Isotopically light carbon dioxide in nitrogen rich gases : the gas distribution pattern in the French Massif Central , the Eifel and the western Eger Rift

Based on characteristics of the distribution pattern of the western Eger Rift spring gases, a distribution pattern is presented for the gases of the French Massif Central. The central parts of these areas with ascending magmatic CO2 are characterised by high gas fluxes, high CO2 contents of up to 99.99 vol% and isotopially heavy CO2. In the peripheries, the decrease of δ C values of CO2 and CO2 contents in the gas phase is compensated by a rise in N2 contents. It can be demonstrated that gas fractionation in contrary to mixtures with isotopically light biogenic or crustal CO2 controls the distribution pattern of gas composition and isotopic composition of CO2 in these spring gases. Dissolution of CO2 results in formation of HCO3 – causing isotope fractionation of CO2 and an enrichment of N2 in the gas phase. With multiple equilibrations, values of about –17 ‰ or lower are obtained. The scale of gas alteration depends on the gas flux and the gas-water ratios respectively and can result in N2-rich gases. Essential for the interpretation are gas flux measurements with mass balances derived for most of the springs. Without such mass balances it is not possible to discriminate between mixture and fractionation. The processes of isotopic and chemical solubility fractionations evidently control the gas distribution pattern in other regions as well.


Introduction
In areas with both low CO 2 abundance and contents in the gas phase as for example in complexes of crystalline basement or in areas with younger volcanic activity, one frequently encounters CO 2 with isotopic compositions of δ 13 C values lesser than -10 ‰.In many cases, such values are interpreted as biogenic/organic CO2 or mixtures between magmatic and biogenic CO2 or respectively mantle and crustal end members (e.g., Griesshaber et al., 1992).It appears that CO 2 gases with low δ 13 C values occurring in the margin areas of regions with ascending magmatic CO2 as for example in mineral springs of the western Eger Rift, the Eifel or the French Massif Central (Batard et al., 1982) confirm this interpretation.
Commonly, the isotopic data are compared and calculated with a single equilibration.However, in the peripheral areas of regions with magmatic CO2 marked by longer migration Mailing address: Dr. Falk H. Weinlich, Referat Gasund Isotopengeochemie, Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, 30655 Hannover, Germany; e-mail: falkweinlich@gmx.deIsotopically light carbon dioxide in nitrogen rich gases: the gas distribution pattern in the French Massif Central, the Eifel and the western Eger Rift Falk H. Weinlich Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Hannover, Germany Abstract Based on characteristics of the distribution pattern of the western Eger Rift spring gases, a distribution pattern is presented for the gases of the French Massif Central.The central parts of these areas with ascending magmatic CO2 are characterised by high gas fluxes, high CO2 contents of up to 99.99 vol% and isotopially heavy CO2.In the peripheries, the decrease of δ 13 C values of CO2 and CO2 contents in the gas phase is compensated by a rise in N2 contents.It can be demonstrated that gas fractionation in contrary to mixtures with isotopically light biogenic or crustal CO2 controls the distribution pattern of gas composition and isotopic composition of CO2 in these spring gases.Dissolution of CO2 results in formation of HCO3 -causing isotope fractionation of CO2 and an enrichment of N2 in the gas phase.With multiple equilibrations, values of about -17 ‰ or lower are obtained.The scale of gas alteration depends on the gas flux and the gas-water ratios respectively and can result in N2-rich gases.Essential for the interpretation are gas flux measurements with mass balances derived for most of the springs.Without such mass balances it is not possible to discriminate between mixture and fractionation.The processes of isotopic and chemical solubility fractionations evidently control the gas distribution pattern in other regions as well.
pathways, it can be assumed that gases migrate in different water systems, for example along various faults where dissolution of CO2 and fractionation with concomitantly formed HCO3 can take place several times.This results in a comprehensive gas fractionation concerning both gas composition and isotopic composition of CO2 and can yield N2-rich gases.

