Specific features of the formation of elementary sedimentary rhythms of rock salt in facies zones of the Permian halogenic basin of the Caspian region

Based on the composition of brine inclusions in halite, the formation conditions of elementary sedimentation rhythms of rock salt were determined in facies zones of the Permian halogenic basin of the Caspian region. The distribution of structural types of halite was analyzed and the rate of brine condensation was estimated by calculating the K⁺ concentration coefficients during the precipitation of 1 m rock salt (K₁ g/l/m) and one elementary rhythm (K₂ g/l/rhythm). It was established that the rock salt of the Karpensk and Lugovsk rhythmic beds (hereafter, rhythmites) at the base and middle part of the halogenic sequence of the Baskunchak structure precipitated in a relatively deep-water lithofacies zone at a constant influx of NaCl-saturated brine and its slow condensation. This is reflected in insignificant variations of the thickness of elementary rhythms. In troughs at the northwestern framing of the depression, rocks salts of the Privolzhsk and partially, Lugovsk rhythmites were precipitated without the condensation of brine owing to the partial dissolution of halite in the coastal zones of the basin. The anomalously thick elementary rhythms (up to 30–40 cm) were composed of coarse-crystalline and water-transparent feather halite deposited at the bottom. A rapid condensation of brine was also noted during the precipitation of rock salt underlying the K-Mg saliferous rocks in the troughs. In this area, the brine experienced density stratification, with the influx of both condensed brine from the central part of the basin and less dense brine from the inlet.     

An integrated methodology for salt damage assessment and remediation: the case of San Jerónimo Monastery (Granada, Spain)

San Jerónimo Monastery (Granada, Spain) was selected as a case study for the investigation of the effect of indoor environmental conditions on salt weathering and for on-site testing of a remediation treatment using crystallization inhibitors on account of the extreme salt damage affecting both the building stone, a biomicritic limestone, calcarenite and wall paintings. A methodology combining several analysis techniques, phenomenological observations, salt and moisture analysis, environmental monitoring and thermodynamic simulations, was adopted in order to study the salt damage problems affecting this building. Within the collected samples, the majority of salts were found to be magnesium sulphate in the form of either hexahydrite or epsomite, depending on the climate conditions, together with minor amounts of gypsum, nitrates and chlorides. Comparison of empirical observations with thermodynamic simulations of the salt mixture behaviour clearly showed that salt-induced damage events take place during the seasonal changes from spring to summer and winter to spring. An aqueous solution of an organic phosphonate, which in laboratory experiments was found to be an effective inhibitor of magnesium sulphate crystallization, was sprayed over a selected test area of unpainted stonework at the site. Preliminary results seem to indicate that after the application of the treatment both the amount of efflorescence and ongoing damage to the stone support is reduced. However, long-term monitoring of the future condition of the test area is needed to confirm whether indeed this treatment is appropriate and effective in reducing salt damage at this case study site. The outcome of this study extends beyond the particular problems at San Jerónimo Monastery, as it demonstrates a methodological approach for the study and evaluation of salt weathering problems affecting cultural heritage.     

Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4-H2O and the generation of stress

Mechanical disintegration by crystal growth of salts in pores is generally considered as an important mechanism of rock breakdown both on Earth and on Mars. Crystal growth is also a major cause of damage in porous building materials. Sodium sulfate is the most widely used salt in accelerated weathering tests of natural rocks and building materials. This paper provides an updated phase diagram of the Na₂SO₄-H₂O system based on a careful review of the available thermodynamic data of aqueous sodium sulfate and the crystalline phases. The phase diagram includes both the stable phases thenardite, Na₂SO₄(V), and mirabilite, Na₂SO₄·10H₂O, and, the metastable phases Na₂SO₄(III) and Na₂SO₄·7H₂O. The phase diagram is used to discuss the crystallization pathways and the crystallization pressures generated by these solids in common laboratory weathering experiments and under field conditions. New crystallization experiments carried out at different temperatures are presented. A dilatometric technique is used to study the mechanical response of sandstone samples in typical wetting-drying experiments as in the standard salt crystallization test. Additional experiments with continuous immersion and evaporation were carried out with the same type of sandstone. Both, the theoretical treatment and the results of the crystallization experiments confirm that the crystallization of mirabilite from highly supersaturated solutions is the most important cause of damage of sodium sulfate in porous materials.     

