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Marine and Petroleum Geology xxx (2017) xxx-xxx

Contents lists available at ScienceDirect

Marine and Petroleum Geologyjournal homepage: www.elsevier.com

Large salt accumulations as a consequence of hydrothermal processes associated with‘Wilson cycles’: A review Part 1: Towards a new understandingMartin Hovland a, ∗, Håkon Rueslåttenb, Hans Konrad Johnsen b

a Tech Team Solutions, Stavanger, Norwayb Independent Consultant, Trondheim, Norway

A R T I C L E I N F O

Keywords:Hydrothermal saltWilson cyclesPhysico-chemical reactionsThermodynamicsLarge salt accumulationsSubductionRiftingOcean spreadingHidden salts

A B S T R A C T

The formation of large salt deposits is observed especially in areas with a geological history of high tectonic activ-ity. Over the last decade it has become a well-established fact that heavy brines form and solid salts precipitate,due to the thermodynamic and physico-chemical properties of seawater at high temperatures and pressures en-countered within hydrothermal systems. This article reviews the modern theoretical and experimental researchbehind these findings, and also describes geological settings that most likely cause brine- and salt-forming hy-drothermal processes to occur. This analysis has led to the identification of a set of specific conditions, properties,and processes (referred to as Conceptual elements) that are used to explain the often complex processes of brinebehavior that leads to hydrothermal formation of solid salt.

The main objective of this review is to present hydrothermal conditions known to occur during Wilson cycles:subduction, collision, and rifting, e.g., zones of repeated tectonic unrest, where brines (commonly derived fromseawater) are concentrated into heavy brines and precipitate solid salts. The internal heat of the Earth and itsinteraction with deeply-circulating seawater in hydrothermal systems and also the immense recycling of crustalmaterials, including porous oceanic crust and serpentinite (hydrated) rocks via mantle processes may lead to theformation of salt accumulations. It is also acknowledged that such brines and solid salts may often be storedsub-surface for long time periods, extending from one Wilson cycle to another. Thus, on the basis of this analysis,it is cautiously suggested that large amounts of salts ‘hidden’ inside subduction zones may appear on the surfaceduring subsequent rifting and oceanization phases.

In Part 2 of this review, the Conceptual elements, which are described and discussed herein (Part 1) are appliedto selected cases, including the Andean Mountains, the East African Rift, the Red Sea Rift, and other locations.

1. Introduction

Over the last two centuries large volumes of salts (mostly chloridesand sulphates) have been discovered both on and below Earth's sur-face. (The term “salt” is here used to indicate a seawater-derived ioniccompound in its solid state). The discovery of these large volumes,however, came much earlier than the realization of how the planet'scrust was dynamically and thermally structured. Therefore, the theoriesfor salt accumulations were established prior to the understanding ofplate tectonics. Being unaware of the forces and heat involved in tec

tonic processes, the only conceivable process for producing solid saltswas solar evaporation of seawater (e.g., Alling, 1928).

As years went by, all the work done on the solar evaporation of sea-water and the formation of salts (evaporites) has formed a large andseemingly consistent knowledge base for the explanation and explo-ration of giant salt bodies (Hsü et al., 1973a,b; Warren, 1999, 2006;2010, 2016). Thus, the solar evaporite theory has become so dominantin the last century that it is more-or-less considered ‘forbidden’ to tam-per with it.

Nevertheless, the large salt accumulations in deep marine basinsare difficult to explain with a surface seawater evaporation model(Scribano et al., 2017, and references therein; Christeleit et al., 2015;

∗ Corresponding author.Email address: [email protected] (M. Hovland)

https://doi.org/10.1016/j.marpetgeo.2017.12.029Received 30 September 2017; Received in revised form 17 December 2017; Accepted 21 December 2017Available online xxx0264-8172/ © 2017.

Review article

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M. Hovland et al. Marine and Petroleum Geology xxx (2017) xxx-xxx

Lugli et al., 2015). The internal heat of the Earth and its interactionwith deeply-circulating seawater in hydrothermal systems, and the im-mense recycling of crustal materials via mantle processes (Wilson cy-cles with subduction, rifting, and collision; e.g., Liou et al., 2014) have,seemingly, been neglected as a potential explanatory model for theformation of large salt accumulations, either above or below ground(Sozansky, 1973; Momenzadeh, 1990; Hovland et al., 2006a,b, c; 2014,2015, 2016; Scribano et al., 2017; Hardie, 1990).

It has long been recognized that salts (evaporites) play economicallyimportant roles in petroleum prospecting and extraction. Thus, accord-ing to Warren (2006, 2010) and Mohriak et al. (2012), they play a ma-jor role in i) facilitating hydrocarbon migration; ii) providing tectonicand sealing control for hydrocarbon trap formation, and, thus, iii) playa role in hydrocarbon reservoir distribution. However, from this currentreview, it becomes evident that the roles of salt formation and accumu-lation may deserve more attention in global tectonic (crustal) research,as salts may produce areas of weakness in and around suture zones thatare later prone to rifting. This is evidenced by the presence of salts inearly rifts, also before the sea has invaded the rift, e.g., the East AfricanRift.

1.1. Objectives and organization

The objectives of this review article are to present hydrothermal con-ditions known to occur during Wilson cycles, where salts may concen-trate into heavy brines and solid salts, starting from seawater. We setforth to identify various stages of the Wilson cycles where such condi-tions are likely to be present. Hydrodynamics, thermodynamics and cer-tain properties of brines, rocks and salts, lead to predictive processesand results. A number of ‘Conceptual elements’ are used to explain the of-ten complex processes of brine behavior and the formation of hydrother-mal solid salts (Table 1).

These ‘Conceptual elements’ are applied in the following chapters,including Part 2 of the article, to explain the new model for hydrother-mal salt formation. In addition, a number of geological cases are dis-cussed in order to substantiate and visualize the model (mainly found inPart 2: ‘Application on selected cases’).

This review is divided into the current Part 1: 'Towards a new under-standing'; and Part 2: 'Application on selected cases'. Part 1 thus, concen-trates on the theoretical basis for the application of the ‘Conceptual ele-ments’ in Table 1 on the selected geological examples, mainly found inPart 2.

Table 1‘Conceptual elements’ applied to explain hydrothermal salt formation. The items refer tothe thermodynamics of physical and chemical reactions, including items that characterizeobservable phenomena.

Conceptual elements

‘Thermo- and fluid-dynamics’ (When seawater encounters heat at high pressures inthe Earth's crust, phase separation may occur; e.g. the formation of heavy brines,solid salt and low-saline vapor in the supercritical and boiling domains of seawaterand derived brines).‘Solubility and precipitation of salts’ (Salts may precipitate over large temperatureintervals due to their wide solubility range at elevated temperatures and pressures).‘Fresh water formation’ (Condensation of vapor in hydrothermal systems producesfresh (‘distilled’) water that can re-dissolve solid salt along its ascending path).‘Formation of leaky salt stocks (domes)’ (Solid salt is permeable at depths,allowing brine to flow through. Solid salt may precipitate upon cooling during theascent, and thus, build the salt stock continuously higher).‘Refining of salts’ (Marine salts have different solubilities and the more soluble saltsachieve higher concentrations than others, before precipitating. Some salts havecrystal water attached to their crystal structure (e.g. tachyhydrite). These salts maymelt in their own crystal water upon heating and leave the solid salt as flowingbrines).‘Salt Preservation’ (Solid salts may be preserved by dry climate, dense brines, orsheaths of anhydrite, gypsum, carbonates, or fine grained clastic or biogenicsediments).‘Hidden salts’ (Salts and brines may form at certain thermodynamic conditionsalbeit not always observable; e.g., in subduction zones, within- or beneath oceanic-and continental crust and within deep seated slab-remnants. This salt may becomevisible at later stages in Wilson cycles).

1.2. The origin of sea salts

Salt accumulations formed by hydrothermal activity, may have twosources of origin; (a) precipitated salt from seawater; and (b) salts de-rived directly from crust and mantle masses. Subgroup (b) may also in-clude previously formed and hidden salts, either in the form of solids, orbrines. Solids and brines encountered by the hydrothermal systems maybe refined relative to seawater composition.

Serpentinite will contain both types of salts. The contribution fromthe two sources is difficult to distinguish from each other, but Braitsch(1971), who performed a thorough study of salt compositions in the gi-ant Zechstein salt accumulations in Europe, concludes that the salt ac-cumulations actually do not have the same composition as that of theoceans. Thus, he found that the salt accumulations could not originatedirectly from the simple evaporation of seawater.

A study aiming at determining the size of different sources of chlo-rine (Graedel and Keene, 1996) concluded that 99.6% of the Earth'schlorine is to be found in the mantle, while 0.3% is found in the crustand only a mere 0,1% resides in the oceans. However, a more modernassessment, by Kendrick et al. (2017), suggests that the budget is muchmore complicated.

According to Wallace and Anderson (2000), the “Yearly amounts ofreactive volatiles H2O and Cl that are subducted are in the range of whatis returned surfaceward by arc magmatism. If there were no surfacewardreturn of subducted H2O and Cl, then the formation, alteration, and sub-duction of oceanic crust would comprise a net drain on the water andsalt in the oceans. The geologic record and understanding of tectonismreveal that continental crust and ocean water have existed for most ofEarth's history and subduction has not caused the oceans to dry up. Thuswater lost to altered oceanic crust is probably largely returned to theoceans via dewatering of subducted crust and subduction-related mag-matism.” This clearly shows that tectonic cycling of elements is activeon Earth, and indicates how important it is to understand the full im-plications of Wilson cycles that cause seawater to interact and exchangeelements with hot, pristine mantle.

1.3. The current evaporite theory

The conventional (solar) ‘evaporite theory’ is, generally, resting onthree main pillars: a) dry climate, b) precipitation in shallow water, andc) a relatively continuous supply of seawater that renews the content ofsalt in the evaporation basin (without diluting the brine more than therate of evaporation). To explain the observed salt thicknesses of severalkilometers often found at great water depths, the solar evaporation the-ory alone does not provide a sound explanation (Selley, 2005; Scribanoet al., 2017). The discussion about the possibility of accumulating suchgiant deposits of ‘evaporites’ with solar evaporation alone, really madethe headlines subsequent to the DSDP (Deep Sea Drilling Project), Leg13 drilling in the western part of the Mediterranean. How was it possi-ble to explain the formation of thick ‘evaporite’ deposits at over 3kmwater depth?

The most cited papers from this scientific drilling were those by Ken-neth Hsü et al. (1973a, 1977), and Ryan et al. (1973), about the historyof the Mediterranean Salinity Crisis. They also provide a rare glimpse ofthe bitter controversy that arose during the interpretation and publica-tion of the drilling results: “The significance of the DSDP discovery was,however, shadowed by controversies. That the Mediterranean evapor-ite is Messinian (Late Miocene) in age seemed to be just about the onlyconsensus after the Leg 13 drilling. Even the shipboard scientific staffcould not reach an agreement on the genesis of this unusual formation…” “The desiccated deep-basin model (e.g., Ch. 43, of the Leg 13 cruisereport) was authored by Hsü, Cita, and Ryan because they were the onlymembers of the shipboard staff totally convinced on its plausibility.”(Hsü et al., 1977).

