Karin Bräuer

An active subcontinental mantle volatile system in the western Eger rift, Central Europe: gas flux, isotopic (He, C, and N) and compositional fingerprints

The composition and flux of gas emanations, and the isotopic ratios of CO2, He and N2 of 74 mineral springs and dry gas vents (mofettes) in the western Eger rift (Czech Republic) have been analyzed. Four geochemically similar, but tectonically separate, gas escape centers are distinguishable, out of which 3 show a free gas flux >85000 dm3 h−1. All gases from the centers are CO2-rich (>99 vol.%) and have δ13C values ranging from −1.8 to −4.0‰. 3He/4He ratios are as high as R/Ra = 5, and are among the highest measured in Europe. The discharge of the gas mixture decreases with distance from the emanation centers with both decreasing fractions of CO2 and δ13C values, whereas the fractions of N2 and trace gases increase. These changes in chemical and isotopic composition are associated by a decrease in R/Ra ratios from about 5 in the centers to <2 in the peripheries. The changes of the contents and isotopic composition of CO2 can be explained by physico-chemical fractionations of CO2 between gaseous and aqueous phases. Towards the periphery, the contents of free CO2 and its δ13C are reduced by dissolution of CO2 in groundwater, whereby the content of N2 increases. 3He/4He ratios give evidence for mixing of He from both a deep-seated magmatic and a crustal source. The gas emanation centers, with their strongly magmatic δ13C value of about −2.7‰, seem to outline the intersections of the Eger rift and the Marianske Lazně fault, which are considered to represent a deep-reaching fracture system that enables the ascent of gases from a magmatic body in the European subcontinental mantle (SCM). Therefore, the European SCM is suspected to be the main source of CO2. The most mantle-like He (and probably N2) occurs in the centers of gas release. The total regional gas flux in the western Eger rift is determined to be 3.6 × 108 mol a−1. When related to the investigated area of 1500 km2, flux densities greater than 0.24 × 106, 52, and 0.65 mol km−2 a−1 for CO2, N2 and He respectively are calculated.

Ammonium concentration and nitrogen isotope composition in metasedimentary rocks from different tectonometamorphic units of the European Variscan Belt

Abstract The content and isotopic composition of ammonium ( NH4+) were measured by using Kjeldahl distillation for 63 samples of predominantly metasedimentary rocks from two different geological settings of the European Variscides, i.e., the Erzgebirge and the Zone of Erbendorf-Vohenstrauss in Germany. The studied Erzgebirge schists represent an early Paleozoic passive margin sequence that reappears in different deep subducted metamorphic units. This allows an efficient examination of nitrogen loss and isotope fractionation during prograde metamorphism. Ammonium is found to be progressively depleted, accompanied by a shift in δ15N from the Low-grade Units (≈2 kbar/300°C) with 638 ± 124 ppm and δ15N = +2.2 ± 0.6‰, to the Garnet-Phyllite Unit (≈9 kbar/470°C) with 621 ± 190 ppm and δ15N = +3.5 ± 0.9‰, the Mica Schist/Eclogite Unit (≈12 kbar/550°C) with 394 ± 113 ppm and δ15N = +3.9 ± 0.8‰ and to the Gneiss/Eclogite Unit (>12 kbar/730°C) with 99 ± 32 ppm and δ15N = +7.7 ± 2.0‰. Using equilibrium models for Rayleigh distillation and batch volatilization suggest that the nitrogen depletion took place by ammonia release. Only for the mica schists, this isotope fractionation can be explained by the loss of molecular nitrogen. In comparison with the early Paleozoic schists, the Proterozoic ortho- and paragneisses of the Erzgebirge Gneiss Unit (≈6–8 kbar/650°C) contain significantly lower amounts of ammonium (≈70 ppm) and reach relatively low δ15N values (+2.5‰ to +3.6‰). The Paragneisses from the Zone of Erbendorf-Vohenstrauss (ZEV) sampled from the KTB pilot hole represents a separate tectonometamorphic unit that has undergone polyphase metamorphic conditions with the major imprint being amphibolite facies (7 kbar/650–700°C). The metagraywackes have NH4+ contents of ≈80 ppm and δ15N of approximately +6‰. Gneisses in the vicinity of a late cataclastic shear zone at 2000 m depth are characterized by a shift in NH4+ (>250 ppm) and δ15N up to +12‰, which suggest late fluid-rock interactions with a nitrogen- and 15N-enriched fluid.