Experimental definition of microclimatic conditions based on water transfer and porous media properties for the conservation of prehistoric constructions: Cueva Pintada at Galdar,Gran Canaria,Spain

Microclimatic parameters and natural materials were studied in order to assess conservation of the cave complex at Galdar, Gran Canaria. Based on the microclimatic data, experimental simulations were carried out to quantify water retention capacity and water vapour transport kinetics under continuously changing extreme temperatures and relative humidity values. The behaviour of natural construction materials is greatly influenced by changes in thermo-hygrometric conditions and is linked to pore structure. The host rock has a complex porous media: high porosity and polymodal pore size distribution, where the smallest pores contribute to water condensation, whilst large pores lead to high water absorption rates. The axial compressive strength of the host-rock decreases between 30 and 70% at water saturation. Stuccos covering cave wall paintings are formed by clay minerals, mainly smectites. These clay minerals cover a large specific surface area, which leads to a high capacity for water condensation and retention. It is also shown that neither water condensation nor vapour transport are noticeably modified by the presence of stucco on the host-rock when rapid, highly variable changes occur in environmental conditions if large shrinkage cracks are present. Results show that safe threshold microclimatic conditions can be found below 75% RH in the natural temperature range and that slight variations in temperature and relative humidity do not modify durability properties.     

Metasomatism and ore formation at contacts of dolerite with saliferous rocks in the sedimentary cover of the southern Siberian platform

The data on the mineral composition and crystallization conditions of magnesian skarn and magnetite ore at contacts of dolerite with rock salt and dolomite in ore-bearing volcanic—tectonic structures of the Angara—Ilim type have been integrated and systematized. Optical microscopy, scanning and transmission electron microscopy, electron microprobe analysis, electron paramagnetic resonance, Raman and IR spectroscopy, and methods of mineralogical thermometry were used for studying minerals and inclusions contained therein. The most diverse products of metasomatic reactions are found in the vicinity of a shallow-seated magma chamber that was formed in Lower Cambrian carbonate and saliferous rocks under a screen of terrigenous sequences. Conformable lodes of spinel-forsterite skarn and calciphyre impregnated with magnesian magnetite replaced dolomite near the central magma conduit and apical portions of igneous bodies. At the postmagmatic stage, the following mineral assemblages were formed at contacts of dolerite with dolomite: (1) spinel + fassaite + forsterite + magnetite (T = 820?740°C), (2) phlogopite + titanite + pargasite + magnetite (T = 600–500°C), And (3) clinochlore + serpentine + pyrrhotite (T = 450°C and lower). Rock salt is transformed at the contact into halitite as an analogue of calciphyre. The specific features of sedimentary, contact-metasomatic, and hydrothermal generations of halite have been established. The primary sedimentary halite contains solid inclusions of sylvite, carnallite, anhydrite, polyhalite, quartz, astrakhanite, and antarcticite; nitrogen, methane, and complex hydrocarbons have been detected in gas inclusions; and the liquid inclusions are largely aqueous, with local hydrocarbon films. The contact-metasomatic halite is distinguished by a fine-grained structure and the occurrence of anhydrous salt phases (CaCl₂ · KCl, CaCl₂, nMgCl₂ · mCaCl₂) and high-density gases (CO₂, H₂S, N₂, CH₄, etc.) as inclusions. The low-temperature hydrothermal halite, which occurs in skarnified and unaltered silicate rocks and in ore, is characterized by a low salinity of aqueous inclusions and the absence of solid inclusions. The composition and aggregative state of inclusions in halite and forsterite indicate that salt melt-solution as a product of melting and dissolution of salt was the main agent of high-temperature metasomatism. Its total salinity was not lower than 60%. The composition and microstructure of magnetite systematically change in different mineral assemblages. Magnetite is formed as a result of extraction of iron together with silicon and phosphorus from dolerite. The first generation of magnetite is represented by mixed crystals, products of exsolution in the Fe-Mg-Al-Ti-Mn-O system. The Ti content is higher at the contact of dolerite with rock salt, whereas, at the contact with dolomite, magnetite is enriched in Mg. The second generation of magnetite does not contain structural admixtures. The distribution of boron minerals and complex crystal hydrates shows that connate water of sedimentary rocks could have participated in hydrothermal metasomatic processes.     

