Geochemistry of ash leachates during the 2021 eruption of Cumbre Vieja volcano, La Palma, Canary Islands

On September 19, 2021, a new eruption began at the west flank of Cumbre Vieja volcano (La Palma, Canary Islands), after an inter-eruptive period of 50 years from the previous eruption (Teneguía, October 1971). The 2021 event was a fissure and powerful strombolian eruption with a magnitude VEI=3 and it has been considered as the most important eruption of Europe during the last 75 years in terms of the significant amount of SO₂ released and the serious damage caused by the lava flows.

In this work we report the leachate analyses of volcanic ash from the beginning of the eruption, focused on determining the relationship between chemical composition of water‐soluble components adhering to volcanic ash and the volcano’s activity episodes. A total of 5 main control sites or ash-collecting stations were established and ash was picked up in a daily basis. These were located around the main eruptive vents at different distances. Water-extractable concentrations of the samples leached at 1:25 for 2 hours were analyzed by ion chromatography (Cl^-, SO₄^2-, F^-, Br^-, NO₃^-, Na⁺, Ca²⁺, Mg²⁺, K⁺) and ICP-MS (Li, B, Al, Si, Sr, Ba, Fe, Ti, Cu, Sb, Rb, Ni, Co, Cd, V). The most abundant components in the leachates were SO₄^2- for the anions and Na⁺ for the cations, with mean concentrations of 854 and 455 mg/kg, respectively. The results showed the following trend, in decreasing order of abundance: SO₄^-2>Na⁺>Cl^->F^->Ca²⁺>Al³⁺>K⁺>Mg²⁺. Fluoride, an element of primary concern for human and animal health, showed a range of 16 and 733 mg/kg and an average of 239 mg/kg, which is relatively higher than global median value (129 mg/kg). The S/Cl molar ratio in the ash leachate presented a ranged of values from 0.16 to 5.9 and the observed values > 3.0 seems to be related to ash-rich phases of the eruption. Preliminary results show significant temporal variations in ash leachate compositions, revealing changes in the eruption dynamics.