Procedia Earth and Planetary Science 13 ( 2015 ) th Applied Isotope Geochemistry Conference, AIG-11 BRGM

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1 Available online at ScienceDirect Procedia Earth and Planetary Science 13 ( 2015 ) th Applied Isotope Geochemistry Conference, AIG-11 BRGM Halogen isotopes ( 37 Cl and 81 Br) in brines of the Siberian Platform Alexeeva L.P. а*, Alexeev S.V. а, Kononov А.М. а, Ma Teng b, Liu Yunde b a Institute of the Earth s crust SB RAS, Irkutsk, salex@crust.irk.ru, b China University of Geosciences, Wuhan, mateng@cug.edu.cn Abstract This paper offers the results obtained through a survey of 37 Cl and 81 Br stable isotopes in ground saline waters and brines in sedimentary and intrusive rocks of the Siberian Platform. Over 60 ground water samples collected in wells and springs have been analyzed. In addition, the chlorine and bromine isotope compositions in deep ground waters of the Siberian Platform have been compared with that of ground waters in crystalline rocks of the Canadian and Fennoscandian Shields, as well as in sedimentary sequences of the North China Plain. Preliminary results showing patterns of distribution and isotope compositions in ground brines of the Siberian Platform confirm the meteoric nature of the chloride Na brines, as well as formation of chloride Ca brines from evaporated paleo-seawater The Authors. Published by Elsevier by Elsevier B.V. B.V. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the scientific committee of AIG-11. Peer-review under responsibility of the scientific committee of AIG-11 Keywords: chlorine and bromine isotopes; ground brines; meteoric waters and evaporated paleo-seawater; Siberian Platform. 1. Introduction The genesis and evolution of brines in platform sedimentary cover and basement highs is of prime interest in hydrogeochemical investigations recently complemented by comprehensive isotope studies. Cl and Br stable isotopes have been broadly utilized as useful tracers to constrain the origin of chloride ground waters or for investigating geochemical evolution of bittern connate waters in sedimentary basins. Studies of processes causing fractionation of halogen isotopes have revealed that Cl isotopes fractionate due to salt precipitation 1, evaporative concentration of brines 2, ion exchange and diffusion 3,4, the latter playing a significant role in bromine isotope fractionation 4. * Corresponding author. Tel.: ; fax: address: lalex@crust.irk.ru The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the scientific committee of AIG-11 doi: /j.proeps

2 48 L.P. Alexeeva et al. / Procedia Earth and Planetary Science 13 ( 2015 ) This paper aims at describing the distribution of 37 Cl and 81 Br stable isotopes in chloride ground waters of the Siberian platform, to enlarge the databank on their isotope features and to assess the feasibility of using halogen isotopes as a critical tool for unraveling the water-rock evolution of the Siberian Platform since Early Cambrian time. 2. Sampling and analysis Research on halogen isotopes in ground waters of the Siberian Platform was initiated in 2005 in collaboration with Canadian scientists from the Waterloo University 5,6. A total of 47 samples of Na-Ca chloride type waters with salinity varying from 38 to 626 g/l were analyzed for their Cl and Br isotopic compositions. Analyses were performed by Isotope Ratio Mass Spectrometry (IRMS) with an analytical precision of 0.1 for 37 Cl and 81 Br. In 2014, the analyses of the State Key Laboratory of Biogeology and Environmental Geology of the China University of Geosciences determined δ 37 Cl and δ 81 Br in 20 samples of Ca-Na chloride brines with salinity of g/l. Water samples were collected through depth m. The analyses were performed using the GasBench II-IRMS by Thermo Finnigan МАТ-253. The 37 Cl and 81 Br isotopes were measured with precision 0.07 and 0.06, accordingly 7,8. The results acquired in this study for the isotope features of ground waters collected in the sedimentary and intrusive rocks of the Siberian platform constitute the foundation of this article. 3. Geology and hydrogeology The Siberian Platform, including the studied hydrogeological formations, occupies the area over km 2, extending from the Laptev Sea in the North to the Baikal Lake in the South (Fig. 1). The geological-tectonic and hydrogeological features of the platform developed since the late Upper Cambrian due to trap and kimberlite magmatism, accumulation of halogen sequences in the southern part and carbonate ones on the northern termination and long-term erosion. Particular conditions of heat exchange led to formation of the unique cryolithozone in the north-eastern part of the craton, which thickness reaches over 1400 m. Groundwaters are found in the sedimentary cover down to depth of 2-3 km, they are saline to strongly saline brines of primarily chloride sodium or calcium (in places mixed) composition. Fig.1. Location of the Siberian Platform.

