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 δ¹⁸O and δ¹³C 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 CO₂ outgassing, as suggested by the geochemical calculations and the δ¹³C 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 δ¹⁸O 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 CO₂ loss is surprising. However, it can be explained by an HCO₃⁻–water oxygen isotopic equilibrium and a direct transfer of the HCO₃⁻ isotope signal to the carbonate without fractionation due to the fast CO₂ 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.

本文研究了Fitero温泉的石灰华，其中大多数样品中的文石含量超过98％。主要目的是提高对文石沉淀的一般理解，因为几乎纯的文石的沉积非常稀少。这项研究提出了一个完整的矿物和同位素特征，包括δ评价¹⁸ O和δ ¹³沉淀C期间分馏，作为古气候古环境和研究的宝贵资料。从几乎纯净的文石构成的层状钙华沉积物样品取自排放40°C的水的管道。水域遭受重要的CO ₂除气，通过地球化学计算和δ的建议¹³钙华和水的C值。这种放气触发了碳酸盐相的过饱和和沉淀。通过研究另一个方解石含量更高的钙华样品，温度似乎是控制文石或方解石沉淀的主要因素。各种δ ¹⁸被用于文石和方解石氧同位素分馏方程。结果表明，根据其中一些方程，降水接近平衡。令人惊讶的是，平衡在具有重要CO ₂损失的自然系统中得以维持。但是，它可以通过一个HCO进行说明₃ ^- -水氧同位素平衡和HCO的直接转移₃ ^-由于快速的CO ₂损失和沉淀，同位素信号将碳酸盐同位素分馏。最后，考虑其他天然文石样品，定义了温度在23至80°C之间的天然文石的分馏方程。