up to 25 mass% NaCl, with probably only small changes in the dP/dT

slope of the liquidus. At 25 mass% NaCl, the hydrohalite liquidus extends from 0.1 MPa to higher pressure and shows achange in the dP/dT slope from negative to positive at ��7.5 �C and �140 ± 40 MPa, which suggests a change in thedynamic structure of the aqueous liquid.

Raman spectra of an unknown phase of columnar crystal habit are reported, which formed upon freezing of 15 or25 mass% NaCl solutions at high pressure. The Raman spectrum of this phase in the O–H stretching region is characterizedby three bands at �3410, �3460, and �3510 cm�1, and does not match any of the ice phases or hydrohalite.

Based on the liquidi in the H2O + NaCl system determined in this study and a calculated temperature profile from the lit-erature, the icy shell overlying an aqueous liquid mantle on the Jovian satellite Europa is estimated to be about 9 km thick,assuming NaCl as the predominant solute. Furthermore, our liquidus data suggest a maximum NaCl concentration of about15–20 mass% NaCl and a minimum subsurface ocean temperature of �260 K.� 2012 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

Water–electrolyte fluids are ubiquitous in the Earth’scrust and in subduction zones, where they play a major rolein transferring mass and energy and in the formation of

many types of magmatic-hydrothermal ore deposits (e.g.,Roedder, 1984; Bodnar, 1995, 2005; Scambelluri andPhilippot, 2001; Touret, 2001; Manning, 2004; Kesler,2005). The binary H2O + NaCl is a widely studied aqueouselectrolyte system and is one of the most representativemodels for Cl-rich fluids in many geologic environments(e.g., Roedder, 1984; Bodnar, 2003; Driesner and Heinrich,2007). Moreover, analyses of brine inclusions in diamondsshow that NaCl and KCl are major components of hydrous

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⇑ Corresponding author.E-mail address: [email protected] (R.J. Bodnar).

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Geochimica et Cosmochimica Acta 92 (2012) 117–128


fluids in the Earth’s upper mantle (Izraeli et al., 2001;Klein-BenDavid et al., 2007). There is also growing evi-dence for the extraterrestrial presence of brines in the inte-riors of icy planetary bodies (Zolensky, 2005). Directevidence for aqueous sodium chloride-rich solutions wasprovided by the discovery of fluid inclusions in halite fromthe H-chondrite regolith breccias Zag and Monahans(1998) (Zolensky et al., 1999; Rubin et al., 2002). Magneto-metric data collected by the Galileo spacecraft on Jupiter’ssatellites Ganymede, Callisto and Europa indicate the exis-tence of subsurface layers of significant electrical conductiv-ity, most likely due to the presence of salty oceans beneaththe icy crusts (Khurana et al., 1998; Kivelson et al., 1999,2000, 2002). Reflectance spectra from Galileo’s near infra-red mapping spectrometer suggest the presence of hydratedsalt minerals on the surface of Europa and Ganymede,which are thought to have formed by evaporation of brinesthat ascended from a fluid layer in the interior (McCordet al., 1998, 2001; Zolotov and Kargel, 2009). The CosmicDust Analyser of the Cassini spacecraft found sodium saltsin plumes of water vapor and ice particles emitted from Sat-urn’s moon Enceladus (Postberg et al., 2009). The evidencepresented above suggests that chloride-rich brines are com-mon not only on earth but also elsewhere in the solarsystem.

Models of the internal structure of planetary bodies con-taining aqueous electrolyte fluids (e.g., Schilling et al., 2004;Hand and Chyba, 2007) require knowledge of phase equi-libria in relevant water–electrolyte systems at high pressureand relatively low temperatures. However, extensive exper-imental data on phase equilibria at such conditions are onlyavailable for a few systems, e.g., H2O + MgSO4 (Hogen-boom et al., 1995). Even for the generally widely-studiedbinary H2O + NaCl, our knowledge of the P–T locationsof liquidi is still fragmentary at high pressures and at rela-tively low salinities, i.e., at conditions relevant to the interi-ors of planetary bodies with thick ice shells. The meltinglines of the end members are known over large ranges inpressure, even if only static experiments are taken into ac-count. The melting curve of NaCl has been determined to6.5 GPa by Clark (1959), Pistorius (1966), and Akellaet al. (1969). It shows a positive dP/dT slope of about4.7 MPa/K at low pressure. Boehler et al. (1997) studiedthe melting of NaCl to 91 GPa and observed two separatecurves owing to the conversion of solid NaCl from the B1to the B2 structure at 29 GPa. Both melting lines displayeda considerable steepening of the dP/dT slope with pressure.Due to polymorphism, phase relationships are more com-plicated for H2O. The melting curve of Ice Ih has a negativedP/dT slope, whereas those of ice III, ice V, ice VI, and iceVII are positive (Wagner et al., 1994 and references there-in). The melting curve of ice VII was determined to 900 Kand 22 GPa (e.g. Bridgman, 1937; Pistorius et al., 1963;Mishima and Endo, 1978; Fei et al., 1993; Lin et al., 2004).

