Fractionation of hydrogen and oxygen isotopes of gypsum hydration water and assessment of its geochemical indications

Abstract

Fundamental knowledge of the isotopic fractionation between the hydration water and the mother solution and whether the primary information recorded in hydration water can be preserved or not in deposits or mines have long been unclear. In order to calculate the accurate hydrogen and oxygen isotopic fractionation factors between gypsum hydration water and its mother solution with new methods, to understand the mechanism of fractionation and synthetically assess the record-keeping abilities of the isotopic composition of hydration water during the process of diagenesis after deposition, experiments on the hydrogen and oxygen isotopic compositions of gypsum hydration water and its mother solution at different isothermal temperatures from 5 to 50°C were systematically conducted. In addition, samples from two typical gypsum deposits formed in different environmental conditions were also determined. Results show that during gypsum crystallisation, both hydrogen and oxygen isotopes show significant fractionation between the hydration water and the mother solution. The calculated hydrogen isotopic fractionation factors are <1, while the oxygen isotopic fractionation factors are >1 at temperatures from 5 to 50°C. The fractionation factors show no functional relationships with temperature. Isotopic compositions of gypsum hydration water in arid lake sediments can be used to trace the source of water and primary deposit environmental information. However, the isotopic composition of the gypsum hydration water can easily be altered by dissolution and secondary precipitation of gypsum during later diagenesis, particularly in areas with humid climate and abundant groundwater. A very careful assessment on record-keeping abilities of the primary isotopic composition of hydration water in gypsum during later diagenesis must be considered before application.

关于结合水与母溶液之间同位素分化的基本知识和记录在结合水中的主要信息是否可以被保存于沉积物中或矿床中，长久以来对此并不清楚。为了用新的方法计算出石膏结合水和母溶液之间准确的氢气和氧气的同位素分化系数，为了了解分化机制和综合评估结合水的同位素组分在沉积后成岩过程中备存纪录的能力，我们对石膏结合水的氢、氧同位素组成和其母溶液在不同等温温度（5〜50˚ C）下进行了系统性实验。此外，还确定了两个典型的形成在不同环境条件的石膏沉积样品。结果表明，在石膏结晶过程中，氢和氧同位素都显示结合水与母溶液之间显著分化。计算出的氢同位素分化因子为<1，而氧的同位素分化因子为>1，其温度范围为5〜50℃。分化系数没有显示任何与温度的功能关系。在干旱地区湖泊沉积物中的石膏结合水的同位素组成，可用于跟踪水源和原生矿床的环境信息。然而，石膏结合水的同位素组成可以很容易地在以后的成岩作用过程中通过溶解和石膏的二次析出而改变，尤其是在潮湿气候和富地下水区域。在运用前必须考虑对在后来成岩过程中石膏内结合水的主要同位素组成的记录保持能力进行非常谨慎的评估。

KEY WORDS:

• gypsum
• hydration water
• hydrogen and oxygen isotopes
• fractionation
• geological indication

ACKNOWLEDGEMENTS

We thank Zhiguo Su and Huifang Zhou, who helped to determine the samples. We also thank Professor David Price, who constructively reviewed the latest version and improved English writings. This study was financially supported by the National Natural Science Foundation of China (Grant Nos. 41073034, 41173049 and 41271041) and the Program for New Century Excellent Talents in Universities.