Gas distribution pattern in European
areas with magmatic CO 2

Western Eger Rift (Czech Republic)
In the western Eger Rift the main release of magmatic CO2-rich gases is bound to gas release-centres in the Cheb Basin and Karlovy Vary north and Mariánské Lázne ˇ south of the main structures of Eger Rift (Weinlich et al., 1998).These structures of the Krus ˇné hory (Erzgebirge) main fault together with the central fault both dipping to south and the Litome ˇr ˇice deep fault dipping to north form a Y-structure.This Y-structure splits the gas flux and forms a shielded gas free zone within the Eger Rift.
The gases of the mofettes and springs in these areas with the highest gas fluxes are characterised by high CO 2 contents of up to 99.99 vol% and δ 13 C values of -3.9 up to -1.9 ‰ (fig.1).In some mofettes and springs the gas flux reach values up to 28 m 3 /h in Bublák and 35 m 3 /h in Soos (Cheb Basin) or about 100 m 3 /h in the Mariiny mofette in Mariánské Lázne ˇ.The magmatic nature of these gases is indicated, besides these δ 13 C values, by high proportions of mantle-derived helium with R/Ra values up to 6 (Weinlich et al., 1999).
With increasing distances from these gas release-centres the gas flux falls and related to it the CO2 becomes isotopically lighter whereas the CO2 contents decrease and are compensated by a rise of the N2 and He contents.To the east and south of Konstantinovy Lázne ˇ the CO 2 contents drop linked with gas fluxes of lower than a half l/h up to values of 87 and 67 vol%, respectively.To the north of the Eger Rift nitrogen contents of 98 vol% are attained apart from 0.7 vol% CO2 as in the Schönbrunn fluorite mine or in other spring gases in the Erzgebirge (Weinlich, 1989).Linked with the decrease of the CO2 contents in the gas phase is a decrease of the δ 13 C values of CO2.South of the Eger Rift, the δ 13 C values fall to -7.4 ‰ in the Konstantinovy Lázne ˇ area and to -8.8 ‰ in Bavaria, respectively.In the same way, the δ 13 C values of CO 2 decrease north of the Eger Rift down to -6.0 ‰ in Bad Elster and in the N 2-rich gases of Schönbrunn down to a value of -17.4 ‰.
The gases in the mofettes and mineral springs in the western Eger Rift migrate upwards along faults in the area with exposed metamorphic rocks of Variscian in the north and of Moldanubian consolidated basement in the south or with exposed Variscan granite intrusions.The metamorphic rocks in this area have very low potential if any for a CO 2 release because all organic carbon is fixed thermodynamically as very stable graphite.In the course of the Variscan, metamorphism mobilised and displaced organic carbon with δ 13 C values of -14 ‰ is present in the form of CO 2 gases among others in the fluid inclusions of granitic quartz in Schönbrunn.With a decrepitation temperature of 800°C (Weise et al., 2001), the release of CO2 gases by mineral waters is today hardly possible.

Eifel (Lower Rhine Graben, Germany)
CO 2-rich gases linked with Quaternary volcanic activities occur also in the Eifel.Griesshaber et al. (1992) report isotopic compositions of CO 2 ranging from -7.8 ‰ up to -3 ‰.The lighter isotopic values are explained with the aid of lower R/Ra values due to mixing of magmatic and biogenic CO 2. May (2002) describes the occurrence of CO2-rich gases linked with higher gas fluxes within the central West Eifel and the decrease of CO2 contents in its margin areas (fig.2).So gases with up to 98.3 vol% CO2 predominate for e.g., in Wallenborn in the central part of the West Eifel and with up to 99 vol% in Laach lake in the East Eifel.The δ 13 C values of CO2 in the West Eifel range from -5.7 up to -2.0 ‰ (Hubberten, 2004, pers. comm.) and in the central parts of the East Eifel from -5.1 up to -3 ‰ (Griesshaber et al., 1992).
In the N 2-rich gas of Bad Bertrich, δ 13 C value of CO2 amounts to -13.9 ‰ (own analysis).The δ 13 C values of CO2 in these marginal springs are significantly lower than in the central parts.

