X-ray microanalysis of fluid inclusions and its application to the geochemical modeling of evaporite basins
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Halite is a major evaporate mineral in the surface environment,and it contains abundant fluid inclusions formed during crystal formation,which can record the brine temperatures of crystal growth in ancient oceanic salt ponds and salt lakes in geological history.Using the cooling nucleation method,homogenization temperature of fluid inclusions in halite can be measured.However,the interpretation and application of the homogenization temperature in palaeotemperature reconstruction is not straightforward.Fluid inclusions in halite can be captured in cumulate halite crystals formed at the air-water interface or in chevron halite crystals formed at the water-sediment interface.Lowenstein et al.(1998)carried out a microthermometric analysis on halite crystals grown in the laboratory through evaporation of halite-saturated brines in flasks submerged in water baths;however how to translate brine temperature into air temperature is the key question of paleoclimate.In this study,we measured a series of homogenization temperature of fluid inclusions in both cumulate and chevron halite crystals grown in the laboratory at a temperature of 40℃(air temperature)in a thermostatic air drying oven in 20cm deep brine in order to reconstruct the paleotemperature using homogenization temperatures directly.The homogenization temperatures range from 10.6℃ to 39.9℃.The results show that only the maximum homogenization temperature of fluid inclusions(Th max)matches brine temperatures.Both cumulate halite and chevron halite have a similar Th max,both types of halite can be used in paleotemperature reconstruction in shallow water.
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Experiments using artificial and seawater brines indicate that gypsum, halite, and sylvite can be precipitated by mixing brines at differing stages of evaporation, as well as by the previously recognized mechanisms of direct evaporative crystallization and crystallization by temperature changes. A modification of existing geologic models is proposed to show how brine mixing might work in an evaporite basin. Conclusions based on the experiments and their relations to the geologic model are as follows: 1. Precipitation of salts can occur in a marine evaporite basin by mixing brines of different composition and specific gravity. 2. Precipitation occurs without further water loss by evaporation. 3. Precipitation can occur from brines that were undersaturated before mixing. 4. Brine mixing could cause different salts to be deposited in different parts of a basin depending on the stage of the evaporite cycle. 5. Sylvite could be precipitated as a primary mineral. 6. Hopper crystals (cubic and tabular) of sodium chloride can form as a result of brine mixing in water of any depth. End_of_Article - Last_Page 647------------
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Analysis of data on the chemical composition of primary fluid inclusions (brine inclusions) in primary‐bedded halite from many evaporite formations of northern Pangea shows that during the Phanerozoic the chemical composition of marine brines was oscillating significantly between the Na‐K‐Mg‐Ca‐Cl type and the Na‐K‐Mg‐Cl‐SO4 type. We regard those changes as corresponding to the chemical evolution of the Phanerozoic ocean. The changes correlate in time with earlier suggested secular changes in the mineralogies of marine nonskeletal limestones and potash evaporites. In addition to those secular changes of seawater chemistry, the concentrations of K, Mg, Ca, and SO4 ions in marine brines underwent important changes under the influence of local factors, including rock‐water interaction. These secondary changes did not influence the principal chemical type of the brine characteristic for a given time interval. In the past concentration of the Ca‐ion did not exceed the present concentration in marine water by a factor of three, and the increase was synchronous with a decrease in the SO4‐ion concentration. This could be as much as three times lower when compared to the present concentration of that ion in seawater.
