Cyclic Compositionally Zoned Explosive Eruptions at an Arc Volcano (Cosigüina, Nicaragua): Implications for Magma Reservoir Processes
Abstract
Intermediate-to-silicic volcanic systems commonly produce compositionally zoned eruption deposits. Inversion of the deposit stratigraphy has typically been interpreted to reflect compositional zoning in the magma reservoir, the origin of which has important implications for magma reservoir dynamics, timescales of magmatic processes, and eruption trigger mechanisms. Accordingly, several models for magma reservoir stratification have been proposed, including incomplete convective mixing of two distinct magma batches and in situ differentiation of an initially more homogeneous magma via crystal-liquid separation. In this work, we focus on a sequence of 10 compositionally zoned pyroclastic fall deposits from Volcán Cosigüina, Nicaragua, and present bulk rock, matrix glass, melt inclusion and mineral compositions to (1) demonstrate the remarkably cyclical behavior of this arc volcano and (2) identify the repeatable conditions that have produced compositional zoning in its magmatic system and destabilized it to cause eruptions. First-erupted and last-erupted tephra samples are consistently crystal-poor dacite pumice (63-67 wt% SiO2) and crystal-rich andesite scoria (56-58 wt% SiO2), respectively. Dacite-hosted plagioclase crystals (An40-45) appear "clean" and show simple and modest oscillatory zoning. In the andesites, in contrast, plagioclases are calcic (An75-90) and typically look "dirty", due to the presence of numerous glass inclusions and channels. Andesite bulk rocks display higher Al2O3 than matrix glasses and melt inclusions, which may indicate that bulk rocks are not true melts but instead mixtures of silicic andesite liquid and a plagioclase crystal mush component. However, all samples have a negative Eu anomaly (the magnitude of which increases with increasing SiO2), suggesting all magmas suffered plagioclase removal. Certain trace element ratios, such as Ba/La, vary significantly throughout the stratigraphy but show a very restricted range within any given eruption deposit, revealing a genetic link within dacite-andesite pairs. We propose that, after influx of new andesitic magma, compositional zoning in Cosigüina's magmatic system is reestablished between each eruption via upward expulsion of silicic liquid from a crystalline mush.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V33D0268L
- Keywords:
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- 1036 Magma chamber processes;
- GEOCHEMISTRYDE: 3653 Fluid flow;
- MINERALOGY AND PETROLOGYDE: 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGY