Sub-surface precipitation of salts in supercritical seawater

Basin Research (2006) 18, 221–230, doi: 10.1111/j.1365-2117.2006.00290.x

Sub-surface precipitation of salts in supercritical seawater Martin Hovland n, Tatyana Kuznetsovaw, Ha˚ kon Ruesla˚ ttenz, Bjørn Kvammew, Hans Konrad Johnsenz, Gunnar E. Fladmark‰ and Andreas Hebachw n

Statoil, Stavanger, Norway wDepartment of Physics and Technology, University of Bergen, Bergen, Norway zNumericalRocks,Trondheim, Norway zStatoil, R&D Department, Rotvoll,Trondheim, Norway ‰Department of Mathematics, University of Bergen, Bergen, Norway

ABSTRACT Extremely low solubility of typical seawater salts within certain supercritical sections of their pressure^temperature composition space is a proven experimental fact. Its consequences are often referred to as either ‘shock crystallization’ or ‘out- salting’. Our alternative model for the formation of salt deposits hypothesizes that high temperatures and pressures characteristic for the high heat- £ow zones of tectonically active basins may bring submarine brines into the out- salting regions and result in the accumulation of geological- scale salt depositions. To con¢rm the laboratory observations, molecular- scale simulations (molecular dynamics) have been employed to study structural changes in a model seawater system where temperature increased from ambient to near-critical and supercritical. Fluid properties and phase transition regions extracted from the simulations were then used as input parameters for a reservoir simulator program to model out- salting in a simple hydrothermal geological system. Both numerical simulations and laboratory experiments con¢rm that supercritical out- salting is a viable process of geological signi¢cance for the formation and accumulation of evaporites.We suggest two regions where hydrothermally associated salts may be depositing today: Atlantis II Deep, in the Red Sea, and Lake Asale, Ethiopia.

INTRODUCTION The widely accepted hypothesis for the formation of geo logically signi¢cant evaporites on Earth (mainly halite, anhydrite and gypsum) is the solar-induced evaporation of seawater. There are, however, numerous paradoxes and unresolved problems associated with this model, as discussed by Warren (1999), Wilson (2003, 2004) and Talbot (2004) that clearly illustrate a lack of fundamental data, especially from the deepest portions of the salt basins, to verify this evaporite hypothesis as the general model for salt formation. The following citation from the most recent and authoritative Geological Encyclopedia (Selley, 2005) also underlines this conundrum: ‘As the name suggests, it was once thought that evaporites formed exclusively from the drying out of enclosed marine basins.This required improbably large volumes of seawater to provide the resultant evaporites. It is now realized that many evaporites actually form in sabkhas (Arabic for ‘‘salt marsh’’) from the replacement of pre- existing rocks, principally carbonates, by circulating brines. Evaporites should thus more correctly be termed ‘‘replacementites.’’ ’ Selley (2005) avoids, however, to suggest a possible origin for the Correspondence: Martin Hovland, Statoil, N-4035, Stavanger, Norway. E-mail: [email protected] r 2006 The Authors. Journal compilation r 2006 Blackwell Publishing Ltd

‘circulating brines’. This problem is addressed in the present work. Salt formation by hydrothermal processes is also known to have contributed salt volumes of geological signi¢cance (Lowell & Germanovich, 1997, in review by Warren, 1999), but the basic hydrothermal processes involved have not been seriously addressed in this context, i.e. (1) salt precipitation when seawater attains supercritical conditions and (2) salt precipitation by submerged or buried boiling of seawater. This work discusses these processes in terms of laboratory experiments and theoretical molecular modelling for water and brines under sub- and supercritical conditions. A basin modelling study has also been carried out to study the behaviour of seawater circulating in the proximity of a shallow heated sill body in the Earth’s crust. These results and facts are discussed in relation to natural environments where analogue processes may occur. It is concluded that hydrothermally associated salts may be more abundant than hitherto realized, and consequently, more signi¢cant from the geological viewpoint.

Supercritical water The critical point (CP) for distilled water is 374.15 1C and 221.2 bar. Beyond this point, the physical and chemical properties of water change fundamentally; e.g. the dielec-

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M. Hovland et al. tric constant declines from the normal-water value of 80 to only 2 at 250 bar and 450 1C (Tester et al., 1993). Similarly, the ionic dissociation constant declines from the normalwater value of10 14 down to10 23. In the near-critical region, the density of water changes rapidly with respect to both temperature and pressure and at CP it attains a value of 0.3 g/cm3 (Tester etal.,1993; Bellissent-Funel, 2001).This strongly in£uences the inter-molecular behaviour of water, and Raman spectra of deuterated water in the supercritical region show only remnants of hydrogen bonding left (Franck, 1976; Kohl et al., 1991). Consequently, the supercritical water behaves essentially as a non-polar low-density £uid, with its solvation properties resembling those of low-polarity organic £uids. Under certain pressure/temperature conditions, supercritical water is also unable to dissolve common sea salts (Tester et al., 1993; BellissentFunel, 2001). These dramatic deviations from ‘normal’ water behaviour make supercritical water highly corrosive to a number of compounds, a property of importance when studying deep hydrothermal systems.

Supercritical brines When salt is added to pure water, one more degree of freedom is added to the system (in accordance with the Gibbs phase rule), with the CP changing along a line connecting the CP of the two pure components (salt and water). Depending on the local salinity, supercritical conditions in seawater and brines will be reached at pressures and temperatures that di¡er signi¢cantly from those of pure water. The onset of critical behaviour in normal seawater with salinity 3.5 wt% occurs at around 300 bar and 405 1C (Bischo¡ and Rosenbauer, 1989). If boiling takes place, a higher £uid column (steam and water) is necessary in order to reach a su⁄cient hydrostatic pressure to attain the CP (Fig.1). Some experimental studies of supercritical brines have been reported in the literature; e.g.Tester et al. (1993), who examined the phase behavior of synthetic brines be-

low and above their CP.The researchers observed the pro cess through a sapphire window in the pressure chamber, and when the brines were passing into the supercritical region at a temperature of 405 1C and 300 bar pressure (the CP of seawater), they could see a ‘cloud’ formed by the onset of ‘shock crystallization’of NaCl and Na2SO4.The sudden phase transition occurred as the solubility of the salts declined to near-zero over a temperature range of only a few degrees C. The resulting solids were found to consist of amorphous microscopic particles of sizes between 10 and100 mm.The reason for the observed shock crystallization is discussed below.

Supercritical brines in nature Natural occurrences of supercritical water and associated processes are well hidden from direct observation by humans, deep below the Earth surface and inside the deepsea hot vents (Von Damm et al., 2002; Kawada et al., 2004). In the ocean, the critical pressure for seawater corresponds to a depth of 42800 m. Given that the world average ocean depth is more than 3000 m, there is a great potential for the formation of supercritical water in hot and fractured oceanic crust. Deep sea drilling of oceanic crust has also shown regional porosities of up to 25% (Becker, 1999; Davis et al., 2004), and consequently, a large potential for the circulation of £uids. At these depths, the exposure of seawater to strong heat sources; e.g. intrusive basalts of typically 800^ 1200 1C, will potentially cause the water to become supercritical (Kawada et al., 2004; Hovland et al., 2005; M. Hovland, H. Ruesltten, H.K. Johnsen, B. Kvamme & T. Kutznetsova, submitted). A fundamental knowledge of the behaviour of water and salt at various temperatures and pressures encountered in the Earth’s crust must be the starting point for understanding the natural processes associated with supercritical water.

SOLUBILITY OF SALT ^ THEORETICAL BACKGROUND The dipolar nature of water

Fig. 1. Location of possible supercritical and boiling environments according to depth (pressure): Far left: fresh water and steam column above a deep-lying heat source (4374 1C) in a lacustrine situation (e.g. on Iceland). Depth corresponding to the critical point exceeds 3 km due to the low density. In the ocean, the potential critical point (PCP) lies at around 2800 m (pressure of about 300 bar), provided that a su⁄cient heat source (4405 1C) is located under the sea£oor.

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The ability of water to dissolve salts is due to its highly dipolar nature, allowing the water molecule to form shelllike structures around the dissociated salt ions. These shells screen the ions’ charges and signi¢cantly attenuate their Coulomb interactions with each other (the e¡ect is not unlike the Debye shielding in plasma, although the screening entities are electroneutral in case of hydration). The dipolar moment of water and its orientational selectivity is directly re£ected in the strength of the hydrogenbonded network.

The SSIP and CIP regimes At low to moderate temperatures, the ions are e⁄ciently screened from each other by water molecules. This is known as the solvent- separated ion pair (SSIP) regime.

r 2006 The Authors. Journal compilation r 2006 Blackwell Publishing Ltd, Basin Research, 18, 221^230

Sub-surface precipitation of salts in supercritical seawater 390 bars, there will be two intersection lines. These lines are called lower and upper solidus lines. At 250 bars, they are located at 450 and 720 1C, respectively.Temperatures in between the two solidus lines will correspond to the region where the only coexisting phases are vapour and solid salt. At lower temperatures, the NaCl^water system is a mixture of liquid and vapour phases.

Solubility of NaCl around the CP

Fig. 2. P^T projection of the monovariant solid^liquid^vapour saturation curve (solidus) for the NaCl^water system based on Hodes et al. (2004a). (Note that pressure increases downwards, as in earth and marine systems.) Arrows indicate the two points of intersection with the section at 250 bar (lower and upper solidus boundaries). See also Fig. 6 and text for further details and discussion.

As the temperature increases, the structure in the solution will gradually decay. The ions’ Coulomb forces are much stronger than the forces acting between the water dipoles; thus, the electrostatic attraction between the ions will persist at temperatures where the hydrogen-bond network has already deteriorated to such a degree that it is no longer able to shield the salt ions from each other.The resulting e¡ect is that the salts are no longer soluble in water (which now is in a vapour state). When the temperatures are elevated even higher, a regime will emerge where the ions are soluble again as electroneutral ion pairs surrounded by water molecules.This regime is called the contact ion pair (CIP) regime (undissociated salt, the upper solidus in Fig. 2). Between these two regimes (SSIP and CIP) of salt solubility, there is a two -phase equilibrium region where solid salt coexists with £uid containing very small amounts of salt: the solid- £uid ‘out- salting’ region.

Lower and upper solidus line Consider the P^T projection of the monovariant solid^ liquid^vapour (S^L^V) saturation curve (‘‘solidus’’) for NaCl^water in Fig. 2 (Hodes et al., 2004b). The S^L^V curve describes a surface in the pressure^temperature^ salt concentration (P^T^x) space where three phases, solid salt, salt- saturated liquid and salt- saturated vapour can coexist. This surface separates several regions where only two phases coexist. If we cut the S^L^V curve horizontally (at a constant pressure), we will obtain a temperature^ concentration (T^x) section. Given the shape of the S^L^V curve in Fig. 2, one can see that at any pressure below

The solubility of NaCl in the liquid close to the CP could be as high as 40 wt%, and consequently, the liquid phase can contain a large amount of salt. When the temperature of the salt- saturated £uid rises above the lower solidus line (450 1C at 250 bars), the £uid can only exist as either vapour or solid salt. Given the extremely low solubility of salt in vapour, almost all the salt previously dissolved in the water will fall out. This is the so - called ‘out- salting’ or ‘shock crystallization’. Temperatures above the upper solidus line (or even lower in case of low salt concentrations) will be able to support a liquid phase again, resulting in sharply increased salt solubility.We should note here, however, that although dissolved in water, this salt remains undisso ciated, a condition that drastically a¡ects its properties.

SALT CROSSOVER BEHAVIOUR AND BRINE PROPERTIES: MOLECULAR SIMULATIONS Model system and parameters State- of-the art models describing atomistic-level interactions between water molecules have been compiled in recent reviews by Nieto -Draghi et al. (2003) and Guillot (2002). The SPC/E model is chosen for describing the water^water interactions in our simulations, and the model by Smith and Dang (1994) is chosen for the ion descriptions. Cross interactions were calculated using the Lorenz^Berthelot mixing rules (Allen & Tildeslay, 1987). Our model system consists of 512 water molecules and six molecules of sodium chloride, corresponding approximately to 3.7 wt% salt. Simulations were performed in a closed system at constant pressure and temperature. The thermostat parameters in the Nose^Hoover formulation were ¢xed at 100 femtosecond (fs). The pressure control parameter was set to 8000 fs, and for the rotational degrees of freedom we used an implicit quaternion scheme (Fincham, 1992). The long-range electrostatic forces were handled using the Ewald summation approach (Allen & Tildesley, 1987). We have found our simulated results to be in excellent agreement with the previous numerical studies of similar model systems (Lyubartsev & Laaksonen, 1996; Driesner et al., 1998; Chialvo & Simonson, 2003) and the experimental investigations of the solubility regions for sodium chloride in water as functions of temperature and pressure (Hodes et al., 2004a being the most recent).


