The Sensitivity of Temperature to Tachyhydrite Formation: Evidence from Evaporation Experiments of Simulated Brines Based on Compositions of Fluid Inclusions in Halite

Cheng, Huaide; Hai, Qingyu; Li, Jun; Song, Jianguo; Ma, Xuehai

doi:https://doi.org/10.1155/2019/7808036

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Research Article | Open Access

Volume 2019 | Article ID 7808036 | https://doi.org/10.1155/2019/7808036

The Sensitivity of Temperature to Tachyhydrite Formation: Evidence from Evaporation Experiments of Simulated Brines Based on Compositions of Fluid Inclusions in Halite

Huaide Cheng,^1,2Qingyu Hai,^1,2Jun Li,^1,2,3Jianguo Song,^1,2,3and Xuehai Ma^1,2,3

Academic Editor: John A. Mavrogenes

Received26 Feb 2019

Accepted11 Jun 2019

Published03 Jul 2019

Abstract

An average of concentrations of Na⁺, Mg²⁺, Ca²⁺, K⁺, and Cl^– in fluid inclusions, from the Khorat Plateau evaporite primary halite, was employed. The evaporation–crystallization sequence and paths were obtained under various temperature conditions for the quinary system, Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O. The results showed (1) a halite, sylvite, and carnallite stage at 25°C; (2) a halite, sylvite, carnallite, and bischofite stage at 35°C; and (3) a halite, sylvite, carnallite, bischofite, and tachyhydrite stage at 50°C. These results indicated that (1) a hot state is favorable for tachyhydrite formation, (2) tachyhydrite occurs in the late evaporation stage, and (3) the stability field of tachyhydrite increases with increasing temperature. The crystallization paths were plotted by the application of Jänecke phase diagram at 25°C, 35°C, and 50°C involving the system Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O. The crystallization sequence predicted on the Jänecke phase diagram showed a good agreement with the experimental crystallization sequences and paths. Tachyhydrite precipitate more easily from a high Ca concentration solution during the late evaporation stage with increasing temperature under the same relative humidity condition. The evaporite mineral succession in the Khorat Plateau, Sergipe, and Congo basins agrees well with the mineral precipitation sequences predicted from their own fluid inclusions in halite. This is confirmed by the simulation of the Jänecke phase diagram at 50°C involving the system Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O. The precipitation of tachyhydrite was sensitive to the temperature, and that the thermal resource may originate from a temperature profile in the solar pond. This study presented a simulated approach that can help in understanding similar cases that studies the sensitivity of temperature to salt formation.

1. Introduction

Only one modern environment, namely, the salt pan of the Gavkhoni Playa Lake, southeast of Isfahan in central Iran, is known to contain tachyhydrite (see Table 1 for mineral formulas). Besides a minor deposit in Germany, tachyhydrite is common in Cretaceous graben and half-graben basin evaporite deposits in Brazil, western African, and Thailand [1]. Because fractional crystallization of seawater in a series of preconcentration brines does not result in a CaCl₂-rich brine and thus complete evaporation of marine water cannot form tachyhydrite [2], tachyhydrite is present mostly in nonmarine settings particularly in hydrothermal regimes [3–7]. Through studies on Khorat basin reformation and its evolution, Vysotskiy [8] proposed that (1) chloride brines became enriched in calcium in late stages mainly through chloride–calcium brine outflow from underlying rocks along faults and weakened zones, (2) the evaporation formation was accompanied by a thermal setting that was favorable to tachyhydrite, and (3) formed tachyhydrite beds are rift grabbers or gentle depressions of the syneclise type. Evidence from a regular upward increase in bromine and a constant strontium concentration through the upper zone of tachyhydrite in the Khorat Plateau [1] showed that (1) the marine brines became enriched in calcium in late stages mainly through the structural setting in fault-bounded troughs with large local relief; (2) tachyhydrite crystallization was uninterrupted by sudden influxes of marine waters and where primary salts were preserved without alteration; and (3) a “dry-lake” stage was never reached even though extreme volume reduction is required for tachyhydrite crystallization. El Tabakh et al. [7] suggested that hydrothermal CaCl₂ waters entered the restricted marine basin and created the right conditions for tachyhydrite precipitation, and they cite granitic intrusions [9] as possible evidence for thermal activity during the time of tachyhydrite formation. Some researchers proposed a tachyhydrite formation mechanism by multicomponent water–salt system theory [1, 6] and a phase theory [5, 10–13]. We have also attempted to characterize the formation of tachyhydrite by evaporation experiments of simulated brines based on compositions of fluid inclusions in halite.

