Evidence for a nitrogen flux directly derived from the European subcontinental mantle in the Western Eger Rift, Central Europe

doi:10.1016/j.gca.2004.05.032

Geochimica et Cosmochimica Acta

Volume 68, Issue 23, 1 December 2004, Pages 4935-4947

https://doi.org/10.1016/j.gca.2004.05.032 Get rights and content

Abstract

In the Czech-German border region of the Vogtland and NW Bohemia (western Eger rift, Central Europe), chemical and isotopic compositions (C, N, He, Ar) of free gas from a thermal water escape (fluorite mine, Schönbrunn), two mineral springs (“Eisenquelle,” Bad Brambach; “Sprudel III,” Bad Elster) and a mofette (Bublak) located along an ∼40-km long traverse are reported. The gases of Bublak and Bad Brambach are CO₂-rich (>99 vol.%) and have δ¹³C values of −1.95 and −4.29‰, respectively. With distance from the center of CO₂ degassing (Bublak) the δ¹³C values decrease, most likely due to physico-chemical fractionation of CO₂ between gaseous and aqueous phases rather than to admixture of organic/biogenic CO₂. The δ¹⁵N values range between −3.2 and −0.6‰, compared to an upper mantle value of −4.0 ± 1.0‰. The four locations are characterized by ³He/⁴He ratios decreasing from 5.9 R_a in the center (Bublak) to 0.8 R_a in the periphery (Schönbrunn) and give evidence for mixing of He from a deep-seated magmatic source with a crustal source. The location with the highest ³He/⁴He ratio (5.9 R_a) is accompanied by the highest ⁴⁰Ar/³⁶Ar (550). We argue that the nitrogen of the Bublak mofette gas is a mixture of predominantly atmospheric and mantle-derived components, whereas at the other three locations crustal nitrogen may also be present. The Bublak δ¹⁵N value of ≈−4.5 ± 1.0‰ represents the first free gas δ¹⁵N reference from the European subcontinental mantle (ESCM) and indicates that, in contrast to the ³He/⁴He ratios, the δ¹⁵N values are equal for ESCM and MORB, respectively.

Introduction

Nitrogen isotope measurements in mantle-derived rocks have established that the upper mantle is depleted in ¹⁵N relative to atmospheric nitrogen. The δ¹⁵N values cluster between −3 and −5‰ (Javoy and Pineau, 1991; Marty and Humbert, 1997; Cartigny et al., 1998a, 1998b, 2001; Marty and Zimmermann).

The isotopic composition of nitrogen from the continental mantle has mainly been derived from diamond investigations. There, both positive and negative δ¹⁵N values were observed, but it was shown that positive δ¹⁵N values are associated with rather negative δ¹³C values, indicating that such signatures are related to subduction processes (Boyd et al., 1987; Boyd and Pillinger, 1994; Sano et al., 1998). Several geochemical and petrological observations support the feasibility of nitrogen recycling into the mantle. There are N₂-rich fluid inclusions in mantle xenoliths (Andersen et al., 1995) as well as high N₂ contents in microdiamonds (De Corte et al., 1998), and N isotopic signatures of fluid inclusions trapped in continental xenoliths from Australia seem to be derived from sedimentary material subducted during the Paleozoic (Matsumoto et al., 2002).

Marty and Dauphas (2003) studied the geological nitrogen cycle from Archean to Present. They found that the development of the biosphere and various processes of nitrogen fixation through time have been responsible for significant changes of the δ¹⁵N signatures. Accordingly, they proposed that light nitrogen was intensely recycled during the Archean whereas from Proterozoic to Present the recycled material carried heavy nitrogen (enriched in ¹⁵N) to the deepest regions of the mantle. As a result, ¹⁵N-enriched nitrogen should be sampled by mantle plumes.