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Pair-correlation functions and hydrogen bonding The existence of the hydrogen-bonded structure discussed in the previous section can readily be inferred from the so - called paircorrelation functions. A pair correlation function describes the probability of ¢nding a pair of particles at a certain distance from each other, relative to the probability expected in a random distribution of particles at the same density. Pair- correlation functions can be directly deduced from experiments probing £uid structures (see for example neutron scattering results: Soper, 1986; Soper & Phillips, 1986) or computed by means of molecular simulations. The average number of water neighbours around a speci¢c ion (the aforementioned co ordination shell) can be estimated by integrating the paircorrelation function. As the oxygen atom dominates the water molecule, a fair approximation of the number of water molecules, na, around a speci¢c ion, a, can be found by naoxygenwater ðr1 ; r2 Þ Zr2 ¼ 4prwater gaoxygenwater ðrÞr2 dr

ð1Þ Fig. 3. Pair- correlation functions for oxygen^ oxygen (a) and oxygen^hydrogen (b) in brine at di¡erent temperatures.

r1

where r1 and r2 de¢ne the boundaries around ion a. Integration from r1 5 0 to r2 corresponding to the ¢rst minimum of the paircorrelation function will yield the average number of water molecules in the ¢rst coordination shell around the ion. The numerically simulated pair correlations for oxygens and hydrogens in water as functions of temperature at 300 bars are shown in Fig. 3a and b. The centre of mass in the water molecule is very close to the geometric centre of oxygen. In the absence of any orientational preference among the water molecules, one would expect to ¢nd the ¢rst peaks of the oxygen^hydrogen pair- correlations at a separation close to the size of the oxygen atom. The fact that the ¢rst peak in the oxygen^hydrogen pair correlation function lies closer than the corresponding peak in the oxygen^ oxygen pair correlation function is the evidence of the hydrogen-bonded structure (preferential orientations of oxygen and hydrogen molecules). As the temperature increases, two phenomena emerge: ¢rstly, the decreasing height of the oxygen^hydrogen peak is directly related to the strength of hydrogen bonding. Secondly, we should note the lack of any longer-ranging structures at high temperatures, meaning that the hydrogen-bond network deteriorates. In this situation, water will essentially behave as a non-polar £uid and loses the ability to dissolve salts as ions (indicated above). The behaviour of the Na1^Cl ion pair in terms of their paircorrelation functions at temperatures outside the two -phase (out- salting) region is described in Fig. 4. Particularly important are the dramatic changes in the heights of the ¢rst (CIP) and second (SSIP) peak of the Na1^Cl correlation function. Supplementary evidence

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Fig. 4. Sodium^ chloride ions (Na1^Cl ) paircorrelation functions for sub- and supercritical temperatures at 300 bar.The inset shows higher temperatures.

for the transition to a di¡erent solvation structure is found in the oxygen^Na1 and oxygen^Cl paircorrelation functions (Fig. 5a and b). One can ¢nd the average number of oxygen molecules surrounding each ion by integrating the pair correlation function up to its ¢rst minimum and then multiplying by the average solution number density. The oxygen coordination numbers around the small Na1 ion decreased from 5.5 to 3.8, whereas the corresponding number for Cl increased from 7.3 to 12.3 for the same range of temperatures. From 980 K and upwards, the individual coordination numbers for both Na1 and Cl decrease, as does the density of water.