Fig. 4.
Rise of N2 (air-free) in gases with decreasing free gas-water ratio due to increased gas fractionation, i.e. selective CO2 solution in water.The unavoidable scattering in the data is due to varying air proportions in the gases by different partial pressures influencing the bubble point pressure of the gas-water systems (data from Weinlich et al., 1998Weinlich et al., , 2003)).

23
Isotopically light carbon dioxide in nitrogen rich gases

French Massif Central
In the French Massif Central CO2 occurs with δ 13 C values ranging from -23 ‰ up to -4 ‰.
The CO2-rich gases of Vichy, Royat or Mont Dore and Cezallier (>99 vol% CO 2) are bound to the area of Limagne depression or its direct vicinity.Matthews et al. (1987) proved the mantle-derived nature of these gases with R/Ra of up to 5.5.Gas flux measurements (Moureu and Lepape, 1912;Batard et al., 1982) carried out on these spring gases in the above region also show that the CO2-rich gases are linked with high gas fluxes.
Apart from the Limagne depression filled with Oligocene -Quaternary sediments where the spring gases migrate upward along marginal faults and the volcanic complex of Mont Dore these spring gases migrate upward along faults in areas with exposed Variscian metamorphic rocks or granites.

The gases in the western Eger Rift
In contrast to other gases, CO 2 is very vulnerable to fractionation processes.Firstly, due to its good solubility in water compared to N2, HC's and rare gases, the gas composition can be altered solely by solubility fractionations.This results in enrichment of the inert gases as observed by an aureole of N2-richer gases in the surroundings of all regions with CO2-rich magmatic gases in Europe.Figure 4 demonstrates that with an ongoing solution of CO2 resulting in a decrease of the gas/water ratios the gases in the western Eger Rift are enriched in N 2 in the gas phase (Weinlich et al., 1998).
Secondly, linked with the solution of CO2 are decreasing pH values of these waters.This results in leaching of cations from the adjacent rocks and formation of HCO 3 -ions.Between the newly formed HCO3 -and CO2 in the gas phase exists an isotope fractionation of about 10 ‰ (at 10°C) (Wendt, 1968).
With ongoing HCO3 -formation, the remaining CO 2 in the gas phase becomes isotopically lighter.Consequently, the decreasing gas flux correlates with decreasing δ 13 C value of CO2 in the gas phase as demonstrated in the distribution pattern for the Eger Rift gases.
However, isotopically lighter CO 2 can be also a result of mixing of biogenic and magmatic CO 2.
The key for discrimination between mixing or fractionation is the compilation of complete mass balances of CO2 for each mineral spring with a scribed by the following equation: HCO gas 13 13 gas total total total HCO free gas phase consisting of gas flux, isotopic and chemical gas composition, contents of HCO3 -and dissolved CO 2 contents and water discharge.
According to Wendt (1968) the isotope balance for CO 2 in the free gas phase can be de-Fig.5. Dependence of δ 13 CCO 2 values in free gas phase from the ratio of HCO3 -transport (mHCO3) in water to total CO2 flux in mineral springs and mofettes in the western Eger Rift with complete mass balances, according to the isotope balance formula (see text) (data of HCO3 -, dissolved CO2 and water discharge are taken from Kolár ˇová and Myslil, 1979and Weinlich et al., 1999, 2003).Lines starting from the scattering range of the dry gas vents (on the y axis) in the mofettes wrap the field of theoretical fractionation according to the fractionation factor ε HCO3-gas at 10°C.Between these lines, the δ 13 C values for the free gas are exclusively a result of fractionation by the means of formed HCO3 -during a single equilibration, without the necessity to assume an additional biogenic carbon.The δ 13 C values below the lines can be explained by twice equilibration.This results in an increase of N2.Multiple equilibrations with solely dissolved CO2 and single equilibration with HCO3 -result in occurrence of N2-richer gases, which fall in between the fractionation lines.