In addition to mantle rocks, a few δ¹⁵N determinations have been performed on free gas exhalations of volcanic or hydrothermal origin (Javoy et al., 1986; Marty et al., 1991; Fischer et al., 1997, 1998, 2002; Sano et al., 2001). However, more investigations are required to better constrain the potential sources of N₂ in free gas exhalations. Such studies could permit a better evaluation of mantle-crust interactions and improve our comprehension of the geodynamic cycle of nitrogen.

In contrast to the δ¹⁵N value of MORB, which has been quite well established at between −3 and −5‰ (Marty and Zimmermann, 1999), the nitrogen isotopic signature of the European subcontinental mantle (ESCM) reservoir has not been characterized so far.

The distinction of various mantle-derived and crustal components in fluids is often achieved using the isotopic compositions of the noble gases, in particular He. An overview of the relationship between the isotopic abundances of nitrogen and the rare gases was given by Tolstikhin and Marty (1998). Recent reviews by Ballentine and Burnard (2002) and Ballentine et al. (2002) have evaluated the current knowledge on origin, release, transport and interaction of noble gases in the continental crust.

The focus of this study is to evaluate the sources of nitrogen from free gas exhalations in a continental rift. We report evidence for an active, mantle-derived nitrogen flux from the ESCM, obtained by detailed monitoring of free gas exhalations in the Vogtland and NW Bohemia (Germany and Czech Republic). The free gas from four locations which are characterized by different contributions of mantle-type He was sampled repeatedly for about 6 months. The gas composition, the isotope ratios (C, N, He, Ar), the relations between inert components (He-Ar-N₂), as well as the CO₂/³He and N₂/³He ratios are used to evaluate the sources of nitrogen. They provide the first evidence for nitrogen in free gas exhalations derived from the ESCM.

Section snippets

Geological, geophysical and geochemical settings

Our investigation area is located at the Czech/German border, close to the crossing of the Eger rift and the Mariánské Láznĕ fault zone at the NW boundary of the Bohemian Massif in Central Europe (Fig. 1).

The European Cenozoic Rift system, which includes the Limagne, Rhone Bresse, Upper and Lower Rhine, Leine, and Eger grabens, developed in the early Cenozoic as a major tectonic feature within western and central Europe (Fig. 1, inset map). Rifting and volcanism in this period has been

Locations, sampling and methods

Gas samples for this study were taken at four locations along a SE-NW directed ≈40-km-long traverse. The Bublak mofette in the SE is located in the northernmost gas escape center (Františkovy Láznĕ/Cheb basin) defined by Weinlich et al. (1999), followed in Germany by the mineral spring “Eisenquelle,” Bad Brambach, the mineral spring “Sprudel III,” Bad Elster, and a thermal water discharge in the Schönbrunn fluorite mine at the NW periphery of the traverse (Fig. 1).

Bublak represents the

Results

The data of the samples which were taken between December 1995 and July 1996 are listed in Table 2. Both the gas compositions and the isotope data at the four locations show clear differences. The greatest scatter in the major components CO₂ and N₂ at one single location is observed for the free gas of Bad Elster, probably influenced by the regime of pumping. In contrast, the δ¹³C and δ¹⁵N values are relatively constant at all locations and do not correlate with fluctuations of the gas

Discussion

The non-atmospheric nitrogen is depleted in ¹⁵N at all four locations (Table 3). The δ¹⁵N value of Bublak is consistent with the MORB range (Marty and Zimmermann, 1999; Nishio et al., 1999; Cartigny et al., 2001; Marty and Dauphas, 2003), but by themselves the N₂ isotopic compositions do not allow a clear classification in terms of crustal or upper mantle-derived nitrogen. Therefore we are going to discuss them in the context of other geochemical indicators in the following sections.

Conclusions

To evaluate the origin of nitrogen in free gas exhalations, a comprehensive study of the chemical and isotopic composition of the gas including the light noble gas isotope ratios (He, Ne, Ar) is necessary. δ¹⁵N values of free gas can prove to be a useful tool for understanding the geochemical N cycle and can contribute to our knowledge about deep earth processes.