For binary mixtures, the vapor-saturated liquidus (e.g.,Driesner and Heinrich, 2007) is well known, and the haliteliquidi have been studied for bulk compositions between 30and 70 mass% NaCl to a few 100 MPa (e.g., Gunter et al.,1983; Chou, 1987; Koster van Groos, 1991; Bodnar, 1994;Becker et al., 2008). Assuming linear behavior, the dT/dP

slope of the liquidus increases with salinity between 26and 100 mass% NaCl and changes from negative to positivevalues above about 50 mass% NaCl (Bodnar, 1994). Thisassumption of linear behavior may be valid only for thehigh-temperature isopleths on the liquidus, as stated byDriesner and Heinrich (2007) based on their analysis ofthe data by Adams (1931) and Bodnar (1994). A curvatureof the 40 mass% NaCl liquidus was also observed bySchmidt et al. (1998). The liquidus of this isopleth steepenssubstantially with pressure, similar to the behavior of theend member melting curves except that of ice I. For compo-sitions <30 mass% NaCl, very little is known about the li-quidi above vapor saturation. Frank et al. (2008) reportthree ice VII melting temperatures at pressures between 4and 16 GPa for a 5 mass% NaCl solution. Manning andDaniel (2008) determined the ice VI and VII melting curvesfor the 5.5 mass% NaCl isopleth at pressures between 1.1and 4.5 GPa. As observed in both studies, addition ofabout 5 mass% NaCl decreases the liquidus temperaturesat high pressures, which in turn places constraints on boththe thickness and composition of hypothetical subsurfaceoceans in the interiors of icy satellites.

In summary, the liquidi in the system H2O + NaCl arelargely unexplored at lower salinities, particularly at pres-sures <1 GPa. However, these conditions are of interestnot only for models of the interiors of icy planetary bodiesbut also in general for the topology of aqueous systems be-cause of the transition from negative to positive dP/dT

slopes of the liquidi. The aim of this study is to better con-strain the P–T location of the liquidi for H2O + NaCl bulkcompositions of 5, 15, and 25 mass% NaCl in the tempera-ture range between �30 and +26 �C and in the pressurerange between 0.1 and 1400 MPa using a hydrothermal dia-mond-anvil cell (HDAC) in combination with opticalmicroscopy, cryometry, and Raman spectroscopy. Further-more, the triple point ice I + ice III + liquid was con-strained for a bulk composition of 5 mass% NaCl, andthe ice I + hydrohalite + liquid triple point for 15 mass%NaCl was determined. Additional experiments were per-formed using a high-pressure optical cell (HPOC) to inves-tigate the ice I liquidus for 10 and 15 mass% NaCl attemperatures between �12 and �7 �C and pressures to25 MPa.

2. EXPERIMENTAL PROCEDURE

2.1. Hydrothermal diamond-anvil cell and Raman

spectroscopy

Liquidi to high pressures were determined using ahydrothermal diamond-anvil cell (HDAC) (Bassett et al.,1993), with modifications to facilitate Raman spectroscopyand accurate temperature measurement. The temperatureswere measured using K-type (Ni–NiCr) thermocouples withtheir tips attached to the diamond anvils. The thermocoupleconnections were placed about 8 cm away from the body ofthe cell to minimize changes in the additive electromotiveforce due to small differences in wire and contact composi-tion during heating. The thermocouples were calibrated bymeasuring the triple point of H2O (0.01 �C, 0.6 kPa) and

118 P. Valenti et al. / Geochimica et Cosmochimica Acta 92 (2012) 117–128


the a–b quartz transition temperature at ambient pressure(574 �C) in the HDAC. Calibrations were performed withthermocouples attached to the diamond anvils in a config-uration identical to that used during the experiments. Tem-perature corrections based on the calibrations ranged from0.1 �C at �42 �C to 0.8 �C at 25.8 �C. The sample chamberof the HDAC consisted of a hole 400 lm in diameter drilledin a doubly polished �125 lm thick Re gasket that wassandwiched between the two diamond anvils. First, aquartz chip (�30 lm thick) was loaded into the samplechamber for later pressure determination using Ramanspectroscopy. Then, sodium chloride crystals (ultrapure,Alfae) were loaded, and subsequently a drop of doubly dis-tilled water, which dissolved the NaCl crystals. Before thesample chamber was sealed, a small air bubble (�5–10 vol.%) was trapped (Fig. 1). After sealing, the actualsalinity of the solution was determined by cryometry. Thecell was connected to a cryostat filled with liquid nitrogen(N2) and cooled to about �70 �C to completely solidify(freeze) the contents. Then, the flow of nitrogen in the cellwas interrupted and the spontaneous heating processstarted. During heating, phase transitions were observedwith an optical microscope equipped with a 20� objective.The salinity of the solution was determined from measure-ments of the ice I melting temperature (for 5 and 15 mass%NaCl) or the hydrohalite (HH) melting temperature (for25 mass% NaCl), all in the presence of vapor (Bodnar,1993).

After the salinity was confirmed, the sample chamberwas then compressed until the vapor bubble disappeared.After compression, the cell was cooled with liquid nitrogento 6�70 �C. After cooling, the flow of cold nitrogen intothe cell was interrupted to allow the temperature to slowly

increase. During warming, phase transitions were observedwith an optical microscope until the last solid phase (otherthan the quartz pressure sensor) in the sample chamber haddissolved. The cell was then cooled again without furthercompression and placed on the sample stage of the Ramanmicroprobe. To determine the P–T path during subsequentwarming, the pressure was determined at numerous temper-atures from the frequency shift of the 464 cm�1 Raman lineof quartz using Eqs. (2) and (3) in Schmidt and Ziemann(2000). The pressure determination has an estimated uncer-tainty of ±25–50 MPa based on the attainable accuracy inthe determination of the line frequency of �±0.2–0.45 cm�1. Additional Raman spectra were collected toidentify solid phases, which nucleated in the sample cham-ber during the cooling and warming cycles. The samplechamber was then compressed further and the experimentswere repeated stepwise, at various bulk densities, until apressure of about 1000 MPa (in the experiments with5 mass% NaCl) or �1400 MPa (in the experiments with15 and 25 mass% NaCl solutions) was attained. In somecases, runs were repeated before further compression ifmetastability was indicated from non-reproducible meltingbehavior and temperatures.