Crystallization behavior of Na2SO4–MgSO4 salt mixtures in sandstone and comparison to single salt behavior

We report on the crystallization behavior and the salt weathering potential of Na₂SO₄, MgSO₄ and an equimolar mixture of these salts in natural rock and porous stone. Geochemical modeling of the phase diagram of the ternary Na₂SO₄–MgSO₄–H₂O system was used to determine the equilibrium pathways during wetting (or deliquescence) of incongruently soluble minerals and evaporation of mixed electrolyte solutions. Model calculations include stable and metastable solubilities of the various hydrated states of the single salts and the double salts Na₂Mg(SO₄)₂·4H₂O (bloedite), Na₂Mg(SO₄)₂·5H₂O (konyaite), Na₁₂Mg₇(SO₄)₁₃·15H₂O (loeweite) and Na₆Mg(SO₄)₄ (vanthoffite). In situ Raman spectroscopy was used to study the phase transformations during wetting of pure MgSO₄·H₂O (kieserite) and of the incongruently soluble salts bloedite and konyaite. Dissolution of kieserite leads to high supersaturation resulting in crystallization of higher hydrated phases, i.e. MgSO₄·7H₂O (epsomite) and MgSO₄·6H₂O (hexahydrite). This confirms the high damage potential of magnesium sulfate in salt damage of building materials. The dissolution of the incongruently soluble double salts leads to supersaturation with respect to Na₂SO₄·10H₂O (mirabilite). However, the supersaturation was insufficient for mirabilite nucleation. The damage potential of the two single salts and an equimolar salt mixture was tested in wetting–drying experiments with porous sandstone. While the high damage potential of the single salts is confirmed, it appears that the supersaturation achieved during wetting of the double salts at room temperature is not sufficient to generate high crystallization pressures. In contrast, very high damage potentials of the double salts were found in experiments at low temperature under high salt load.1     

Salt Crystallization Sequences of Nonmarine Brine and Their Application for the Formation of Potassium Deposits

The salt assemblages precipitated during evaporation of concentrated brine collected from Gasikule Salt Lake (GSL) were studied to better understand the formation of potassium deposits in the Qaidam Basin. The study included isothermal evaporation at 25 °C in the laboratory and solar evaporation in the ponds at GSL field. Brines increased in density and became moderately acidic (pH?≈?5.30) while major ion geochemistry and precipitate mineralogy all showed broad agreement between both systems. Four salt assemblages were identified in the isothermal evaporation experiment: halite?→?halite?+?hexahydrite?→?halite?+?bischofite?+?carnallite?→?bischofite. Alternately, three salt assemblages were recognized in the solar evaporation: halite?→?halite?+?epsomite?+?carnallite?→?halite?+?carnallite?+?bischofite. The key difference in salt assemblages between the two systems is attributed to differences in relative humidity and temperature conditions. Although the GSL has deep spring inflow recharge, the high abundance of MgSO₄ salts demonstrates that the salt assemblages are similar to normal seawater evaporation. Thus, different proportions of deep spring inflow and river water could form both MgSO₄-deficient potassium evaporite and normal seawater potassium evaporites. Therefore, nonmarine water may form diverse potassium evaporite deposits in continental basins when the geological structure as well as hydrogeological and climatic conditions is appropriate.     

Crystallization behavior of NaNO3–Na2SO4 salt mixtures in sandstone and comparison to single salt behavior

We report on the crystallization behavior and the salt weathering potential in natural rock and porous stone of single salts (NaNO₃, Na₂SO₄) and salt mixtures in the ternary NaNO₃–Na₂SO₄–H₂O system. Geochemical modeling of the phase diagram of the ternary NaNO₃–Na₂SO₄–H₂O system was used to determine the equilibrium pathways during wetting (or deliquescence) of incongruently soluble minerals and evaporation of mixed electrolyte solutions. Experiments were carried out in order to study the phase changes during dissolution either induced by deliquescence or by the addition of liquid water. In situ Raman spectroscopy was used to study the phase transformations during wetting of pure Na₂SO₄ (thenardite) and of Na₃NO₃SO₄·H₂O (darapskite). In both experiments crystallization of Na₂SO₄·10H₂O (mirabilite) from highly supersaturated solutions is demonstrated confirming the high salt weathering potential of thenardite and darapskite wetting. In order to study the damage potential of darapskite experimentally, wetting–drying experiments with porous sandstone with the two single salts (Na₂SO₄, NaNO₃) and two NaNO₃–Na₂SO₄ salt mixtures were carried out. Different destructive and non-destructive techniques were tested for damage monitoring. NaNO₃ was found to be the least damaging salt and Na₂SO₄ is the most damaging one. The classification of the two salt mixtures was less obvious.     

