Fluoride distribution in volcanic rocks of the Aluto volcano complex, Main Ethiopian Rift

High fluoride in groundwaters of the Main Ethiopian Rift is of geogenic origin. Although the involvement of volcanic rocks in fluoride enrichment is well documented, the specific mineralogical sources and dissolution mechanisms remain incompletely understood. This study characterized the mineralogical sources of fluoride in volcanic rocks of the Aluto volcano complex, identifying the specific mineral phases and their fluoride concentration that control fluoride availability in volcanic aquifer systems. Pyrohydrolysis was employed for precise fluoride determination, while X-ray fluorescence and electron microprobe analyses enabled phase-specific fluoride distribution and quantification of fluoride-bearing minerals. Water-rock leaching experiments were performed to quantify fluoride release and characterize associated ion dissolution patterns for mineral phase identification. The study shows that: (1) The younger volcanic deposits like pumice and obsidian generally contain high fluoride concentrations of up to 0.32 wt.%, while the older ignimbrites show lower concentrations between 0.11 and 0.15 wt.%. (2) Fluoride behaves as an incompatible element in the post-caldera rocks showing strong linear correlations with other incompatible elements such as Zr, Nb, and Y (R 2 > 0.98). (3) Microprobe analyses of thin sections indicate that fluoride is incorporated into the glassy rock matrix, into apatite, and into the sodium amphibole riebeckite (the latter two occur only as accessory minerals). Mass calculations reveal that the largest proportion of fluoride is bound in the fine-grained matrix, as this phase represents 40 to 100 wt.% of each rock sample. Thus, volcanic glass – particularly pumice – and glass-rich rocks have the highest potential to release fluoride, indicating that this phase controls fluoride availability in the rift groundwater system. Highlights : • Fluoride (up to 0.32 wt.%) varies with lithology and volcanic age • Young glass-rich rocks have highest fluoride content • Fluoride mainly occurs in volcanic glass, apatite, and riebeckite