Unpolarized Raman spectra of the solids were acquiredusing a HORIBA Jobin Yvon LabRAM HR800 UV–visRaman microprobe (gratings 1800 lines/mm, focal length800 mm) equipped with a CCD-detector (1024 � 256 pixel,cooled by a Peltier element). The spectra were recorded inbackscattering geometry with a Nikon SLWD Planachro-mat 20� objective (numerical aperture 0.35, working dis-tance 20.5 mm) and a confocal pinhole aperture of100 lm. The 488.0 nm line of an Ar+ laser was used forexcitation. The laser power was 230 mW at the source.The collection time was 10 accumulations of 15 s each forquartz and two or five accumulations of 10 s each forhydrohalite and the ice polymorphs. The interference filterwas removed to monitor the position of two plasma linesfrom the argon ion laser at 351.60 and 529.51 cm�1. Theplasma lines were recorded at the same conditions andsimultaneously as those of quartz spectra to achieve a highaccuracy in the determined wavenumber of the Raman line.Furthermore, reference Raman spectra of quartz were col-lected at about 23 �C and vapor pressure at the start of eachexperimental series. All Raman bands and plasma lineswere fitted with the software PeakFit v4.11 fromSYSTATe using the probability density function of thePearson type IV.

2.2. High-pressure optical cell

Additional experiments using a high-pressure optical cell(HPOC) (Lin et al., 2007) were performed for two solutions(10 and 15 mass% NaCl) to determine the ice I liquidus atpressures below 25 MPa with a better resolution and accu-racy in pressure compared to the HDAC experiments. Thecell was modified by insertion of a second silica glass win-dow beneath the sample chamber to allow optical observa-tion in transmitted light. The HPOC was coupled with acooling system, which consisted of a temperature-control-ling circulating bath filled with a dry ice (solid CO2) and

Fig. 1. Photomicrograph of the HDAC sample chamber at thestart of an experiment (room temperature) after loading. A smallair (vapor) bubble was trapped intentionally to ensure that theconcentration of the solution can be determined by cryometry atvapor-saturated conditions. A chip of quartz was loaded as aRaman spectroscopic pressure sensor.

P. Valenti et al. / Geochimica et Cosmochimica Acta 92 (2012) 117–128 119


acetone mixture. Both aqueous solutions were prepared bymixing NaCl (99.95% purity) and Millipore water in a flaskusing a thermal magnetic stirrer for 2–4 h. After mixing, theflask containing the solution was connected to a vacuumsystem for less than a minute to remove the air from theflask without changing the salinity. The sample chamberof the cell was loaded by flushing the saline solution intothe capillary tubes, which connect the flask with the cell.A small air bubble (�10 vol.%) was left upon loading, sim-ilar to the experiments performed using the HDAC. Then,the cell was cooled and phase transitions were observedwith a microscope. Temperature and pressure were mea-sured directly using a K-type thermocouple and a pressuretransducer. The accuracy of temperature measurements was<±0.05 �C, while the accuracy of the pressure transducerwas ±0.10% of the pressure output (Lin et al., 2007). Thethermocouple was calibrated, by measuring the meltingtemperatures of mercury (�38.8 �C) and of ice I (0.01 �C)at atmospheric pressure, before being placed inside the cell.The maximum pressures of the experiments were 20 MPafor 10 mass% NaCl and 25 MPa for 15 mass% NaCl solu-tions. For each loading of the cell, the cell was cooled to��70 �C to solidify the contents. Then the cell was allowedto warm slowly while monitoring the sample chamber con-tinuously using a binocular microscope and recording thetemperature and pressure in the cell when a phase disap-peared. After all phases had melted, the cell was cooledagain and the measurement repeated at this same pressure.After the second heating cycle, the pressure in the cell wasincreased in �5 MPa increments, and the process was re-peated. The pressure was increased using a manual pressuregenerator. The experiment terminated when at least one ofthe two silica glass windows of the cell fractured upon in-crease in pressure. Before a new saline solution was loadedinto the HPOC, the capillary tubes and all parts of theapparatus that were in contact with the salt solution werecleaned by flushing with Millipore water. This was doneto completely remove the previously used NaCl solutionand to minimize corrosion.

3. RESULTS

3.1. HDAC experiments on a 5 mass% NaCl solution

Six different ice polymorphs were observed during theexperiments with this composition. Five of these (ice I, iceII, ice III, ice V and ice VI) are stable phases, while iceIV is metastable (Chou et al., 1998). The identification ofthese phases was based on optical observation of the crystalmorphology (Haselton et al., 1995), their density relative toliquid (ice I accumulated at the culet face of the top anvilbecause of its lower density) and on Raman spectra col-lected in the frequency range between 2800 and3600 cm�1 (Chou et al., 1998). Fig. 2 shows Raman spectraof ice polymorphs collected during experiments with pureH2O by Chou et al. (1998) compared to Raman spectra col-lected in our study during the experiments with 5 mass%NaCl. It should be noted that we were not able to distin-guish ice IV from the new ice phase reported by Chouet al. (1998), and have therefore assigned all such spectra

to ice IV. This uncertainty, however, is irrelevant becausewe only report melting curves of stable phases.

