Chemical And Stable Isotope Composition Of Recent Hot-water Travertines And Associated Thermal Waters, From Egerszalok, Hungary: Depositional Facies And Non-equilibrium Fractionation

摘要

Combined petrographical, mineralogical, geochemical and stable isotope analyses were conducted on an actively forming Egerszalok Travertine mound to determine the factors that govern carbonate precipitation and thus influence the use of travertines in paleoclimate reconstruction. Stable isotope analyses of oxygen, carbon and hydrogen demonstrate both biological and abiological factors to control travertine deposition. Processes of morphology-related outgassing had major effects upon isotopic composition. Continuous CO_2 degassing and temperature change of the thermal water between the spring orifice and distal parts of the system, caused stable carbon and oxygen isotope compositions of the precipitating travertines to increase (from +2.7 to +4.3‰ relative to V-PDB and from +10.5 to + 14.7‰ relative to V-SMOW, respectively). The travertines at the spring orifice (T～67 ℃) are composed of almost pure calcite with a δ~(13)C value of +2.7‰ (relative to V-PDB) enabling classification as intermediate between thermometeogene and thermogene. In lower temperature water, away from the spring orifice, various amounts of aragonite (5-35%) with higher Sr concentrations precipitate from solution. This phenomenon is exceptional in hot spring carbonate deposits and could be explained by fast carbonate precipitation due to the morphology-related outgassing. Oxygen and hydrogen isotope measurements of the thermal water confirmed evaporation to be causing limited kinetic isotope fractionation. However, δ~(18)O values of the travertine do show isotope shifts away from the equilibrium fractionation curve, which is most probably related to the rapid calcite precipitation and transportation along the flow path. Our data closely follow the temperature-Δ~(18)O_(calcite-water) relationship observed for other travertine localities. This is slightly displaced from the experimental curve and determines an empirical 'travertine curve'. Our study shows that this shift may result in an approximately 8 ℃ difference in paleotemperature calculations depending upon which fractionation curve is used.