Table I.
Measured and calculated δ 13 C values of CO2 for springs in the vicinity of mofettes in North Bohemia demonstrate that the differences between mofettes and springs are solely caused by HCO3 --fractionation.For the δ 13 Cprimary (total) -value the isotope signature of the neighbouring mofettes, the HCO3 -as well as dissolved CO2 and the water discharge of the respective spring were used according to isotope balance formula (data from Weinlich et al., 1998Weinlich et al., , 1999)) where m is the amount of CO2 and ε the fractionation factors of -1.3 ‰ for ε diss-gas and 9.6 ‰ for ε HCO 3 -gas at 10°C (Wendt, 1968;Mook et al., 1974;Zhang et al., 1995).
In case of the mofettes the water discharge is 0, i.e. the ratios mdiss/mtotal and mHCO 3 /mtotal are 0. Consequently, the measured δ 13 C value of the free gas phase is identical to the total, i.e. the primary isotopic composition of magmatic CO2 in this area.Therefore mHCO 3 /mtotal ratios near 0 can be used to identify the primary isotopic composition.
In the case of mineral springs with a continuous transport of leached cations, mainly Ca ++ and Mg ++ , isotopic fractionation occurs with contemporaneously formed HCO3 -whereas remaining CO2 in the gas phase becomes isotopically lighter and CO 2 contents can decrease.Figure 5 exhibits this dependency of the δ 13 C value of CO2 in the gas phase from the mHCO 3 /mtotal ratios in the springs.In cases of spring gases in the close vicinity of mofettes it can be demonstrated that the differences in the isotopic composition are solely caused by HCO3 -fractionation (table I).The spring gases, which are further away from the main gas release-centres, can be transported within more than one fault system and thus in different waters.Therefore, the equilibration between CO2 in the gas phase and HCO 3 -can occur several times and the calculated δ 13 CCO 2 -values are consequently higher than expected in a single equilibration.
Changes in isotopic composition can also be explained by mixing with lighter biogenic CO2.However, the common change in isotopic and chemical composition (fig.6) points to fractionation processes caused by multiple equilibrations as increasing nitrogen contents are not linked with biogenic CO2 admixtures.In the gases of the western Eger Rift a correlation between N2 and He contents can be observed (fig.7) which indicates one source and In the case of waters with low TDS contents and without HCO3 -formation, the fractionation can only take place with dissolved CO2 and the isotopic heavy CO2 remains therefore in the gas phase.a continuous enrichment by solubility fractionation (Weinlich et al., 1998).The assumption of an additional N2-source in case of the N2-richer gases is therefore not necessary.Compared to the gas release centres the nitrogen flux decreases in the springs in the marginal areas.An exception is the water inflow about 500 m below the surface with 100 l/h N2 in the Schönbrunn fluorite mine, caused by pressure release in this mine.
A special case of these fractionations are springs with very low Ca-Mg-HCO3 contents and with isotopic composition of CO2 being still nearly unchanged due to lack of extensive HCO 3 fractionation and where the CO2 contents are solely decreased by the solution of CO 2. This can be explained by fractionation processes.However, in case of an interpretation of isotopically lighter CO2 in the gas distribution pattern of the western Eger Rift caused by mixing with lighter biogenic CO 2 there should be no reason to elucidate why the mixing should not occur in springs with low Total Dissolved Solids (TDS) contents.
Regarding longer migration pathways as mentioned above it is considered that the gas migration occurs within different hydrological systems and therefore these fractionations can take place repeatedly during the migration.This results in a drastic drop of the CO2 contents and the δ 13 C values in the remaining gas phase.
Figure 5 demonstrates the isotopic composition of the N2-rich gases in the Fluorite mine of Schönbrunn (one of the most northern springs shown in fig. 1) with a δ 13 C value of -17.4 ‰ which can be explained alone under the assumption that the CO 2-HCO3 -system is equilibrated twice.Certainly, an admixture of biogenic CO2 cannot be excluded but in this mine about 3.6 m 3 of CO2 gas and 3664 m 3 of dissolved CO2 per year were released.Facing the fact that the granite surface is located only about 650 m below both the thermal water and gas inflows (Kuschka and Hahn, 1996) this amount is hardly explainable with noticeable proportions of biogenic CO2.It is problematic to derive the nitrogen from crustal sources in terms of the 100 l/h N2 in the free gas phase and about 450 l/h dissolved N 2 (air-free over dissolved Ar; procedure in Weinlich et al., 1998) accompanied by 0.45 l/h He and 5.72 l/h dissolved He (in total 54 m 3 /yr He).It should be considered that due to the intrusion of Variscian granites the metamorphic rocks were exposed to far higher temperatures, as is the case today.Therefore, the N2 is probably also mantle derived, because the N 2 gas release including metamorphic CO 2 sourced from these crustal rocks took place during the Variscian intrusions.The nitrogen isotope composition with δ 15 N 0.7 (Weinlich et al., 1999) exhibits a tendency to more positive values of a plume-like mantle (Marty and Dauphas, 2003) which occurs in Central Europe (Wilson and Downes, 1992).
Recalculating the CO2 and N2 in the gas phase together with the dissolved N 2, and CO2 including the HCO3 -the whole fluid system contains about 90 vol% CO2 (possible CaCO3 precipitations would additionally increase this CO2 content) and about 10 vol% N2 and thus this system is comparable with the Eger Rift gases.

