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- Hydrochemistry and gas geochemistry of the northeastern Algerian geothermal waters
- Collection and analysis of geothermal and volcanic water and gas discharges.
- Collection and Analysis of Geothermal and Volcanic Water and Gas Discharges
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Hydrochemistry and gas geochemistry of the northeastern Algerian geothermal waters
Forty-five gas samples have been collected from natural gas manifestations at the island of Kos—the majority of which are found underwater along the southern coast of the island. On land, two anomalous degassing areas have been recognized. These areas are mainly characterized by the lack of vegetation and after long dry periods by the presence of sulfate salt efflorescence.
Significant contents of H 2 up to 0. Only one of the underwater manifestations is generally rich in N 2 up to Isotope composition of He ranges from 0. C-isotope composition of CO 2 ranges from Isotope composition of CH 4 ranges from The dataset presented in this work consists of sites that were repeatedly sampled in the last few years, with some of which being also sampled just before and immediately after the magnitude 6. Changes in the degassing areas along with significant variations in the geochemical parameters of the released gases were observed both before and after the seismic event; however, no coherent model explaining those changes was obtained.
CO 2 flux measurements showed values of up to about in both the areas of Volcania and Kokkino Nero, at Paradise beach, and at Therma spring. CO 2 output estimations gave values of The southern Aegean Sea is one of the most tectonically active regions of western Eurasia, where fast convergence of the Aegean microplate and the Eastern Mediterranean lithosphere the front part of the African plate occurs.
During the Lower Pliocene, the rate of convergence between the two continental margins increased and initiated the volcanism and the development of several hydrothermal systems at the SAAVA e. The geologic units of Kos consist of alluvial deposits with greenschists and flysch in the northern part of the island, lacustrine and terrestrial deposits of the Pliocene age in the central part with tuffs, and ignimbrites of the Quaternary age that cover the southern part of the island [ 14 ].
The tectonic evolution of the island is controlled by the dominant WNW-ESE and NE-SW faults, which are related to extensional processes and volcanic activity that took place during the Pleistocene and Pliocene [ 16 , 17 ].
The volcanic island of Kos has been active for at least 3 million years [ 18 ] and continued to be active until recent times e. Some geothermal areas of particular interest have been identified in the island. Furthermore, hydrothermal activity is noticeable along the island with the most important sites being i the thermal spring of Therma, which is emerging on the beach close to Cape Fokas [ 20 ], ii the ferruginous spring of Kokkino Nero rich in CO 2 [ 21 ], and iii the intensively degassing area of Paradise beach at Kefalos Bay.
Geogenic carbon emissions have a critical impact on the carbon cycle [ 22 , 23 ] and are regarded as one of the reasons of global climate changes on long time scales. Gas emissions from such sources strongly contribute to the increasing concentrations of greenhouse gases in the atmosphere, with methane and carbon dioxide playing a fundamental role [ 24 , 25 ].
Moreover, earth degassing defines relations among flux, tectonic structures [ 26 , 27 ], and volcanic activity [ 28 , 29 ]. The aim of this study is to estimate the total CO 2 output of Kos island and to investigate the possible relation of these manifestations and anomalous areas with the geodynamically active area of Greece. This work presents new chemical and isotope data about the main gas manifestations of the island both on land and underwater.
The collected data are interpreted together with literature data in order to determine the origin of the gases and the postgenetic processes that affect them. Finally, some considerations about possible variations on gas geochemistry induced by the earthquake on the 20 th of July with an epicenter close to the island are made. Dry gases were collected in glass flasks equipped with two stopcocks. A single amount of gas sample is simultaneously split into the loops of the two combined systems.
The analytical results of the collected samples as well as their coordinates are presented in Table 1. Flux maps were drawn according to the dataset obtained from the three field campaigns that took place in the period from October to October Table 2. Measurements were always made during dry and stable weather conditions. Flux values were determined at each site from the rate of CO 2 concentration increase in the chamber and are expressed in grams per square meter per day after conversion from volumetric to mass concentrations considering atmospheric pressure and temperature values.
Chamber A has an area of 0. This method consists in the definition of single populations through the inflection points main populations or changes in direction secondary populations of the curvature on the probability plot by visual analysis. Following the stochastic simulation approach, CO 2 flux maps were drawn. The data were converted by normal score transformation to follow a Gaussian distribution.
The grid resolution was. The final maps were produced averaging the results of the realizations, using the E-type postprocessing method. In the period from to , 10 sampling campaigns took place in the island of Kos and 45 gas samples were collected.
Literature data were also taken into consideration for comparison [ 14 , 35 , 36 ]. Names, sampling date, coordinates, chemical concentrations, and isotope values are presented on Table 1.