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For salt thicknesses of some tens of metres, the ‘evaporite’ explana-tion may be realistic. But, according to Warren (2006): “… the scale oflacustrine evaporite development, past and present, pales in comparisonto some ancient marine-fed saline systems. There are no modern coun-terparts to such ancient marine-associated systems.” This is because, ac-cording to Warren (2006, 2010), there are no places on Earth whereseawater is evaporating at a high enough rate to satisfy the climaticand shallow-water requirements of the conventional ‘evaporite’ theory:“Our inability to compare all aspects of evaporite deposition, past andpresent, reflects a present-day lack of both longterm shallow epeiric seasand of large drawdown basins fed by seawater seeps.” (Warren, 2006).

These disagreements and difficulties to give proper explanations tothe processes of evaporite formations show all too clearly that a revi-sion of the traditional ‘evaporite’ theory is required. For example, theterm lacustrine evaporites, used above, easily brings ambiguities into thediscussion. This is because the evaporation of fresh water (from rivers,etc.) gives negligible amounts of salts upon evaporation, but hydrother-mal meteoric water may give significant amounts of salts upon evapora-tion like we see in the East African Rift. In most cases it seems this termis applied to evaporation of hydrothermal brines which reaches the sur-face; for example, in Danakil (Afar), Ethiopia, where dense brines em-anate onto the surface, dry out and become preserved in the dry and hotclimate (Talbot, 2007).

2. Formation of hydrothermal salts

2.1. Salts forming in hydrothermal systems

Besides plate tectonics, two of the last century's perhaps most impor-tant discoveries within marine geology, were indeed related to salt andhydrothermal processes: 1) The discovery of hot brines in the AtlantisII Deep of the Red Sea (e.g., Charnock, 1964); and 2) The discovery atthe East Pacific Rise of hot vents, so-called ‘black smokers’ (Lonsdale,1977). Both discoveries are considered ‘game-changers’ in the quest forunderstanding seawater circulation in the oceanic crust. It is now wellunderstood that the driving force for this circulation is heat from the un-derlying mantle; so-called ‘forced convection’, with auto-circulation ofseawater when the temperature gradient between a hot (>1100 °C) ris-ing pluton (igneous intrusion) and the ocean floor (at typically 2–4°C)is adequately steep. Hydrothermal circulation of seawater in the Earth'scrust at temperatures above 400°C and >3km below sea surface, willinevitably lead to the formation of concentrated brines and the precipi-tation of solid salt. It is suggested that hydrothermal processes may ex-plain both the location and the amount of salt deposits in a better waythan solar evaporation. Even so, this does not prevent solar evaporationfrom being an active contributor to brine densification and precipitationof solid salt in many cases (Schreiber et al., 2007).

2.2. Energy considerations

Attempts to explain observed evaporites based on mantle heat act-ing on seawater in a static setting, will not produce the accumulationsobserved in nature. The concept of producing evaporites from seawa-ter based on the heat from stationary magma/mantle bodies is not astraightforward evaporation process. This is demonstrated by a simplecalculation: A 1km cube of hot mantle contains approximately the heatnecessary to evaporate a 1km cube of seawater, which leads to a 1kmsquare of salt of ∼15m thickness; i.e., far from being comparable to thevast amounts of salt observed in some geological settings. However, thepremises for this calculation are wrong. A more correct calculation musttake into account several other factors than the heat-transfer betweentwo static objects; e.g., water-rock interactions.

When seawater encounters hot mantle, the water-rock interactionsare causing dissolution and hydrolysis of minerals in the rock, resultingin uptake of a significant amount of water in the form of OH-groups innew formed minerals. Simultaneously, the brine becomes denser due toits uptake of salts and other dissolved elements from the mantle rocks.These reactions are exothermic in nature, and extra heat is producedduring the reactions, especially if serpentinization is involved, whichis often the case. In addition, during serpentinization, hydrogen is alsoproduced, which represents a very strong reduction agent in chemicalreactions involving iron-bearing minerals.

These reactions form brines that are much denser than the ini-tial brine (seawater), without consuming large amounts of heat fromthe surroundings. Also, in contrast to the static example above, thehydrothermal processes are extremely dynamic. The initially formedbrines may indeed be subjected to heat from other parts of the system asthey move around, or by dynamic hot magma bodies (volcanism). Thisis likely to happen in both rifting and subduction systems, and providesthe necessary energy to produce accumulations of salts.

If we accept the concept that salts and brines may survive over ge-ological time spans in the mantle/deep crust, they may also experienceenergy supply from several stages of the Wilson cycle before appear-ing as observable salt accumulations. The Earth has undergone severalperiods during which supercontinents have assembled and rifted apartagain. Therefore, the supply of energy and the availability of seawaterduring billions of years in the Earth's history make deep salt and brineproduction unavoidable. In short, the salt accumulations we observe to-day may be the result of processes that date much further back in timethan the latest stage in the Wilson cycle of that area.

Bearing in mind all the Wilson cycles encountered during the Earth'shistory the question whether the mantle has provided the necessaryamounts of energy to produce the observed salt is not an issue. It shouldalso be noted that Taylor and McLennan (1985) estimated that in total2% of the continental crust consists of salts (in sedimentary and pale-osedimentary units). This is an enormous amount, which is undergoingtransitions during rifting, subduction, and continental collisions (e.g.,during Wilson cycles).

2.3. Serpentinization of mantle rocks

Over the last ~40 years, there has been a revolution in our under-standing of how the oceanic lithosphere has evolved through interac-tion with seawater. Along slow-spreading ridges there are areas whereseismic profiling shows more than 15km thick ‘amagmatic’ zones repre-senting relatively cool lithosphere, where seawater has reacted with theupper mantle to form serpentinite rocks (Pearce, 2002; Dick et al., 2003;Mével, 2003; Snow and Edmonds, 2007; Ildefonse et al., 2007; Mirandaand Dilek, 2010 ). This zone overlies almost unaltered ultramafic mantlerocks. On the seafloor above such areas the serpentinization processesare recognized by the occurrences of hydrothermal venting, both hot(∼360°C) black smokers and low-temperature (40–90°C) white smokers.The black color of black smokers is due to the emittance of abundantsulfide particles, while the ‘white smokers’ are characterized by havinghigh pH-values (9–10), abundant CO2 and methane contents (e.g. Mével,2003), and are precipitating carbonate and magnesium hydroxide min-erals onto the seafloor. The water that comes up in the hot springs iscommonly found to have significantly lower salinity than seawater.

It is well known that the upper mantle consists of dry magne-sium-rich silicate rocks (‘peridotite’), containing abundant iron-bear-ing olivine ((Mg,Fe) 2SiO4). These minerals always coexist with orthopy-roxene (MgFeSiO3)±Ca-rich clinopyroxene±Al-Cr spinel and ± Ni-Fesulfides. When seawater encounters peridotite through deep cracks inthe hot oceanic crust, spontaneous water-rock reactions start. These re-actions are referred to as serpentinization, because peridotite is trans

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formed into serpentinite by pervasive hydrolysis reactions (Scribano etal., 2017).

Experimental reactions show that olivine is preferentially dissolvedat temperatures around 250–300°C, while orthopyroxene is rapidly dis-solved at temperatures above 400°C (Martin and Fyfe, 1970). Sev-eral accessory minerals may also form in the serpentinization process,such as brucite or talc±magnetite±FeNi sulfides±FeNi alloys±chlo-rite± tremolite, depending on the physico-chemical conditions of thesystem (Früh-Green et al., 2004). In particular, the formation of NiFe al-loys (taenite and awaruite) demand strongly reduced conditions. Theseconditions are best achieved where serpentinization is at a non-equi-librium stage; i.e. at the reaction front where olivine is present. Onceolivine is completely altered, the oxygen fugacity associated with therock transformation will increase markedly .

The formation of NiFe-alloys in addition to the formation of hydro-gen, demonstrates that the serpentinization process also includes reduc-tion processes that are balanced out by the oxidation of iron (Fe2+ →Fe3+) to form magnetite and also serpentinite containing some ferric iron.A general formula, including both olivine and pyroxene with their con-tent of magnesium and iron, is suggested by Evans (2004). This equa-tion also illustrates the redox reactions in the serpentinization process.The valences of iron in the minerals are indicated by (II) for ferrous and(III) for ferric (oxidized) iron in the equation:1.2 Mg 1.8Fe(II)

0.2SiO4 + 0.76 Mg 0.9Fe(II)0.1SiO3 + 2.088

H2O→Mg2.85Fe(II)0.11Fe(III)

0.08Si1.96O5(OH)4 + 0.042Fe(II)O*Fe(III)

2O3 + 0.176 H 2

Olivine + Pyroxene + Water → Serpentine + Magnetite + Hydrogen(+Heat)

The reaction is strongly exothermic, causing the serpentinizationprocess to increase its longevity due to its own heat production.

2.3.1. Serpentinization and saltThe average density of a fresh peridotite is approximately 3.3g/cm3,

while serpentine minerals have much lower density (∼2.5g/cm3), due tothe uptake of OH-groups in the new-formed crystal lattices. This causesup to 40% swelling of the rock at complete alteration. Consequently,any brine (seawater at sub-critical conditions) will increase its salinity inthe reaction zone. However, some substitution of chloride for hydroxylis found in the new-formed minerals (e.g. iowaite, brucite, and serpen-tine) in addition to other sea-salt elements incorporated in the crystallattices of the serpentinite minerals (Sharp and Barnes, 2004; Manuellaet al., 2016; Vogel et al., 2014).

According to Sharp and Barnes (2004) a significant amount of Mg2+

and Na+ water-soluble chlorides are hosted in fresh serpentinite. Byleaching of serpentinites with distilled water they extracted an amountof salts corresponding to more than 0.5wt% Cl. They suggest thatCl-bearing minerals are incorporated in the serpentine both as inter-stitial water soluble solid salts (e.g. halite and bischofite) and alsoas insoluble salt components incorporated in the crystal structure offine-grained serpentinite (Sharp and Barnes, 2004). Because Cl consti-tutes 55% by weight of sea salt, 0.5wt% of Cl in the serpentinite indi-cates a total salt content of about 0.9wt%.

Fluid flux rates for water that enters into serpentinization reactionzones in the Earth's crust are calculated to be in the range of 60–600molm−2 a−1 . This can be translated into water volumes of up to 10,000m3

per km−2 a−1. If it is assumed that this is mainly seawater, a corre-sponding amount of 300 tons per km−2 a−1 of sea salt is enriched inthese reaction zones, either as heavy brines or as precipitated solidsalt (mainly magnesium and sodium chlorides), as shown by Sharp andBarnes (2004).

According to Scribano and Viccaro (2014), the theoretical amountof salt left or “produced” by changing one m3 peridotite to serpentinite,

is 10.5kg: “Since the serpentinization process requires pure H2O, theinvolved seawater-derived serpentinization fluid will undergo totalout-salting.” However, the difficulty is to document this, in nature.

A common feature of serpentinite rocks are their patterns of perva-sive fracturing (Malvoisin et al., 2017). The swelling of the rock exertsstrong mechanical forces on the rocks surrounding the reaction zones,causing fracturing on several scales. Even fractures on the micrometrescale will immediately be filled with brine due to the high capillaryforces involved. These fine fractures provide large interaction surfacesbetween the brine and the rock and chemical reactions will cause fastconsumption of the pore water. Thus, the salinity of the brine may reachhigh saturations and eventually also precipitate solid salts (Sharp andBarnes, 2004).

The mechanisms for the migration of ‘hidden salt’ from serpentiniza-tion zones were discussed by Scribano et al. (2017). Two mechanismswere suggested; (a) the hydrothermal salt plume; and (b) the buoyantdiapir. The first model depends heavily on a continued flow of hot hy-drothermal water from the serpentinization zone that has sufficient en-ergy to move such heavy brines and even slurries of solid salt parti-cles in heavy brines. Dewatering of serpentinite at higher temperatures(>400° C) may constitute sufficient energy to move such salt plumes.