For the first time, mantle-derived nitrogen isotope data have been reported for free gases from a European continental rift area

Acknowledgments

We would like to thank J. Tesar̆ for measuring the gas composition and for assistance during sampling at the location in the Czech Republic. For sampling assistance on the German side we thank U. Koch (Saxon Academy of Sciences), P. Kern (Laborunion Bad Elster) as well as W. Netzel and fellows (Hartsteinwerke Vogtland GmbH). Technical assistance was provided by R. Meinert, N. Kadlec, and W. Staedter. Thanks are due to Chris Ballentine, Bernard Marty, Tobias Fischer, Takuya Matsumoto, Rainer

References (69)

W. Aeschbach-Hertig et al.
Quantification of gas fluxes from the subcontinental mantleThe example of Laacher See, a maar lake in Germany
Geochim. Cosmochim. Acta
(1996)
F.T. Aka et al.
Noble gas compositions and water/gas chemistry of soda springs from islands of Bioko, São Tomé and Annobon with Cameroon Volcanic Line, West Africa
Appl. Geochem.
(2001)
C.J. Ballentine
Resolving the mantle He/Ne and crustal ²¹Ne/²²Ne in well gases
Earth Planet. Sci. Lett.
(1997)
C.J. Ballentine et al.
Regional groundwater focusing of nitrogen and noble gases into the Hugoton-Panhandle giant gas field, USA
Geochim. Cosmochim. Acta
(2002)
A. Battani et al.
Basin scale natural gas source, migration and trapping traced by noble gases and major elementsThe Pakistan Indus basin
Earth Planet. Sci. Lett.
(2000)
S.R. Boyd
Nitrogen in future biosphere studies
Chem. Geol.
(2001)
S.R. Boyd et al.
A preliminary study of ¹⁵N/¹⁴N in octahedral growth from diamonds
Chem. Geol.
(1994)
S.R. Boyd et al.
Multiple growth events during diamond genesisAn integrated study of carbon and nitrogen aggregation state in coated stones
Earth Planet. Sci. Lett.
(1987)
P. Cartigny et al.
Subduction-related diamonds?—The evidence for a mantle-derived origin from coupled δ¹³C- δ¹⁵N determinations
Chem. Geol.
(1998)
P. Cartigny et al.
Volatile (C, N, Ar) variability in MORB and the respective roles of mantle source heterogeneity and degassingThe case of the Southwest Indian Ridge
Earth Planet. Sci. Lett.
(2001)

Cited by (52)