Sub-surface precipitation of salts in supercritical seawater

SIMULATION OF OUT-SALTING IN SEDIMENT BASINS The ‘ATHENA’ reservoir simulator The reservoir simulation program ATHENA (see detailed description in Garrido et al., 2004) was applied to model seawater £ow and out- salting e¡ects in the supercritical (SC) region of a simpli¢ed hypothetical sediment basin. Primary input parameters to the ATHENA simulator are temperature, water pressure and molar masses for each £uid component, including dissolved salts. ATHENA uses ¢nite-volume space discretization and a backward Euler scheme for time to determine the £uid pressure and temperature. The mass-balance equations are solved using an implicit scheme. The objectives of the basin modelling were to:

Fig. 5. Oxygen^ sodium (a) and oxygen^ chloride (b) paircorrelation functions for 300 bar at various temperatures.

(1) provide a ¢rst approximation of the produced salt volumes (anhydrite and halite) in a very simple hydro thermal system; (2) establish constraints on the geometry, mass balance and circulation processes in the system; and (3) suggest a three-dimensional deposition model for the salts produced by boiling and passage of brines and seawater through the CP.

The basin model used The geological basin model used for this simulation is to model out- salting after a hot sill has been injected into sediments at high pressure, i.e., more than 300 bars, total static pressure. For this purpose, we consider a static, hot sill at a constant temperature of 1400 K and of unspeci¢ed thickness (more than about 10 m). The following simple one-dimensional geological model has the following geo logical constraints: Fig. 6. Density of water and brine as a function of temperature along the 300 bar isobar. Legend: 1 5 IAPWS-95 (Wagner & Pruss, 2002); 2 5 EoS from Phillips et al. (1981); 3 5 solidus boundaries; 4 5 MD results for pure water; and 5 5 MD results for brine.The two-phase region (or ‘out- salting region’) is between the two vertical lines (3).

Density of brine as a function of temperature The density of brine as a function of temperature at 300 bars, determined by means of constant T- constant P molecular dynamics, is shown in Fig. 6. Also shown are the corresponding data from Phillips et al. (1981) and the out- salting region (solidus lines from Hodes et al., 2004b). The modelled densities agree very well with the experimental values. This information, combined with the correct behaviour of the paircorrelation functions, allowed us to use the results of the simulations as input parameters for the ‘ATHENA’ reservoir simulation program.

Water depth 3000 m (corresponding to a pressure of 30 MPa). A hot sill at 1400 K, buried 3220 m below sea level. Three layers of sediments resting upon the sill: Two sand layers with a shale layer ‘sandwiched’ in between. Thicknesses: upper 20 m, sand layer, 100 m shale and lower sand layer, 100 m thick. The sand was assigned a porosity of f 5 0.35 and permeability of k 5 230 mD.The shale was assigned a porosity and a permeability of f 5 0.1025 and k 5 2.9 mD. The boundary conditions included a constant pressure of 32 MPa at the sill intrusion level, and 30 MPa at the sea£oor. The simulation added or removed brine from the top or bottom to keep the pressure gradient constant.This resulted in an upward £uid £ux. Temperatures were maintained ¢xed at roughly1400 K at the sill, and 2801K on the sea£oor. The simulation model was subdivided into cells of 5 m height.


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M. Hovland et al. As the model was treated as a single-phase model, there was no need for relative permeability and capillary pressure data. The basin model allows only vertical £uid £ow.

Input parameters to the modelling and simulation The ‘ATHENA’ reservoir simulator needs densities, speci¢c enthalpies and bulk viscosities of the £uid phases as input parameters to calculate the mass and heat balances. The values are tabulated, and the simulator performs linear interpolations to ¢nd the intermediary values. Pure water was used in the ¢rst simulation run, and the results were analysed for consistency, and subsequently compared with those for brine £ow to study the in£uence of out- salting. The input properties for pure water were obtained from equations presented by the International Association for the Properties of Water and Steam (aka IAPWS-95 formulation; Wagner and Preuss, 2002), and were tabulated at temperature steps of 50 K. The brine enthalpy and density data were extracted from the numerical simulations that also yielded the pair correlation data. It should be noted, however, that although the numerical simulations provide brine densities that can be used directly, only the residual enthalpy given by the molecular dynamics can be related to the experimental values in a straightforward manner. To ¢nd the brine- speci¢c enthalpies required by the ‘ATHENA’ simulator, we have calculated the enthalpy di¡erences between simulated pure water and brine, and adjusted the IAPWS values accordingly. Pure water viscosity was used for both water and brine. A quasi- single-phase system was simulated instead of a complicated two -phase system. The following scenario was therefore proposed and backward-validated: out- salting may start to occur only at temperatures lower than the upper solidus line for pressures equal to 30^32 MPa (980^ 1020 K, see Fig. 6). Salt is soluble above this line but remains largely non-dissociated. Thereafter, the brine will be salt-free (pure water) until the gradual cooling of the upward £ow crosses the lower solidus line and water regains the ability to dissolve salts (conventional solubility). The salt content of brine in this region will be restored due to mixing with the existing brines in the sediments and/or dissolving existing salt deposits. The precipitation of salt at the upper solidus line will block the pores and result in decreased porosity and permeability of the given cell.To incorporate the e¡ect of out- salting, the cell porosity and permeability were scaled up by constant factors as described in the captions of Figs 7^9. Allowing the porosity to vary during the simulation required a slightly longer time to reach a steady state.

