Lithium isotopes in waters of the tropical volcanic island of Guadeloupe: A proxy of high and low-temperature water-rock interactions

This study examines the lithium isotope (δ 7 Li) geochemistry of waters flowing through andesitic rocks in order to better constrain the dissolved lithium dynamic in a tropical volcanic island context. We report the first measurements of δ 7 Li for eleven of the main rivers in Guadeloupe and four thermal springs on the slopes of La Soufrière volcano. We have measured a large range of δ 7 Li values (3.0–31.6‰) that we explained by different water–rock interaction processes at the scale of this small island. First, the rivers affected by hydrothermal inputs are the most concentrated and isotopically lighter (δ 7 Li from 3.0–12.3‰). This is partly attributed to the leaching and dissolution of clay-rich, hydrothermally-altered rocks which are presumed to have low δ 7 Li value and to the discharge from the hot-springs on La Soufrière volcano. The rivers not impacted by hydrothermal inputs are more diluted and isotopically heavier (δ 7 Li from 16.0–31.6‰), highlighting two distinct weathering processes in the regolith: the precipitation and dissolution of secondary minerals. In the youngest part of the island, the rivers are characterized by high δ 7 Li values, emphasizing that dissolved lithium is mainly controlled by the dissolution of primary andesitic minerals and the incorporation of 6 Li into secondary minerals. While in the oldest part of the island, the rivers are characterized by lower δ 7 Li values, which are partly attributed to a low δ 7 Li source from the dissolution of secondary minerals in the regolith. We also show a positive correlation between δ 7 Li in river waters and the chemical weathering rates (CWR), related to the weatherable primary minerals content in watersheds. We note that this content is linked to rock age, precipitation, regolith type/thickness and geomorphologic parameters such as elevation and slope. The Basse-Terre Island has a wide range of watersheds, making it possible to highlight the different alteration processes influencing the lithium signature of rivers in volcanic and tropical context. These results have important implications for characterizing the mean riverine flux of lithium coming from the weathering of continental volcanic rocks, understanding the global oceanic budget of lithium, and finally, for interpreting the δ 7 Li of past seawater.