Diffuse soil CO<inf>2</inf> emissions at rift volcanoes: Structural controls and total budget of the Olkaria Volcanic Complex (Kenya) case study
2023, Journal of Volcanology and Geothermal Research
Continental rift systems are first-order emitters of deep-sourced CO₂ but their impact on the global budget is largely unknown. For instance, the estimate of the total amount of deep-sourced CO₂ released by the East African Rift (EAR) is largely undocumented due to the small number of direct measurements available and the impracticability of remote sensing methods induced by atmospheric noise. This study explores the degassing budget and the structural control of soil CO₂ emissions at an active rift volcano aiming to implement the database of available direct measurements of EAR's volcanoes. We investigated soil CO₂ degassing at the Olkaria Volcanic Complex, a large, multicentred, geothermally exploited volcanic complex in the southern portion of the Kenyan Rift. A total of 1158 soil CO₂ flux measurements were collected on fumarolic fields or crosscutting unaltered major faults. The contribution of multiple sources of CO₂ (biogenic background vs magmatic/hydrothermal) was tested using both graphical statistical analysis and the carbon isotopic signature (¹³C-CO₂). Soil CO₂ fluxes reached up to ∼5500 gm^-2d^-1. The emission of deep-sourced CO₂ coupled with the structurally controlled hydrothermal fluid circulation, sums up to ∼280 tCO₂ released per day by the entire complex. Finally, we provide an estimate of the total budget for soil CO₂ of the rift, summing up the contributions of all volcanoes in the EAR.
Deep CO<inf>2</inf> and N<inf>2</inf> emissions from Peruvian hot springs: Stable isotopic constraints on volatile cycling in a flat-slab subduction zone
2022, Chemical Geology
Gas-rich hot springs throughout the Peruvian Andes contain a surprising contribution of mantle and crustal volatiles (CO₂ and N₂) despite being located along a volcanic gap associated with modern flat-slab subduction. Similar mantle and crustal volatile degassing is observed in springs from the backarc region of the northern Altiplano Plateau which experienced flat-slab subduction in the Oligocene. We constrain the sources of deeply-derived volatiles using C, N, O and H stable isotope analyses alongside previously reported helium isotope (³He/⁴He) and gas abundance data. δ¹⁸O and δ²H values of spring waters are consistent with regional meteoric water, with deviations towards higher values due to fluid mixing and high-temperature fluid-rock interaction. δ¹⁵N values of N₂ gas range from −0.5 to +4.4‰ (vs AIR), indicating contributions of mantle and crustal (sedimentary) nitrogen. δ¹³C values of dissolved inorganic carbon (DIC) and CO₂ gas from bubbling springs are used to model initial δ¹³C values before degassing, with resulting values ranging from −13.6 to −0.3‰ (vs VPDB) and an average value of −6.9‰. DIC concentrations range from 1 to 57 mmol/kg, of which carbonate dissolution accounts for only a small fraction (average of 15%). The remaining carbon (1 to 44 mmol/kg) is derived from a mixture of deep CO₂ sources with endmember δ¹³C values of −7‰ and −14‰ which, in light of relationships with ³He/⁴He ratios, are interpreted to be the subcontinental lithospheric mantle (SCLM) and metamorphic crustal carbon, respectively. These results are consistent with previous interpretations that slab-to-lithosphere fluid transfer is mobilizing volatiles from the SCLM to the continental crust above the flat slab and confirms that mantle CO₂ is migrating with mantle helium. Metamorphic CO₂ and N₂ are mobilized from the continental crust and entrained by mantle fluids on their way to the surface. In the backarc region, similar geochemical patterns suggest that volatiles are released by dehydration and/or partial melting of previously hydrated lithosphere. We estimate a CO₂ flux of ~3 × 10⁸ mol yr⁻¹ (34 t d⁻¹) from 45 springs with available discharge values. If scaled to all reported thermal springs in Peru, then the total CO₂ emissions from Peruvian thermal springs may approach 2 × 10⁹ mol yr⁻¹ (203 t d⁻¹), or ~ 0.2% of global emissions from subaerial volcanism. Regional CO₂ emissions may be 10 to 100 times greater when considering CO₂ diffusely lost along fault zones or temporarily dissolved near-surface aquifers. These results demonstrate that flat-slab subduction leads to an efficient and unexpected transfer of mantle and crustal volatiles to Earth's surface, and more generally, that deep volatile fluxes in subduction zones are not limited to active volcanism.
An evaluation of the C/N ratio of the mantle from natural CO<inf>2</inf>-rich gas analysis: Geochemical and cosmochemical implications
2020, Earth and Planetary Science Letters
