Datation U/Th et comportement en système ouvert: Implications pour les travertins fondées sur l'étude des sites de Süttő (Hongrie) et Ballık (Turquie)

Reliable dating is essential in order to put palaeoclimatological, palaeoenvironmental and archeological information from continental carbonates in the correct time frame. This study evaluates U/Th dating of travertines based on two case studies, i.e., in the Ballık (Denizli, SW Turkey) and Süttő (N Hungary) areas. Previous petrographic, mineralogical and elemental characterization of the travertines enabled sampling of, purely calcitic, micrite or spar dominant, zones that are non-, or solely weak, cathodoluminescent. Despite adapted sampling and detrital thorium corrections, U/Th ages of the Ballık and the Süttő travertines are inconsistent with their stratigraphic position. Based on the observation of wavy extinction affecting micritic fabrics and ghost structures, the studied travertines behaved as an open system for uranium after deposition through recrystallisation. Their U/Th ages, consequently, can only be considered as minimum ages. The Ballık travertines precipitated earlier than 0.55-1.31 Ma ago, when late-stage cementation along fractures and in cavities took place, and form the oldest travertines deposited in the Denizli basin. The formation of the Süttő travertine area started at the earliest around 2.5 Ma with precipitation of the Cukor travertine. With some short interruptions, active travertine formation in the area took place maximally until 120 ka. The decreasing ages of the Süttő travertines towards the Danube, prove the complex nature of travertines both during deposition as well as during diagenesis, which leads to anomalies with regard to the law of superposition. The diagenetic susceptibility of continental carbonate deposits poses problems in terms of accurate and precise dating. U/Th dating of the intrinsically heterogeneous fabrics of travertines and tufa, as presently applied, should be ceased. Travertines and tufa up to ~100 ka might be the exception due to the limited extent of recrystallisation. Sparite cements and veins can be dated more accurately but, even with multiple samples from the same study site and inclusion of coeval samples, give only minimum ages for the whole deposits. To better constrain the time frame of the whole bodies, results from independent dating methods and from the surrounding deposits are necessary. In addition, it is important to keep in mind that the open-system behavior of continental carbonates has implications for diagenetic, palaeoenvironmental and palaeoclimatological studies based on geochemical proxies.