All experimentally obtained P–T data for the liquidi forH2O + 5 mass% NaCl are listed in Table 1. Fig. 3 showsthese data in a P–T diagram, along with additional infor-mation on the stability of ice phases. The topology of thephase diagram for the H2O + 5 mass% NaCl compositionwas found to be very similar to that of the H2O unary sys-tem (Fig. 3). Moreover, the ice liquidi display dP/dT slopessimilar to the ice melting curves of H2O (Wagner et al.,1994). However, addition of 5 mass% NaCl shifts the stabil-ity fields of the ice polymorphs to higher pressure (Fig. 3).We note that the ice phases coexist with liquid above thesolidus temperature. For example, first melting occurredat �31.6 �C at 1000 MPa, which is �57 �C below the iceVI liquidus temperature at this pressure. At 260 MPa, thefirst liquid appeared at �30.5 �C, which is �5 �C belowthe ice I liquidus temperature at this pressure.

The ice I liquidus for 5 mass% NaCl determined in thisstudy shows a negative slope (��11 MPa/�C) and extendsfrom 0.1 MPa and �2.7 �C to the ice I + ice III + liquid tri-ple point at �300 MPa and �27.7 �C. This triple point wasidentified based on optical observation of melting of twodifferent ice phases at the same temperature, the fact thatone of these ices (ice I) was less dense than the solution,and the pressure obtained from Raman spectroscopy. Allobserved segments of the ice III, ice V, and ice VI liquidihad positive dP/dT slopes (Fig. 3, Table 1). None of theexperiments intersected the triple points ice III + ice V + li-quid and ice V + ice VI + liquid.

Fig. 2. Raman spectra of different ice polymorphs; (a) for purewater, from Chou et al. (1998) and (b) for H2O + 5 mass% NaClcollected in this study.



3.2. Ice I liquidi for 10 and 15 mass% NaCl compositions

from HPOC experiments

Ice I liquidi for 10 and 15 mass% NaCl were determinedup to 25 MPa using a HPOC because of the better accuracyand resolution in pressure compared to a HDAC. In the

HPOC experiments the pressure was increased in 5 MPaincrements during each run. This small pressure increment,compared to that of the HDAC runs (at least �20 MPa),allowed us to collect more P–T data points along the ice Imelting curve, which provided better constraints on its loca-tion. Moreover, the precision of pressure determinations inthe HPOC is about an order of magnitude better than in theHDAC. The ice I liquidus was determined from 1.6 to20 MPa for H2O + 10 mass% NaCl, and from 0.1 to25 MPa for H2O + 15 mass% NaCl. The individual P–T

points along these liquidi are listed in Table 1. Fig. 4 com-pares ice I liquidi from the HPOC technique with the datafor 5 mass% NaCl obtained using a HDAC and with the iceI melting curve of pure H2O (Wagner et al., 1994). The dP/dT slope of these curves is always negative but clearly be-comes steeper with increasing salinity (Fig. 4). The resultsobtained for the 15 mass% NaCl solution are also consis-tent with the data collected at higher pressure for the samecomposition using the hydrothermal diamond anvil cell (seeSection 3.3). The ice I liquidus for 10 mass% NaCl deter-mined in this study shows a negative slope (��11 MPa/�C) and extends from 1.6 MPa and �7.2 �C to 20 MPaand �8.6 �C. The data for 15 mass% NaCl show that theice I melting temperature decreases from �10.3 �C at0.1 MPa to �11.7 �C at 25 MPa (Table 1), which gives anegative dP/dT slope of about �15 MPa/�C. This slope issignificantly steeper than that of the ice melting curve forpure water, which has an overall dP/dT slope of about�9 MPa/�C (Wagner et al., 1994).

3.3. Liquidi of 15 and 25 mass% NaCl compositions from

HDAC experiments

The experimentally obtained P–T data for the liquidi of15 and 25 mass% compositions are reported in Table 1 and

Table 1Experimentally-determined P–T locations of the liquidi in the H2O–NaCl system.

5 mass% NaCl 10 mass% NaCl 15 mass% NaCl 25 mass% NaCl

Liquidus T (�C) P (MPa) Liquidus T (�C) P (MPa) Liquidus T (�C) P (MPa) Liquidus T (�C) P (MPa)

Ice I �2.7 0.1 Ice I �7.2* 1.6* Ice I �10.3* 0.1 HH �6.4 0.1Ice I �4.5 40 Ice I �7.3* 5* Ice I �10.1* 0.2 HH �6.9 45Ice I �4.9 50 Ice I �7.9* 10* Ice �10.8* 5* HH �7.1 65Ice I �5.6 69 Ice I �8.1* 15* Ice I �11.1* 10* HH �7.3 100Ice I �8.6 90 Ice I �8.6* 15.5* Ice I �11.2* 15* HH �7.1 180Ice I �11.5 125 Ice I �8.6* 20* Ice I �11.6* 20* HH �6.4 180Ice I �14.8 150 Ice I �11.7* 25* HH �6.4 200Ice I �25.7 260 HH �11.2 82 HH �5.7 230Ice I + ice III �27.7 300 HH �10.4 150 HH �2.4 500Ice III �21.0 380 HH �6.8 385 HH �0.7 670Ice III �20.3 400 HH �6.5 420 HH 1.2 920Ice V �10.5 600 HH �4.8 545 HH 3 1040Ice V �2.4 720 HH �4.2 640 Ice VI 12.8 1400Ice VI 17.7 950 Ice VI 13.1 1280Ice VI 25.8 1080 Ice VI 13.8 1290

Ice VI 14.2 1310Ice VI 16.8 1360

The reported P–T data were obtained during HDAC experiments, except for the values for the ice I liquidus of 10 and 15 mass% NaCl (from0.2 to 25 MPa) indicated with “*”. These latter data were obtained with HPOC experiments. HH – hydrohalite.