The sequences and types of crystallized salts and changes and relative changes in brine composition with the amount of water evaporated during isothermal brine evaporation are theoretical bases for our study on geochemical brine evolution, and the fundamental information that they provide allows us to explain the origin or formation condition of evaporites. We employed an average of concentrations of Na⁺, Mg²⁺, Ca²⁺, K⁺, and Cl^– in fluid inclusions, from the Khorat Plateau evaporite primary halite (39 fluid inclusions) [14]. The Na⁺, Mg²⁺, Ca²⁺, K⁺, and Cl^– concentrations and the Jänecke coordinates () of the fluid inclusion are listed in Table 2. The purpose of this study is to (1) obtain a crystallization sequence from evaporation experiments of simulated brines based on the major-ion composition of fluid inclusions at 25°C, 35°C, and 50°C; (2) simulate a crystallization path by the application of the Jänecke phase diagram at 25°C, 35°C, and 50°C for the quinary system, Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O; and (3) discuss the formation condition of tachyhydrite. In addition, we paid more attention on the sensitivity of temperature to tachyhydrite formation and attempted to elucidate the relationship between tachyhydrite occurrence and temperature.

2. Experimental

2.1. Reagents and Materials

Sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl₂·6H₂O), and calcium chloride (CaCl₂) were of reagent grade and were purchased from Sigma-Aldrich. All reagents were used as received without any further purification. Double-distilled water (18 MΩ cm) was used in all experiments and for analytical measurements. Standard glassware was used in all experiments.

2.2. Evaporation Procedure

A synthetic brine sample based on compositions of fluid inclusions in halite from the Khorat Plateau primary halite was used as a starting point for experimental isothermal evaporation at 25°C, 35°C, and 50°C, and its average concentrations were given in Table 2. During evaporation, brine samples were taken in sequence at different densities for chemical analysis. The precipitated salt in the saturated brine was filtered and stored for X-ray diffraction and chemical analysis. X-ray diffraction analysis was carried out on the same day as the solid samples were collected to avoid any alteration by atmospheric conditions. Herein, tachyhydrite is hardly to be kept in solid at room temperature during the preparation of X-ray diffraction sample, because tachyhydrite will be transformed to bischofite and a Ca-rich mother liquor once it is exposed at atmosphere during the process of its crystallization. Moreover, tachyhydrite will be precipitated in the late of evaporation. Therefore, the indirect way to identify tachyhydrite mineral is to determine the compositions of terminal precipitated minerals and its saturated mother liquor. Because the tachyhydrite occurs in the late stage of evaporation and the Jänecke coordinates of the terminal saturated mother liquor locate on the tachyhydrite stability zone of the phase diagram involving the system Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O, it can be considered that tachyhydrite should exist in the precipitated salts. Then, the tachyhydrite can be identified indirectly by determining the ionic compositions of terminal precipitated minerals, especially Ca and Mg. Density measurements were carried out using a specific gravity balance.

Evaporation experiments were conducted in a constant temperature water bath with an accuracy of ±0.1°C. There was no stirring of solution. The evaporation vessels were 1000 ml, 500 ml, 250ml, and 100 ml beakers. The masses of solution at the beginning of the experiment were 1000.00 g. When a kind of new mineral was precipitated out, the solution and solid would be completely separated. Then, the evaporation of filtered solution continued. The entire evaporation process lasted 80 days. In the absence of saline mineral precipitation, the evaporation process will continue for another 10 days to ensure complete evaporation. Our experiment was conducted in March and April at an experimental site in Xining, Qinghai, China. According to local climatic conditions, the local air relative humidity ranges from 36% to 42% in the fall. The solid-phase minerals and the liquor-phase solutions were separated using a Sartorius Polycarbonate Filter Holder (8 μm membrane).

The saturated initial brine used for evaporation was prepared in deionized water using NaCl, KCl, CaCl₂, and MgCl₂·6H₂O salts based on the compositions of fluid inclusions in halite (see Table 2). Because of the temperature dependence of the salt solubility, a small variation in temperature will change the saturated brine concentration. Therefore, saturated brines were stirred for 24 h to achieve saturation at room temperature. All saturated brines were filtered using an 8 μm membrane filter to remove fine salt particles and other insoluble material prior to use.