The second model refers to the formation of salt stocks by hot brinesmigrating in the middle of the salt body; provided that the salt stockis situated within the ‘Holness zone’, where salt is permeable (see Ch.2.4.7 in the current paper). This assumes subcritical conditions and flowof hot brines that reach saturation upon cooling at the upper part ofthe salt body, where solid salts are precipitating according to their spe-cific solubility at each particular temperature and pressure interval. Thismodel thus, includes a refining process of the salt, where e.g. halite pre-cipitates long before calcium and magnesium chlorides, upon cooling.

Sharp and Barnes (2004) conclude that subduction of serpentinitemay be the most important contributor to bring Cl down to mantledepths, thereby being a main contributor to the global cycle of Cl. Ahigh-salinity fluid plume is suggested to evolve in the subduction zoneduring serpentinite dehydration of a subducting slab that will affect andintensify water-rock interactions and melting properties above the slab.

2.4. The hydrothermal salt model

This chapter provides some more background information pertinentto further explain the ‘Conceptual elements’ in Table 1.

2.4.1. Model descriptionBased on previous work in rifted salt basins, mainly the Red Sea

(Hovland et al., 2015), a total of five relevant steps or phases have beenidentified, whereby salt is naturally processed in one way or another toeventually end up as salt accumulations. Different types of salts are in-cluded in various structures and accumulations, including 'diapirs' (saltstocks) and layered deposits.

These steps are:

1) Seawater encounters hot rocks at moderate depths in the fault zones.Salt precipitation takes place due to boiling. Upon cooling of this sys-tem, salts are dissolved and most of them are returned to the sea.

2) As fracture volume expands and hydrothermal brines move furtherdown towards the hot igneous body, high-pressure ‘supercritical’phase separation occurs, resulting in salt precipitation and the for-mation of low density (0.3g/cc at the critical point) water vapor andheavy brines that migrate further down and concentrate beyond sat-uration.

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3) The circulating fluids cool the system further, and the upwards mi-grating low salinity vapor is condensing, and starts to dissolve pre-viously deposited salts. The different solubilities of sea salts lead toa refining of the salt types, whereby the most soluble ones are pre-dominantly dissolved and displaced first. When these brines migrateupwards due to fluid and/or tectonic pressures, they are cooled andsome salts are re-precipitated before the brine reaches the seafloor.

4) Upon reaching the seafloor, the brines are cooled further, and willeasily become supersaturated with respect to some of the salt types.The remaing brine creates ponds of heavy brines along the seafloorwhen topography and sea currents allow. Salts will then precipitatefrom these heavy brines on the seafloor (ponding/mini-basin forma-tion) and form stratified deposits. This process includes another re-fining of the salts.

5) In the continued subsurface processes, brines will migrate throughsalt deposits as they build up. This brine migration also includesa) dissolution and re-precipitation, b) pressure build-up due tosalt-clogging and release, and c) extrusion of salts/salt slurries andbrines (e.g., the formation of salt glaciers, salt walls, salt injectites,and ‘diapirs’).

These steps are not to be understood as completely isolated or se-quential, as several of the steps may occur simultaneously, although onestep may at a certain time be more dominant than the other. Further-more, it must be noted that the accumulation of salt in specific regions,is fully dependent on the local conditions of ‘salt preservation’, as wediscuss in Ch 2.4.12. It should therefore not come as a surprise that al-though large quantities of salt are produced in a particular subsurface ornear-surface area, the salt may not accumulate due to poor local preser-vation conditions.

2.4.2. Solubility and precipitation in P/T-spaceThe common sea salts are polar ionic compounds. When precipitated

as solid salts the ions are mainly bound together by electrostatic forces.When these salts are immersed in water, which has a very large dielec-tric constant, the water molecules are weakening the electrostatic bond-ing. This is causing the ions, both cations and anions, to lose contactwith the solid salt and brought into solution. For this reason, the dielec-tric constant of water has been regarded as a proxy for the solubility ofpolar ionic salts, while non-polar compounds (like oil) are less solublein water.

However, while most of the common sea salts increase their solubil-ity at higher temperature (with the exception of calcium sulfate, anhy-drite), the dielectric constant of water decreases with increasing temper-ature; e.g., from 88 units at 0°C and down to 73 units at 100°C (Pan etal., 2013). Furthermore, the dielectric constant of water is reduced rightdown to only 2 units, at the critical point (CP) (Hayashi and Hakuta,2010) and, thus, water is losing most of its solubility for the commonsea salts at the transition to the superctitical domain (Liebscher andHeinrich, 2007).

Each mineral has its own thermodynamic behavior according to theavailability of water, and according to temperature, and pressure vari-ations. This is normally described by a phase diagram for each type ofsalt. But, because the sea salts interact with each other in solution, thephase diagrams will be dependent on the total ionic composition, mak-ing the phase diagrams even more complex than shown in Fig. 1, whichis only valid for one type of salt, NaCl (halite).

The reverse process, - precipitation of salts, is a consequence oftheir solubility (Table 2). Upwards directed brine flow inside salt struc-tures effectively maintains the salt concentration at the saturation levelas it flows and cools at the same time. This leads to precipitation ofsolid salts all along the flow path; e.g., saturated NaCl brine at ∼100 °Cand ambient pressure dissolves up to 390g of NaCl per liter of water.

Fig. 1. Phase diagram for the system H2O-NaCl showing the distribution of the three components liquid solution (L), vapor (V) and solid halite (H) in the P/T-domain. It illustrates wherethe various components (phases: L, V and H) occur at different pressures and temperatures typically encountered in hydrothermal systems associated both with rifting and subduction.(Based on Driesner and Heinrich, 2007). The blue-colored points, A, B, and C, refer to Example 1 in the text. (For interpretation of the references to color in this figure legend, the readeris referred to the Web version of this article.)

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Table 2Solubility at 20 and 100 °C for some pertinent salt minerals (CRC Handbook of Chemistryand Physics: Lide, 1990).

SaltsSolubility at 20 °C(g/100g)

Solubility at 100 °C(g/100g)

NaCl (halite) 35.7 39.1Na2SO4*10H2O(mirabilite)

11.0 92.7

KCl (sylvite) 34.7 56.7KMgCl3*6H2O(carnallite)

64.5 decomposes

K2SO4*MgSO4*6H2O(schoenite)

19.2 59.8

MgSO4*7H2O (epsomite) 71 912K2CO3*3H2O 129.4 268.3MgCl2*6H2O (bischofite) 167 367CaCl2*6H2O 279 536CaSO4 (anhydrite) 0.21 0.16

Upon cooling the brine to 20 °C, solid salt will precipitate, as the solubil-ity is reduced to 357g per liter (Table 2). Thus, salt structures, like saltstocks (domes) or salt walls, may build up.

Gypsum and anhydrite have retrograde solubilities, i.e., their solubil-ities decrease with increasing temperature (Table 2). Furthermore, an-hydrite has a lower solubility than gypsum, especially at higher temper-atures, and will precipitate from seawater at temperatures above 130°C.

It is also pertinent to note that gypsum, which is essentially the samemineral as anhydrite, but with some additional crystal water, is rarelyobserved in salt accumulations. Gypsum is the low-temperature mineralof the two, and should be abundant in salts formed at low tempera-ture, provided it does not lose its crystal water rapidly after deposition(Braitsch, 1971).

Halite is actually the least soluble chloride and does not have theability to bind crystal water. Chlorides of magnesium, calcium andpotassium, as well as more exotic salts, such as tachyhydrite, have theability to bind crystal water and, therefore, tend to stay in solution longafter halite saturation is reached. Thus, in hot water-deprived systems,brines of magnesium and calcium may escape the system as very hotliquids. Upon cooling, the water turns into crystal water and the saltsbecome solid. This is a viable explanation for the formation of tachyhy-drite, a salt so hygroscopic that it does not survive as a solid when ex-posed to normal air humidity. Thus, the source for tachyhydrite wouldmost probably be from a mixed salt accumulation that is occasionallyexposed to more heat, which enables intergranular magnesium and cal-cium chlorides in halite to liquefy in their crystal water (e.g., at temper-atures above 120 °C) and escape out of the salt masses in hot liquid form.To be preserved and solidify upon cooling, these brines must be some-how protected by equally saturated brines, a situation to be discussedlater.

2.4.3. Salt-formation by boiling – ‘wet evaporation’A prerequisite for the formation of solid salt and observable salt

accumulations associated with hydrothermal systems is the supply ofseawater. At ocean depths of less than 2800m (i.e., pressures below300bars), seawater will not reach its CP. However, at sufficiently hightemperatures, seawater will boil, even when confined in sediments orin fractures of the crust. Although it is well known that the boiling ofseawater at surface may produce large quantities of salts (e.g., Talbot,2007), it is not readily clear what happens when seawater boils underwater.

This was, however, demonstrated by a boiling experiment in a7m×7m and 3m deep pool filled with seawater, where the boiling sys-tem included an open 200L steel container with a 5kW heating ele-ment buried in gravel. The boiling was controlled by three thermome-ters and continued for 11 days. An artificial ‘salt stock’ consisting of

anhydrite and finely disseminated halite developed around the heatingelements. The ‘salt stock’ made its own accommodation space by push-ing away the gravel (Hovland et al., 2006a). The experiment demon-strates that anhydrite and halite precipitates even in wet environments,where conditions are favorable, e.g., close to strong natural heat-sourceslocated where seawater can freely circulate (‘wet evaporation’).

2.4.4. The discovery of high-pressure phase separationWhen seawater is heated and pressurized to near the CP a ‘two-phase

mode’ occurs, i.e., most of the volume becomes a vapor (about 95vol%)with a density of 0.3g/cm3, and the remaining liquid becomes a densebrine as shown in the phase diagrams (Figs. 1 and 2) to be discussed inthe next chapter.

The first to study phase separation of seawater at high pressuresand temperatures, e.g., at supercritical conditions (T=∼407°C,p=∼300bar), were Bischoff and Rosenbauer (1989). However, becausethey used pressure cells without visual access, they were unable to vi-sually observe the phase transitions. Such experiments were later car-ried out by Tester et al. in the early 1990's: “When a temperature ofaround 405°C was attained, a ‘cloud’ formed in the chamber and wewitnessed a sudden transition from one-phase flow to a two-phase flow,as solid salt ‘shock-crystallized’ when the solubility of NaCl dropped tonear-zero over a temperature range of only a few degrees C at the crit-ical point of brine.” (J. Tester, pers. com., 2002). The resulting NaCland Na2SO4 solids inside the reaction chamber were found to consistof 10–100μm ‘highly amorphous kernel-shaped particles’ (Tester et al.,1993).

The reason why salts are practically insoluble in supercritical va-por is the extreme lowering of the dielectric constant, from ∼80 to ∼2units. This causes a reduction of the ionic dissociation constant for thedissolved salts to such a degree that the salts precipitate. Other impor

Fig. 2. Images showing the salt formation, from Simulation case #6 of Coumou et al.(2009). The figure shows the results from a simulation where sea water meets hot mantle,after a period of 4000 years. Color coding reflects salinity. The grey areas indicate wheresolid salts are formed and fresh vapor leaves the system. Depth is indicated in metres be-low sea level. a) Shows the temperature distribution. b) Shows the established water flowlines. (For interpretation of the references to color in this figure legend, the reader is re-ferred to the Web version of this article.)

