Mobilizing mush: assembly and eruption of modern monotonous basaltic andesite magma at Momotombo Volcano, Nicaragua

Documenting erupted products and estimating pre-eruptive magmatic conditions is critical in assessing future volcanic hazards, since the physical and chemical conditions of magma assembly influence eruption style. Here, we present a petrologic examination of eruptive products at Momotombo Volcano, Nicaragua, with a focus on the 2015–2016 eruption, to identify the dominant magmatic processes that modulate modern eruptive history. Basaltic andesite lavas and tephras from the 2015–2016, 1905 and earlier eruptive episodes largely show monotonous major and trace element bulk rock compositions. Erupted products contain abundant (34–48 vol.%) and texturally diverse crystals of plagioclase and pyroxenes, commonly forming glomerocrysts, suggesting that the incorporation of a crystal mush dominates the bulk rock composition. In contrast, the matrix glasses of tephra samples and melt inclusions from the opening phase of the 2015–2016 eruption cover a wider compositional range, from basaltic andesite to dacite. We apply plagioclase–melt K₂O partitioning and thermodynamic models to demonstrate that diverse melt compositions (basalt to dacite) are required to produce the observed range in plagioclase compositions (An47 to An95). Together, we interpret (1) relatively evolved tephra glass compositions, (2) thin, reversely zoned rims of pyroxene crystals, and (3) low volatile saturation melt inclusion pressures to record an influx of new intermediate melt to the shallow crust (0.5–2 km) within days of the 2015–2016 eruption, with precursory seismic activity enabling magma movement and extending timescales of unrest back to 2013. Our results provide context for the locally high geothermal potential and silicic magma generation and assembly underneath Momotombo.