2.3. Analytical Methods

Na⁺, K⁺, Mg²⁺, Cl⁻, and Ca²⁺ concentrations in brine samples were determined at the Qinghai Institute of Salt Lakes, Chinese Academy of Sciences. All analyses for major in this study followed the procedures of the Qinghai Institute of Salt Lakes [15]. Mg²⁺ and Ca²⁺ ion concentrations were determined by complexometric titration with ethylenediaminetetraacetic acid (EDTA) standard solution. K⁺ ion concentration was determined by gravimetric methods using sodium tetraphenylborate [KB(C₆H₅)₄]. Cl⁻ ion concentration was determined by Hg(NO₃)₂ titration. Na⁺ ion concentration was calculated by charge balance (N represents ionic equivalent value). The analytical precision for cations and anions is better than ±2%.

2.4. Jänecke Phase Diagram Analysis for the Quinary System, Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O

In the quinary system diagram, Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O, the Jänecke coordinates are expressed as where “” is the molar number and .

In the application of the Jänecke phase diagram, it is assumed that the solution is characterized by (1) halite saturation, which means that the solution will coprecipitate halite jointly with other salt phases that have separated during evaporation, and (2) sulfates and carbonates that are considered unimportant constituents because their concentration in the saturated solution is very low [16, 17].

3. Results and Discussions

3.1. Crystallization Path during Evaporation

Ionic compositions and Jänecke coordinates of the studied brine sample, measured at different densities during experimental evaporation at 25°C, 35°C, and 50°C, are given in Table 3.

The crystallization path is the progress of chemical transformations by the loss or addition of a constituent through a given solubility phase diagram. It can define the number, nature, composition, and relative quantity of different condensed phases that precipitate or disappear during a system’s evolution. In our case, water leaves as steam at constant pressure and temperature. Such a graphic representation shows the reactions that occur by changing an intensive variable, mainly composition. The crystallization path construction demonstrates the sequence of salt precipitation. In this study, the fluid inclusion composition showed that the studied brine is composed mainly of Na⁺, K⁺, Mg²⁺, Ca²⁺, and Cl^– (Table 2). The crystallization path of such a studied brine can be predicted using the quinary system diagram (Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O). The crystallization path during evaporation of the studied brine is based mainly on the location of Jänecke coordinates () for the quinary system, Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O.

3.1.1. Predicted Crystallization Path

The predicted crystallization path during evaporation of the studied brine is based mainly on the location of the Jänecke coordinates of the initial studied brine on the Jänecke phase diagram for the Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O system. Figures 1(a)–1(c) showed that all the initial studied brines (point “O”) lie on the stability field of sylvite at different temperatures. If the solution reaches the drying-up point, the predicted crystallization path (pink dotted line in Figure 1) is “O-E-F-G-H-R” at 25°C (Figure 1(a)), “O-E-F-G-J-K” at 35°C (Figure 1(b)), and “O-E-F-G-J-K-M” at 50°C (Figure 1(c)).

(a)

(b)

(c)

In Figure 1(a), upon evaporation of the initial studied brine at 25°C, halite will be precipitated. When the brine is saturated with sylvite at point E, sylvite+halite will be precipitated along O-E. At point E, sylvite will be consumed, and carnallite+halite will be formed along E-F. At point F, bischofite will become stable and will be precipitated together with carnallite and halite along F-G. At point G, bischofite disappears and tachyhydrite+carnallite+halite are precipitated along G-H. At point H, tachyhydrite will be consumed, and antarcticite+halite will be formed along H-R. Evaporating to dryness leads to the final assemblage antarcticite+halite. Along the crystallization path “O-E-F-G-H-R,” further evaporation should make the brine evolve from its initial location at point “O” toward point “R.” Point “R” is a drying-up point at which the crystallization process terminates. The predicted crystallization path of 35°C and 50°C for the same processes as 25°C can be seen in Figures 1(b) and 1(c), respectively. We found that the predicted crystallization path become long with increasing temperature indicating that more types of crystalline minerals would occur.

3.1.2. Experimental Crystallization Path

The ionic compositions (mmol/kg H₂O) and Jänecke coordinates of the studied brine sample measured at different densities during experimental evaporation at 25°C, 35°C, and 50°C are given in Table 3. Brine evolution during evaporation is presented graphically on the Jänecke phase diagram for the Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O system (see Figure 1). Under this experiment condition (relative humidity, 36-43%), the experimental crystallization path (black solid line in Figure 1) is “O-e-f” at 25°C (Figure 1(a)), “O-e-f-m” at 35°C (Figure 1(b)), and “O-e-f-g-h” at 50°C (Figure 1(c)).

