RAPID REMOBILIZATION OF INTERMEDIATE ARC MAGMA: INSIGHTS FROM TEXTURAL ANALYSIS, MINERALOGY AND GEOCHEMISTRY OF LAVA SAMPLES FROM MOMOTOMBO VOLCANO, NICARAGUA

Momotombo is a basaltic andesite stratovolcano in the Central American Volcanic Front, where the pre-eruptive processes have not been extensively documented in published literature. To constrain timing of key stages in the leadup to eruption and magma storage conditions, our petrologic study analyzes textures, phenocryst compositions and bulk-rock geochemistry of lava samples from 2015, 1905 and the centuries before 1905. All samples in this study are basaltic andesites with an exceptionally limited range of SiO₂ content. Based on Spider Plot and REE similarities, Momotombo appears to draw from a long-lived source in the upper crust or mantle, and is characterized by high levels of slab-derived fluid contribution like other continental arc volcanoes. The crystal-rich magmas contain ~30-50% phenocrysts by thin section area, and their high crystal content makes them significantly more viscous than dry basalts. Crystal size distribution (CSD) is bimodal, as observed in other arc volcanic systems, consistent with the presence of two mixing magma populations or multiple phases of recharge. Plagioclase phenocrysts contain high An (>95%) cores, with variable oscillatory zoning in their rims – suggesting temperature stratification in the magma reservoir shortly before eruption. CSD combined with homogeneity of bulk rock indicates rapid homogenization of the reservoir within decades to years before eruption, possibly by felsic and mafic mingling or assembly of the magma chamber from a trans crustal mush system. Our results highlight the importance of continual monitoring on Momotombo’s slopes to detect magma movements on short timescales for timely forecasting of eruptions.