PRE-ERUPTIVE PROCESSES AND MAGMA STORAGE CONDITIONS AT MOMOTOMBO VOLCANO, NICARAGUA

Momotombo is a little-studied basaltic andesite stratovolcano in the Central American Volcanic Front, with the potential to endanger two of Nicaragua's largest cities. Our petrological study focuses on 2015 and 1905 lava flows plus analysis of bulk rock geochemistry, offering preliminary insights on magma origins and the leadup to eruptions. Image processing of thin sections reveals crystalline magmas of over 30% phenocrysts, and bimodal plagioclase crystal size distribution (CSD). Estimates of phenocryst residence time from CSD suggest two phases of magma mobilization punctuated by a period of storage, all occurring on a scale of years before eruption. Over the past century and into prehistory, Momotombo magmas demonstrate remarkable consistency in trace element profiles and major element composition of bulk rock samples. Geochemical similarities between samples indicate a homogenized reservoir in steady state over thousands of years. Trace element enrichments originate in superheated fluids rising from the subducting Cocos Plate, with strong signals from marine sediments. Electron probe micro-analyzer profiles and back-scatter electron images show oscillatory zoning in rims of large plagioclase phenocrysts, consistent with periodic influxes of high temperature mafic magma into the reservoir. Zoning trends vary widely — with examples of both normal and reverse zoning, evidence that viscous, crystalline magma remained poorly mixed on meter scales. Conflicting findings from EPMA and bulk rock geochemistry raise questions about small scale magma movements in the reservoir. The mechanism behind magma homogenization also remains unsolved, given the slow rate of convection in high viscosity magmas. CSD of Momotombo samples is consistent with timescales and patterns of magma mobilization observed at other continental arc volcanoes, bolstering the case for rapid magma chamber assembly from crystalline mush. Like other Nicaraguan volcanoes, Momotombo appears to draw from a long-lived magma source in the upper mantle and experience high levels of subduction-related contamination. Our results highlight the importance of continual monitoring on Momotombo’s slopes, to detect magma movements on short timescales for timely forecasting of eruptions.