Stable isotope geochemical study of Pamukkale travertines: New evidences of low-temperature non-equilibrium calcite-water fractionation

摘要

In this paper we present the first detailed geochemical study of the world-famous actively forming Pamukkale and Karahayit travertines (Denizli Basin, SW-Turkey) and associated thermal waters. Sampling was performed along downstream sections through different depositional environments (vent, artificial channel and lake, terrace-pools and cascades of proximal slope, marshy environment of distal slope)-δ~(13)C_(ravertine) values show significant increase (from +6.1‰ to +11.7‰ PDB) with increasing distance from the spring orifice, whereas the δ~(18)O_(travertine) values show only slight increase downstream (from -10.7‰ to - 9.t‰ PDB). Mainly the CO_2 outgassing caused the positive downstream shift (-6‰) in the δ~(13)C_(travertine) values. The high δ~(13) C values of Pamukkale travertines located closest to the spring orifice (not affected by secondary processes) suggest the contribution of CO_2 liberated by thermometamorphic decarbonation besides magmatic sources. Based on the gradual downstream increase of the concentration of the conservative Na~+, K~+, Cl~-, evaporation was estimated to be 2-5%, which coincides with the moderate effect of evaporation on the water isotope composition. Stable isotopic compositions of the Pamukkale thermal water springs show of meteoric origin, and indicate a Local Meteoric Water Line of Denizli Basin to be between the Global Meteoric Water Line (Craig, 1961) and Western Anatolian Meteoric Water Line (Simsek, 2003). Detailed evaluation of several major and trace element contents measured in the water and in the precipitated travertine along the Pamukkale MM section revealed which elements are precipitated in the carbonate or concentrated in the detrital minerals.Former studies on the Hungarian Egerszalok travertine (Kele et al, 2008a, b, 2009) had shown that the isotopic equilibrium is rarely maintained under natural conditions during calcite precipitation in the temperature range between 41 and 67 ℃ In this paper, besides the detailed geochemical analyses along downstream sections, we present new evidences of non-equilibrium calcite-water fractionation in lower temperature range (13.3 to 51.3 ℃). Our measurements and calculations on natural hot water travertine precipitations at Pamukkale and Egerszalok revealed that the δ~(18)O_(travertine) is equal with the δ~(18)O_(HCO3) at the orifice of the thermal springs, which means that practically there is no oxygen isotope fractionation between these two phases. High rate of CO_2 degassing with rapid precipitation of carbonate could be responsible for this as it was theoretically supposed by O'Neil et al. (1969). Thus, for the determination of the deposition temperature of a fossil travertine deposit we propose to use the water-bicarbonate oxygen isotope equilibrium fractionation instead of the water-travertine fractionation, which can result 8-9 ℃ difference in the calculated values. Our study is the first detailed empirical proof of O'Neil's hypothesis on a natural carbonate depositing system. The presented observations can be used to identify more precisely the deposition temperature of fossil travertines during paleoclimate studies.