Table 4 shows the evaporation-crystallization path and mineral phase of predicted and experimental results. We found that the types of mineral phase increases with increasing temperature indicating that salt minerals are sensitive to temperature especially calcium-bearing mineral-like tachyhydrite. Figure 1 shows that the experimental crystallization path agrees well with the predicted crystallization path. However, in this case, the evaporation process stopped near point “F” at 25°C, point “G” at 35°C, and point “J” at 50°C in the experimental evaporation. This may be because of some extremely high salinity. Furthermore, relative humidity condition (in this study, 36-43%) prevents further evaporation. It can be seen that the crystallization path becomes long which is more beneficial to the tachyhydrite formation with the increase of evaporation temperature (from 25 to 50°C).

3.2. Evolution of Initial Studied Brine during Evaporation

The evolution of major ions during progressive evaporation is based on the principle of “chemical divide.” The basic idea of the chemical divide rule is “whenever a binary salt is precipitated during evaporation, and the initial molar proportion of the two ions forming this salt is not equal in solution, further evaporation will result in an increase in the concentration of the ion present in greater relative concentration in solution, and a decrease in the concentration of the ion present in lower relative concentration” [18].

A plot of the major ion concentrations as a function of density during evaporation at 25°C, 35°C, and 50°C is given in Figure 2. In Figure 2(a), for evaporation process at 25°C, at the beginning of evaporation, and at a starting density of 1.2043, the initial Cl^– ion concentration is greater than that of the Na⁺ and K⁺ ions. Therefore, during the evaporation of this brine, it is expected that halite precipitation should cause an increase in concentration of Cl^– and a depletion in Na⁺ concentration. The same effect will occur when sylvite and carnallite start precipitating, where Cl^– should build up in solution with decreasing K⁺ during progressive evaporation. The concentrations of Mg²⁺ and Ca²⁺ show a steady increase, for Ca²⁺ ions without any depletion. In Figure 2(b), for evaporation process at 35°C, the change in the concentrations of Na⁺, K⁺, Mg²⁺, Ca²⁺, and Cl^– is consistent with the change of these ions at 25°C evaporation process before bischofite start precipitating. Due to the precipitation of bischofite, the Mg²⁺ and Cl^– concentrations show a slightly decrease. In Figure 2(c), for evaporation process at 50°C, the change in the concentrations of Na⁺, K⁺, Mg²⁺, Ca²⁺, and Cl^– is consistent with the change of these ions at 25°C and 35°C evaporation process before tachyhydrite starts precipitating. Tachyhydrite precipitation should cause a decrease in concentration of Mg²⁺ and a depletion in Na⁺ and K⁺ concentration. Nevertheless, the Ca²⁺ concentration still shows an increasing trend.

(a)

(b)

(c)

3.3. Crystallization Sequences and Evaporation Stages

Based on the mineral types during experimental evaporation at 25°C, 35°C, and 50°C (see Table 4), the experimental crystallization sequences (see Figure 2) can be divided into (1) three sequences, namely, the halite, sylvite, and carnallite stages, at 25°C; (2) four sequences, namely, the halite, sylvite, carnallite, and bischofite stages, at 35°C; and (3) five sequences, namely, the halite, sylvite, carnallite, bischofite, and tachyhydrite stages, at 50°C.

3.4. The Relationship between Major Concentration and Density

In Figure 2(a), the first sequence was observed at densities from 1.2043 to 1.2745. In this sequence, the Na⁺ concentration decreases significantly once evaporation commences. This may occur because halite precipitation is accompanied by the effect of the principal of chemical divide where the initial Cl^– concentration is greater than the Na⁺ concentration. K⁺ increased steadily with evaporation until the K⁺ ion reached a maximum concentration at a density of 1.2745. The second sequence in experimental evaporation started at densities from 1.2745 to 1.2935. In this sequence, the Na⁺ ion concentration continued to decrease because of halite precipitation whereas the Cl^– ion concentration continued to increase. The K⁺ concentration after it reached a maximum at a density of 1.2745 started to decline at the next densities, whereas the Cl^– solution concentration continued to increase. This may be attributed to sylvite precipitation. The third sequence was defined at densities from 1.2935 to 1.3106. In this sequence, the K⁺ concentration decreases significantly because carnallite starts precipitating where Cl^– should continue to increase. Although the Mg²⁺ ion coprecipitated with the K⁺ ion during carnallite precipitation, the concentration of Mg²⁺ in solution continues to increase with evaporation. The Ca²⁺ ion showed a steady increase during evaporation in the three sequences without any depletion.

