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Title

The stable H and O isotope analysis of small highly saline water samples: A methodological study

Authors
Smuda, J.
Koroleva, M.
Published in Goldschmidt Conference Abstracts 2009. Davos (Switzerland) - 21-26 Juin 2009 - . 2009, p. A274
Abstract The isotope diffusive exchange technique is used to determine isotopic composition of pore water and the water content of low permeability rocks. In the case of highly saline pore water, salt (NaCl or CaCl2) has to be added (up to 6 molal) to the test water (≤5 ml) to avoid water transfer during equilibration with rock samples having low water activity. Quantitative distillation of the solution prior to analysis is the only way to eliminate the salt-induced isotopic fractionation [2] when using usual analytical methods (Cr or Zn reduction for δ2H, CO2 equilibration for δ18Ο). Here we report the results of a methodological study exploring different synthetic salt (NaCl, CaCl2, Na2SO4 and NaF) solutions prepared at different concentrations (0 to near saturation) with five waters of known isotopic composition (δ2H = -275.6 to 3.4 ‰, δ18O = -35.3 to 0.4‰). Two distillation procedures were tested. Low-temperature distillation (LTD) was performed in closed system by heating at 90°C for >4 h followed by 1 h at 130°C. For high temperature distillation (HTD), the sample was heated in a closed system to 500°C for 1 h, and the residual salt to ~600°C for 10 min. The water sample was recovered cryogenically. The δ2Η and δ18O values of all the water samples were determined using Cr-reduction in a ThermoFisher H-Device and CO2-equilibration in a GasBench II, respectively. For the moderately-hygroscopic salt solutions (NaCl, NaF and Na2SO4), the LTD gave results undistinguishable within the analytical error (±0.3‰ δ2H, ±0.1‰ δ18O) for pure water. The HTD gave good δ2H but inaccurate δ18O values for CaCl2 solutions and is not suited for routine analysis. The addition of NaF to CaCl2 solutions scavenges the Ca by CaF2 (fluorite) precipitation, thus allowing the routine use of LTD on CaCl2 solutions with results as good as for pure water.
Keywords Water isotopeBrineCaCl2NaClNaFDistillation
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DE HALLER, Antoine et al. The stable H and O isotope analysis of small highly saline water samples: A methodological study. In: Goldschmidt Conference Abstracts 2009. Davos (Switzerland). [s.l.] : [s.n.], 2009. p. A274. https://archive-ouverte.unige.ch/unige:101084

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Deposited on : 2018-01-03

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