3 L.P. Alexeeva et al. / Procedia Earth and Planetary Science 13 ( 2015 ) Results and discussion Figure 2A provides generalized data on the distribution of Cl isotopes in the geological section. The range of Cl isotopes in natural systems generally covers about 15 : from 8 in pore waters of subduction zones, to +7 Fig. 2. Natural variations in Cl and Br isotope compositions: А - δ 37 Cl (SMOC) in natural waters, rocks and gases of the Earth 5, 9,11,12 ; B - δ 81 Br (SMOB) in ground saline waters and brines of crystalline and sedimentary rocks in some regions of the world 10,13,14. in volcanic gases. In ground waters of different regions the δ 37 Cl value varies from 2.5 to +2.5 (SMOC). In the ground saline waters of the North China Plain (maximum salinity of 9.3 g/l) the Cl isotope variations were established within the narrow range: viz In the ground waters of the Canadian and Fennoscandian Shields with salinity varying from 1.9 to 258 g/l the range is broader, going from 0.78 to and 0.54 to +1.52, respectively 10. The few data on δ 81 Br value in the ground waters of crystalline and sedimentary rocks are also depicted on the diagram (Fig. 2В). In ground saline waters of the North China Plain occurring in the largest oil-gas field of the Hebei province (composed of Mesozoic-Cenozoic sandstones and argillites) the δ 81 Br values are positive: going from to (SMOB). In the ground waters of the crystalline rocks within the shields they vary differently: in the Canadian Shield from to +1.29, in the Fennoscandian Shield they are to 2.04 and in the oil-gas field Oseberg (Norway) the values amount to to (SMOB). Based on available data, we have defined ranges for δ 37 Cl and δ 81 Br variations in ground waters of the Siberian Platform; these are 1.67 to (SMOC) and 0.80 to (SMOB), respectively. The δ 37 Cl values are within the range ( 2.5 to +2.5 ) reported for ground waters of different regions of the world 11. The range of δ 81 Br values in ground waters of the platform sediments is close to that in ground waters of the Williston sedimentary basin (the North America). In chloride calcium brines the values of Cl and Br stable isotopes show greater variations (Fig. 2): δ 37 Cl ranges from 1.67 to (scatter 2.93 ), and δ 81 Br from 0.8 to (scatter 2.27 ). Salinity of very

4 50 L.P. Alexeeva et al. / Procedia Earth and Planetary Science 13 ( 2015 ) concentrated brines reaches 626 g/l, bromine content is 8.5 g/l and genetic coefficient Cl/Br amounts to Chloride calcium brines are referred as the formation water. The chloride sodium brines are characterized by a smaller range of Cl isotope ratios and larger range for Br isotopes against calcium brines: δ 37 Cl varies from 0.29 to (1.83 ), δ 81 Br varies from 0.07 to (3.42 ). Salinity of these waters changes from 44 to 318 g/l, Br content is not high (on average 380 mg/l), and Cl/Br coefficient reaches These are typical leaching brines. Variations of halogen isotopes in chloride calcium brines may indicate a common source of the elements supplied into ground waters and an equal impact of evolutionary geochemical processes (diffusion, ion filtration, etc.) of buried bittern connate waters. This observation is also confirmed by the positive correlation (R 2 =0.62) between δ 37 Cl and δ 81 Br (Fig. 3). Enrichment of chloride sodium brines with heavy-br isotopes proceeded through the process of chloride salt leaching and subsequent evaporative concentration of brines at the stage of their formation. 5. Conclusion Fig.3. δ 37 Cl vs. δ 81 Br in brines of the Siberian Platform (SMOW - Standard Mean Ocean Water). The δ 37 Cl and δ 81 Br values in ground waters of the Siberian Platform fall within the range currently documented for ground waters in different regions of the world. The range of δ 37 Cl values virtually coincides with that of Cl isotope in ground waters of crystalline shields and North China Plain. The δ 81 Br in ground waters of the platform in many cases turns out to negative, as in the sedimentary Williston basin (North America). This may indicate quite different ways of ground water evolution in the Siberian Platform and crystalline shields, on the one hand, and similar processes controlling the isotope composition of brines in sedimentation basin, on the other hand. The application of stable halogen isotopes with the other hydrogeochemical parameters is useful for constraining ground saline water and brine origin. Preliminary results showing patterns of distribution and isotope compositions in ground brines of the Siberian Platform confirm the meteoric nature of the chloride Na brines, as well as formation of chloride Ca brines from evaporated paleo-seawater.