The gases of the French Massif Central
The distribution pattern and the isotopic signature of -23 up to -12 ‰ of the N 2-rich spring gases of the French Massif Central could be explained in the same way.As in the western Eger Rift, the CO2-rich gases with δ 13 C values of -7 up to -5 ‰ are linked with high gas fluxes in the concerned mineral springs.Figure 8 demonstrates this correlation between δ 13 C values and the gas composition.Batard et al. (1982) calculated initial isotope composition for some gases in this area with mass balances according to a single equilibration CO 2-HCO3 -.The authors concluded a biogenic or mixed origin for the CO2 because the isotopic composition of the total carbon ranges between -16 and -11 ‰.However, due to multiple equilibrations, it cannot be excluded a priori but only the m HCO 3 /mtotal ratios near 0 should be used to avoid misinterpretations (the calculated δ 13 C value of CO2 for Schönbrunn assuming a single equilibration is also -11 ‰).
For the gases of the French Massif Central, it can be assumed that the gas of Royat with a δ 13 C value of -6.4 ‰ is unfractionated owing to its mHCO 3 /mtotal ratio of 0.007 and displays the primary composition.Further, geothermometer calculations (Pauwels et al., 1997) Batard et al., 1982 andMatthews et al., 1987).-transport (mHCO3) to total CO2 flux (m0) for gases of mineral springs in the French Massif Central (data from Batard et al., 1982).these waters emerging in the Mont Dore area reach temperatures of about 100-130°C at depth and in Saint-Nectaire (δ 13 C value CO2 -7.0 ‰) temperatures of 160-175°C at depth respectively.According to Mook et al. (1974), the fractionation between CO2 and HCO 3 -at temperatures of around 120°C is zero.Based on the isotope composition of Royat it can be shown that the low values of -12 and -23 ‰ can be reached (fig.9) under the assumption Fig. 10.Long-term observation of isotopic composition of the gas of the Eisen spring in Bad Brambach, Eger Rift (Weise et al., 2002) and typical annual changes in the CO2 production in soils (Andrews and Schlesinger, 2001).Growing seasons -light grew.