The terrestrial carbon to nitrogen ratio is a key geochemical parameter that can provide information on the nature of Earth's precursors, accretion/differentiation processes of our planet, as well as on the volatile budget of Earth. In principle, this ratio can be determined from the analysis of volatile elements trapped in mantle-derived rocks like mid-ocean ridge basalts (MORB), corrected for fractional degassing during eruption. However, this correction is critical and previous attempts have adopted different approaches which led to contrasting C/N estimates for the bulk silicate Earth (BSE) (Marty and Zimmermann, 1999; Bergin et al., 2015). Here we consider the analysis of CO₂-rich gases worldwide for which a mantle origin has been determined using noble gas isotopes in order to evaluate the C/N ratio of the mantle source regions. These gases experienced little fractionation due to degassing, as indicated by radiogenic ${}^{4}H e /^{40} A r^{⁎}$ values (where ⁴He and ⁴⁰Ar* are produced by the decay of U+Th, and ⁴⁰K isotopes, respectively) close to the mantle production/accumulation values. The C/N and $C /^{3} H e$ ratios of gases investigated here are within the range of values previously observed in oceanic basalts. They point to an elevated mantle C/N ratio (∼350-470, molar) higher than those of potential cosmochemical accretionary endmembers. For example, the BSE C/N and ${}^{36}A r / N$ ratios (160-220 and $75 \times 10^{- 7}$ , respectively) are higher than those of CM-CI chondrites but within the range of CV-CO groups. This similarity suggests that the Earth accreted from evolved planetary precursors depleted in volatile and moderately volatile elements. Hence the high $C / N$ composition of the BSE may be an inherited feature rather than the result of terrestrial differentiation. The $C / N$ and ${}^{36}A r / N$ ratios of the surface (atmosphere plus crust) and of the mantle cannot be easily linked to any known chondritic composition. However, these compositions are consistent with early sequestration of carbon into the mantle (but not N and noble gases), permitting the establishment of clement temperatures at the surface of our planet.
CO<inf>2</inf> degassing and melting of metasomatized mantle lithosphere during rifting – Numerical study
2019, Geoscience Frontiers
Citation Excerpt :
The isotope composition (He and C) of CO2-rich gas emanations of mineral springs and mofettes from the French Massif Central and the Eger Rift (Weinlich et al., 1999; Geissler et al., 2005) is consistent with the ascent of gases from fluid reservoirs in the (SCLM). Extensive isotopic studies supporting this idea can be found for example in Dunai and Baur (1995), Bräuer et al. (2004), Buikin et al. (2005), Day et al. (2015). Though mantle-plume activity is not considered to drive the rifting, evolution of the ECRIS was accompanied by the development of major volcanic centres in the French Massif Central and in the Rhenish and Bohemian Massifs (Ziegler and Dèzes, 2006).
Reactivation of metasomatized mantle lithosphere may occur during continental extension, which is an important component of plate tectonics. The lower most part of the metasomatized domains in the subcontinental mantle lithosphere can be locally enriched in CO₂. Therefore, partial melting of these metasomatized domains may play a crucial role in the global carbon cycle. However, little is known about this process and up until now few numerical constraints are available. Here we address this knowledge gap and use a 2-D high resolution petrological-thermomechanical model to assess lithospheric rifting, CO₂ degassing and melting. We test 4 lithospheric thicknesses: 90, 110, 130 and 200 km with a 10 km thick metasomatized layer at the base using CO₂ of 2 wt.% in the bulk composition. The carbonate enriched layer is stable below ∼3 GPa (>110 km) for a temperature of 1300 °C; therefore, we only observe degassing patterns for lithospheric models that are 130 km and 200 km thick. The metasomatized layer for the 130 km thick lithosphere mostly comprises carbonatite melting, whereas in the 200 km thick scenario propagation of melt development from kimberlites to carbonatites occurs as the metasomatic mantle is exhumed during extension. The numerical models fit well into natural rifting zones of the European Cenozoic Rift System for young (shallow) and of the North Atlantic Rift for old (thick) lithosphere.
Spatial distribution of metal(loid) depletion and accumulation zones around a natural carbon dioxide degassing site
2019, Chemical Geology