5 L.P. Alexeeva et al. / Procedia Earth and Planetary Science 13 ( 2015 ) Acknowledgements The work was supported by RFBR grants (projects GFEN and ). The authors are grateful to Tatiana Bounaeva for providing the Russian to English translation. References 1. Eggenkamp HGM, Kreulen R, Koster Van Groos AF. Chlorine stable isotope fractionation in evaporates. Geochimica et Cosmochimica Acta 1995; 59 (24): Luo CG, Xiao YK, Ma HZ, Ma YQ, Zhang YL, He MY. Stable isotope fractionation of chlorine during evaporation of brine from a saline lake. China Science Bulletin 2012; 57 (15): Eastoe CJ, Long A, Land LS, Kyle JR. Stable chlorine isotopes in halite and brine from the Gulf Coast Basin: brine genesis. Chemical Geology 2001; 176: Eggenkamp HGM, Coleman ML. The effect of aqueous diffusion on the fractionation of chlorine and bromine stable isotopes. Geochimica et Cosmochimica Acta 2009; 73: Alekseev SV, Alexeeva LP, Borisov VN et al. Isotope composition (H, O, Cl and Sr) of ground brines of the Siberian Platform. Geology and Geophysics 2007; 48, 3: Shouakar-Stash O, Alexeev SV, Frape SК, Alexeeva LP et al. Geochemistry and stable isotopic signatures, including chlorine and bromine isotopes of the deep ground waters, of the Siberian platform, Russia. Applied Geochemistry 2007; 22, 3: Liu Y, Zhou A, Gan Y, Liu C, Yu T, Li X. An online method to determine chlorine stable isotope composition by continuous flow isotope ratio mass spectrometry (CF-IRMS) coupled with a Gasbench II. J. Central South University 2013; 20: Dua Y, Ma T, Yanga J, Liua L, Shana H, Caia H, Liua C, Chen L. A precise analytical method for bromine stable isotopes in natural waters by GasBench II-IRMS. International Journal of Mass Spectrometry 2013; 338: Chen L, Ma T, Du Y, Yang J, Liu L, Shan H, Liu C, Cai H. Origin and evolution of formation water in North China Plain based on hydrochemistry and stable isotopes ( 2 H, 18 O, 37 Cl and 81 Br). Journal of Geochemical Exploration 2014; 145: Stotler RL, Frape SK, Shouakar-Stash O. An isotopic survey of δ 81 Br and δ 37 Cl of dissolved halides in the Canadian and Fennoscandian Shields. Chemical Geology 2010; 274: Heofs J. Stable isotope geochemistry. 6 ed. Springer; Stotler RL, Frape SK, Shouakar-Stash O. An isotopic survey of δ 81 Br and δ 37 Cl of dissolved halides in the Canadian and Fennoscandian Shields. Chemical Geology 2010; 274: Eggenkamp HGM, Coleman ML. Rediscovery of classical methods and their application to the measurement of stable bromine isotopes in natural samples. Chemical Geology 2000; 167: Shouakar-Stash O, Frape SK, Rostron BJ, Drimmie RJ. Variations of the δ 81 Br and δ 37 Cl stable isotope signature for pre-mississippian formation waters of the Williston Basin. Goldschmidt Conference Abstracts Geochimica et Cosmochimica Acta 2006; 70, 18: А589.

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