Simulation results A wide variety of combinations of porosity and permeability of sandstone and shale layers are tested. None of these

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Fig. 7. Simulated injection of supercritical brine into a threelayer reservoir: temperature pro¢le for di¡erent combinations of scaling factors for porosities and permeabilities relative to original sandstone values. Legend: 1 5 porosity of 10 3 and permeability of 10 6; 2 5 porosity of 1 and permeability of 10 6; 3 5 pure water.

Fig. 8. Simulated injection of supercritical brine into a threelayer reservoir: pressure pro¢le for di¡erent combinations of scaling factors for porosities and permeabilities relative to original sandstone values. Legend: 1 5 porosity of 10 3 and permeability of 1; 2 5 porosity of 10 3 and permeability of 10 2; 3 5 porosity of 10 3 and permeability of 10 5; 4 5 porosity of 1 and permeability of 10 6; 5 5 porosity of 10 3 and permeability of 10 6; 6 5 porosity of 10 3 and permeability of 10 8; and 7 5 pure water.

Fig. 9. Simulated injection of supercritical brine into a threelayer reservoir: £ow velocity pro¢le for di¡erent combinations of scaling factors for porosities and permeabilities relative to original sandstone values. Legend: 1 5 porosity of 10 3 and permeability of 1; 2 5 porosity of 10 3 and permeability of 10 2; 3 5 porosity of 10 3 and permeability of 10 5; 4 5 porosity of 1 and permeability of 10 6; 5 5 porosity of 10 3 and permeability of 10 6; 6 5 porosity of 10 3 and permeability of 10 8; and 7 5 pure water.


Sub-surface precipitation of salts in supercritical seawater a¡ected the temperature pro¢le; it remained essentially linear and identical to that of pure water as shown in Fig. 7. No drift of the out- salting front has been observed either, validating our earlier assumptions. On the other hand, we found that unlike temperature, the pressure and £ow velocity are very sensitive to variations in porosity and permeability, as shown by the reservoir simulation results displayed in Figs 8 and 9. The e¡ects of the out- salting are best illustrated in Fig. 8: the blocking of pores caused by salt deposition leads to the establishment of a stable lowpermeability layer within the sandstone. This layer will give rise to a substantial pressure rise over a relatively small region (20 m) and its existence may have drastic practical consequences; e.g. unexpected pressure build-up during exploration drilling. The £ow velocity pro¢le of Fig. 9 is also quite instructive. It demonstrates that although the out- salting clearly has an impact on the velocity gradient, the e¡ect will manifest itself further downstream from the out- salting front.

DISCUSSION OF CONSEQUENCES Manifestations of supercritical brines in nature The fact that boiling and supercritical conditions a¡ect the salinity distribution in hydrothermal systems has been observed at hydrothermal vents (Ramboz et al., 1988; Lowell & Germanovich, 1997). One good example shows a change in chlorinity over time. During the period from 1991^1994,Von Damm (1995) observed a change in chlorinity from £uids emanating at the same deepsea vent: from having been very low in chlorinity in 1991, directly after a volcanic eruption, to attain about 1.5 times seawater chlorinity 3 years later.These observations were explained by the venting of vapour-phase £uids subsequent to the volcanic eruption, followed by brine-dominated £uids that at ¢rst were stored within the oceanic crust, and that vented when the hydrothermal system had cooled down 3 years later (Von Damm, 1995). Similar observations are reported by Butter¢eld et al. (1997) from the Juan de Fuca ridge and the East Paci¢c Rise. These observations can easily be interpreted from the theoretical discussions above: The venting of low- salinity water indicates that seawater coming into contact with the hot rock is brought into the supercritical ‘two -phase region’ between the upper and lower solidus line (SSIP and CIP). Salt precipitates, and the supercritical vapour £ows upwards. At some point along the migration route, the vapour temperature drops below the CP, and conventional boiling occurs. A further cooling will lead to condensation, and this water is venting on the sea£oor. When the hydrothermal system cools down (after 3 years) the CP is displaced downwards to where the salt was precipitated initially. Salt will be re-dissolved and