Results of the flux measurements are presented in Table S1. Gases collected from Kos island show that CO 2 is the prevailing gas species generally more than and up to ; Figure 2. However, few samples present high concentrations of atmospheric gases O 2 up to and N 2 up to , pointing to a strong contamination by an atmospheric component Figure 2 b. Helium ranges from 0. Hydrogen and H 2 S, typical hydrothermal gases, present significant concentrations up to and , respectively in the soil gases, whereas in the underwater emissions, they are mostly below detection limits.
The isotope composition of He shows values from 0. Carbon isotope composition of CO 2 in the island is in the range of V-PDB, although almost all samples fall within a narrower range The isotope composition of CH 4 varies from The highest values were measured in Paradise beach and Therma spring median and ; maximum and , respectively with the use of the floating chamber method.
The two soil degassing areas of Volcania and Kokkino Nero showed the following median values: and , respectively, whereas the maximum values were and , respectively. Due to failure of the temperature probe, the temperature was not measured in the first campaign. Only few measurements were made in the second and third campaigns, and they were mainly concentrated in the highest CO 2 flux measuring points.
The portioning method of Sinclair [ 33 ] was applied to extract data populations from the dataset. Three main populations, i. Table 2 summarizes the number of points contributing to each population of the dataset and the statistical parameters.
According to the identification of background population from the probability plot, the threshold values used for estimation of the CO 2 flux from the Volcania area were and for Kokkino Nero, while no background value was used for Paradise beach and Therma. Carbon dioxide is the carrier phase for mantle-derived fluids. The combination of C and He isotope ratios is a useful tool to evaluate the origin of fluids. Helium isotope ratios are used to distinguish between crustal and upper mantle-derived components.
In order to identify general controls on the CO 2 characteristics of Kos island, we plotted the data on a CO 2 - 3 He- 4 He ternary diagram Figure 3 ; [ 40 ]. The binary mixing trajectories between MORB-type mantle fluids , and various crustal volatile endmembers 0. In the aforementioned diagrams, the binary mixing curves that display the trends drawn by mixtures of the atmospheric component with different mantle and crustal sources [ 30 ], as well as the typical values of sediment, limestone, and mantle-derived CO 2 [ 41 ], are also plotted.
Literature data of gas manifestations along the SAAVA [ 14 , 35 , 36 , 42 — 44 ] are plotted for comparison. Only few samples show an important atmospheric contribution for He, as they plot close to the atmospheric endmember Figure 4 a.
The distribution of the values seems to follow a geographical distribution with the highest values collected in the western part of the island, while the lower ones are concentrated in the eastern part Figure 5. Many volcanic systems show an approximately regular radial distribution of the values with the highest values being found close to the main volcanic or geothermal vents.
Examples can be found at Nevado del Ruiz [ 45 ], Mt. Ontake [ 46 ], Lesser Antilles islands [ 47 ], Cascades [ 48 ], and Mt. Elbrus [ 49 ]. Such geographical pattern is generally explained with an increase in the contribution of both crustal radiogenic and atmospheric He components when the aquifer water reequilibrates with air, going from the main magmatic feeding system towards the peripheral areas [ 50 ]. However, in the present case, no active or recent volcanic conduit can be recognized on the island.
Nevertheless, it is worth noting that the westernmost sampled gas manifestations are found along the supposed margin of the caldera that formed after the KPT explosive eruption. Along the margin of this structure, many other volcanic systems have been grown since that eruption i.
It may be therefore hypothesized that the westernmost gas manifestations of Kos are connected to a deep mantle source by the presence of the ring faults of the caldera. At present, in the area, there is no surface sign of thermal anomalies that could point to the presence of a geothermal system possibly fed by an ascending magma batch. On the contrary, the easternmost sampling sites are found away from the caldera margins, mostly along the main tectonic structures, and are, at least in the case of Therma, related to thermal water circulation.
Deep-rooted faults frequently constitute channels of high permeability that facilitate the migration of mantle fluids [ 51 ]. The strong difference in He isotope composition between the two groups of sampling sites may be explained also with the different geologic substrate of the two areas.
In the western part of the island, young volcanic formations prevail, while in the eastern part, mostly older metamorphic rocks crop out. The latter could be the source of the crustal component that lowers the values of the gases collected in the eastern part of Kos. Moreover, gases collected in Kos along with those of SAAVA are found in the mixing line between mantle and limestone endmembers, while the contribution of the organic sediments is trivial Figure 4 b.
Hydrothermal hydrocarbon production can be described by two main mechanisms that deal with the biotic and abiotic origins of methane [ 56 ]. Considering this, the origin of CH 4 can be investigated using the classification diagram of Schoell [ 57 , 58 ]. Samples collected in Kos island as well as samples of the SAAVA [ 43 ] plot in the field of volcanic geothermal systems and thus, a geothermal origin is suggested for CH 4 Figure 6.