Geochemical characteristics and controlling factors of chemical composition of groundwater in a part of Guntur district,Andhra Pradesh,India

A survey on quality of groundwater was carried out for assessing the geochemical characteristics and controlling factors of chemical composition of groundwater in a part of Guntur district, Andhra Pradesh, India, where the area is underlain by Peninsular Gneissic Complex. The results of the groundwater chemistry show a variation in pH, EC, TDS, Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃ ^?, Cl^?, SO₄ ^2?, NO₃ ^? and F^?. The chemical composition of groundwater is mainly characterized by Na⁺?HCO₃ ^? facies. Hydrogeochemical type transits from Na⁺–Cl^?–HCO₃ ^? to Na⁺–HCO₃ ^?–Cl^? along the flow path. Graphical and binary diagrams, correlation coefficients and saturation indices clearly explain that the chemical composition of groundwater is mainly controlled by geogenic processes (rock weathering, mineral dissolution, ion exchange and evaporation) and anthropogenic sources (irrigation return flow, wastewater, agrochemicals and constructional activities). The principal component (PC) analysis transforms the chemical variables into four PCs, which account for 87% of the total variance of the groundwater chemistry. The PC I has high positive loadings of pH, HCO₃ ^?, NO₃ ^?, K⁺, Mg²⁺ and F^?, attributing to mineral weathering and dissolution, and agrochemicals (nitrogen, phosphate and potash fertilizers). The PC II loadings are highly positive for Na⁺, TDS, Cl^? and F^?, representing the rock weathering, mineral dissolution, ion exchange, evaporation, irrigation return flow and phosphate fertilizers. The PC III shows high loading of Ca²⁺, which is caused by mineral weathering and dissolution, and constructional activities. The PC IV has high positive loading of Mg²⁺ and SO₄ ^2?, measuring the mineral weathering and dissolution, and soil amendments. The spatial distribution of PC scores explains that the geogenic processes are the primary contributors and man-made activities are the secondary factors responsible for modifications of groundwater chemistry. Further, geochemical modeling of groundwater also clearly confirms the water–rock interactions with respect to the phases of calcite, dolomite, fluorite, halite, gypsum, K-feldspar, albite and CO₂, which are the prime factors controlling the chemistry of groundwater, while the rate of reaction and intensity are influenced by climate and anthropogenic activities. The study helps as baseline information to assess the sources of factors controlling the chemical composition of groundwater and also in enhancing the groundwater quality management.     

Chemical weathering in the Three Rivers region of Eastern Tibet

Three large rivers - the Chang Jiang (Yangtze), Mekong (Lancang Jiang) and Salween (Nu Jiang) - originate in eastern Tibet and run in close parallel over 300 km near the eastern Himalayan syntaxis. Seventy-four river water samples were collected mostly during the summer season from 1999 to 2004. Their major element compositions vary widely, with total dissolved solids (TDS) ranging from 31 to 3037 mg/l, reflecting the complex geologic makeup of the vast drainage basins. The major ion distribution of the main channel samples primarily reflects the weathering of carbonates. Evaporite dissolution prevails in the headwater samples of the Chang Jiang in the Tibetan Plateau interior, as evidenced by the high TDS (928 and 3037 mg/l) and the Na-Cl dominant major element composition. Local tributary samples of the Mekong and Salween, draining the Lincang Batholith and the Tengchong Volcano, show distinctive silicate weathering signatures. We used five reservoirs - rain, halite, sulfate, carbonate, and silicate - in a forward model to calculate the contribution from silicate weathering to the total dissolved load and to estimate the consumption rate of atmospheric CO₂ by silicate weathering. Carbonate weathering accounts for about 50% of the total cationic charge (TZ⁺) in the samples of the Mekong and the Salween exiting the Tibetan Plateau. In the “exit” sample of the Chang Jiang, 45% of TZ⁺ is from halite dissolution inherited from the extreme headwater tributaries in the interior of the plateau, and carbonates contribute only 26% to the TZ⁺. The net rate of CO₂ consumption by silicate weathering is (103-121) × 10³ mol km⁻² year⁻¹, lower than the rivers draining the Himalayan front. GIS-based analyses indicate that runoff and relief can explain 52% of the spread in the rate of atmospheric CO₂ drawdown by silicate weathering, but other climatic (temperature, precipitation, potential evapotranspiration) and geomorphic (elevation, slope) factors also show collinearity. Only qualitative conclusions can be drawn for the significance of lithology due to lack of digitized lithologic information. The effect of the peculiar drainage pattern due to tectonic forcing is not readily apparent in the major element composition or in increased chemical weathering rates. The ⁸⁷Sr/⁸⁶Sr ratios and the silicate weathering rates are in general lower in the Three Rivers than in the rivers draining the Himalayan front.     

