Lithium isotopes systematics in Geothermal systems
Résumé
Assessing the origin and behaviour of lithium and the distribution of Li isotopes in geothermal systems is of major importance in order to increase our knowledge of the lithium cycling in the Earth's crust. Lithium is a fluid-mobile element and due to the large relative mass difference between its two stable isotopes, it is subject to significant low and high temperature mass fractionation which provides key information on the nature of water/rock interaction processes. The main objective of the present work is to constrain the behaviour of Li and its isotopes in geothermal systems both for geothermal water samples representing deep circulation in the crust and by using an experimental approach. The behaviour of Li and its isotopes (7Li) have been characterized in geothermal systems located in volcanic island arc areas: Guadeloupe, Martinique islands and in New Zealand. Moreover, one particularly important aspect of this work was to establish the nature, extent and mechanism of Li isotope fractionation as a function of temperature during water/rock interaction. Therefore, we also report results of Li isotope exchange experiments during seawater/basalt interaction (from 25 to 250°C). These results confirm that Li isotopic exchange is strongly temperature dependent, and demonstrate the importance of Li isotopic fractionation during the formation of Li-bearing secondary minerals. The conclusions of the present work could be of great interest in the regional context of South America. More precisely, this isotopic tool could be applied to the characterization of lithium enrichment in evaporite basins of the Central Andes. Future work will aim to explore the use of Li isotopes to constrain the origin, the mobility, the transfer and the deposition of lithium in an evaporitic regime under arid climatic conditions.