Volatile evolution of silicic magmatic systems in the central Main Ethiopian Rift: insights from melt inclusions and apatite crystals of Tullu Moye and Boset volcanoes

Magmatic volatile elements play a critical role in magma reservoir and ascent processes, from influencing the liquid line of descent to controlling magma storage dynamics and eruption style. However, constraining volatile behaviour prior to eruption remains challenging. This study presents a detailed model of pre-eruptive volatile content and evolution in a peralkaline magmatic system, based on integrated analyses of apatite crystals and glass (matrix and melt inclusions) from four explosive eruptions sourced from the Tullu Moye and Boset-Bericha volcanoes in the Main Ethiopian Rift. Apatite records from these eruptions indicate prolonged volatile-undersaturated conditions during magma crystallization at shallow crustal levels (~4 – 6 km depth). Thermodynamic modelling of measured apatite and melt inclusion compositions suggest that magmatic differentiation was the dominant control on volatile evolution during eruption. Apatite and melt inclusions from the Older Tullu Moye Pumice are compositionally distinct, exhibiting higher Cl and minor element contents (i.e., MgO) than those from subsequent comenditic eruptions at Tullu Moye and Boset-Bericha volcanoes. This likely reflects crystallization from a more primitive melt, associated with early-stage clinopyroxene phenocrysts of the Older Tullu Moye Pumice. Continued crystallization and differentiation in the Tullu Moye reservoir led to volatile and trace element enrichment, ultimately feeding the Younger Tullu Moye Pumice eruption. In contrast, the Boset-Bericha deposits show no comparable inter-eruption variation in magma composition, suggesting limited temporal evolution. Our integrated approach can be broadly applied to reconstruct the temporal evolution of pre-eruptive volatile behaviour in other volcanic systems.