Geochemistry of modern seawater and brines from salt pans: Main components and bromine distribution

Seawater at different stages of evaporation from the salt works of Seovlje near Portoro (Yugoslavia) was analyzed geochemically. The seawater there passes through 20 stages of concentration until the first halite crystallizes. All important parameters were determined at all stages: concentrations of Cl, SO₄, Na, K, Ca, Mg, and Br, temperature, pH, Eh, oxygen content and titration alkalinity. With increasing evaporation calcium carbonate crystallizes first followed by calcium sulphate as gypsum and after these halite. All three components crystallize from supersaturated solutions. The pH of the initial seawater is 8.32; it falls abruptly to 6.65 when the first calcium carbonate precipitates. Eh in the original seawater is +393 mv; negative values were found in the halite crystallization pans, which contain an anaerobic mud as a reducing agent. The oxygen content of the solution decreases parallel to the drop in Eh. In the pans in which NaCl crystallizes Eh is zero. Apparently the mud also adsorbs K, as can be inferred from a change in the Mg/K ratio.The bromine partition between crystallizing halite and the brine in the salt pans of Seovlje is discussed with regard to some genetic problems of marine salt deposits. In the conditions in the salt pans the bromine partition coefficient at the beginning of NaCl crystallization from seawater—expressed as b=wt.-% Br (mineral)/wt.-% Br (solution)—is 0.12 to 0.14 or—expressed as D=Br/Cl(mineral)/Br/Cl(solution)—0.030 to 0.034 at temperatures between 33° C and 42° C. The conclusion is that a Br content of about 60 to 75 ppm is to be expected for the first halite that crystallizes from evaporating seawater.     

Mineral equilibria in a six-component seawater system,Na-K-Mg-Ca-SO4-Cl-H2O,at 25°C

Phase relations in the 6-component system Na-K-Mg-Ca-SO₄-Cl-H₂O have been calculated for halite saturation, 25°C and 1 atm pressure. Using a Jänecke projection with the apices Ca-Mg-K₂-SO₄, 27 stable invariant points have been located which are connected by 69 univariant curves. Polyhalite is the only quaternary solid, but anhydrite occupies the bulk of the interior tetrahedral space. Consequently, 24 of the invariant points lie very close to the Ca-free base, Mg-K₂-SO₄. The remaining three points involve tachyhydrite and/or antarcticite. All points but two (20,27) represent peritectic conditions. Metastable equilibria have been calculated for the Ca-free system and yield relations corresponding to the solar diagram.Seawater lies in the subspace anhydrite-halite-carnallite-kieserite-bischofite (point 20) and its evaporation has been discussed for conditions of equilibrium and fractional crystallization. After gypsum is converted to anhydrite, halite precipitates. The next phase, under equilibrium conditions, is glauberite, crystallizing at the expense of anhydrite. Continued evaporation leads to glauberite resorption and eventual replacement by polyhalite. Then follow the magnesium sulfates epsomite, hexahydrite and kieserite, which are joined by carnallite. Polyhalite is replaced by anhydrite and bischoflte is added at the final invariant condition. Kainite does not appear as a primary phase under equilibrium conditions, but it is an important phase during fractional crystallization, where Ca-phases are not allowed to back-react with the brine.Up to the appearance of glauberite, thickness ratios of halite: anhydrite couplets (equilibrium or fractionation) can vary from 0 to 7, the relative amount of halite increasing with more intense evaporation. During evaporation, the activity of H₂O decreases from 0.98 (seawater) to 0.34 (final invariant brine). The data provided can be used to evaluate the effects of mineral precipitation, evaporation and brine mixing for a wide variety of natural brines.     