Fig. 3. P–T diagram showing the experimental data obtained inthis study for the liquidi of H2O + 5 mass% NaCl (filled symbols)and P–T points along the heating path at which ice coexisted withliquid based on optical microscopy and Raman spectroscopy (opensymbols). I, II, III, V and VI represent the different polymorphs ofice, and L represents liquid. The filled black star indicates thelocation of the observed ice I + ice III + liquid triple point at�300 MPa and �27.7 �C. Black dashed lines represent boundariesof ice + liquid stability fields for a bulk composition ofH2O + 5 mass% NaCl as inferred from the data obtained in thisstudy. Grey dashed lines labelled “pure water” represent phaseboundaries for H2O (Chou et al., 1998; ice melting curves fromWagner et al. (1994)). The dashed line labeled L/VI + L representsthe ice VI liquidus for a 5.5 mass% NaCl solution determined byManning and Daniel (2008).



shown in Fig. 5. Within the range in pressures of this study,three different segments of the liquidus were observed forthe 15 mass% NaCl composition. The ice I liquidus is lo-cated at the lowest pressures. Based on the HPOC experi-ments (see Section 3.2), its dP/dT slope is negative (about�15 MPa/�C). In the HDAC experiments using the15 mass% NaCl composition, hydrohalite (HH) was the lastsolid phase (other than quartz) observed to melt at P–T

conditions from 82 MPa, �11.2 �C to 640 MPa and�4.2 �C (Table 1). This equates to a positive dP/dT slope

of �76 MPa/�C for the HH melting curve. Both liquidiintersect at �25 MPa and �11.7 �C, which gives theapproximate P–T location of the ice I + hydroha-lite + liquid triple point for 15 mass% NaCl. Hydrohalitewas clearly identified based on its optical properties andthe Raman spectrum. Hydrohalite is characterized by ahigher density and a markedly higher index of refractioncompared to the coexisting liquid, and by its crystal habit(Fig. 6a and b). At the temperatures of our study, the Ra-man spectrum of this phase shows two bands in the O–Hstretching region at �3540 and �3420 cm�1. The latterband has shoulders at �3405 and �3435 cm�1 (Fig. 6c).This agrees well with Raman spectra of HH reported inthe literature (Dubessy et al., 1982; Samson and Walker,2000; Bakker, 2004; Baumgartner and Bakker, 2010). Aportion of the ice VI liquidus for 15 mass% NaCl was foundbetween 2.7 �C, 1090 MPa and 16.8 �C, 1360 MPa. The dP/dT slope of this segment of the liquidus is positive(�19 MPa/�C), but considerably less steep than that ofthe HH liquidus for this composition. The approximatelocation of the HH + ice VI + liquid triple point for15 mass% NaCl is about 1.7 �C and 1.07 GPa, based onthe intersection of linear equations fit to the P–T data ob-tained for these liquidi.

For a composition of 25 mass% NaCl, the ice I liquiduswas not observed. Hydrohalite was the last solid to melt be-tween 0.1 MPa, �6.4 �C and 1040 MPa, 3 �C (Table 1,Fig. 5). However, the dP/dT slope of the HH liquidus forthis composition changed from negative to positive andwas not associated with a change in the phase assemblage.This change in dP/dT slope occurred at about �7.5 �C and140 ± 40 MPa, as estimated from a P–T plot of the data(Fig. 5). At 1400 MPa and 12.8 �C, ice VI was the last solidto melt. The dP/dT slope of the ice VI liquidus of 25 mass%NaCl could not be quantified owing to lack of data at high-er pressures. Because the data for the HH liquidus for25 mass% NaCl extend to at least 1.04 GPa (Table 1), theHH + ice VI + liquid triple point is likely located at asomewhat higher pressure compared to that of the corre-sponding triple point at 15 mass% NaCl, which is at�1.07 GPa (Fig. 5).

3.4. Observation of an unidentified solidus phase

During cooling of the 15 and 25 mass% NaCl solutionsat elevated pressure in the HDAC, the liquid crystallized toa single phase of columnar habit (Fig. 7a). For 15 mass%NaCl, this phase was observed at pressures from about1000 to 1200 MPa, and from about 150 to 900 MPa forthe 25 mass% NaCl composition. No other phase was ob-served to form during cooling at these conditions. Thisobservation was confirmed by subsequent Raman analysesat subsolidus conditions at several locations within the sam-ple chamber. Evidence for the presence of aqueous liquid,water ices, or HH was not observed in the spectra. TheRaman spectrum of the unidentified columnar phase inthe O–H stretching region is characterized by three bandsat �3410, �3460, and �3510 cm�1 (Fig. 8).

Fig. 7 shows the melting relationships that occurredupon warming, after a 15 mass% NaCl solution was

Fig. 4. Experimental data for the ice I liquidus at different bulkcompositions. Black filled diamonds are data for 5 mass% NaCl (asin Fig. 3) obtained in the HDAC. The large black diamond at�300 MPa and �27.7 �C represents the ice I + ice III + L triplepoint (see Fig. 3). Open and filled grey diamonds are data collectedusing the high-pressure optical cell (HPOC) for compositions of 10and 15 mass% NaCl, respectively. The grey dashed line labeled“pure water” is the ice I melting curve of pure H2O (Wagner et al.,1994).

