Kinetic isotope fractionation in rich CO2 spring travertine – An experimental study

Léonora Fleurent, Elisabeth Gibert-Brunet, Florent Barbecot

In the proceedings of: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Isotope Tracing II

ABSTRACT: Travertine are supposed to archive environmental conditions with high temporal resolution up to seasonal, monthly or even daily when finely laminated. Therefore, they may be useful to consider the influences of recent climate trends on regional aquifer dynamics. Travertine are calcareous deposits that precipitate from thermal water rich in CO2 and supersaturated with respect to calcite. Hence, if the close relationships between geochemical contents recorded in travertine and environmental parameters are commonly accepted (1), the details of the processes involved as well as their respective weights in the paleorecord conditions are not clearly established and often poorly discussed, mainly because of the kinetic process that could modify the geochemical record. Thus, the isotopic fractionation is rarely maintained under natural condition due to rapid CO2 degassing that influence the precipitation rate. The first results obtained on the travertine precipitating from a gaseous spring (i.e. the Ours Spring, French Massif Central), on both 18O and 13C on the water phase and freshly precipitated travertine, corroborate a non-equilibrium, kinetic fractionation between these two phases. In order to understand how this kinetic fractionation could modify geochemical records in travertine, laboratory experiments were conducted in which CaCO3 was precipitated (20°C) from natural gaseous water in a close system. The N2 gas was injected at different pressures to create different kinetic of degassing and precipitation. Indispensable parameters such as pH, pCO2 and 13Cco2 were analyzed during experiment, while 13CHCO3 and 13CCaCO3 were measured through SIRA mass spectrometer, at the end of the experiment. This allows us (i) to recognize the effect of degassing on the fractionation factor between the bicarbonate / carbonate /carbon dioxide system when precipitation occurs and (ii) to model the evolution of calco-carbonic system during degassing. The results obtained on the sample can be thus reinterpreted with a new geochemical understanding. (1)Pentecost (2005) Travertine

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