transported upwards as brines that vent on the sea£oor. Another observation by Von Damm et al. (2002) at the East Paci¢c Rise showed extraordinary segregation in £uids from a multi-funneled vent complex, the Brandon Vent: ‘‘Of the ¢ve ori¢ces on Brandon, sampled, three were venting £uids with less chlorine than average seawater, and two were venting £uids with greater content of chlorine than average seawater. The liquid-phase ori¢ces contain 1.6^1.9 times the chloride content of the vapors.’’ The di¡erent £uids were venting within metres of each other, with the densest high- chlorinity £uids emanating from the lowermost chimneys. These observations can also be explained similar to those above. An interesting issue is the short distance between vents with very di¡erent salinities. This may be explained by the precipitation of salt (or other minerals e.g. silica and sulphides) along the migration routes of the high- salinity brines that block communication to the neighbouring vents.

Environments for hydrothermal salt formation The processes we study here have very much in common with mineral chimney formation and layered ore-generation by hydrothermal processes. The necessary prerequisite conditions for such structures and deposits are: long-lived subsurface-focused heat sources hydrothermal circulation of water and brines. During extensive studies of buried hydrothermal systems, such as those conducted in several Ocean Drilling Pro gram (ODP), ‘sedimented ridges’ campaigns, it has been clearly demonstrated that seawater circulates deep into the sediments, down towards the buried heat sources that may lie within the oceanic crust (Becker, 1999; Galley & Koski, 1999; Davis et al., 2004). This circulation occurs despite the intervening sediments consisting mainly of low-permeable shales. In their work on hydrothermal venting, Goodfellow and Zierenberg (1999) discussed £uid recharge and discharge through low-permeable sediments.They conclude that recharging of seawater (in the Escabana trough) takes place either locally associated with a heat source, or regionally near the margins of the rift. Fluid discharge is in£uenced by the position of the heat source, but lateral £ow may occur along fractures in the basaltic basement or along more permeable sand and silt layers in the sediment package. They also suggest that £uid discharge through lowpermeable sediments takes place in the form of hydrothermal £uid diapirs that migrate upwards along zones of structural weakness.

Preservation of salts For the preservation of precipitated salts, there is another important prerequisite: the produced salts have to be pro tected from re-dissolution by seawater.We therefore theo -


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M. Hovland et al. suggested that salt can precipitate in the following do mains:

Fig. 10. Oceanographic and physical conditions at the Atlantis II Deep, Red Sea, according to Zierenberg and Holland (2004), Faber et al. (1998), Hartmann et al. (1998a,b), Zierenberg (1990), Danielsson et al. (1980). Note the assumption that hot brine vents exist, indicated to the left.The arrows pointing down indicate mineral (mainly sulphides and metal) precipitation onto the sea£oor (see referred literature).

rize that tectonically active environments, with high sedimentation rates, may o¡er such protection where sediments e⁄ciently cap the salts. Thus, marine and even continental rift zones, for example the Atlantis II Deep in the Red Sea, represent a suitably generative and protective environment (Fig. 10) (M. Hovland, H. Ruesltten, H.K. Johnsen, B. Kvamme & T. Kutznetsova, submitted). Another interesting tectonic setting with a modern accumulation of salt is Lake Asale in Ethiopia. Unlike the Red Sea, this is a terrestrial hydrothermal setting, and the hot and dry climate ensures the evaporation of venting brines and the preservation of precipitated salts.