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tant properties of the supercritical vapor are its low viscosity and highdiffusivity (Tester et al., 1993; Bellissent-Funel, 2001), which facilitatesfast diffusion into porous rocks (‘stockwork’ veins/fissures) and sedi-ments.

Butterfield (2000) shows a scanning electron microscopy (SEM) im-age of halite-coated basalt. His interpretation of this image is that asub-seafloor volcanic eruption (in deep water) “… caused immediatephase separation and halite precipitation, followed by a high-temper-ature reaction period.” Furthermore, he concludes that: “A brine pro-duced by condensation from seawater-derived fluid at 450°C wouldhave a salinity five times greater than that of seawater. Brines producedby exsolution of an aqueous phase from a magma at temperatures in ex-cess of 700°C have salinities of more than 10 times that of seawater.”(Butterfield, 2000).

This realization, lead Lecumberri-Sanchez et al. (2015) to concludetheir world-wide study of rising upper crustal plutons with the state-ment: “Salt precipitation appears to be a ubiquitous feature in hydrousvolcanic and geothermal environments, invoking dynamic behavior dueto rapid multiphase reactions involving liquids, vapor, and solids.”

The implications of these observations are that the interaction be-tween seawater and hot igneous bodies at sufficiently large water depth(>300bar pressure) unequivocally produces concentrated brines andlocal precipitation of halite and other salts in pores and fractures(Hovland et al., 2006a, 2016, b, c; Gruen et al., 2014; Scott et al., 2017;Scribano et al., 2017). It is this fundamental revelation that forms thebasis for the current hydrothermal salt theory, where the fact that sea-water normally phase separates somewhere within every conceivabletype of hydrothermal circulation cell, due to high temperatures, oftencombined with high pressures, plays the central role.

2.4.5. Application of thermodynamicsHydrothermal systems occur in highly fractured, permeable and

porous rocks that typically occur above strong heat-sources, such ashot oceanic crust, rising igneous bodies (‘magma chambers’ or plutons).Water trapped in the fissures, fractures, and cracks will be forced by thesteep temperature gradient to convect, often vigorously. Such locationson the deep ocean floor are also described as: ‘zones of high heat-flow’.

If brines are present in hot fractured fault zones, thermo- andfluid-dynamics will control the brine behavior. As it most often leads todynamic fluid flow (forced convection), interaction with side wall rocks,and precipitation of salts, will occur, as documented by numerical mod-elling (Hovland et al., 2006a; Driesner and Heinrich, 2007; Coumou etal., 2009; Gruen et al., 2014; Lecumberri-Sanchez et al., 2015; Scott etal., 2017).

Scott et al. (2017) studied the exploitation of geothermal energy onIceland, and state: “Numerical simulation of subaerial, magma-driven,saline hydrothermal systems reveals that fluid phase separation nearthe intrusion is a first order control on the dynamics and efficiency ofheat and mass transfer. Above shallow intrusions emplaced at <2.5kmdepth, phase separation through boiling of saline liquid leads to ac-cumulation of low-mobility hypersaline brines and halite precipitation,thereby reducing the efficiency of heat and mass transfer. Above deeperintrusions (>4km), where fluid pressure is>30 Mpa, phase separationoccurs by condensation of hypersaline brine from a saline intermedi-ate-density fluid. The fraction of brine remains small, and advective, va-por-dominated mass and heat fluxes are maximized. We thus hypothe-size that, in contrast to pure water systems, for which shallow intrusionsmake better targets for supercritical resource exploitation, the optimaltargets in saline systems are located above deeper intrusions.” This isfully in line with the hydrothermal salt theory applied here.

To understand the transition and transformation of NaCl-solutionsin high-temperature, high-pressure hydrothermal systems, it is neces

sary to use phase diagrams (Fig. 1). With such diagrams it is possibleto state where and when (in the T/P space) salt will become saturatedand precipitate (to solid halite) and when vapor (low salinity to fresh‘steam’) will form.

The phase diagram in Fig. 1 shows the regions in P/T-space wherethe various phases occur, including the concentration of salt (% NaCl).In the figure, occurrence of the liquid phase is marked with (L), the va-por phase (V), and the solid state, e.g., halite with (H).

As simulated numerically by Coumou et al. (2009) (Fig. 2), salinewater may experience a series of phase separation events during itsmovement downwards through the fractured rock. Consequently, thebrine is subject to phase separation, with the partitioning of low-salin-ity vapors and dense (high salinity) brines. Physical movement of thebrine into domains of slightly higher pressures and temperatures leadsto the next stage, resulting in increased brine density. Retaining thebrine in the fracture system depends on overcoming the lifting capacityof the escaping vapors. Hence, pressure (hydraulic head) plays a role inregulating vapor volume and, therefore, hydrodynamic lifting capacity.Coumou et al. (2009) state that both simulations and evidence from na-ture suggests that brines with higher salinities than ∼12% will tend tosink in and not be lifted out of the system, provided that phase sepa-ration takes place deep enough, e.g. below 3,5km in their simulations.The salts and brines formed at such conditions may, however, be ‘minedout’ by later influx of colder waters, or by mechanical (tectonic) forcesacting on the rocks. Such waters might be the result of condensing va-pors from deeper phase separation processes.

To explain the phase behavior (Fig. 1), three simplified examples areincluded below. Only Example 1 with lines in blue, and points A, B, andC,- is illustrated in Fig. 1.

2.4.6. Example 1In Fig. 1, a brine of 10% NaCl concentration encounters a pressure of

350bars and a temperature of 450°C (point A). Here, the brine will sep-arate into a vapor containing traces of salt (B) and a denser brine having40% of salt (C). In a rift system, given the right hydro-dynamical condi-tions, this brine may now migrate downwards (due to its higher density)and encounter even more extreme conditions leading to another stage inthe phase separation. This may eventually lead to solid salt precipitationand accumulation. In real systems, the fluid flow is tortuous and convo-luted, depending on local conditions within the rock fracture network.The thermodynamically (PT)-determined alterations and phase separa-tion stages in real systems are therefore gradual and infinitive.

If conditions take the fluids outside, or above the critical conditionsfor any salinity, represented by the “critical curve” in Fig. 1, only freshsupercritical vapor and solid salt may exist.

2.4.7. Example 2If subsurface brines are exposed to a pressure drop due to, for ex-

ample rifting, the conditions may move below the V + L + H surface ofthe phase diagram. This will lead to boiling and instantaneous solid saltprecipitation.

2.4.8. Example 3In a subduction setting, the slab with its contained seawater is ex-

posed to gradually higher pressures while maintaining a relatively low(inside) temperature compared to its surroundings (thermal inertia).However, - since the fluids will experience compaction of the slab, theymay also be squeezed out and meet the conditions in the warmer hang-ing wall above the slab. This may move brines with certain salinitiesinto warmer regions where they separate into vapor and even denserbrines.

Coumou et al. (2009) provide ample evidence for deep salt forma-tion in the Earth's crust/mantle during stages of the Wilson cycles wheresaline waters meets a heat source. See Fig. 2 below.

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2.4.9. Fresh water formation (vapor condensation)It should come as no surprise that when brines or seawater are in-

creasingly concentrated in salinity, some fresh water must leave the sys-tem and may interact with adjacent salts or rocks on its way. Even if thewater leaves the hydrothermal system as vapor with little or no salt-car-rying capacity, it eventually cools and condenses to pure (distilled) wa-ter and may start dissolving (‘picking up’) salts again. Wherever de-posited salts are exposed to this flow of fresh water, they may be re-dis-solved along the flow path of this ‘reflux’ water. This may lead to theformation of brines that move on and even deposit salts further up inthe stratigraphy. In the hydrothermal salt model, the flow of fresh wa-ter is not located arbitrarily in relation to the circulation cell, and veryoften, it is located immediately above the centre of the hottest portions.Numerical simulations also confirm this (e.g., Gruen et al., 2014).

Depending on the relative amount of water and salt, lean or richbrines will be the result. At the seabed, salt ‘glaciers’, as seen in theRed Sea, may seemingly end their movement towards deeper parts ofthe rift basin. This is also where this fresh water rises upwards andemanates. During oceanic rifting and subduction, the only water in-jected into the hydrothermal system, is seawater. This implies that anyfresh-water flows observed, must be the result of loss of salt from theseawater. Even if the water has been involved in hydroxylation of min-erals, it still came from seawater that lost its salt somewhere.

There are, however, exemptions to this ‘rule’ in shallow, meteoricsystems involving ground water and in early stage continental riftingwhere water input is often meteoric. The output, however, is rarelyfresh, as it usually leaches and transports different solutes upon return-ing to the surface. Rift lakes of East Africa provide evidence for this.The returning meteoric water has picked up salts from deeper down,and, therefore, several saline (alkaline) lakes exist along the rift. Eachlake tells a story of what substances the water encountered in the deep(Warren, 2006).

2.4.10. Conditioned permeability of haliteWhen hot saturated brines migrate upwards from the deep, they will

be cooled and salts may precipitate from the solution. This crystalliza-tion of salt will exert a pressure on the overburden and is suspectedto be an important displacement mechanism for deep salt and for theformation of salt domes. Salt formations are traditionally treated as be-ing nearly impermeable for brines and hydrocarbons. However, accord-ing to experimental work first carried out by Lewis and Holness (1996),halite buried at depths greater than ∼3km and elevated temperaturesbecomes permeable due to alteration of the dihedral angles of the halitecrystals.

When water is present in the salt, which is always the case in nat-ural salt deposits, the dihedral angle of the salt crystals is determinantfor the connectivity of this water (Holness and Lewis, 1997). When thedihedral angle is less than 60° the water is spreading out on the crys-tal surfaces as continuous liquid films and the salt becomes permeable(Fig. 3). Vice-versa, when the dihedral angle is larger than 60° the wa-ter film becomes disconnected and the salt is impermeable: “Our resultsexplain recent observations of major salt-fluid interactions at depth,and suggest that deep-rooted salt diapirs may act as conduits for basi-nal formation water” (Holness and Lewis, 1997). We are referring tothis PT-dependent permeable zone as the Holness Zone (Hovland et al.,2015). In the hydrothermal model for salt formation, one of the con-sequences of the Holness Zone is that any upward-migrating brine inthis zone will transfer the pressure in the liquid further upwards andmay cause hydraulic fracturing of the above lying impermeable rocksalt, as also pointed out by Schoenherr et al. (2007a, b), Ghanbarzadehet al. (2015), Warren (2016, 2017). Hot brines at depth can flowthrough ‘solid halite’ and displace salt from the high temperature re-gion to a cooler region where dissolved salts precipitate. Furthermore,

Fig. 3. Diagram modified from Lewis and Holness (1996) showing the equilibrium halite/water dihedral angles (θ) as a function of P and T and the corresponding distribution ofpore water. In the white area (upper right), θ is less than 60°, resulting in the formationof continuous liquid films in the halite-water system. The light grey area (middle), definesthe P-T region in which the permeability of halite may be significant. The dark grey area(lower), θ >60°, represents the region with (‘normal’) impermeable halite. The dashedline indicates a geothermal P-T gradient. Drawings of halite crystals illustrating continu-ous liquid films (upper) and disconnected liquid (lower) are also shown.

this mechanism allows refining of salt, by the fact that the various salttypes are precipitating according to their solubility at each specific tem-perature interval.

Another effect of the water saturated inter-connected pore system ofsalt is the reduction of the effective stresses in the salt body, i.e., the to-tal stress on the salt crystals from the overburden pressure is now partlysupported by the pore liquids.