In Figure 2(b), the first sequence was observed at densities from 1.2008 to 1.2787. In this sequence, the Na⁺ concentration decreases significantly once evaporation commences. This may occur because halite precipitation is accompanied by the effect of the principal of chemical divide where the initial Cl^– concentration is greater than the Na⁺ concentration. K⁺ increased steadily with evaporation until the K⁺ ion reached a maximum concentration at a density of 1.2787. The second sequence of the experimental evaporation started at densities from 1.2787 to 1.3055. In this sequence, the Na⁺ ion concentration continued to decrease because of the halite precipitation whereas the Cl^– ion concentration continued to increase. After reaching its maximum value at a density of 1.2787, the K⁺ concentration started to decline at the next densities, whereas the Cl^– solution concentration continued to increase. This may be attributed to sylvite precipitation. The third sequence was defined at densities from 1.3055 to 1.4025. In this sequence, the K⁺ concentration decreases significantly because carnallite starts to precipitate where Cl^– should continue to increase. Although the Mg²⁺ ion coprecipitated with the K⁺ ion during carnallite precipitation, the concentration of Mg²⁺ in solution continues to increase with evaporation. The fourth sequence was defined at densities from 1.4025 to 1.4153. In this sequence, the Mg²⁺ and Cl^– concentrations decrease because bischofite starts to precipitate where Na⁺ and K⁺ should remain unchanged. The Ca²⁺ ion concentration showed a steady increase during evaporation in the four sequences without any depletion.

In Figure 2(c), the first sequence was observed at densities from 1.2019 to 1.2876. In this sequence, the Na⁺ concentration decreases significantly once evaporation commences. This may occur because halite precipitation is accompanied by the effect of the principal of chemical divide where the initial Cl^– concentration is greater than the Na⁺ concentration. K⁺ increased steadily with evaporation until the K⁺ ion reached its maximum concentration at a density of 1.2876. The second sequence of experimental evaporation started at densities from 1.2876 to 1.3438. In this sequence, the Na⁺ ion concentration continued to decrease because of the halite precipitation whereas the Cl^– ion concentration continued to increase. After reaching its maximum value at a density of 1.2787, the K⁺ concentration started to decline at the next densities, whereas the Cl^– solution concentration continued to increase. This may be attributed to sylvite precipitation. The third sequence was defined at densities from 1.3438 to 1.3594. In this sequence, the K⁺ concentration decreases significantly because carnallite starts to precipitate where Cl^– should continue to increase. Although the Mg²⁺ ion coprecipitated with the K⁺ ion during carnallite precipitation, the concentration of Mg²⁺ in solution continues to increase with evaporation. The fourth sequence was defined at densities from 1.3594 to 1.4266. In this sequence, the Mg²⁺ and Cl^– concentrations increase because bischofite starts to precipitate where Na⁺ and K⁺ should remain unchanged. The fifth sequence was defined at densities from 1.4266 to 1.4404. In this sequence, the Mg²⁺ concentration decreases because tachyhydrite starts to precipitate where Cl^– and Ca²⁺ should continue to increase. The Ca²⁺ ion concentration increased steadily during evaporation in the five sequences without any depletion.

3.5. Composition of Halite Inclusion from Cretaceous Evaporites on Jänecke Phase Diagram at 25°C

To obtain the location of halite inclusion from Cretaceous evaporites on the Jänecke phase diagram at 25°C for the quinary system, Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O, the concentrations of Na⁺, Mg²⁺, Ca²⁺, K⁺, and Cl^– in fluid inclusions from the Sergipe basin (13 fluid inclusions) and the Congo basin (16 fluid inclusions) were also used as shown in Table 5 [14]. Figure 3 shows that the Khorat Plateau brines lie on the stability field of sylvite, the Sergipe basin brines lie on the phase boundary of sylvite and carnallite, and the Congo basin brines lie on the stability field of carnallite. We found that (1) the concentration of Ca²⁺ increases with time (from Late Cretaceous to Early Cretaceous), in this case, MgCl₂- and CaCl₂-bearing salts such as bischofite and tachyhydrite are formed easily; (2) the concentration of K⁺ decreases slightly with an increase in time, in this case, sylvite does not occur; and (3) the MgCl₂/CaCl₂ ratio decreases with an increase in time. This result was consistent with the occurrence of the extremely soluble bischofite and tachyhydrite in a thickness of several tens of meters in the evaporite deposits of Congo basin [19].

