Anniina Kittilä, ETH Zürich, Institute of Geophysics, Geothermische Energie u. Geofluide. Sonneggstrasse 5, 8092 Zürich, Switzerland e-mail: anniina.kittila(at)erdw.ethz.ch
The major role of strontium isotope studies in hydrogeological investigations is in revealing water-rock interactions and in investigations of groundwater flow path and origin. The method is based on the 87Sr/86Sr ratio, because it varies with rocks of different ages. Significant portion of 87Sr originates from the decay of 87Rb, so rocks rich in Rb (usually associated with K-rich rocks) tend to have higher 87Sr/86Sr ratios (Clark & Fritz 1997). Strontium has four stable isotopes: 84Sr, 86Sr, 87Sr and 88Sr, from which only 87Sr is radiogenic. This means that when a system is created the amount of 84Sr, 86Sr and 88Sr is set, but the amount of 87Sr increases from the initial amount as 87Rb decays. In other words, the 87Sr/86Sr ratio is based on the amount of the 87Rb decay, but also on the initial Rb/Sr ratio of the rock, which determines the rate at which 87Sr/86Sr ratio will increase. 87Sr/86Sr ratio is used because 86Sr has a constant abundance (SAHRA 2015).
In magma differentiation strontium enriches in minerals that crystallize late in the cooling process. For example, Na-rich plagioclases have more strontium than Ca-rich plagioclases that have crystallized earlier. The amount of strontium is also low in K-rich micas (Lahermo et al. 2002). In general, strontium is common in felsic minerals; alkaline rocks and especially in magmatic carbonate rocks, but almost absent in mafic minerals and in micas. Rubidium, on the other hand, is common in feldspars and micas, and it is also found in pegmatite minerals. During magmatic differentiation rubidium becomes enriched in granitic rocks and pegmatites (Koljonen 1992). So, mantle derived rocks have low rubidium content, and their Rb/Sr, and 87Sr/86Sr ratios tends to be lower, whereas crustal rocks are rich in rubidium, and they have higher Rb/Sr ratio. This leads to higher 87Sr/86Sr ratio, as 87Rb decays into 87Sr.
Description of the method
Strontium isotopes have proved useful especially in determining weathering processes and quantifying end-member mixing processes (Shand et al. 2009). In shallow fresh groundwaters strontium is a minor element, but becomes more concentrated in deeper, saline waters and brines. When assessing 87Sr/86Sr ratios, certain factors should be remembered, as they affect the strontium isotope composition of the groundwater: i) the mineralogical composition of the host rock, because the bigger the Rb/Sr ratio, more the amount of 87Sr increases over time; ii) the dynamics of the water system, especially the duration the water has been in contact with a host rock, and the possibility of mixing of different water masses; iii) the kinetics of mineral alteration, dissolution and precipitation, because, for example, dissolution of a mineral has significant impact on the strontium isotope fingerprint of the groundwater that is in contact with that mineral; iv) the concentration and chemical composition of the water mass before it came into contact with the new host rock, particularly the amount of total dissolved solids affects the equilibrium process; v) the separation of 87Sr from the other Sr isotopes by geochemical processes, which might give ages less than the true age of the rock; and vi) the water-rock ratio, because especially in water-dominated systems, such as fracture zones, the equilibration is not complete, and the water will most likely only reflect the isotopic composition of the more soluble minerals (Frape et al. 2005).
Groundwaters that have evolved in geochemically different systems will have contrasting strontium isotope ratios (Clark & Fritz 1997), and because the Sr isotopes are not fractionated by any natural processes (Négrel & Casanova 1997, Shand et al. 2009), the 87Sr/86Sr ratio will describe the origin and evolution of the groundwater, in addition to the mixing of waters with different isotopic compositions (Négrel & Casanova 1997). Different waters can have a very large range of 87Sr/86Sr ratios, but even weak variations can be interpreted because of the high precision of measurement, approximately 20 x 10-6 (Négrel et al. 2000). Strontium isotope data is often presented according to the classic diagram, where the 87Sr/86Sr ratio is in the vertical axis and 1/Sr is in the horizontal axis (e.g. Faure 1986, Négrel et al. 2000, Négrel et al. 2005).