This may, in fact, well cause a dramatic change in seismic imped-ance. Thus, when the seismic waves pass from the upper ‘brittle salt’with higher sonic velocities to a more ductile and permeable salt deeperdown with lower sonic velocities, the impedance shift will be clearlyseen on the seismic records. The so-called decollement seen on many in-terpreted seismic images of deep-lying salt deposits (‘evaporites’) mayactually represent the ‘Holness Zone’, which causes a seismic responseto the distribution of pore water.

2.4.11. Refining of saltsThe various solubilities of sea salts, which also change significantly

at higher temperatures and pressures, may result in sorting of the salttypes in the hydrothermal circulation of sea water. A water-deprivedsalt accumulation in a hydrothermal system will, therefore, lose themore soluble salts, e.g. magnesium and calcium chlorides, in contactwith any available water. Thus, refined halite may precipitate and ac-cumulate, e.g. i) inside salt stocks (domes/diapirs); ii) by venting intoponds of concentrated calcium and magnesium chloride brines on thesea floor; or iii) by venting up onto the surface where it evaporates ina hot and dry climate, as reported from Dallol (Talbot, 2007). In thisway salt accumulations found as salt stocks or other salt formations havecompositions significantly different from sea salt.

Another plausible scenario is if an accumulation of different saltsis reheated in the subsurface, e.g. by volcanism or sill intrusions, thenthe salts containing crystal water have a tendency to melt, forming hot,

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dense brines that may migrate out, driven by tectonic forces or lifted byvolcanic gases. An example of such a hydrated salt is bischofite, whichmelts at temperatures above 120 °C (Schofield et al., 2014). Upon cool-ing, these brines will again crystallize to form solid salts. Such salts willaccumulate stratigraphically above the halite and anhydrite where theyinitially were deposited. It should be noted, however, that if volcanicgases like CO2 or SO2 are exposed to the brine, new salt types may pre-cipitate; e.g. carbonates and sulfates.2.4.11.1. Refining examples It is very difficult to directly observe refin-ing processes in the subsurface, but the results of refining can be ob-served some places on the surface, for instance in the Mediterranean(Cita et al., 1988; Woodside et al., 2001). The Mediterranean Sea isunique in that it overlies a zone of mantle convection and has under-gone a long period of tectonic deformation which includes slab roll-back and the collision of two large, slowly moving plates (Faccenna etal., 2014). Several brine ponds are located on the seabed, which con-tain concentrated brines of high-solubility salts (magnesium and cal-cium chlorides), e.g. the brine ponds Bannock, Urania, Nadir, and Dis

covery (Corselli et al., 1996; Winckler et al., 2001; Wallmann et al.,2002).

These ponds are located on top of an oceanic subduction zone (theMediterranean Ridge), and the brines are venting out of these zones(Figs. 4 and 5). It is most likely that these brines were originally partof the salty pore water going down with the oceanic slab and thereforelikely the result of refining processes in the subduction zone, leaving be-hind major parts of the sea salt, mainly halite, below ground.

2.4.12. Preservation of saltsThe previous explanation given for the formation of high-magnesium

salts is an example of solubility-based refining of salts, but also pro-vides an example of a mechanism for the preservation of salt. Obser-vation of seafloor hypersaline ponds (or ‘lakes’) in depressions on theMediterranean seafloor, and elsewhere, shows that lack of mixing en-ergy in combination with gravitational stability, prevents mixing of thetwo. Over time, supersaturation as a result of cooling may occur in thebrine lakes, leading to precipitation of solid salts that are protected byalready saturated brines above.

Fig. 4. Image modified from Huguen et al. (2006). It clearly shows the topography of the Mediterranean Ridge accretionary wedge and its location in relation to the deep hypersalinebrine ponds: Bannock (B), Atlantis (A), Urania (U), and Discovery (D), which reach nearly 4000m water depth. The brines are magnesium rich, and halite depleted, signifying refinedsalts, due to hydrothermal processes, see text.

Fig. 5. Geological profile across the Mediterranean Rige, modified from Westbrook and Reston (2002). The red arrow indicates rising warm hydrothermal hypersaline brines feeding intothe anomalous seafloor brine ponds. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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In more demanding environments, dry climate may protect the saltsfrom dissolving, even if the salts were not the result of this same climate(but of hydrothermal activity). In the ocean, salts may also be protectedby a cover of anhydrite between seawater and other salts. This anhy-drite might be the result of brine outflow carrying particulate anhydritewith it, or anhydrite remaining from dissolution of anhydrite stainedsalts on top of salt structures. Normal hemipelagic sediments may alsocover a salt structure and thereby protect it from the actions of the sea-water. Furthermore, some salt structures are located next to leakage ofhydrocarbons into the sea. The biologic community may then interactand provide the CO2 necessary to produce a protective cover of authi-genic carbonate (Judd and Hovland, 2007).

2.4.13. Formation of leaky salt stocks (domes)As explained in Chapter 2.4.6, saline brines will occupy the contin-

uous pore space of the permeable salts within the ‘Holness Zone’. Inthis zone the salt will be more plastic than the salt above (due to lessgrain-to-grain contacts) and will be pressurized by the lithostatic con-fining pressure from the surrounding rocks. This pressure will also betransferred to the pore water, leading to higher pore pressure in the saltbody, causing also the ‘Holness zone’ to move upwards. This increasedinternal pressure is causing expansion of the salt body and may alsolead to salt extrusion/intrusion, as often observed in nature (Schoenherret al., 2007a; Hovland et al., 2015; Warren, 2017). Eventually, this in-creased pore pressure may reach the solid salt above the ‘Holness zone’with sufficient energy to cause hydraulic fracturing and allow periodicleakage of brine, as stated previously (Table 1) and discussed in moredetail in Part 2.

If the hydraulic fracturing happens at great depth, the fractures tendto be vertical, while fracturing at shallow depths is likely to causesemi-lateral fractures, creating ‘mushroom like’ salt bodies towards thesurface (or sea floor).

Hot brines coming from the base of the salt stock (dome) will cooland reach over-saturation during the ascent and salts will precipitate ac-cording to their specific solubilities at the different temperature/pres-sure conditions. However, this precipitation of salt will not necessarilyblock the flow completely; instead the entire salt body is likely to ex-pand, and new flow paths are formed. Some salts with higher solubil-ities, e.g. calcium chloride, may dissolve in the migrating brine, to betransported further up; i.e. a refining of the salt body is taking place.The entire process works in concert with the driving pressure that feedsbrine from below, into the salt accumulation (e.g., Holness and Lewis,1997).

2.4.14. The Dead Sea diapirsThe Dead Sea is a well-known site for ongoing precipitation and ac-

cumulation of salts in a non-marine environment. This saline lake is sit-uated in the large fault system referred to as the ‘Dead Sea – JordanTransform rift’; a pull-apart basin, where two overlapping faults (or afault bend) creates an area of crustal extension, causing large scale sub-sidence of the basin, thus, providing accommodation space for the depo-sition of thick sequences of sediments and salts. The current elevation ofthe lake surface is ∼400m below mean sea level. A map of the southernpart of the Dead Sea shows the location of the main tectonic and saltfeatures (Fig. 6).

Rabinowitz and Mart (1999) were able to reveal the occurrence ofshallow mantle masses under the southern Dead Sea using seismic to-mography, and discloses that: “… the Moho under the Dead Sea is foundat a depth of (only) 22km, and the seismic velocity in the upper lithos-pheric mantle is anomalously low”. The data actually suggest that hotmagmatic diapirs ascend along the boundary faults of the rift into theintermediate crust, giving a heat-flow in the basin of 40–50mW/m2, ac-cording to ten Brink et al. (2006).

The Dead Sea Basin (DSB) structures were further investigated byAl-Zoubi and ten Brink (2002), and their seismic surveys also showed a

Fig. 6. The southern Dead Sea located 400m below sea level. SD=the Sedom diapir;LD=the Lisan diapir. The fault-lines are: JF=Jerico Fault; SF=Sedom Fault; BF=Boqeqfault, and AF=Arava Fault. Based on Gradmann et al. (2005). (Modified from GeoMa-pApp, e.g., www.Geomapapp.org; Ryan et al., 2009).

low velocity zone extending to a depth of 13–18km under the basin,while the lower crust and Moho was not perturbed. These observa-tions are supposed to be due to strain softening in the middle crust, in-voked to explain the isostatic compensation and the rapid subsidence ofthe basin during the Pleistocene (Al-Zoubi and ten Brink, 2002). Theyalso produced a conceptual model of the crustal deformation during theopening of the basin, illustrating the subsidence of the crustal blocksalong listric faults and the sedimentary filling (Fig. 7).

The Lisan salt diapir (in the east) and the Sedom diapir (in the west)are the largest salt structures in the DSB (Figs. 6 and 8), up to 7km deep,and occur as buried structures in the sediments. According to Al-Zoubiand ten Brink (2001) the formation of these structures may have beenas short as 0.2 million years. They also report the composition of the Se-dom diapir to be ∼80% halite, the remaining being anhydrite/gypsum,marls, chalk, dolomite and shale.2.4.14.1. The Dead Sea: accumulating salts in a non-marineenvironment As pointed out by Hardie (1990) the bulk of the solutes inthe lake brines are added by the upwelling of saline springs around andup through the lake bottom: “Thus, the Dead Sea stands as a clear-cutcase of a non-marine evaporite depositional site where the evaporatingbrines are CaCl2-rich and SO4-poor because the inflow-waters areCaCl2-rich and SO4-poor” (Hardie, 1990). These features are illustratedin Fig. 8, with the sedimentary stratigraphy and the two main saltdomes, Sedom and Lisan. The structural features are also confirmed byWeinberger et al. (2006).

Talbot et al. (1996) made a study of the more than 600m thick saltbeds in the basin and focussed on the large, bulbous halite and carnallitebodies on the bottom. These types of anomalous salt bodies had neverbeen described before and they suggested a formation caused by theslow seepage of groundwater brines through cracks, inferring conduitsthrough the Dead Sea lake bottom. They surmised that the band of dens-est bulbous salt growths (‘reefs’) occurred in the area between the Lisanand Sedom diapirs (Fig. 8) and were most probably fed by “groundwa-ter rising from sources deeper than 50m, …” “… along the Boqeq fault,one of the en-echelon transverse fault zones that account for the rhom-bic shape of the southern basin”. (Talbot et al., 1996).

This suggestion was further strengthened by the finding of arte-sian pressurized groundwater brines that caused precipitation of whitehalite by natural springs along the “… brown floor of the Lisan Straits.”

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Fig. 7. Conceptual model of the crustal deformation associated with the opening of the Dead Sea Basin (DSB), indicating a shear zone with fluid flow and earthquake epicentres at thebase of the upper crust. (From Al-Zoubi and ten Brink, 2002).

Fig. 8. The southern Dead Sea Basin: Illustration of the stratigraphy and the salt domesLisan and Sedom. See text for further information (Based on Gradmann et al., 2005).

(Talbot et al., 1996). Here, groundwater with a constant temperature of29°C rises to about the level of the Jordanian salinity ponds on the east-ern flank of the straits.

