INSIGHTS ON VOLCANIC EVOLUTION OF LAS SIERRAS-MASAYA, NICARAGUA, VOLCANO BY MELT INCLUSIONS

Las Sierras-Masaya Volcano is a persistently active basaltic caldera complex in Nicaragua. While there has been almost no juvenile material erupted since 1772, it has been persistently degassing for at least 150 years. One estimate of the total amount of un-erupted magma required to have degassed over the last 300 years is 10 km³. Where does the degassed magma go and what does this mean for the system as a whole are two perplexing problems. An additional unusual behaviour for Las Sierras-Masaya volcano, is that the complex is capable of large caldera forming basaltic plinian eruptions with the most recent occurring at 1.8 ka. With no historical data chronicling the events leading up to and following the last plinian eruption, the rock record can be used to investigate the volcano's evolution.

Here we present melt inclusion analyses from 8 different locations/eruptions within the caldera. The majority of samples are comprised of scoria and were taken from cinder cones, 7 samples were taken from cones located on an annular ring structure within the caldera. Melt inclusion bearing plagioclase and/or olivine were analyzed for major and volatile elements (S, Cl, F and H₂O). The age of the eruptions that created the majority of the samples are unknown but field relationships together with historic data suggests that the melt inclusion data likely spans from the 1.8 ka basaltic plinian eruption to present. Interestingly, the K₂0 vs CaO plot for all melt inclusions, whether hosted in plagioclase or olivine, show a simple linear trend. This may suggest that each sample originally came from the same source or parental magma and that the compositional variability can be explained by simple factional crystallization. This agrees with previous whole rock geochemical work showing that the overall chemical signature of volcanic activity at Masaya has remained largely unchanged for ~ 30,000 years and that shallow processes dominate. This data leads us to suggest that Masaya has one very large deep magma reservoir with either no phenocryst formation or that the processes which bring them to the surface destroy any evidence of the deeper chamber.