Fig. 5. P–T diagram of liquidi and columnar phase meltingconditions for H2O + 15 mass% NaCl (open symbols) andH2O + 25 mass% NaCl (filled symbols) determined in this study.Diamonds, circles, and triangles indicate P–T conditions on theliquidus (diamonds – ice I, circles – ice VI, triangles – hydrohalite(HH)). Squares indicate P–T conditions at which the unidentified

columnar phase melted upon heating according to the reaction(columnar phase + HH + liquid = HH + liquid).



Fig. 6. (a) Photomicrograph of the HDAC sample chamber during an experiment with H2O + 15 mass% NaCl. The euhedral crystal ofhydrohalite (HH, shown enlarged in (b)) was grown by slight heating just before the liquidus temperature was attained. The focus is on thebottom of the sample chamber, i.e., the culet of the lower diamond anvil. (b) The same hydrohalite crystal as shown in (a) at highermagnification. (c) Raman spectrum of hydrohalite at 3.5 �C and 1.1 GPa, for a bulk composition of 15 mass% NaCl. The bands at �3540 and�3420 cm�1 with shoulders at �3405 and �3435 cm�1 are characteristic for hydrohalite. The spectrum also contains a broad feature betweenabout 3100 and 3600 cm�1, probably from O–H stretching vibrations in the aqueous liquid above the HH crystal.

Fig. 7. Series of photomicrographs of the HDAC sample chamber at different pressures and temperatures during a cooling and heatingexperiment with 15 mass% NaCl. (a) Following initial cooling only the quartz pressure sensor and the unidentified columnar phase arepresent. (b) With increasing temperature the columnar phase decomposes with simultaneous formation of hydrohalite (HH) and liquid. (c)Hydrohalite, ice VI and aqueous liquid (L) are present at 3.4 �C and �1.2 GPa (HH is probably metastable at these conditions). (d)Hydrohalite melted when the assemblage shown in (c) was heated to 14.2 �C, leaving only liquid, ice VI and the quartz pressure sensor.



completely converted into a solid of columnar habit. First,hydrohalite and liquid formed as the columnar phase rap-idly disappeared (Fig. 7a and b), and produced the assem-blage HH + ice VI + aqueous liquid at supersolidustemperatures (Fig. 7c). Further warming resulted in thedecomposition of HH (Fig. 7d) and finally, at a slightlyhigher temperature, in complete melting of ice VI at the liq-uidus. Temperatures and pressures at which the columnarphase disappeared are reported in Table 2. This same phasebehavior was observed for the 25 mass% NaCl solution.

4. DISCUSSION

4.1. Topology of the liquidi to 25 mass% NaCl

Pronounced topological changes with pressure and com-position were observed for the system H2O + NaCl. Thephase diagram for the 5 mass% NaCl composition is similar

to that of pure water, with ice stability fields shifted to dis-tinctly higher pressure and slightly lower temperature(Fig. 3). For example, the location of the ice I + ice III + -liquid triple point for H2O + 5 mass% NaCl was found at�300 MPa and �27.7 �C, whereas that of pure H2O is at209.9 MPa and �21.985 �C (Wagner et al., 1994). Thehydrohalite liquidus was not observed for theH2O + 5 mass% NaCl composition. At 15 mass% NaCl,HH stability is enhanced at elevated pressure at the expenseof ice III and ice V. In addition, the HH liquidus decreasesthe maximum pressure along the ice I liquidus and increasesthe minimum pressure along the ice VI liquidus (Fig. 5).Stated differently, the stability range of ice polymorphsshrinks at the expense of the hydrohalite. At 25 mass%NaCl, ice I was not stable at any condition, and the HH liq-uidus extends to the HH + ice VI + liquid triple point,likely at a somewhat higher pressure than that of this sametriple point for 15 mass% NaCl (Fig. 5).

Overall, addition of NaCl appears to steepen dP/dT

slopes of liquidi (Fig. 4). This is significant for the ice Imelting curves, and likely also for the ice VI melting curves.The ice I melting curve of pure water has a dP/dT slope ofabout ��9 MPa/�C (Wagner et al., 1994), and the slopes ofthe ice I liquidi determined in this study are ��11 MPa/�Cat 5 and 10 mass% NaCl and �15 MPa/�C at 15 mass%NaCl. The ice VI melting curve of pure water is not linear(Wagner et al., 1994), and has a dP/dT slope of �29 MPa/�C at its upper pressure end at 81.85 �C and 2216 MPa, anda slope of �11.5 MPa/�C at its lower pressure limit at0.16 �C and 632 MPa. The average dP/dT slope is�19 MPa/�C if the data are fitted to a first-order polyno-mial. The low-pressure portion of the ice VI liquidus of15 mass% NaCl was found to have a dP/dT slope of�22 MPa/�C. The small portion of the ice VI liquidusdetermined in this study for 5 mass% NaCl is parallel tothe melting curve of ice VI in the pure water system(Fig. 3). However, the dP/dT slope differs significantly fromthe steeper ice VI melting curve for H2O + 5.5 mass% NaClreported by Manning and Daniel (2008) (Fig. 3). Thisinconsistency may be related to the limited amount of datain our study and/or to the different accuracies of the pres-sure sensors used at these P–T conditions, or to the slightlydifferent salinities.