The most spectacular and obvious seepage-related salt structures arethe polygonal networks of salt walls on the seafloor they document:“Thus, patches of distinct grey halite associated with wafts (smell) of hy-drogen sulphide probably signal venting of foetid groundwater in smallgroups of reef compartments hundreds of metres apart. More spectac-ularly, a particular screen of halite, a few decimetres thick and 1mhigh, separates brines that crystallize halite for kilometres on one sidefrom brines that crystallize carnallite for kilometres on the other side.”(Talbot et al., 1996). These features demonstrate that the seepage ofbrines on the seafloor is derived from different compartments in the sub-surface, where refining processes have isolated various types of salts.2.4.14.2. Where is the source of the brines? The pattern of the ventingbrines into the Dead Sea strongly indicates that the sources are ofdeep-seated hydrothermal origin, where the circulation is driven by themagmatic activity under the basin. The 3D model of the fault zone(Fig. 7) from Al-Zoubi and ten Brink (2002) actually indicates a shearzone

with fluid flow at the base of the upper crust, at 13–18km depth. It issuggested that the hydrothermal brines are coming from this zone, as itmay explain the low temperature and high salinity of the venting brines(29 °C), which is an effect of cooling along long migration conduits dur-ing the ascent. The temperature in this deep shear zone is not known,but global average vertical temperature gradients are 20–30 °C per km(Levitte and Greitzer, 2000), indicating temperatures above 300 °C at15km depth.

This concept of a deep hydrothermal source is supported by the find-ings of Eckstein and Simmonsi (1977): “Deep faults associated with theJordan-Dead Sea Rift system act as conduits for hot waters ascendingfrom deep confined aquifers …” “Most of the ascending thermal watersare absorbed by shallow aquifers with lower hydraulic potential. Suchregions are characterized by anomalously high heat-flow; several valuesexceed 2 and one value is 11 μcal/cm2s”.

However, the extensive hydrothermal system which is filling thebasin with salts must also be sourced from large accumulations of ‘hid-den salt’ deep in the subsurface; probably below the upper crust. Suchoccurrences of ‘hidden salt’ below the upper crust, interacting with anextensive hydrothermal system must have affected the entire system sig-nificantly, both with respect to the tectonic processes and also the seis-mic properties; i.e. lower seismic velocities. This is actually indicatedon the model of crustal deformation associated with the opening of theDead Sea Basin, given by Al-Zoubi and ten Brink (2002) (Fig. 7). Here,a shear zone at the base of the upper crust with fluid flow is indicated ataround 14km depth, and they conclude: “The lower lithostatic pressure(rock overburden) at mid-crust levels under the basin relative to the sur-rounding crust and the fracturing of rock under the basin due to earth-quakes at that depth (13–18km) promote fluid flow from the surround-ing crust into the middle crust below the basin. The presence of fluidswithin the fractured crust helps create weak shear zones along whichthe opening is concentrated.”

The large scale hydrothermal system driven by the heat of shallowmantle masses beneath the basin and the occurrences of large scale ac-cumulations of ‘hidden salt’ below the upper crust can also explain thevarious types of salts depositing on the seafloor: halite and carnallite.It is cautiously suggested that such accumulations of ‘hidden salts’ maybe the result of subduction in a previous Wilson cycle. Furthermore, it

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is suggested that such salt accumulations will form zones of rock-me-chanical weakness in the underground, thus, promoting new rifting topreferably propagate through such zones. These items will be discussedfurther in the following chapter (Chapter 3).

3. Relationships between salt formation and Wilson cycles

3.1. Introduction

Following Alfred Wegener's assertion that the Earth's continentshad fitted together, and that they later had split up and drifted apart(Wegener, 1915, 1922), geophysicists and geologists took another 40years to resume this model. It was the unequivocal inference from mag-netometer transects across mid ocean-ridges with the vessel Eltanin in1966 that turned the table and provided the ‘smoking gun’ that Wegen-er's model was, indeed, generally correct (Pitman and Talwani, 1972).The magnetic anomalies on both sides of the mid-ocean ridges came asproof that the ocean bottom was young and continuously was renewedalong the ridges (Talwani and Eldholm, 1977).

During the same year (1966), John Tuso Wilson also published hisbenchmark paper: “Did the Atlantic close and then reopen?” Here, hepresented the question: “How can the plate-tectonic history of the Earthbe deciphered for times earlier than the oldest in situ Ocean floor?”(Burke, 2016). His answer was “In terms of rocks and structures char-acterizing stages in the life cycle of the ocean basins” (Wilson, 1968).Soon this definition was termed “The Wilson cycle” (Dewey and Burke,1974; Burke, 2016) and today, the concept of Wilson cycles has becomea fundamental term in plate tectonics, despite still being debated.

Questions about the start of the Wilson cycles were addressed byPiper (2013) who suggests a concept of Lid tectonics before Plate tec-tonics. This suggestion is based on the fact that mobile tectonic platesare recognized by their paleomagnetic patterns of polar wander paths,while stagnant tectonic lids are not. These paleomagnetic patterns canbe mapped, and according to Piper (2013) the change from ‘Lid tecton-ics’ to ‘Mobile plate tectonics’ was transitional, and the Wilson cycle styleplate tectonics developed mainly in the Proterozoic eon that began at2.5 Ga.

3.2. What are Wilson cycles?

The stages in the Wilson cycles in their simplest form are, generally,as follows (Wilson, 1968; Torsvik et al., 2010; Burke, 2011, 2016): StageA – a stable craton of continental crust exists. Due to its lower specificgravity, it floats on top of the mantle.

Stage B – the craton experiences uplift from a rising hot-spot andstarts to rift. A system of branched rifts occurs, thereby creating a4–5km relief between the edges and the bottom of the rift. Eventually,hot, mafic or ultramafic magma flows in through its bottom.

Stage C – an early divergent margin evolves. The dense magmastarts forming oceanic crust as it cools. Its density prevents it from ris-ing above sea level. As extension continues, the margins become moredistant from the rift. The oceanic crust cools with time and becomesdenser, sinking deeper into the mantle, thus forming the deep oceans.

Stage D – a fully divergent margin develops. By now, the firstoceanic crust is too dense to be supported by the underlying mantle. Itstarts to sink in, and thereby, starts pulling the rest of the oceanic platewith it. As it subducts, it brings water with it that feeds volcanism at theplate boundary.

Stage E – a volcanic margin starts building mountains, either as anisland arc (if the subduction is oceanic), or as a mountain arc if the sub-duction is of Cordilleran type.

Stage F – eventually, the subducting plate has brought the two con-tinents on either side of the oceanic crust into contact with each other,and collisional mountain building starts. Continental crust from one

plate is being pushed on top of the other. Eventually, the subductingslab tears off and the mountains stop growing.

Stage G –brings us back to a stable craton, where erosion eventuallygrinds down the mountains and the stage is prepared for a new riftingepisode.

3.2.1. Salt-forming stages in the Wilson cyclesThe stages that provide the opportunity for water or seawater to con-

tact hot mantle, are the most important stages for salt formation. Theinitial continental rifting (Stage B) is usually involving influx of mete-oric water into deep faults within the rift system. This may easily mobi-lize soluble substances in the mantle/deep crust and bring them to thesurface via hydrothermal processes. Water may leach out elements con-stituting salts when interacting with pristine mantle and other rocks, orbring out salts from previous stages of the Wilson cycle. Burke et al.(2003) have used the presence of obducted carbonatites to show thatsuture zones in the vicinity of the East African Rift represent older sub-duction zones that are now located at the center of rifting activities. Theimplication of their work is that old salts accumulated during much ear-lier stages, may be mobilized and re-appear on the surface, from deep,hidden sources.

When seawater starts filling the rift during Stage C, the sameprocesses as above will continue and, in addition, the seawater bringsin more salts that may form heavy brines and precipitate solid salts inthe hydrothermal processes. An example of halite production from LakeMagadi in the East African Rift clearly represents evidence of halite be-ing accumulated despite occurring far away from current seawater.

During Stage D, the cooling oceanic crust is continually being ex-posed to seawater down to considerable depth due to contraction andfaulting. Oceanic crust is known to have porosities in the range 20–25%(Becker and Fisher, 2000). While still remaining hot, ‘off-axis’ hy-drothermal processes continue at depth within the oceanic crust, thus,making it ready for the next stage, Stage F. The oceanic crust is nowloaded with seawater, brines, and even some solid salts (Butterfield,2000).

During Stage F, the oceanic crust and its content of components fromformer seawater is subducted. It is heated and pressurized, thereby al-lowing a new set of chemical and physical reactions to take place.

A subset of the Wilson cycles is commonly referred to as the “Su-percontinent cycle”. During the history of Earth, several supercontinentshave assembled large landmasses, for thereafter to rift and drift apartagain. From complete assembly of landmasses to rift, it has taken on av-erage ∼250 Ma.

These events are of great importance to the total energy input tothe formation of solid salts from dilute brines such as seawater, andthere is evidence that supercontinents are an integral part of Wilsoncycles, involving rifting, sea floor spreading, and subduction, and thatthese processes have been going on for at least 2.7 Ga (Piper, 2013;Buiter, 2016). This provides ample evidence for the existence of numer-ous stages of Wilson cycles and vast areas of Earth that potentially havebeen involved in salt formation.

3.3. Subduction and salt

3.3.1. Characteristics of the oceanic crustThe oceanic crust has an average thickness of ∼8km and a regional

porosity of up to 25%, a porosity, which is considered mainly to befilled with seawater (Becker and Davis, 1998). Whereas it has provedrelatively easy to find ocean floor vent sites for hydrothermal flow,it is much more difficult to document where seawater enters into theoceanic crust, the ‘recharge’ zones. However, one such site was foundby chance, during the drilling of DSDP Hole 395A, at Site 395 in theAtlantic Ocean. The hole is located at an isolated sediment pond, the

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‘North Pond’, measuring 8 by 15km. The hole was drilled ∼70km westof the Mid Atlantic Ridge (MAR) crest, at 4440m water depth. Afterpenetrating 92m into the sediments and over 500m into the oceaniccrust, mainly consisting of basalts, to their surprise, the researchersfound drawdown of water instead of the expected over-pressure (Beckerand Davis, 1998).

During a re-visit to this hole, they measured that seawater was dis-appearing into the open hole at a rate of ∼1000L per hour. The holewas then fitted with a CORK (Circulation Obviation Retrofit Kit, e.g., aninstrumented borehole seal) and monitored: “In the 21 years since theinitial drilling, Hole 395A has been re-entered 4 times: first during Leg78B, and most recently, during Leg 174B. Each time, repeat temperaturelogs, fluid samples, and flowmeter logs clearly demonstrated that oceanbottom water was flowing down the hole at a consistent rate of about1000L/hr” (Becker and Davis, 1998). Results from similar settings, i.e.,the flank of the Endeavour Ridge (ODP Leg 168), strongly supports thenotion that there are huge fluxes of low-temperature seawater flowingthrough very transmissive upper basement sedimented young oceaniccrust, regardless of whether the sediment cover is continuous or patchyand regardless of spreading rate.

Oceanic/continental subduction occurs where parts of the oceaniclithosphere descends into the mantle beneath an overriding continentalplate. During subduction, the oceanic plate becomes dewatered as itshydrous fluids are released and permeate the overlying mantle wedge(Fig. 9). Similar dewatering also occurs for intra-oceanic subductions,where oceanic lithosphere descends below another oceanic lithosphere.

Even if the subducting slabs release some of their pore water con-tent early in the subduction process, large masses of water and salts arebrought down to great depth and may undergo hydrothermal conver-sion. To illustrate the potential for salt formation in just one subductionzone, the Andean subduction can provide some hints. Faccenna et al.(2017) have calculated the length of subducted lithosphere in the An-dean subduction zone to be in the range of 2200km to 7800km, overa period of 25–80 Ma. This subduction zone stretches along the entireSouth America over a width of more than 15,000km. With a suspectedporosity of 5–10% (from initially 20–25%), filled with saline porewa-ters within the subducted, multiple kilometres thick slabs of the PacificOcean crust, the amount of potentially formed salt will be in the size ofmillions of cubic kilometres.

