Stable isotope characterisation of recent aragonite travertine deposits associated with the Fitero thermal waters (Spain)

The travertines of the Fitero thermal springs, with more than 98% of aragonite in most of the samples, are studied in this paper. The main objective is to improve the general understanding of aragonite precipitation, since the deposits of almost pure aragonite are very scarce. This study presents a complete mineralogical and isotopic characterisation, including the evaluation of the δ 18 O and δ 13 C fractionation during precipitation, as valuable information for paleoclimate and paleoenvironmental studies. Samples of a laminated travertine deposit constituted by almost pure aragonite were taken from a pipe discharging water at 40 °C. Waters suffered an important CO 2 outgassing, as suggested by the geochemical calculations and the δ 13 C values of travertines and waters. This outgassing triggers the oversaturation and precipitation of carbonate phases. Temperature seems to be the main factor controlling the precipitation of aragonite or calcite, as checked by studying another travertine sample with higher calcite content. Various δ 18 O isotope fractionation equations for aragonite and calcite were used. The results indicate that precipitation took place close to equilibrium according to some of these equations. The fact that the equilibrium is maintained in a natural system with an important CO 2 loss is surprising. However, it can be explained by an HCO 3 − –water oxygen isotopic equilibrium and a direct transfer of the HCO 3 − isotope signal to the carbonate without fractionation due to the fast CO 2 loss and precipitation. Finally, considering other natural aragonite samples, a fractionation equation is defined for natural aragonite in the temperature range between 23 and 80 °C.