3.6. Prediction of Mineral Precipitation Sequences in Halite Inclusion from Cretaceous Evaporites on Jänecke Phase Diagram at 50°C

There are three evaporite deposits in Cretaceous: the Early Cretaceous of the Sergipe basin, Brazil, the Congo basin, Republic of the Congo, and the Early to Late Cretaceous of the Khorat Plateau, Laos, and Thailand. The potash deposits of the Khorat Plateau include only two potassium minerals, sylvite (Sy.) and carnallite (Car.), and the latter is by far the most abundant. A common constituent of carnallite deposits is the mineral tachyhydrite. Although tachyhydrite generally is in amounts of less than 30 percent of the total carnallite deposits, it locally forms nearly pure layers as much as 16 m thick [20]. Tachyhydrite is present with halite and carnallite and is concentrated mostly in the basin centers [7]. The vertical evaporite mineral succession in the Ibura member of the Muribeca Formation in the Sergipe basin is anhydrite, halite, carnallite, and tachyhydrite, with beds of tachyhydrite as thick as 100 m [1]. Tachyhydrite is located within the central and deepest portions of the Sergipe basins [1, 21]. The basic salt cycle of the Congo basin is made up from bottom to top by (1) a thin black shale, (2) a layer of halite, (3) a mixture of halite and carnallite, and (4) bischofite and/or tachyhydrite [19]. In general, mineral succession in the Khorat Plateau, Sergipe, and Congo basin are “Ha, Ha+Sy,” “Ha, Ha+Car, Ha+Car+Tac,” and “Ha, Ha+Car, Ha+Bis+Tac,” respectively (see Figures 4(d)–4(f)). The mineral precipitation sequences predicted from their own fluid inclusion in halite, depending on the quinary system, Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O at 50°C, are “Ha, Ha+Sy, Ha+Car, Ha+Bis+Car,” “Ha, Ha+Car, Ha+Car+Tac,” and “Ha, Ha+Car, Ha+Car+Tac,” respectively (see Figures 4(a)–4(c)). The evaporite mineral succession in the Khorat Plateau, Sergipe, and Congo basins agrees well with the mineral precipitation sequences predicted from their own fluid inclusion in halite, depending on the quinary system, Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O, at 50°C. These simulated evolution paths by the application of Jänecke phase diagram for fluid inclusion brines indicated that a hot state is favorable for tachyhydrite formation.

3.7. Formation Conditions of Tachyhydrite

3.7.1. Calcium Source

CaCO₃ and CaSO₄ are relatively insoluble salts, and because of an excess of sulfate in normal seawater relative to calcium, the small amount of calcium present tends to be removed completely from the solution at an early evaporation stage. Calcium is no longer available for calcium salt formation, such as tachyhydrite formation, during the later stages of brine concentration. Tachyhydrite, a rare mineral, occurs in Cretaceous evaporites such as marine evaporites from the Khorat Plateau, Sergipe, and Congo evaporites. At an advanced stage of evaporation, brines must have been enriched significantly in CaCl₂ and depleted in SO₄, and the question arises as to how these brines become enriched in calcium and depleted in sulfate. Possible processes are (1) a release of calcium during dolomitization [22–25], (2) a liberation of calcium through ion exchange with clays [26], (3) an addition of groundwater that may be meteoric, connate, or juvenile in origin, but which has become enriched in calcium [27], and (4) a reduction in sulfate by bacteria or by other processes [28–30]. These alternatives are difficult to evaluate because evaporite basins tend to be open systems with in- and out-flowing brine steams [1]. Deposition can occur from brine which is flowing rather than static. With pronounced lateral fractionation, it would be possible for basal carbonates and sulfates to exist in one portion of an evaporite basin but not in another. Incomplete information on the lateral distribution of evaporite facies in connected subbasins and a lack of knowledge on how brines flowed or were decanted from one basin to another makes it difficult to reconstruct the factors responsible for the progressive change in brine composition. In previous work, the source of calcium from brines enriched in CaCl₂ is still being debated. Wardlaw [1] argued that brines enriched in CaCl₂ and depleted in sulfate may have resulted in a modification of marine brines by CaCl₂-rich groundwater discharge. Brines with these characteristics exist in the Red and Dead seas today and provide an interesting analog. Vysotskiy [8] also argued that chloride brines became enriched in calcium in late stages mainly through the outflow of CaCl₂-rich brines from underlying rocks along faults and weakened zones. Hardie [31] proposed that midocean ridge hydrothermal brines became enriched in CaCl₂ and impoverished in MgSO₄ compared with parent seawater. Particularly extensive carbonate platforms existed during periods of high sea level during the Cretaceous, and these platforms were conducive to widespread dolomitization during seawater evaporation, which would take up Mg and release calcium [32–35]. Some evidence of hydrothermal activates and a thermal event during the Cretaceous exists in the Khorat Plateau, which may have supplied CaCl₂-rich waters into the basin [9]. Hite and Japakasetr [20] suggested that a Khorat sea that could produce tachyhydrite deposits is compatible with a lack of magnesium sulfate minerals because the necessary concentration of Ca²⁺ should have kept the brine purged of SO₄^2- because of the reaction, . EI Tabakh et al. [7] argued that CaCl₂-rich brines enter the waters of the basin and modify the chemistry of the surface waters and of the associated groundwater because of early halite replacement of primary gypsum. Variations in major-ion chemistry of Cretaceous seawater may explain the “anomalous” evaporites from the Khorat Plateau [1, 14, 20, 36–39]. This result is consistent with our observation that tachyhydrite will be precipitated by a simulation of evaporation of fluid inclusion in halite from the Khorat Plateau. Therefore, to form tachyhydrite, the calcium source may be mainly from CaCl₂-type hydrothermal brines because of the link between seawater chemistry and the midocean ridge hydrothermal brine flux.