3.3.2. Hidden- and sometimes visible salts in subduction zonesA study by Scribano et al. (2017) states that serpentinites may act as

reservoirs for dense brines and salts due to the mineralogical processesinvolved in serpentinization of hot peridotite in contact with seawater.This salt may later be driven out to become visible salt deposits.

Similarly, a study on the fate of salts in peridotites and serpentinitesexposed to seawater was undertaken by Sharp and Barnes (2004). Someof their conclusions are:

(1) Serpentinite may be the major conduit for surficial chlorine transferto mantle depths and an important part of the chlorine cycle. TheCl flux in serpentinites into the mantle is larger than all other previ-ously identified fluxes.

(2) Cl is sited in both water-soluble and –insoluble components.(3) A high-salinity fluid “plume” is evolving during serpentinite dehy-

dration of a subducting slab. The high-salinity fluid affects meta-morphic reactions and melting properties above the slab and cancause intense metasomatism.

Deeper processes within subduction zones involving granites havebeen investigated by Srikantappa et al. (1992). They observe fluid in-clusions with halite contents of up to 50–60% held in rocks having beenexposed to 500–550°C. Based on appearance and orientation of the in-clusions, the authors suggest that their content is of magmatic origin,and not meteoric.

Studies of Eclogites from even deeper portions of subduction zoneshave been performed by numerous authors, including i) Svensen et al.(2001), and their study of the eclogites of western Norway, where theyobserved salinities in eclogite inclusions of 15–25% salt content; ii)Xiao et al. (1998) who studied eclogites in eastern China and foundhigh salinity inclusions in rocks having been exposed to more than700°C; and iii) Alexeev et al. (2017) who report from a study on dis-posal of highly saline brines flowing into a diamond mine in an eclog-ite hosted kimberlite, situated within the Siberian platform. Inflow ofsalt brines into the mine exceeded 75m3 per day of brines with solidcontents of more than 350g/litre. These examples leave no doubt that

Fig. 9. Left, Zellmer et al.’s figure, showing subduction-related arc volcanism that is triggered by fluids and minerals ‘squeezed’ out of the down-going oceanic lithosphere under the sub-duction wedge of the overlying continental plate. ‘AW’=Accretionary wedge; ‘OC’=Oceanic crust; ‘CC’=Continental crust. Right, diagram from Manning (2004), which schematicallyshows the path of a slab-derived fluid, with mantle H2O contents (wt%). The fluid migrates into the mantle wedge (solid orange arrows), where it is absorbed through formation of hydrousminerals. Downward flow of solid mantle (dashed arrows) causes dehydration. After multiple hydration/dehydration steps, the fluid enters a region where it is stable with anhydrousminerals, which allows greater travel distances. Both images illustrate important aspects of the inner ‘anatomy’ of subductions. These processes occur within the green rectangular zoneindicated on the lefhand figure (Modified from Zellmer et al., 2015; Manning, 2004). (For interpretation of the references to color in this figure legend, the reader is referred to the Webversion of this article.)

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salts are transported to great depths in subduction zones, from wherethey may reappear during later stages of the Wilson cycle.

It might be stated that the fraction of salt within inclusions is rela-tively small compared to the volume of the solid rocks and hence thatthey may only represent a minor source of salt. This view does not cap-ture the full extent of the processes behind the formation of these rocksand their inclusions. There is ample evidence that the trapped inclusionsrepresent just a minute part of the salts and fluids that were present dur-ing crystallization of the rocks. The relatively low crystallizing tempera-tures tell us that these rocks were not formed in dry melts and that muchmore water was present before hardening. This is being recognized inseveral studies, for example Alderton and Harmon (1991), O'Reilly et al.(1997) and Harlov et al. (2012).

They all observe how different fluids, including brines, are trappedduring the crystallizing of the rocks, thereby proving presence of fluxesof brines within the subducting system. When quartz is crystallizingfrom solution, it produces water that tends to dilute the brines pre-sent. It is therefore likely that many inclusions in granites and eclogiteshave captured slightly diluted brines, relative to those present at the on-set of crystallization. Regarding the question of melting versus solutionof minerals in the subduction zones, Manning (2004) states that: “Un-der appropriate conditions, silicate solubilities may become so high thatthere is complete miscibility between hydrous melts and dilute aqueoussolutions.”

Laumonier et al. (2017) confirm that the quantity of brines in rockswithin the deeps of subduction zones is in the order of 8–10% relativeto rock volume. They have located a vast volume of water (that origi-nally subducted as seawater) beneath one volcano - the Uturuncu vol-cano on the Bolivian Altiplano. The volume of fluids beneath this sin

gle volcano is equivalent to 500,000km3. The volcano is located southof Lake Uyuni.

This is what Laumonier et al. (2017) state: “Similar high conductiv-ity features are observed beneath the Cascades volcanic arc and TaupoVolcanic Zone. This suggests that large amounts of water in super-hy-drous andesitic magmas could be a common feature of active continen-tal arcs and may illustrate a key step in the structure and growth of thecontinental crust.” The investigation by Laumonier et al. rests on the in-terpretation of water as the cause for the high conductivity. The authorsperformed high pressure, high temperature experiments with relevantrocks and water to establish the water content needed to produce theobserved conductivity. It may also cautiously be suggested that water atthese depths is most likely saturated with salts, e.g., it is likely that hid-den salts or brines play a yet undetected but important role here.

During later tectonic events, e.g., rifting (like in the East AfricanRift), the hidden salts may become activated and mixed with newlyformed salts from rift-related hydrothermal processes (e.g., Lake Mag-adi), although, however, it is not straightforward to distinguish betweenthe two generations of salt. Generally, however, the most soluble salttypes, e.g., magnesium chloride, disappear out of the system.

3.4. Rifting and salt

3.4.1. IntroductionIn the Wilson cycle salt model, rifting represents the ‘second phase’,

following that of subduction. Rifting occurs where parts of the continen-tal or oceanic crust is stressed to such an extent that it is pulled apart,and causes hot mantle to be drawn upwards, so that it partially melts(Fig. 10) (Asimow, 2017).

Fig. 10. Conceptual drawing showing the geology of mid ocean rifting, modified from Asimow (2017). The age (millions of years) of the oceanic crust is indicated on the horizontalaxis. “The solid-state flow induced by plate spreading brings hot material upward. Upwelling mantle first crosses the hydrous solidus, where melting begins as a result of trace water inthe mantle, and subsequently the dry solidus, where melting accelerates. The thickness and composition of the crust, geophysical probes, and experiments locating the solidus constrainestimates of mantle temperature. Sarafian et al. infer that the mantle is 60K hotter than previously thought.” (Asimow, 2017).

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This chapter reviews how brines and solid salts may become integralin the rifting process in both continental (non-marine) and oceanic rift-ing.

3.4.2. Hot-spots and mantle plumes: precursors for riftingAccording to Bosworth (2015) and references therein, the Red Sea

rift occurred between two hot-spots, or hot mantle plumes, the ‘CairoMini-Plume’ and the ‘Afar Plume’. What do such plumes represent inthe current context?

Hot-spots arise as a consequence of physical instabilities in mantlematerials. During subduction, the first instability to appear is the com-pression of the subducting slab, leading to the escape of some of thesaline fluids. During oceanic subduction, the fluids have a relativelyeasy access to the surface - or seabed, where they escape to the watercolumn. This especially occurs along fault planes leading down to thesubduction zone.

As the slab decends the saline brines are causing basaltic volcanismand the build-up of island arcs, by penetrating the hanging wall (Fig. 9).During Cordilleran subduction, the situation is similar, but the porewa-ter contains some extra elements due to slightly different chemistry inthe system. Some saline fluids also escape the subduction zone by flowto the surface via hydrothermal activity and conductive faults. Brinesescaping the subduction zone in this situation have to penetrate muchthicker crust in order to emanate on the surface. Salty brines emanatingonto surfaces with arid climate may lead to accumulation of salt, whilethose venting out on locations with wet climate will be washed back tothe sea.

Hot brines emanating on the bottom of deep oceans tend to be bet-ter protected against dissolution than brines ending up on land. This isbecause brines that are venting on the sea floor will cool and precipitatesolid salts that may later be protected permanently by sediments or ma-terial of biogenic origin such as carbonates.

During the next step of development in the Cordilleran subduction,the brines interact with the hanging wall to produce the physical in-stability that leads to granitic plutons rising to the surface. The plutonsalso act as a transport mechanism for saline brines. If the subduction hasbeen flat for extended periods of time, the original hanging wall mantlemay be pushed away, and a different form of magmatism is seen. Butthe water and accompanying salts are still important components of thisinventory.

During the last stages in the life of a subducting slab it sinks intothe so-called ‘slab graveyard’ at the lower boundaries of the outer man-tle (Fig. 11). Here, the relatively cool remains of the slab start heat-ing up again, and with its higher content of remaining hydroxyl-bear-ing minerals may become buoyant and form a volcanic hot-spot thatrises to the surface. During ascent, it will interact with the surround-ing mantle and become slightly mixed with it. This does not precludethe fact that the first parts of the hot-spot magma arriving at the

surface must contain a higher portion of materials from the slab grave-yard itself. This would imply a certain content of water and salts.

As a hot-spot develops into a spreading oceanic crust, the initialsurge of volatile materials may become less dominant and it releasesmagma with a typical basaltic content of chlorides. The difference insalinity between hot-spots and oceanic basalts has been documented byStroncik and Haase (2004). They state: “The majority of hotspot lavashave higher Cl/K ratios than depleted mid-ocean ridge basalts, con-sistent with the presence of recycled crustal components in the man-tle-plume sources of hotspots.”

Kovalenko et al. (2006) studied the zonal differences within threemantle hot-spots. They report on the source of the plumes as well as thedistribution of water, chlorine and other elements within it. They con-clude: “The plume mantle was formed mainly by the mixing of threesources: ultradepleted mantle, moderately enriched relatively dry man-tle, and moderately enriched H2O-rich mantle. In addition to the threemain components of the plume mantle, there are probably minor com-ponents enriched in chlorine and depleted in fluorine. It is supposed thatall these components are entrained into the plume mantle through themantle recycling of components of the oceanic and continental crust.The established relationships are in agreement with the zonal model ofa mantle plume, which includes a hot central part poor in H2O, Cl, andS; an outer part enriched in volatile and nonvolatile incompatible ele-ments; and enclosing mantle material interacting with the plume.” Thisdescription of mantle plumes is in line with our model, where watercontained in the outer parts of the plume most likely originates fromthe subducted oceanic plate and that chlorine and sulfur are remnantsof marine salts associated with pore water in the oceanic slab.

Both of these two studies on chloride contents and origin of hot-spotmagmas are basing their results on studies of inclusions trapped in vol-canic glass as the magma crystallized. It is well known that water andbasalts exist in full miscibility at elevated pressures and temperatures,as the water lowers the melting temperature of the mixture. However,during ascent the magma is gradually depressurized and ultimately ahigher percentage of water escapes and may transport salts with it asthe lava solidifies. Thus, the amount of water and salts found trappedin inclusions probably represents the later stage amount and concentra-tions, rather than the initial stage.