The garnets of the eastern area of the Sierra de Guadarrama,Sistema Central,Spain

Electron-probe microanalysis of a series of garnets in metapelitic rocks of the chloritoid staurolite, kyanite and sillimanite metamorphic zones, eastern area of the Sierra de Guadarrama, Sistema Central, Spain, manifest the well-known cryptozonation commonly observed in these minerals, with MgO and FeO increasing and MnO and CaO decreasing from the center to the outer rim of the crystals.The differences in composition of the garnets, from one metamorphic zone to another, is mainly a result of small differences in composition of the host-rock, since: (1) the amounts of MnO in the garnet are controlled by the amounts of SiO₂, Al₂O₃ and FeO present in the host-rock; and (2) the percentages of MnO and MgO of the parent-rock influence in some way the concentration of CaO in the garnet, and those of MnO, Al₂O₃ and CaO influence the concentration of FeO. Nevertheless, the amount of FeO in the garnet is finally controlled, due to the diadochy, by the concentration of MnO + CaO in this mineral.     

硫酸盐对麦积山砂砾岩风化影响的试验研究

麦积山石窟砂砾岩在盐分参与下出现粉化、剥落等现象,这大大加速了石窟岩体的风化进程。通过对砂砾岩进行毛细迁移及循环劣化试验,研究了硫酸盐在砂砾岩中的运移规律,分析了盐分参与下砂砾岩的破坏特征,并得出了其产生的结晶压力。结果表明：硫酸钠结晶使砂砾岩发生显著破坏;硫酸盐在砂砾岩中的运移呈现一定的规律性;硫酸钠溶液的浓度达到0.95～1.13 mol/L区间时,砂砾岩开始破坏,并且得出试验条件下结晶压力理论最大值可以达到33 MPa。     

Climatic and lithologic controls on the temporal and spatial variability of CO2 consumption via chemical weathering: An example from the Australian Victorian Alps

A detailed geochemical study on river waters of the Australian Victorian Alps was carried out to determine: (i) the relative significance of silicate, carbonate, evaporite and sulfide weathering in controlling the major ion composition and; (ii) the factors regulating seasonal and spatial variations of CO₂ consumption via silicate weathering in the catchments. Major ion chemistry implies that solutes are largely derived from evaporation of precipitation and chemical weathering of carbonate and silicate lithologies. The input of solutes from rock weathering was determined by calculating the contribution of halite dissolution and atmospheric inputs using local rain and snow samples. Despite the lack of carbonate outcrops in the study area and waters being undersaturated with respect to calcite, the dissolution of vein calcite accounts for up to 67% of the total dissolved cations, generating up to 90% of dissolved Ca and 97% of Mg. Dissolved sulfate has δ³⁴S values of 16 to 20‰_CDT, indicating that it is derived predominantly from atmospheric deposition and minor gypsum weathering and not from bacterial reduction of FeS₂. This militates against sulphuric acid weathering in Victorian rivers. Ratios of Si vs. the atmospheric corrected Na and K concentrations range from ~ 1.1 to ~ 4.3, suggesting incongruent weathering from plagioclase to smectite, kaolinite and gibbsite.Estimated long-term average CO₂ fluxes from silicate weathering range from ~ 0.012 × 10⁶ to 0.039 × 10⁶ mol/km²/yr with the highest values in rivers draining the basement outcrops rather than sedimentary rocks. This is about one order of magnitude below the global average which is due to low relief, and the arid climate in that region. Time series measurements show that exposure to lithology, high physical erosion and long water–rock contact times dominate CO₂ consumption fluxes via silicate weathering, while variations in water temperature are not overriding parameters controlling chemical weathering. Because the atmospheric corrected concentrations of Na, K and Mg act non-conservative in Victorian rivers the parameterizations of weathering processes, and net CO₂ consumption rates in particular, based on major ion abundances, should be treated with skepticism.     