3.7.2. Climate and Extreme Temperature

Ancient marine evaporite deposition required particular climate, eustatic, or tectonic juxtapositions. When megaevaporite settings were active within appropriate arid climatic and hydrological settings, huge volumes of seawater were drawn into the sub-sea-level evaporitic depressions. These systems were typical of a region where the evaporation rates of ocean waters were at a maximum and thus were centered on past latitudinal equivalents of today’s horse latitudes [40]. The term horse latitudes encompass the regions beneath the north and south subtropical high atmospheric pressure belts [41]. The subtropical high atmospheric pressure belts are marked by dry sinking air masses that form part of the world’s Hadley cells. They are centered around 35°N and 30°S and extend to ~25°N to 40°S, respectively [40]. The paleolatitudes of ~21.1–21.3°N for the Cretaceous Indochina Block (Vientiane) and ~20.9–27.6°N for the Cretaceous-Paleocene Lanping-Simao Blocks (Jiangcheng) indicate that these regions are situated in the subtropical high atmospheric pressure belts [42]. The estimated palaeoposition of the Khorat Plateau at ~21–26°N during the Jurassic to Cretaceous also supports that these regions are situated in the subtropical high atmospheric pressure belts [43]. Accordingly, the extreme arid state of the subtropical high atmospheric pressure belts is also favorable for tachyhydrite formation.

The abovementioned discussion indicates that the formation of tachyhydrite is sensitive to temperature. The Cretaceous was a time of hothouse climate, elevated atmospheric CO₂, relatively warm surface and deep ocean waters, and high sea levels [44–56]. Apparently, the mid-Cretaceous experienced the highest temperatures in the past 100 Ma [57–60]. Evidence from reconstructed temperatures for the mid-Cretaceous Khorat Plateau [61] (62.1°C), the Silurian Michigan Basin [62] (59°C), and the Quaternary Lop Nur [63] (35.6-43°C) showed that hot conditions were probably a prerequisite for the formation of potash deposits. Li et al. [64] also proposed that hydrothermal fluids may have played an active role in the formation of the Mengye potash deposits in the Lamping-Simao Basin. Accordingly, a hot state is favorable for evaporite formation.

3.7.3. Thermal Resource

A salinity gradient solar pond (SGSP) is a simple and effective way to capture and store solar energy. A SGSP consists of three distinct zones, namely, an upper convecting, lower convecting, and nonconvecting zone (see Figure 5). Lake brines typically absorb solar energy as heat. Normally, this heat is lost as warm water rises to the surface of the lake and cools by radiation, convection, and evaporation. Water, however, is a poor conductor of heat, and therefore, if this natural circulation is stopped by the presence of a salinity gradient that causes the fluid density to increase with depth, then heat can become trapped at the bottom of the lake. The temperature profiles are correlated positively with SGSP salinity [65]. The temperature in the storage zone can exceed 90°C during the summer whereas it can exceed 50°C in the winter for SGSPs in the Middle East [66]. In Cretaceous, therefore, a SGSP will occur in the Khorat Plateau, because a paleolatitude of the Cretaceous Indochina Block lies on the subtropical high atmospheric pressure belts and tachyhydrite-saturated brine has a high density (1.4266-1.4404 in this study). Tachyhydrite crystallization occurs under extreme dryness and high temperature, and the thermal resource may result from the temperature profile in a SGSP.