Because hot spot plumes from deep in the mantle (e.g. 3000km) in-clude subducted oceanic crust, also sea salts are found in these lavas.Sobolev et al. (2011) report the analysis of strontium isotopes(87Sr/86Sr) in lava from Mauna Loa, Hawaii, which matches the com-position of 0.3–0.5 Ga old seawater. Seawater older than 0.5 Ga is sig-nificantly less radiogenic. Direct contact with modern seawater is notconsistent with the low values of B and Cl. Thus, they conclude thatseawater must have ‘contaminated’ the Mauna Loa source rock prior tosubduction (Sobolev et al., 2011). These results indicate that sea salts

Fig. 11. Cartoon modified from Svensen et al. (2017), showing subducted lithospheric slabs that may interact with the margins of thermochemical provinces in the deep mantle, including‘TUZO’ (in red), located beneath much of Africa and India. This may trigger the formation of plumes, as discussed in the text. (For interpretation of the references to color in this figurelegend, the reader is referred to the Web version of this article.)

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used 0.5 Ga to re-appear at surface; i.e., much shorter time than earlierexpected. Based on their study, Sobolev et al. (2011) also calculated theaverage circulation rate of crust through the mantle to be in the orderof 1–2cm/year.

The differences in salinity of hot-spot volcanism, therefore, may beexplained by the origin of their magma. Hot-spots that arise as a resultof subducted materials being recycled should have higher salinity thanothers. However, this salinity may not last forever as the more volatilecomponents must be expected to arrive early and then diminish in con-centration over time. Initial hot-spot volcanism of recycled mantle ma-terials should therefore be expected to produce salts with the magma.This salt comes in addition to other salts produced by hydrothermal ac-tivity from heat and rock-water interaction.

The composition of evaporites and various rock types were investi-gated by Kendrick et al. (2017) and compared to seawater. Some of thedata are shown in Table 3 together with calculated ratios between thethree elements Cl, Br, and F. Interestingly, these data show very largedifferences between the ratios of these elements for the various sources.The Cl/Br ratios for evaporites are one order of magnitude higher thanfor seawater, while the corresponding Cl/F ratios for evaporites arenearly four hundred times higher than for seawater. The Cl/Br ratiosfor ‘primitive mantle’ are comparable to those of seawater while theCl/F ratio of ‘primitive mantle’ is two orders of magnitude lower thanseawater. These elemental ratios demonstrate that evaporites have gonethrough pervasive refining processes and interactions with the side-wallrocks in the hydrothermal system and are no longer similar to seawatercomposition.

It is important to be aware that even if concentrated brines areformed from seawater circulating in ascending mantle plumes, specialgeological conditions are required for this salt to precipitate as solidsalt and to be preserved. Such relationships cannot be found every-where. Along the MAR, for example, there is no concentration of salt,because the seawater-rock interactions are near the seabed and the saltis washed away. This will often be the case if the serpentinization takesplace near the seabed.

3.4.3. The South Atlantic rift basinsIn their analysis of the opening of the South Atlantic Ocean and the

dissection of an Aptian salt basin, Torsvik et al. (2009), argue for a con-ventional evaporite formation: “ … that the Aptian salt accumulationsbelonged to a single pre-breakup (syn-rift) basin confined to continen-tal and/or subaerial basaltic substrates.” However, water depths of upto 500m in this single syn-rift basin are not excluded: “It is widely ac-cepted that the salt was deposited in a shallow-water environment, andpalaeontological evidence from the Aptian to earliest Albian dolomiteand sapropel sequence of the Angolan margin, deposited just above theAptian salt, suggests no more than 500m paleo-water depth (Marton etal., 2000)”.

In order to explain evaporation in such a deep basin, Torsvik et al.(2009) refer to work by Nunn and Harris (2007): “These authors pro-posed a model of subsurface seepage of seawater across a barrier to ex-plain this apparent dilemma, which also would constitute a possible ad-ditional mechanism for the syn-rift deposition of the great evaporite se-quence.” In our opinion, however, the potential effects of hydrothermalprocesses associated with high heat-flow are more plausible mechanismsfor explaining the salt accumulation in the basin.

3.4.4. Numerical modeling of the South Atlantic riftIn a recent numerical modeling of the ‘rift-to-drift phase’ of the

South Atlantic salt basin, Norton et al. (2016) mapped the limits ofoceanic crust and salt (Fig. 12). In accordance with the model proposedby Torsvik et al. (2009), they suggest that salt was deposited when theseafloor spreading began, that is, when the South Atlantic opened. Inaddition, Norton et al. (2016), suggest that precipitated salt flowed overand covered the spreading ridge axis, thus, sealing off the ‘extrusivecomponents’ of the oceanic crust, resulting in the formation of intrusiveoceanic crust: “Seafloor spreading eventually broke through the thin-ning salt, forming breakthrough volcanoes preserved today as basementramps at the distal limits. These ramps formed diachronously, so the dis-tal salt limits are not isochrones, explaining the poor fit of these featuresin plate reconstructions.” (Norton et al., 2016). Their results are actu-ally much in line with recent descriptions from the Red Sea by Augustinet al. (2016), where giant salt flows seal off large tracts of the activelyrifting central axis of the Red Sea (to be discussed in Part 2).

Norton et al. (2016) actually suggest that the salt formation, in sce-nario C, of Fig. 12, is more-or-less ‘instantaneous’: “Estimates of du-ration of salt deposition are poorly constrained, ranging from 600 k.y.to 5 m.y. (Davison et al., 2012). Given Aptian plate spreading rates of50–60km/m.y. (Heine et al., 2013), longer intervals of salt depositionwould mean that the conjugate salt basins separated before the end ofsalt deposition; this is implausible. We thus prefer a short period ofsalt deposition. We also assume that isostatic loading, occurring as itdoes over thousands of years as opposed to hundreds of thousands ofyears for salt deposition (van den Belt and De Boer, 2007), was instan-taneous”. However, to us, this seems very improbable, especially whenwe know that isostatic forces will not be powerful enough to provide theneeded subsidence.

Thus, a short period of salt deposition is likely, combined with in-stantaneous isostatic compensation. According to van den Belt and DeBoer (2007) this scenario seems improbable, because the weight ex-certed by the low density salt is not sufficient to isostatically push downthe heavy rocks beneath the basement floor. However, according to ourmodel the salt filling the basin originates from ‘hidden salt’ beneath thebasin-floor, and the removal of this salt up into the basin will give aninstantaneous subsidence of the basin floor due to mass balance condi-tions. This process is referred to as subrosion, e.g., below-ground salt dis-solution (Ehrhardt and Hübscher, 2015). Furthermore, the circulation ofseawater during the transportation phase added more salt to the salt al-ready accumulated beneath the basin, as is also seen in the Red Sea, tobe described and discussed in more detail in Part 2.

Thus, it is suggested that the salt basin was filled by salt depositsduring periods of high heat-flow, e.g., during SDR-formation, and thatsalt was brought into the basin by hydrothermal saturated brines frombelow.

3.4.5. Indications of the same process other placesIt is inferred that a similar process is currently taking place in the

northern Red Sea and Afar regions (see Part 2) and is thus, demonstrat-ing the close relationship between hydrothermal salt production andvolcanism, as well as brine migration and refining of salts, whether theyare deposited on the surface in a dry climate or on the seafloor by pre-cipitation in high-density brine pools. However, the most interesting as-pect of the conclusions of Norton et al. (2016) is their transition from

Table 3Contents of Cl, Br and F in seawater, evaporites and ‘primitive mantle’. Ratios between the three elements are also shown. (From Kendrick et al., 2017).

Cl (ppm) Br (ppm) F (ppm) Cl/Br Cl/F

Seawater 19,300±970 66±3.3 130±0.07 292 148Evaporites 550,000±50,000 150±100 1000±300 3667 55,000Primitive mantle 5±2 0.076±0.025 17±6 342 1.5

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Fig. 12. Sketch, modified from Norton et al. (2016), represents a modern view, based on numerical geo-modeling, on how and when salt was deposited in the Central Atlantic, Camposand Kwanza basins. ‘RA’ is the depth of the Rift Axis below sea level. The figure text of Norton et al. (2016) is as follows:”The case of seaward-dipping reflector (SDR) margins like partof the Campos Basin is illustrated. A: Rift basin is subaerial and transitioning to oceanic crust formation. Rift axis is ∼1km below sea level. B: Same time as in A, but now seawater hasflooded the basin. Rift axis is ∼1500m below sea level. C: Salt has filled the basin to sea level; rift axis is ∼3500m below sea level. D: 3–4 Ma after C, with ∼100km total motion since C.Intrusive oceanic crust is being created under salt. E: Shortly after D, with volcanism breaking through the salt, forming the breakthrough volcanoes. F: A few million years after E, withnormal oceanic spreading”.

Stage B to Stage C (Fig. 12), where enormous salt deposits are formed“instantaneously” (Norton et al., 2016). This is indeed parallel to whatseems to have occurred also in the Mediterranean Sea (e.g., during orprior to the Messinian Salinity Crisis) and in the Red Sea a few millionyears earlier.

But even though the bulk of the salt was deposited in the late Ter-tiary era, salt formation continues. In the Red Sea basin, precipitationof salt occurs in the subsurface, and slurries of solid salt and satu-rated brines rise up and are observed venting out along the sides ofthe basin, forming “namakiers” of salt that move down the slopes to-wards the embryonic spreading centers of the Red Sea. This is in spiteof the fact that the basin water has salinities just above that of seawa-ter (35,000–37,000ppm). A completely different scenario is seen in theDead Sea, where the salinity in the water exceeds 300,000ppm, andhalite and carnallite is observed to precipitate in the margins and at theseabed (Talbot et al., 1996).

4. Concluding remarks

The formation of large marine salt deposits is mainly registered inancient basins, especially in areas with a geological history of high tec

tonic activity preceding salt deposition. Over the last decade it has be-come a well established fact that heavy brines form and solid salts pre-cipitate within hydrothermal systems, due to the thermodynamic andphysico-chemical properties of seawater at high temperatures and pres-sures. It is suggested that volcanism and hydrothermal activity eventsassociated with different stages of the Wilson cycle are the driving forcesbehind salt accumulations in giant basins.

The Wilson cycle is an expression of the geodynamics of Earth,and based on the conceptual elements highlighted in this work, it issuggested that the formation of hydrothermal salt is associated withdeeply circulating seawater within the hydrothermal systems in theEarth's crust and upper mantle, driven by the internal heat of the Earth.These seawater-based processes also include serpentinization of mantlemasses and water-rock interactions by the circulating hot seawater inthe oceanic crust. The immense recycling of crustal materials, includ-ing the subduction of the porous oceanic crust charged with brines andprobably also solid salt, brings water and salt deep down into the man-tle.

The tectonic activity, including volcanism and associated hydrother-mal processes bring water and salts upwards towards the surface wherehuge amounts of salts may precipitate; e.g. in the high An

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des Mountains. It is also acknowledged that such brines and salts may bestored sub-surface for long periods of time, e.g. extending from one Wil-son cycle to another. Thus, it is cautiously suggested that large amountsof salts remain hidden inside the subduction zone including the volcanicand sedimentary rocks above the subducted slab. During later riftingand oceanization processes, the subducted rocks, including their con-tents of hidden salts, may again be mobilized and deposited at the sur-face.

Uncited references

Bird, 2003; Burke, 1975; Burke et al., 1977; Ehrhardt et al., 2005;Engvik et al., 2011; Garfunkel et al., 2014; Gutscher et al., 2000;Hannington et al., 2001; Pirajno, 2009; Searle and Ross, 1975; Wilson,1966.

Acknowledgements

We would like to thank editors Massimo Zecchin and TimothyHorsecroft for suggesting publishing this review article. Also we thankV. Scribano and W. Mohriak for their thorough and constructive re-views.

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