Vol 54, No 8 (2016)

An empirical computer model was developed to describe granite magma degassing and the partitioning behavior of Cl between melts and aqueous chloride fluids that formed during eutectic isobaric crystallization of magmas at pressures from 4 to 0.4 kbar and a temperature of 800 ± 25°C. This model is the extensions of the earlier model describing the decompression degassing of granite melts (Lukanin, 2015). The numerical modeling was performed for both closed-system conditions, when fluid remains in the system, and open-system conditions, when fluid is removed from the system. The results of numerical modeling revealed the main factors controlling the behavior of Cl during crystallization-induced degassing, such as the initial contents of Cl and H₂O of the melts, pressure, and the degree of system openness. At high pressures (>1.6 kbar), isobaric crystallization is accompanied by a decrease in the concentrations of Cl in the melt (C_Cl^m) and fluid phase (C_Cl^fl). This tendency becomes even more pronounced in an open-system with increasing pressure and initial Cl content. A decrease in pressure in the range of 1.62–0.85 kbar results in a drastic change in the Cl behavior: the trend of C_Cl^fl and C_Cl^fl decrease dominating during crystallization at high pressures changes to the opposite. At low pressures (<0.85 kbar), the enrichment of the residual melts and released fluids in Cl leads at a certain stage of crystallization to the formation of a heterogeneous fluid consisting of two immiscible aqueous chloride phases, a waterdominated aqueous phase and a chloride-rich liquid (brine).

We generalize, for the first time, published and original data on the gallium concentrations in natural magmatic melts and fluids obtained by studying quenched glasses in volcanic rocks and inclusions in minerals. Based on 2688 determinations, gallium concentrations in magmatic melts vary between 0.47 and 495 ppm at average content of 18.0 ppm (+4.2/–3.4). Gallium concentrations in magmatic melts generated in different geodynamic settings show different distribution. Minimum concentrations (on average, 16.0 ppm, +3.6/–2.9) are typical of the island-arc melts, while maximum contents were determined in melts of oceanic islands (on average, 29.1 ppm, +13.4/–9.2) and intracontinental rifts and hot spots (26.5 ppm, +25.4/–13.0). Published and new 339 determinations of gallium concentrations in natural fluids indicate the wider range of their variations as compared to those of melts: from 0.02 to 11260 ppm, at average 1.6 ppm (+10.8–1.4). The possible gallium fractionation in fluid—magmatic systems is discussed.

Computer simulations of carbon dioxide leaching of Aptian–Albian sandstone at the Nagutskoe groundwater field, Caucasian Mineral Waters, are compared with laboratory experimental data obtained using a high-pressure autoclave under parameters close to conditions under which mineral waters are formed at the Nagutskoe and Essentuki fields (temperatures 20–25 and 65–70°C, carbon dioxide pressure up to 4.04 MPa). The solvents were distilled water and naturally occurring groundwaters from the Caucasian Mineral Waters (CMW) area, individual experimental runs lasted for 2 h, the starting material (rock) was crushed to 0.25 mm, and the gas phase was carbon dioxide. In most of the experiments, the solid: liquid phase (R/W) ratio was 1: 5 and was varied from 1: 10 to 1: 100 in other experiments. Our simulation results indicate that multiple-cycle (10 cycles) leaching leads to an increase in mineralization from 1.3 g/L to 4 g/L and transformation of the geochemical type of the waters from the hydrocarbonate calcic–sodic one (leaching cycle 1) to chloride–hydrocarbonate sodic (cycles 5 and later). The mineralization increased mostly because the and Na⁺ ions are transferred into solution at an insignificant increase in the Cl concentration and a practically unchanging concentrations of the sulfate, calcium, and magnesium ions. With regard for the averaged mineralogical composition of the sandstone (quartz, feldspars, mica, glauconite, magnetite, ilmenite, garnet, rutile, zircon, and tourmaline) used in our thermodynamic simulations, we arrived at the conclusion that the chemical compositions of the waters, including their minor-element compositions, are controlled by (i) the composition of the cement (clay, calcareous, siliceous, limonitic, chloritic, zeolitic, phosphate, sulfate, or mixed) of the rocks, (ii) weight percentages of minerals containing certain elements, and (iii) temperature, at a given composition of the gas phase of the simulated system (silty sandstone–rainwater–CO₂ gas phase).