View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Bern Open Repository and Information System (BORIS) 1 On the use of Li isotopes as a proxy for water-rock interaction in 2 fractured crystalline rocks: a case study from the Gotthard rail 3 base tunnel 4 Christoph Wanner1*, Kurt Bucher2, Philip A. E. Pogge von Strandmann3, H. Niklaus 5 Waber1, Thomas Pettke1 6 1Rock Water Interaction Group, Institute of Geological Sciences, University of Bern, 7 Baltzerstrasse 3, CH-3012 Bern, Switzerland. 8 2Institute of Geosciences, Mineralogy and Geochemistry, University of Freiburg, Germany. 9 3Institute of Earth and Planetary Sciences, University College London and Birkbeck, University 10 of London, UK. 11 *corresponding author (email:
[email protected], phone: +41316314023, fax: 12 +41316314843) 13 14 ABSTRACT | downloaded: 6.1.2020 15 We present Li isotope measurements of groundwater samples collected during drilling of 16 the 57 km long Gotthard rail base tunnel in Switzerland, to explore the use of Li isotope 17 measurements for tracking water-rock interactions in fractured crystalline rocks at temperatures 18 of up to 43°C. The 17 groundwater samples originate from water-conducting fractures within 19 two specific crystalline rock units, which are characterized by a similar rock mineralogy, but 20 significantly different fluid composition. In particular, the aqueous Li concentrations observed in 21 samples from the two units vary from 1-4 mg/L to 0.01-0.02 mg/L. Whereas δ7Li values from the https://doi.org/10.7892/boris.91642 22 unit with high Li concentrations are basically constant (δ7Li=8.5-9.1‰), prominent variations are source: 1 23 recorded for the samples from the unit with low Li concentrations (δ7Li=10-41‰).