Magma storage, evolution, and degassing at Mombacho (Nicaragua): New insights to decipher the current state of activity of a long-dormant volcano

The Mombacho is a basaltic-andesitic composite volcano of Nicaraguan Quaternary volcanic chain. As no historical eruption has been recorded at Mombacho, there is no available information is about the depth of potential magma storage zones, or about the evolution of ascending magmatic fluids. However, this information is urgently needed given the current state of the volcano, which is showing signs of unrest. To acquire this knowledge, melt inclusions and pyroxene crystals in various Mombacho tephras were analyzed, along with the volcanic gases emitted by fumaroles in the crater. The major element compositions of the melt inclusions support, to a first order, a magmatic evolution controlled by the fractional crystallisation of olivine, pyroxene, plagioclase and titanomagnetite crystals. However, the compositional variability of the most mafic samples can only be explained by a source heterogeneity. Additional trace element analyses agree with this hypothesis but seem to highlight a migration of the most mobile elements with an aqueous phase. The composition of the clinopyroxene crystals and the H 2 O and CO 2 contents of the melt inclusions reveal the presence of a complex main storage zone between 8.2 ± 1.6 and 3.1 ± 0.4 km. Degassing modelling also supports this architecture and indicates that the gases sampled in 2024 in the Mombacho fumarole come from depths similar to this large storage zone. In addition, the 2024 fumarole gas contents in SO 2 , H 2 O and CO 2 are similar to those sampled around twenty years ago, highlighting the stability of the degassing and therefore of the underlying magmatic system. • A main shallow storage zone has been identified between 3.1 and 8.2 km. • Mixing, crystallisation & fluids explain Mombacho products' chemical diversity. • The current surface gas emissions come from the large shallow storage zone. • The renewed activity is not due to a resumption of magmatic activity.