Depositional environments of marine-dominated bedded halite, Permian San Andres Formation, Texas

The interpretation of the depositional environments in which bedded halite is formed is a frontier of evaporite sedimentology. Facies studies, supplemented by geochemical data, define an evaporite shelf depositional environment for the halite of the San Andres Formation of the Palo Duro Basin, Texas Panhandle, that is significantly different from modern and previously described ancient halite-precipitating environments such as playas, salinas, sabkhas, and barred basins. A coastal palaeoenvironmental setting for the San Andres halite is defined by its facies position between normal marine-shelf carbonates and aeolian deposits. The San Andres brine pool extended over more than 10 000 km² but was very shallow. Perennial brine-pool conditions (thick sequences of halite) alternated with ephemeral brine-pool conditions (halite, haloturbated mudstone-halite, mudstone interbeds, and microkarst). The depositional sequence in the perennial brine pool includes (1) flooding and minor dissolution of previously deposited halite, (2) precipitation of a thin lamina of gypsum, and (3) evaporation and precipitation of halite. Bottom-growth forms of halite, including chevrons and vertically elongated crystals, are well preserved. Cumulates of foundered, floating crystals and rafts were probably abundant but have been almost completely recrystallized. Mudstone interbeds in halite and microkarst features cut into bedded halite record the episodic interruption of brine-pool deposition and the establishment of subaerial conditions. Vertically elongated karst pits originate at the palaeosurfaces, cross-cut primary brine-pool fabrics, and are filled with geopetal sediment and coarse halite cement. Mudstone interbeds and mixtures of mudstone and halite have been haloturbated by the alternating growth and dissolution of halite reflecting conditions of fluctuating salinity. The geochemistry of halite from both ephemeral and perennial brine-pool environments reflects the frequent influx of marine brine, indicating that communication with the marine environments to the south was ‘good’ considering that restriction was sufficient to precipitate halite.     

Geochemical zones and reconstruction of late Holocene environments from shallow core sediments of the Pachapadra paleo-lake, Thar Desert, India

Lithostratigraphy, mineralogy, major and trace element concentrations, carbon and sulphur contents are investigated from a shallow depth profile from Pachapadra paleo-lake, Thar Desert, north-western India, to understand the phases of paleo-hydrology and paleo-limnology. Based on the geochemical proxies (Na/Al, Si/Al, Zr/Al and Ca/Mg) and evaporite mineralogy, the depth profile is divided into three geochemical zones of variable sediment-water interaction, evaporation and aeolian activity. The sub-recent zone (I) enriched in halite (NaCl) indicates low chemical weathering and higher aeolian input. The intermediate relatively humid zone II is enriched in major elements, trace elements and calcite (CaCO₃) and reflects higher chemical weathering in the catchments. Zone III is enriched in gypsum (CaSO₄·2H₂O) and characterised by lower chemical weathering, higher aeolian activity and evaporation.     

Atmospheric gas in modern and ancient halite fluid inclusions: A screening protocol

Halite possesses great potential for hosting and storing information vital to the reconstruction of Earth's ancient climate, seawater chemistry and evolving atmosphere. Here, we propose a screening protocol that not only distinguishes between primary and secondary halite, but also identifies fluid inclusions that carry original gas trapped during the primary crystallization process. An integrated multi-analytical protocol is presented for sample preparation, petrographic evaluation, analytical measurements and distinguishing original gases and contents from contaminants. The screening protocol starts with the visual inspection of halite for primary chevron and hopper features and/or milky appearance. Then, petrography is used to distinguish between primary and secondary (diagenetic) crystal fabrics and inclusions. The trace element and isotope geochemistry speak directly to the composition of the depositional and/or diagenetic fluids and to the formation in marine, non-marine or diagenetic settings. Furthermore, rare earth elements and redox sensitive elements may address the redox conditions of the salt pan/flat brines, whereas microthermometry helps characterize the depositional/diagenetic environment's temperature. Finally, gas extracted by quadrupole mass-spectrometer from gas bubbles in fluid inclusions is screened with concomitant CH₄, CO₂ and Ar contents to distinguish and quantify organic matter decomposition contributions and with ⁴⁰Ar/³⁶Ar to monitor modern atmospheric gas diffusion/leakage issues, and thus, contamination of the primary gas contents.Our test case shows that halite collected from cores is well-suited in maintaining its original mineralogical texture, chemistry and gas content. Our integrated screening protocol suggests that the 815 Myr old halite from the Tonian Browne Formation of the Officer Basin, Australia formed under atmospheric conditions of about 10.9% pO₂ (back calculation) or 9.6% pO₂ (mean calculation). The coeval Neoproterozoic gypsum collected in outcrop from Minto Inlet, Victoria Island, Canada underwent some mineralogical alteration. However, experiments suggest that depositional gypsum from outcrop/core may have potential of retaining vestiges of original gas in fluid inclusions.     