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Halite is a major evaporate mineral in the surface environment, and it contains abundant fluid inclusions formed during crystal formation, which can record the brine temperatures of crystal growth in ancient oceanic salt ponds and salt lakes in geological history. Using the cooling nucleation method, homogenization temperature of fluid inclusions in halite can be measured. However, the interpretation and application of the homogenization temperature in palaeotemperature reconstruction is not straightforward. Fluid inclusions in halite can be captured in cumulate halite crystals formed at the air-water interface or in chevron halite crystals formed at the water-sediment interface. Lowenstein et al. (1998) carried out a microthermometric analysis on halite crystals grown in the laboratory through evaporation of halite-saturated brines in flasks submerged in water baths; however how to translate brine temperature into air temperature is the key question of paleoclimate. In this study, we measured a series of homogenization temperature of fluid inclusions in both cumulate and chevron halite crystals grown in the laboratory at a temperature of 40 degrees C (air temperature) in a thermostatic air drying oven in 20cm deep brine in order to reconstruct the paleotemperature using homogenization temperatures directly. The homogenization temperatures range from 10. 6 degrees C to 39. 9 degrees C. The results show that only the maximum homogenization temperature of fluid inclusions (T(h max)) matches brine temperatures. Both cumulate halite and chevron halite have a similar T(h max), both types of halite can be used in paleotemperature reconstruction in shallow water.
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Evaluation of data sets on inclusion brine compositions in halite from the Phanerozoic marine evaporite deposits used for the reconstruction of ancient seawater chemistry shows that brine analysis of primary inclusions from primary marine halite (in the case of proper genetic type determination) undoubtedly indicate two megacycles in secular variation of seawater chemistry during the Phanerozoic. It is also shown that inside primary halite, inclusions formed at later stages of deposit formation locally occur. Erroneous attribution of such inclusions to primary ones is the main reason for deviations observed in most data sets. It is also obvious that fluid inclusions in clear (recrystallized) halite are unsuitable for the reconstruction of ancient seawater chemistry. Brines from inclusions properly determined as primary in primary bedded halite are micro-droplets of concentrated ancient seawater.
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The fluid inclusions in the marine Middle Ordovician halite of the Majiagou Salt Formation of the Ordos Basin (China) have been investigated. In addition to the primary inclusions the secondary ones of several generations were also detected. The fluid inclusions brine chemistry of halite was studied using an ultramicrochemical (UMCA) method, and the homogenization temperature of fluid inclusions was determined in a special thermal chamber designed by V. A. Kalyuzhny At the post-sedimentation stage, the studied salt strata were exposed to high temperature (58–72 °C) and high (up to several tens of MPa) pressure. Although there are opinions of the inability of primary inclusions in such halite to determine the physical and chemical conditions of sedimentation, however, the informative value of primary inclusions in halite of the Majiagou Formation has remained. The preservation of the integrity (and thus the informative value) of primary inclusions in halite is evidenced by the same chemistry of their brines, which differs from that of secondary inclusions The sedimentation brines of the basin were concentrated to the middle of halite stage and points to the Na-K-Mg-Ca-Cl seawater. The physical and chemical conditions of evaporites formation are not known enough. Currently, the results of the brine chemistry of primary fluid inclusions in marine halite are the best indicators of seawater composition in the Phanerozoic. It is established that the magnesium content in the brines of the Lower Paleozoic basins is lower comparing to modern seawater of the corresponding concentration, and the potassium ion concentration is higher. The chemical composition of the concentrated seawater from which the halite was crystallized in the Ordovician salt basin of Ordos, with the exception of the calcium ion content, is similar to the seawater chemistry of the Cambrian and Silurian basins, which indicates the relative constancy of Early Paleozoic seawater chemistry. Age-related changes in the chemical composition of seawater are always consistent with many quantitatively or qualitatively characterized processes of the Earth’s crust evolution. So we believe that the causes that led to more than twice the potassium content of Riphean-Devonian clays, unlike the younger ones, it were also the reason for the increase in potassium content in the Lower Paleozoic marine brines. The studies conducted also clarify the limits of oscillation of calcium ion content, which determines the type of seawater. Its content in the sedimentary brines of the Ordos basin of the Middle Ordovician reaches 66 g/l at the middle of halite stage. Therefore, at the beginning of the stage of halite precipitation, its content should be approximately 20 g/l (considering its theoretical content of 10 g/l with the modern composition of the atmosphere). Apparently, the cause of the abnormally high calcium content in the early Paleozoic Ocean was the direct flow of it with hydrothermal solutions into the ocean during the activation of global tectonics of the Earth and the increase of solubility of carbonates of continents and ocean floor due to high carbon dioxide atmospheric content.
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