Earthquake swarms in non‐volcanic regions: What fluids have to say

The detailed processes generating earthquake swarms are complex and not fully understood. Most earthquake swarms occur in volcanic regions and mid-ocean rifts. Here, we report new 3 He/ 4 He data of free gases monitored at CO 2 -rich degassing locations close to the Nový Kostel focal zone (NKFZ) located in the western Eger rift. The NKFZ is known for the recurrence of earthquake swarms at which the focal zone ranges between 6 and 12 km depth. At degassing locations neighboring to the NKFZ a progressive increase of mantle-derived helium has been observed during the last 15 years - actually the highest 3 He/ 4 He ratios (>6 Ra) in Central Europe. The 3 He/ 4 He anomalies indicate hidden magmatic activity. We assume that the latest strong earthquake swarm in October 2008 was initiated by a hidden magma intrusion process from the upper mantle into the lower crust that has been indicated by a three month lasting increase of the 3 He/ 4 He ratios in spring 2006 at all degassing locations near the NKFZ.

Natural laboratory NW Bohemia: Comprehensive fluid studies between 1992 and 2005 used to trace geodynamic processes

Comprehensive studies of CO2-rich fluids close to the swarm earthquake region Nový Kostel at the Czech-German border have been started 15 years ago and have in particular included two extended chemical and isotope monitoring studies lasting for several years each. The regional surface distribution patterns of the fluid signatures including the identification of the origin of fluid components are the focus of the detailed studies. Three degassing centers (Cheb basin, Marianske Lazně, and Karlovy Vary) with high CO2 flux and the same level of δ13C values, but different levels of 3He/4He ratios, have been identified. The studies have located the CO2 source and have investigated seismically induced changes in fluid characteristics on the basis of unique weekly sampling campaigns at selected locations. A seismically triggered release of crustal helium was confirmed by both monitoring campaigns. Finally, indications for a presently active magmatic process beneath the Cheb basin have been found. In contrast to volcanically active regions, magma accumulation in the study area takes place at the crust-mantle boundary and is not yet accompanied by heat transfer to the surface. Likewise, reactive magma-derived components are absent in the degassing fluids. The area of investigation has the potential to be a natural laboratory for fundamental studies of active geodynamic processes. The results of our fluid monitoring, including the stunning observation of mantle-derived free fluids marked by 3He/4He ratios within the subcontinental mantle range, are supported by geophysical findings from seismic studies and geologic indications.

Combined Gas-geochemical and Receiver Function Studies of the Vogtland/NW Bohemia Intraplate Mantle Degassing Field, Central Europe

The study area — the western part of the Eger (Ohře) Rift (Fig. 1) — belongs to the European Cenozoic rift system (ECRIS) (Ziegler 1992, Prodehl et al. 1995, Dezes et al. 2004). This system of graben structures and intraplate volcanic fields spreads over a distance of some 1100 km from the Mediterranean to the North Sea coast, including the French Massif Central, the Upper Rhine Graben, the Eifel, the North Hessian Depression, the Vogelsberg, and the Eger Rift (Fig. 1, inset map). There are different models to explain the widespread rifting and associated volcanism in the foreland of the Alpine orogen. Most of them are related to the effects of Alpine collision (e.g. Ziegler 1992). However, there also exist ideas of a mantle plume or several small mantle plumes (mantle fingers) as the source of the magmatic activity (e.g., Granet et al. 1995, Goes et al. 1999).

Seismically triggered anomalies in the isotope signatures of mantle‐derived gases detected at degassing sites along two neighboring faults in NW Bohemia, central Europe

The Vogtland and NW Bohemia region is known for its earthquake swarms; the most intensive swarm since 1985/86 occurred in October 2008. To find further indications for the interaction of ascending mantle-derived fluids and the occurrence of earthquake swarms, detailed fortnightly studies of gas compositions (CO2, N2, Ar, He, H2, and CH4) and isotope ratios (δ13C, δ15N, and 3He/4He) were carried out between October 2008 and April 2011 at four locations close to the Nový Kostel focal zone and at the Wettinquelle spring (Bad Brambach). From the start of the 2008 earthquake swarm seismically induced isotope-geochemical anomalies were recorded at locations along the Pocatky-Plesna fault zone (PPZ) and were, for the first time, also found at degassing locations on the Marianske Laznĕ fault zone (MLF). Variations were observed in both the temporal and spatial distributions of the anomalies as well in anomaly strengths, probably due to the positions of these fault zones relative to the focal zone, and to differences in fluid migration pathways. Prior to both the 2000 and 2008 swarms, 3He/4He ratios > 6 Ra were recorded at the Bublak mofette. These anomalous pre-seismic 3He/4He ratios suggest that both the 2000 and 2008 swarms may have been associated with the supply of fresh magma from a less degassed reservoir in the lithospheric mantle. The temporal δ13CCO2 pattern from detailed studies at Bublak between 2005 and 2011 indicates progressive magma degassing, as well as seismically induced variations in the δ13C, providing additional support to the interpretation derived from the 3He/4He ratios.