29
Isotopically light carbon dioxide in nitrogen rich gases that the equilibration between CO2 and HCO3 takes place only twice and in two cases three times.Therefore, it is not absolutely necessary to assume biogenic contributions in the region as well as in the Eger Rift.Just two isotope values, which represent gases of Santenay and Saint Honoré, situated at the edge of the Morvan horst and which are associated with Na-SO4 waters are lighter than those lying in the field of gases equilibrated three times.However, an uncertainty of these mass balances lies within the possibility of influence of non-mineralised groundwaters and/or mineral waters of different type, which «dilute» the HCO 3 --rich mineral water in the respective springs.
In the Cézallier area, Négrel et al. (2000) demonstrate such mixtures of mineral waters with meteoric and different mineralised waters in line with the REE distribution and strontium isotope ratios.Pauwels et al. (1997) state similar processes in the Mont Dore region on the basis of the main element distribution in spring waters.This effect of «dilution» of these mineral waters can produce lower m HCO 3 /mtotal ratios, present during equilibration in deeper regions as higher m HCO 3 /mtotal ratios.Since the CO2 of these N2-rich gases is completely fixed as HCO 3 -, it is no longer possible to form new HCO 3 -in these waters.The δ 13 C values were altered by the formed HCO 3 -and remained unchanged in less mineralised waters.This results in a shift of these gases in plot of the ratio of HCO3 -versus total CO2 flow (fig.9) and acts as if a thrice equilibration took place.According to Schoeller and Schoeller (1979) the TDS contents and especially the HCO3 -contents decrease with increasing distances from the area of Vichy-Cantal-Devès.
As in the western Eger Rift, the N2-rich gases in the Massif Central are enriched in helium.The extreme enrichment of helium, whose contents are the highest in Europe, points rather to a complete fractionation than to a simple mixing with biogenic components.According to the gas flux measurement of Batard et al. (1982) 30 l/h of N2 are also released in the CO2rich gases of Royat.On the other hand, N 2 release in the case of N 2-rich gases are ca.2.8 l/h in Bourbon-Lancy, 29.6 l/h in Evaux-les-Bains, 0.5 l/h in Sail-les-Bains and 1.7 l/h N2 in the Lithium spring in Santenay.Thus only the gas composition is fractionated and it is not necessary to assume additional N2 sources.
An additional argument contradicting the influence of mixing processes is that outside these areas with magmatic CO2 there are no springs with biogenic/organic CO2 in the gas phase.The production rates of biogenic CO 2 in soils (Andrews and Schlesinger, 2001) are too small to nourish a free gas phase.A long-term measurement of isotope composition of CO 2 in the gas phase of the Wettin spring in Bad Brambach (Weise et al., 2001) compared with the biogenic CO 2 production rates (Andrews and Schlesinger, 2001) demonstrate that there is no influence (fig.10).
It is also problematic to derive biogenic/organic CO 2 from sedimentary rocks, especially in areas of metamorphic rocks, since these waters and gases circulate within fault systems.There, either a far-reaching CO2 exchange between the gases migrating along fault pathways and the surrounding country rocks is impeded or the ascending magmatic CO 2 saturates the groundwater with CO 2 gas, as it is the case in the Cheb Basin.The CO2 concentration gradient in the close vicinity of the faults prevents the admixture of CO2 from other sources like for example the biogenic/organic CO2.

Conclusions
As demonstrated, it is possible to elucidate low δ 13 C values with gas fractionation, i.e. by isotope fractionation with formed HCO 3 -and not necessarily and exclusively by mixing with biogenic or organic CO2.However, without complete mass balances it is not possible to dis-criminate between either or give reasons to prefer one of the interpretations.In some cases, it will not be possible to educe the «last proof» for the interpretation.Therefore, it should always be considered that even enhanced CO2contents in the soil air encountered in the vicinity of fractured rocks can also represent completely fractionated magmatic CO2.
However, if we have to assume that the isotopic composition and contents of CO 2 in the gas phase can be alternated by fractionation processes, an influence on the C/ 3 He ratios should also be assumed.Marty et al. (1989) described abating C/ 3 He linked with reduced CO2 contents.

Fig. 6 .
Fig. 6.Plot of N2 content versus δ 13 C values displays the common variations in gas and isotope composition of gases from the Eger Rift area, e.g., from the Cheb Basin/South Vogtland area (CB-SV), Konstantinovy Lázne ǎrea (KL) and Bavaria (BY).These variations are caused by fractionation (CO2 solution and HCO3 -formation).In the case of waters with low TDS contents and without HCO3 -formation, the fractionation can only take place with dissolved CO2 and the isotopic heavy CO2 remains therefore in the gas phase.

Fig. 7 .
Fig. 7. Correlation between nitrogen and helium contents in gases of the western Eger Rift (data from Weinlich et al., 1998).