Over the entire range of bulk compositions studied here,the dP/dT slope of the liquidi changes from negative at thelowest pressures to positive for all liquidi at high pressures.For the 5 and 15 mass% NaCl compositions, ice I was thesolid phase in equilibrium with the liquid along the liquiduscurves with negative dP/dT slopes. Based on the Clausius–Clapeyron relationship, solids such as ice I with a largermolar volume at the melting point than that of the coexis-ting liquid display negative dP/dT melting curve slopes, be-cause the entropy of melting is positive for all substancesexcept helium at low temperatures (Bilgram, 1987). Thetransition to positive dP/dT liquidi slopes is marked by aphase change, i.e., the ice I liquidus ends at triple points,which are the ice I + ice III + liquid triple point for5 mass% NaCl and the ice I + HH + liquid triple pointfor 15 mass% NaCl. At 25 mass% NaCl, the hydrohaliteliquidus also displayed a negative dP/dT slope at the lowest

Fig. 8. Raman spectrum at -30 �C and 1.3 GPa of the unidentifiedcolumnar phase that formed during cooling of a 15 mass% NaClsolution. During cooling, the solution crystallized to a single solidphase with a columnar habit. No other phase (except quartz) wasdetected optically or by Raman spectroscopy. This phase assem-blage is shown in Figure 7a.

Table 2P–T conditions of disappearance of the columnar phase uponwarming during HDAC experiments with 15 and 25 mass% NaClsolutions.

15 mass% NaCl 25 mass% NaCl

T (�C) P (MPa) T (�C) P (MPa)

�4.1 1100 �30.2 1501.7 1200 �24.6 300

�15.0 520�10.4 700�6.4 900



pressures. This portion of the HH liquidus is very steep,and the temperature decreases only by about 1 K, from�6.4 �C at 0.1 MPa to �7.3 �C at �100 MPa. This decreasein temperature is thought to be real, given the accuracy ofthe temperature measurement in the HDAC. At higherpressures, the HH liquidus shows normal behavior, i.e., ithas a positive dP/dT slope. As a consequence, the changefrom negative to positive dP/dT slopes was not coupledwith a change in the phase assemblage at a triple point. Thisis difficult to explain on the basis of the Clausius–Clapeyronrelationship. It cannot be reasonably attributed to ananomalous change in the molar volume of hydrohalite.As expected a priori, the hydrohalite crystals did not floatin the aqueous liquid at the lowest pressures. The hypothe-sis of metastable melting of HH relative to ice I can be re-futed because a metastable solid must melt at a lowertemperature than the stable solid at constant pressure andbulk composition. This behavior can also not be ascribedto a change in composition of the solid with increasing pres-sure. The mutual solubilities of ice I, halite and hydrohaliteare nil, and there was no indication for the presence ofeither ice I or halite. A hypothetical change in the HHhydration state is not supported by the Raman spectra.Moreover, and importantly, the halite liquidi for bulk com-positions <50 mass% NaCl also display negative dP/dT

slopes (Bodnar, 1994), at least at relatively low pressures(Schmidt et al., 1998). There are no known significantchanges in the halite composition, and the molar volumeof halite (and hydrohalite) at the melting point should besignificantly smaller than that of the coexisting aqueous li-quid. In summary, the experimental evidence that haliteand hydrohalite liquidi can display negative dP/dT slopesis confirmed by independent studies using different tech-niques, but it is in conflict with behavior predicted by theClausius–Clapeyron relationship.

One possible explanation for this apparent discrepancy isthat changes in the dP/dT slopes of liquidi in water–salt sys-tems reflects a change in the dynamic structure of the aque-ous liquid. Experimental work on pure H2O suggests theexistence of a structural transition from low-density water(LDW) to a less compressible high-density water (HDW)structure (e.g., Okhulkov et al., 1994; Mishima and Stanley,1998; Soper and Ricci, 2000; Kawamoto et al., 2004;Mirwald, 2005). The liquid–liquid transition is also reportedin saline aqueous solutions (e.g., Cavaille et al., 1996;Mirwald, 2005; Schmidt, 2009) at P–T conditions similarto those observed in this study for the change in the dP/dT

slope of the hydrohalite liquidus from negative to positivevalues (�7.5 �C, �140 ± 40 MPa, 25 mass% NaCl).Mirwald (2005) concluded that such inflections in the Cla-peyron slope are mainly attributed to the change in entropyof the aqueous solution, which is likely related to a structuralchange. However, the question remains open as to whetherthe effect of volume (density) of the aqueous liquid on entro-py is large enough to produce a negative dP/dT slope.

4.2. Unidentified columnar solidus phase

The unidentified columnar phase formed upon coolingof the 15 and 25 mass% NaCl solutions at the highest bulk

densities studied here. The optical properties and theRaman spectrum in the O–H stretching region do not matchthose of ices (e.g., Haselton et al., 1995; Chou et al., 1998)or hydrohalite (e.g., Dubessy et al., 1982; Bakker, 2004) ora mixture of these phases. The Raman spectrum suggeststhat the solid may be a previously unidentified high-pres-sure NaCl-hydrate other than hydrohalite. In contrast,the cooling behavior indicates that it is a solid solution. Ifso, it is likely not identical to NaCl-bearing ice VII studiedby Frank et al. (2006) at bulk compositions of 5 and10 mass% NaCl, or with the NaCl-bearing ice VI or iceVII solid solutions inferred by Manning and Daniel(2008) based on their experiments with a bulk compositionof 5.5 mass% NaCl. This conclusion is based on the obser-vation that the Raman spectrum of the unidentified colum-nar phase lacks the fairly intense Raman band at�3200 cm�1 or somewhat lower wavenumbers that is char-acteristic for ice phases (e.g., Chou et al., 1998; Frank et al.,2006) (Figs. 2 and 8).