Comparative study of geophysical and soil–gas investigations at the Hartoušov (Czech Republic) natural CO2 degassing site

Abstract Our study at this natural analog site contributes to the evaluation of methods within a hierarchical monitoring concept suited for the control of CO2 degassing. It supports the development of an effective monitoring concept for geological CO2 storage sites—carbon capture and storage as one of the pillars of the European climate change efforts. This study presents results of comprehensive investigations along a 500-m long profile within the Hartoušov (Czech Republic) natural CO2 degassing site and gives structural information about the subsurface and interaction processes in relation to parameters measured. Measurements of CO2 concentrations and investigation of the subsurface using electrical resistivity tomography and self-potential methods provide information about subsurface properties. For their successful application it is necessary to take seasonal variations (e.g., soil moisture, temperature, meteorological conditions) into consideration due to their influence on these parameters. Locations of high CO2 concentration in shallow depths are related to positive self-potential anomalies, low soil moistures and high resistivity distributions, as well as high δ13C values and increased radon concentrations. CO2 ascends from deep geological sources via preferential pathways and accumulates in coarser sediments. Repetition of measurements (which includes the effects of seasonal variations) revealed similar trends and allows us to identify a clear, prominent zone of anomalous values. Coarser unconsolidated sedimentary layers are beneficial for the accumulation of CO2 gas. The distribution of such shallow geological structures needs to be considered as a significant environmental risk potential whenever sudden degassing of large gas volumes occurs.

An alternative procedure for the 18O measurement of nitrate oxygen

Abstract A new method for the preparation of nitrate samples for 18 O measurement is presented. The technique utilizes guanidine hydrochloride to convert the nitrate oxygen to CO 2 . The δ 18 O values of our KNO 3 laboratory standard (K1) obtained by the guanidine method are compared with the results of the reaction of the potassium nitrate laboratory standard K1 with mercury cyanide and with the silver nitrate-graphite method. The δ 18 O data produced by the high temperature (900°C) cyanide and graphite technique are comparable to the guanidine method only when the potassium nitrate is converted to silver nitrate. The reproducibility for the guanidine method is ±0.2‰ based on the tests carried out using KNO 3 (laboratory standard, K1) and the IAEA-N3 reference material. The yield of the conversion of nitrate oxygen to CO 2 is 95–100%.

Deformation structures in Plio- and Pleistocene sediments (NW Bohemia, Central Europe)

Abstract The small intracratonic Cheb (Eger) Basin in NW Bohemia (Central Europe) is characterized by swarm earthquakes, many mineral springs and mofettes with upper mantle CO2 degassing and by neotectonic graben and basin structures. Especially in non-lithified Upper Pliocene clay formations of the basin, a variety of deformation patterns is exposed. They include non-tectonic and tectonic activity and comprise faulting and folding from µm- to km-scale. Previously unrecognized N-S- and ENE-striking faults are sites of mantle degassing and seismic activities. Confined-layer deformation and liquefaction structures hint to palaeoseismic events and gas escape activity. Cleavage-like arranged clay mineral plates represent the microfabric of clay within fault zones. For the first time the degassing channels of Upper Mantle fluids/gases through the Pliocene clay sediments can be documented: µm-scale micro-tubes were produced by the opening of Riedel shear planes induced by fault movements.

MONACO—Monitoring Approach for Geological CO2 Storage Sites Using a Hierarchical Observation Concept

The reliable detection and assessment of potential CO2 leakages from storage formations require the application of assurance monitoring tools at different spatial scales. Such tools also play an important role in helping to establish a risk assessment strategy at carbon dioxide capture and storage (CCS) facilities. Within the framework of the MONACO project (“Monitoring approach for geological CO2 storage sites using a hierarchical observation concept”), an integrative hierarchical assurance monitoring concept was developed and validated with the aim of establishing a modular observation strategy including investigations in the shallow subsurface, at ground surface level, and in the atmosphere. Numerous methods and technologies from different disciplines (such as chemistry, hydrogeology, meteorology, and geophysics) were either combined or used complementarily to one another, with results subsequently being jointly interpreted. Patterns of atmospheric CO2 distributions in terms of leakage detection can be observed on large scales with the help of infrared spectroscopy or micrometeorological methods, which aim to identify zones with unexpected or anomalous atmospheric CO2 concentrations. On the meso-scale, exchange processes between ground surface level and subsurface structures need to be localized using geophysical methods and soil gas surveys. Subsequently, the resulting images and maps can be used for selecting profiles for detailed in situ soil gas and geophysical monitoring, which helps to constrain the extent of leakages and allows us to understand controlling features of the observable fluid flow patterns. The tools utilized were tested at several natural and industrial analogues with various CO2 sources. A comprehensive validation of the opportunities and limitations of all applied method combinations is given and it shows that large spatial areas need to be consistently covered in sufficient spatial and temporal resolutions.