Long-term influence of geogenic CO₂ affects soil conditions and pedogenesis. Mobilization of metals and metalloids from soil to solution has been reported to occur in natural CO₂ degassing sites, so-called mofettes. We determined metal(loid)-specific spatial distribution patterns in soil around a mofette as well as metal(loid) pore water concentrations along a CO₂ gradient (0–98% CO₂ in soil air). Depletion of Mn, Co, and Ni in soil in the mofette center was caused by leaching due to the long-term soil acidification leading to correspondingly low pore water concentrations and thus indicating no recent influence of CO₂ on metal(loid) mobility. Iron and As were also depleted in soil within the mofette center where pedogenic Fe (oxyhydr)oxides could not form due to absence of oxygen. Small-scale variations in redox conditions lead to ongoing Fe cycling and the repeated reduction of Fe (oxyhydr)oxides resulted in increased Fe and As pore water concentrations at high CO₂. Precipitation in form of sulfide minerals caused immobilization and accumulation of Cd, Cu, and Zn directly in the degassing center. The highest mobilization risk occurred within 2–4 m distance from the degassing center, where complexation with dissolved organic matter (DOM) increased the mobility of Al, As, Cr, Cu, Fe, and Zn. Our results show that CO₂ as soil-forming factor influences the spatial distribution of metal(loid)s. The highest metal(loid) mobilization risk after long-term CO₂ influence arises from accumulation of scarcely degraded organic matter, which can easily dissolve and form mobile metal(loid)-DOM complexes.
Monitoring of helium and carbon isotopes in the western Eger Rift area (Czech Republic): Relationships with the 2014 seismic activity and indications for recent (2000–2016) magmatic unrest
2018, Chemical Geology
We report new data of the regional distribution pattern of total gas compositions as well as He and CO₂ isotopic compositions from 25 gas exhalations in the western Eger Rift and its surroundings. Additionally, the first time-series data from gas exhalations in a clay pit within the Cheb Basin (CB) are given. At 21 degassing locations, the first data were obtained >20 years ago. From 7 locations within the degassing center CB and from 3 degassing sites belonging to the Mariánské Lázně (ML) degassing center, neon and argon isotope compositions were determined also.
CO₂ is the major component at all degassing sites. The δ¹³C values display a small range (−1.7 to −5.1‰) and the ³He/⁴He ratios vary from 1.9 to 5.9 R_a. The highest ³He/⁴He ratios are found at locations along the Počatky-Plesná Fault Zone, followed by the degassing site in the clay pit on the Nová Ves Fault and the locations on the ML fault at the edge of the CB. Although gas flow and CO₂ concentrations in all degassing centers are very high, the fractions of mantle-derived helium are different, with presently up to 94% (in relation to the SCLM ³He/⁴He of 6.32 R_a) in the CB, up to 73% in the ML and up to 35% in the Karlovy Vary degassing center. At the locations in the eastern part of the CB a clear, progressive increase of the ³He/⁴He ratio has been observed since the first sampling campaigns there in 1993 and 1994, whereas at the other degassing sites the helium isotope ratio remained essentially the same. The progressive increase of the ³He/⁴He ratio in the eastern part of the CB, together with further short-time increases up to 6.3 R_a at one location (Bublák) before both the 2000 and 2008 earthquake swarms, indicate an ongoing magmatic process beneath this area, which seems to be associated with the occurrence of seismicity. The CB is located close to the Nový Kostel focal zone where since the beginning of our investigations four strong periods of seismicity (with magnitudes >3) occurred. The latest gas data confirm our earlier findings: time-series studies showed that in relation with seismic events, decreased ³He/⁴He ratios were repeatedly observed due to admixed seismically released crustal helium. Presently, the eastern part of the CB is the most active non-volcanic region in the European Cenozoic Rift System, with gas signatures similar to those found in free mantle-derived gases from the East African Rift system.

View all citing articles on Scopus

View full text

Evidence for a nitrogen flux directly derived from the European subcontinental mantle in the Western Eger Rift, Central Europe

Abstract

Introduction

Section snippets

Geological, geophysical and geochemical settings

Locations, sampling and methods

Results

Discussion

Conclusions

Acknowledgments

Geochim. Cosmochim. Acta

Appl. Geochem.

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Chem. Geol.

Chem. Geol.

Earth Planet. Sci. Lett.

Chem. Geol.

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

J. Volcanol. Geotherm. Res.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Chem. Geol.

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Chem. Geol.

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Chem. Geol.

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Chem. Geol.

Geochim. Cosmochim. Acta