3.7.4. Preservation

The hygroscopic and extremely deliquescent nature of tachyhydrite means that tachyhydrite could not have been exposed to the atmosphere during or after deposition because, under such conditions, it alters rapidly to a bischofite residue. Wardlaw [1] proposed that there must always have been a covering of brine, and a “dry-lake” stage could not have been reached during tachyhydrite deposition, even though extreme evaporation and volume reduction are required for its formation. Hardie [6] also argued that tachyhydrite must always precipitate subaqueously, and thus, its rarity in nature may be a factor in preservation potential and not a lack of precipitation. Tachyhydrite is located within the central and deepest portion of the Sergipe basins [1, 21]. The very regular upward increase in bromine (from 3000 to 3700 ppm) and the constancy of strontium (average value of 1800 ppm) through the upper tachyhydrite zone could only have been achieved where crystallization was uninterrupted by sudden influxes of marine water and where crystals were not subjected to later solution or diagenetic alteration [1]. At this deposition stage, the basin may have been closed completely to marine influxes and tachyhydrite could have crystallized from tachyhydrite-saturated brine, although the brine depth may have been considerably greater [1]. A deep brine circumstance, therefore, is probably a prerequisite for tachyhydrite preservation.

4. Conclusion

Tachyhydrite occurs in Cretaceous graben and half-graben basin evaporite deposits in Brazil, western Africa, and Thailand. At an advanced stage of evaporation, the brines which produced these deposits must have abnormally enriched CaCl₂. Brines with these characteristics have been verified by the available data for the composition of fluid inclusion in halite from these evaporite deposits [14, 37, 39, 67–69]. However, the mechanism of tachyhydrite formation in those evaporites is still being debated. The present work dealt with studying the mineral crystallization sequences during the experimental evaporation of an average concentration fluid inclusion involving the system Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O employed from the Khorat Plateau evaporite primary halite. The results showed (1) a halite, sylvite, and carnallite stage at 25°C; (2) a halite, sylvite, carnallite, and bischofite stage at 35°C; and (3) a halite, sylvite, carnallite, bischofite, and tachyhydrite stage at 50°C. The crystallization paths were simulated by the application of the Jänecke phase diagram at 25°C, 35°C, and 50°C involving the system Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O. The crystallization sequence predicted on the Jänecke phase diagram showed a good agreement with the experimental crystallization sequences and paths. These results indicated that the precipitation of tachyhydrite is sensitive to the temperature.

A simulated evaporation-crystallization path on the Jänecke phase diagram at 50°C involving the system Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O model was carried out to predict the mineral precipitation sequences for fluid inclusion in halite from the Khorat Plateau, Sergipe, and Congo basins. The model showed that the evaporite mineral succession in the Khorat Plateau, Sergipe, and Congo basins agrees well with the mineral precipitation sequences predicted from their own fluid inclusion in halite. We found that a hot state is favorable for tachyhydrite formation.

Because of the occurrence of tachyhydrite precipitation in the late evaporation stage and its saturated brine with a high density, we proposed that the temperature profile in the solar pond may have played an active role in the formation of tachyhydrite. In addition, tachyhydrite could not have been exposed to the atmosphere during or after deposition. The surface layer of highly concentrated brine acted as a screen protecting the tachyhydrite from the atmosphere. So, tachyhydrite could have crystallized from tachyhydrite saturated brine and been preserved in a deep brine.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by the applied Basic Research Project of Qinghai Province (Grant No. 2016-ZJ-781) and the National Program on Key Basic Research Project of China (973 Program) (Grant No. 2011CB403004) for financial support.

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Copyright © 2019 Huaide Cheng et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Geofluids

The Sensitivity of Temperature to Tachyhydrite Formation: Evidence from Evaporation Experiments of Simulated Brines Based on Compositions of Fluid Inclusions in Halite

Abstract

1. Introduction

2. Experimental

2.1. Reagents and Materials

2.2. Evaporation Procedure

2.3. Analytical Methods

2.4. Jänecke Phase Diagram Analysis for the Quinary System, Na+, K+, Mg2+, Ca2+//Cl–-H2O

3. Results and Discussions

3.1. Crystallization Path during Evaporation

3.1.1. Predicted Crystallization Path

3.1.2. Experimental Crystallization Path

3.2. Evolution of Initial Studied Brine during Evaporation

3.3. Crystallization Sequences and Evaporation Stages

3.4. The Relationship between Major Concentration and Density

3.5. Composition of Halite Inclusion from Cretaceous Evaporites on Jänecke Phase Diagram at 25°C

3.6. Prediction of Mineral Precipitation Sequences in Halite Inclusion from Cretaceous Evaporites on Jänecke Phase Diagram at 50°C

3.7. Formation Conditions of Tachyhydrite

3.7.1. Calcium Source

3.7.2. Climate and Extreme Temperature

3.7.3. Thermal Resource

3.7.4. Preservation

4. Conclusion

Data Availability

Conflicts of Interest

Acknowledgments

References

Copyright

2.4. Jänecke Phase Diagram Analysis for the Quinary System, Na⁺, K⁺, Mg²⁺, Ca²⁺//Cl^–-H₂O