A NEW MODEL FOR HYDROTHERMAL SALT FORMATION The new proposed model for hydrothermally induced salt formation and accumulation can be summarized as follows (M. Hovland, H. Ruesltten, H.K. Johnsen, B. Kvamme & T. Kutznetsova, submitted): it is based on ¢ve main prerequisites: 1. A source of seawater or brine. 2. A buried, strong, local heatsource, which is either a shallow magma chamber (To1200 1C), or a set of intersecting high-heat£ow crustal faults (T4405 1C), or a combination of both (12004T4405 1C). 3. A system of intersecting fractures and faults above the heat source(s) that allows the circulation of water. 4. A sedimentary unit located above 3 that allows the convection of £uids, i.e. that the pore-pressure is near-hydrostatic from the surface down towards the heatsource along the ‘£anking recharge zones’, and slightly over-pressured along an upwards ‘central re£ux zone’ located above the heatsource(s). 5. An environment that protects the salts from re-dissolving. With these prerequisites satis¢ed, and where the sedimentary pile is su⁄ciently thick (up to several kilometres), it is

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At the surface, by evaporation of supersaturated brines. On the sea£oor, by precipitation from supersaturated brines in shallow pools on the sea£oor of deep con¢ned basins (generally 41km water depth). Within the central re£ux zone, immediately above the buried heatsource(s), by ‘surpercritical out- salting’. In the £anking recharge zones and central re£ux zone of the sedimentary package (at temperatures between 130 and 405 1C). Furthermore, we hypothesize that salts precipitating under supercritical conditions in the subsurface will accumulate in the pore spaces and fractures above the heat source(s) or (depending on accommodation space) move upwards as supersaturated brines or a slurry of salt and brine in a gaseous water phase. If these salts and £uids reach the CP upon entering the sea£oor, the venting will be similar to a ‘black smoker’ discharge, and the deposits will be spread along the axial rift, controlled by the heat source(s) (Goodfellow & Zierenberg, 1999). However, if the migrating £uids reach the CP within the sedimentary sequence that acts as an overburden to the hydrothermal system, boiling may occur (depending on whether it is pressure or temperature that ¢rst reaches the CP) and the salts will precipitate within the sedimentary sequence. Upon further cooling of the hydrothermal system, the CP (i.e. the boiling point) will be displaced downwards, and the condensation water will start to dissolve salt that was previously precipitated, and subsequently transport it upwards until it ¢nally discharges onto the sea£oor or on the terrestrial surface.This is in accordance with the observations of hydrothermal venting made by Butter¢eld et al. (1997).

CONCLUSIONS Laboratory experiments performed in the early 1990s demonstrated that supercritical water behaves like a non-po lar £uid, and that salts dissolved in the water ‘shock crystallized’ upon passage into the supercritical P^Tregion (Tester et al., 1993). Molecular theory con¢rms why water loses its ability to dissolve salts on passage into the supercritical region. We provide the theoretical basis for this fact. One of the natural consequences of this revelation is that brines and seawater that circulate into deepseated hydrothermal systems will become supercritical and that the salts will precipitate, although this has not been directly observed in any natural environment, so far. We predict that ‘shock crystallization’ (or supercritical out- salting) occurs during the intrusion of magma bodies in the oceanic crust and during sill intrusions into deepseated water- saturated sediments. Numerical basin modelling and simulation demonstrates the e¡ects on


Sub-surface precipitation of salts in supercritical seawater temperature-, pressure- and £uid velocity ¢elds of such a small- scale hydrothermal system. By referring to ¢eld observations from two large- scale, deep - seated hydrothermal rift-related systems, the Atlantis II Deep, of the Red Sea, and the Dallol area and Lake Asale, Ethiopia, it is suggested that much of the accumulated salts and brines observed in these systems stems from hydrothermally associated boiling and supercritical out- salting at depth. For this reason, it is concluded that hydrothermally associated salt formation and accumulation is under-rated as a contributor to evaporite depositions, especially in rift-related tectonic settings.

ACKNOWLEDGEMENTS We would like to thank Statoil for giving the necessary support to carry out this work and for permission to publish the results. We would also like to thank B. Charlotte Schreiber and one anonymous reviewer for their constructive and encouraging comments.

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Manuscript received12 August 2005; Manuscript accepted 3 April 2006.