We did not observe the unidentified columnar solidusphase during the study of the liquidus for theH2O + 5 mass% NaCl solution. This may be because thepressures reached in our experiments were not high enoughfor its formation, or it does not form at all with this com-position, or the amount formed was too small for Ramanspectroscopic detection. To our knowledge, this solid hasnot been previously observed in the system H2O–NaCland additional studies are required for better characteriza-tion and identification.

4.3. Application to Europa

There is considerable evidence for the existence of sub-surface aqueous liquid layers in some large icy satellitesof the solar system, particularly on Europa and Enceladus,and perhaps also in Ganymede, Callisto, Titan and Tritonand few others planetary bodies (e.g., Pappalardo et al.,1999; Grasset et al., 2000; Hussmann et al., 2002; Schubertet al., 2004, 2007; Zolensky, 2005; Postberg et al., 2009).The aqueous liquids can contain significant amounts of dis-solved salts from fluid–rock interaction and/or from evap-oration of H2O from an originally lower salinity liquid inthis low pressure atmospheric environment. For Enceladus,NaCl is the likely most common solute, with concentrationsof 0.29–1.15 mass% NaCl (Zolotov, 2007; Postberg et al.,2009). Data from the Galileo spacecraft magnetometer forEuropa and Callisto indicate the existence of an electricallyconducting layer below the ice shell, which has been inter-preted as a salty subsurface ocean (e.g., Khurana et al.,1998; Kivelson et al., 2000). Hand and Chyba (2007) com-bine spacecraft data-derived conductivities for Europa withelectrical conductivity data for aqueous MgSO4 solutionsand terrestrial seawater, and conclude a thin ice shell of lessthan 15 km in thickness (best match at about 4 km) andnear-saturation salt concentrations in the subsurface oceanat Europa. Other models support a considerably thicker iceshell for present-day Europa (Hussmann et al., 2002; Huss-mann and Spohn, 2004). Zolotov and Shock (2001) report,based on geochemical modeling, that as the inferredEuropean ocean water is cooled, the Cl/SO4 ratio of the



residual brine changes from its initial value of <1 to a valueof approximately 100, i.e., it becomes a chloride-rich brine.At the same time, the Na/Mg ratio remains �1. Theseworkers further state that “freezing oceanic water on Europa

changes its composition, leading to sequential salt deposition

and to uneven enrichment in electrolytic solutes. Hydrated

sulfates of Mg and Na can dominate among precipitated

salts. However, the remaining solution becomes enriched in

Cl, and chloride salts of K, Na, and Mg can precipitate from

deeply fractionated brines.” More recently, Zolotov andKargel (2009) noted that “. . .. Europa’s ocean likely contains

sulfate, Mg, Na, and Cl as major solutes” and that “The

ocean could have evolved from a reduced Na–Cl solution to-

ward a Mg sulfate ocean”. Thus, it is appropriate to modelthe overlying ice shell and sub-surface ocean on Europabased on phase equilibria in the H2O–NaCl system.

In a simple approximation, the thickness of ice I shellsabove denser aqueous liquids can be obtained from theintersection of the temperature–depth relationship in theshell with the liquidus of a model fluid composition usingthe appropriate conversion of pressure to depth. Fig. 9shows this approach for the ice shell of the Jovian satelliteEuropa using the temperature–depth relationship calcu-lated by Nimmo and Manga (2009) for a tidally heated con-ductive icy shell (thermal gradient of about 15 K km�1) anda pressure gradient of 1.3 MPa km�1 (Schubert et al., 1986).The temperature profile modeled for the icy shell of Europaintersects the ice I melting curve of pure water (Wagneret al., 1994) at a depth of �10 km. Addition of NaCl de-creases the thickness of the ice I shell, but not by much.At 15 mass% NaCl, the shell thickness is about 9 km. Fur-thermore, the pressure of the ice I + hydrohalite + liquidtriple point for 15 mass% NaCl at �25 MPa inferred fromthe data of our study shows that the hydrohalite liquidusexpands with increasing salinity at the expense of the ice I

liquidus. Because hydrohalite is denser than the coexistingliquid, this places an upper limit for the maximum NaClconcentration, which cannot be much greater than about15 mass% NaCl. If so, this in turn constrains the minimumsubsurface ocean temperature to about 260 K.

Our results indicate that the NaCl concentration has lit-tle effect on the icy shell thickness, which is much moredependent on the modeled temperature profile. Thus, itwould be largely circular reasoning to state a very goodagreement with thicknesses of the icy shell calculated byNimmo and Manga (2009). However, a thickness of�9 km is still consistent with other models, e.g. by Handand Chyba (2007), but a thin ice shell of about 4–5 km(Zimmer et al., 2000; Hand and Chyba, 2007) is not sup-ported. Even if a low salinity of about 1 mass% NaCl is as-sumed, at which ice I + liquid coexist over a temperatureinterval of about 20 K, the corresponding thickness of aice I + liquid layer is only about 1 km, so that the thicknessof the uppermost, electrically insulating, icy shell would stillbe �10 km.

ACKNOWLEDGMENTS

P.V. gratefully acknowledges financial support of the GermanScience Foundation (DFG) through Grant SP1216/1-1. We thankC. Farley (Virginia Tech) and R. Schulz (GFZ) for their technicalassistance. Reviews by I-Ming Chou, Mark R. Frank and Jean Du-bessy of an earlier version greatly improved the manuscript. P.V. isgrateful to Vincenzo Stagno for improving the quality of the fig-ures. Experiments conducted at Virginia Tech were supported bythe National Science Foundation under Grant No. EAR-1019770to RJB.

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o nacl liquidi to 25 mass% - virginia tech geosciences ... 16 gpa for a 5 mass% nacl...

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