Variations in weathering rates of different minerals, mineral heterogeneity, and differences in methodologies are major sources of difficulties and errors in determining the weathering components of different aquifer systems. However, combining the strontium isotopes together with other hydrogeochemical data is a very useful tool, especially in revealing water-rock interactions, mixing, and other geochemical processes such as ion-exchange (Shand et al. 2009). Frape et al. (2005) also write that the measurement of the 87Sr/86Sr ratio is a very useful tool, particularly brine studies is mentioned, but the values should be evaluated properly taking into account all the variables that affect both water chemistry and strontium isotope composition. In short, application of strontium isotopes helps in distinguishing different water bodies, examining mixing relationships, and identification of allochthonous vs. autochthonous sources of salinity (Clark & Fritz 1997).
Studying water-rock interactions and determining mixing processes by using strontium isotope ratios is a well-established method, as indicated by a large amount of journal publications and articles (e.g. Scholtis et al. 1996, Blum & Erel 2005, Négrel et al. 2005, Négrel & Petelet-Giraud 2005, Shand et al. 2009, Kietäväinen et al. 2013).
A case study of the use of strontium isotopes in assessing impacts of mining on natural waters is presented in the following Closedure web page:
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Clark, I.D. & Fritz, P. 1997. Environmental Isotopes in Hydrogeology. Lewis Publishers, Boca Raton, 328 p.
Faure, G. 1986. Principles of Isotope Geology. John Wiley and Sons, New York, 608 p.
Frape, S.K., Blyth, A., Blomqvist, R., McNutt, R.H. & Gascoyne, M. 2005. Deep fluids in the continents: II. Crystalline rocks. In: Drever, J.I. (Ed.) Surface and Ground Water, Weathering, and Soils. Treatise on Geochemistry. Elsevier, 541-580.
Kietäväinen, R., Ahonen, L., Kukkonen, I.T., Hendriksson, N., Nyyssönen, M. & Itävaara, M. 2013. Characterisation and isotopic evolution of saline water of the Outokumpu Deep Drill Hole, Finland – Implications from water origin and deep terrestrial biosphere. Applied Geochemistry 32, 37-51.
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Négrel, P. & Casanova, J. 1997. Strontium isotopic characterisation of the Palmottu hydrosystem. The Palmottu Natural Analogue Project, Technical Report 97-07, 19 p.
Négrel, P., Guerrot, C., Cocherie, A., Azaroual, M., Brach, M. & Fouillac, C. 2000. Rare earth elements, neodymium and strontium isotopic systematics in mineral waters: evidence from the Massif Central, France. Applied Geochemistry 15, 1345-1367.
Négrel, P., Casanova, J. & Blomqvist, R. 2005. 87Sr/86Sr of brines from the Fennoscandian Shield: a synthesis of groundwater isotopic data from the Baltic Sea region. Canadian Journal of Earth Sciences 42, 273-285.
Négrel, P. & Petelet-Giraud, E. 2005. Strontium isotopes as tracers of groundwater-induced floods: the Somme case study (France). Journal of Hydrology 305, 99-119.
SAHRA. 2015. Strontium. Site visited 21.4.2015. http://web.sahra.arizona.edu/programs/isotopes/strontium.html
Scholtis, A. Pearson, F.J. Jr., Loosli, H.H., Eichinger, L., Waber, H.N. & Lehmann, B.E. 1996. Integration of environmental isotopes, hydrochemical and mineralogical data to characterize groundwater from a potential repository site in central Switzerland. In: Isotopes in Water Resource Management, IAEA Symposium 336, Vienna, 263-280.
Shand, P., Darbyshire, D.P.F., Love, A.J. & Edmunds, W.M. 2009. Sr isotopes in natural waters: Applications to source characterisation and water-rock interaction in contrasting landscapes. Applied Geochemistry 24, 574-586.