The geochemical and isotopic record of evaporite recycling in spas and salterns of the Basque Cantabrian basin, Spain

Evaporite outcrops are rare in the Basque Cantabrian basin due to a rainy climate, but saline springs with total dissolved solids ranging from 0.8 to 260 g/L are common and have long been used to supply spas and salterns. New and existing hydrochemistry of saline springs are used to provide additional insight on the origin and underground extent of their poorly known source evaporites. Saline water hydrochemistry is related to dissolution of halite and gypsum from two evaporitic successions (Triassic “Keuper” and Lower Cretaceous “Wealden”), as supported by rock samples from outcrops and oil exploration drill cuttings. The δ³⁴S value of gypsum in the Keuper evaporites and sulfate in the springs is δ³⁴S_SO4 = 14.06 ± 1.07‰ and δ¹⁸O_SO4 = 13.41 ± 1.44‰, and the relationship between Cl/Br ratio of halite and water shows that waters have dissolved halite with Br content between 124 and 288 ppm. The δ³⁴S value of gypsum in the Wealden evaporites and sulfate in the springs is δ³⁴S_SO4 = 19.66 ± 1.76‰, δ¹⁸O_SO4 = 14.93 ± 2.35‰, and the relationship between Cl/Br ratio of halite and water shows that waters have dissolved halite with Br content between 15 and 160 ppm. Wealden evaporites formed in a continental setting after the dissolution of Keuper salt. Gypsum δ³⁴S_SO4 and δ¹⁸O_SO4 modification from Keuper to Wealden evaporites was due mainly to bacterial SO₄ reduction in an anoxic, organic matter-rich environment. Saline springs with Wealden δ³⁴S_SO4 values are present in a 70 × 20 km wide area. Saline water temperatures, their δ²H_H2O and δ¹⁸O_H2O values, and the geological structure defines a hydrogeological model, where meteoric water recharges at heights up to 620 m above spring levels and circulates down to 720 m below them, thereby constraining the height range of evaporite dissolution. Groundwater flow towards saline springs is driven by gravity and buoyancy forces constrained by a thrust and fault network.     

Geochemical Evolution of Great Salt Lake, Utah, USA

The Great Salt Lake (GSL) of Utah, USA, is the largest saline lake in North America, and its brines are some of the most concentrated anywhere in the world. The lake occupies a closed basin system whose chemistry reflects solute inputs from the weathering of a diverse suite of rocks in its drainage basin. GSL is the remnant of a much larger lacustrine body, Lake Bonneville, and it has a long history of carbonate deposition. Inflow to the lake is from three major rivers that drain mountain ranges to the east and empty into the southern arm of the lake, from precipitation directly on the lake, and from minor groundwater inflow. Outflow is by evaporation. The greatest solute inputs are from calcium bicarbonate river waters mixed with sodium chloride-type springs and groundwaters. Prior to 1930 the lake concentration inversely tracked lake volume, which reflected climatic variation in the drainage, but since then salt precipitation and re-solution, primarily halite and mirabilite, have periodically modified lake-brine chemistry through density stratification and compositional differentiation. In addition, construction of a railway causeway has restricted circulation, nearly isolating the northern from the southern part of the lake, leading to halite precipitation in the north. These and other conditions have created brine differentiation, mixing, and fractional precipitation of salts as major factors in solute evolution. Pore fluids and diagenetic reactions have been identified as important sources and especially sinks for CaCO₃, Mg, and K in the lake, depending on the concentration gradient and clays.