Exceptions are the samples collected at Kefalos, Agia Irini 2, and Therma, which sometimes present extremely sample no. Inorganic oxidation of CH 4 [ 63 ] in some samples cannot be ruled out. However, it is noticeable that the isotope fractionations of organic oxidation and inorganic oxidation of CH 4 follow different fractionation paths. The former follows slopes ranging from 5. Daskalopoulou et al.
Based on that, the values, comprised between 3. It is worth noting that the sites showing signs of methane oxidation are those presenting the lowest gas emission fluxes. The slower uprise of the gases allows a longer interaction with the methanotrophic microorganisms before bubble emission and therefore a higher consumption and a consequent fractionation of methane.
Fluids play an important role in earthquake generation by reducing the friction between the fault blocks [ 66 — 68 ] and transporting upper mantle energy with geochemical anomalies that occur before, during, and after earthquakes [ 69 ]. Therefore, heat flow and tectonics are related to both active faults and geothermal anomalies, and for that reason, many earthquake epicenters occur worldwide in areas with elevated heat flow [ 66 , 67 ].
Many researchers have studied the relations between seismicity and geochemistry and have noticed changes in the physicochemical parameters, the ground deformation, the gas flow rate, and the isotope composition of the gases e.
Our campaigns included the systematic gas collection in the period from to , thus including the seismic event. Three of the sampling sites were specifically resampled on the 26 th of July , 6 days after the main shock.
Collection and analysis of geothermal and volcanic water and gas discharges.
The deepest aquifer embodies the low-enthalpy thermal resources of Southern Tunisia. The thermo-mineral surface manifestations of the conductively heated waters reveal the complexity of the geotectonic history of the region and create oases systems in net contrast to the desertic landscape of the Chotts region. As agriculture constitutes the principal economic activity in the studied area and as it is currently threatened by thermal water quality degradation, the present hydrogeological investigation aims for a characterization of the CI thermal resources on the basis of a critical quality evaluation. The water quality can change along fluid migration pathways expressed by scattered isotopic and chemical data and a number of mixing ratios. Most changes in the background CI composition result from continuous pumping that modifies natural flows and increase quality degradation. Generally, the geothermal systems are studied in regards to the diversification of their utilization Arnorsson et al.
This study focuses on the water and gas chemistry of the northeastern Algerian thermal waters. The helium gas was used to detect the origin of the geothermal fluid. The geothermal conceptual model suggests that a geothermal system was developed by the deep penetration of infiltrated cold waters to a depth of 2. The thermal waters rise up to the surface through the deep-seated fractures. During their ascension, they are mixed with shallow cold groundwater, which increase the Mg content and cause the immature classification of the water samples.
Collection and Analysis of Geothermal and Volcanic Water and Gas Discharges
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The Changbai Mountain volcanic region, eastern Songliao Plain, China, is considered a potential target development area for geothermal water exploitation. To assess the feasibility of geothermal development, we applied integrated multicomponent geothermometry IMG in the program GeoT to estimate the geothermal reservoir temperatures for four major thermal springs in this area.
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Institute of Geological and Nuclear Sciences. Geothermal Department, Geological Survey of Japan. The chemical and isotopic D, 18 O, 34 S compositions were determined for 19 fumarolic gases, 11 hot spring waters and 3 meteoric waters collected from Satsuma-Iwojima volcanic island, Japan. The variation in gas composition from different fumaroles may largely be explained in terms of re-equilibration at lower temperature, addition of earlier deposited elemental S and mixing with meteoric waters at shallow depths. The chemical and isotopic composition of the highly acid hot spring discharges, pH 1.
Magmatic settings involving active volcanism are potential locations for economic geothermal systems due to the occurrence of high temperature and steam pressures. Indonesia, located along active plate margins, hosts more than volcanoes and, therefore, belongs to the regions with the greatest geothermal potential worldwide. However, tropical conditions and steep terrain reduce the spectrum of applicable exploration methods, in particular in remote areas.
This study reports the chemical and isotopic compositions of hot-spring waters and fumarolic gases sampled in the Jigokudani Valley of Tateyama Volcano central Japan in and to reveal the state of the underlying hydrothermal system. We discuss the cause of temporal variations in geochemical data in terms of temperature change in the hydrothermal system and clarify the relationship between hot-spring waters and fumarolic gases. The volcanic gas supplied from deep-seated magma was separated into liquid and vapor phases when it reached a shallow depth. The formation of type-1 and type-2 hot springs was influenced by magmatic components such as HCl and SO 2. We consider that type-1 hot springs are derived from the liquid phase while type-2 hot springs are derived from the vapor phase of the two-phase zone.
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