Microanalytical Investigations of Magma Sources, Evolution, Storage, and Ascent at Nyiragongo and Nyamulagira

Abstract

Processes occurring on the macro scale can be informed by processes occurring on the micro scale. In my dissertation, I show how we can understand volcanism through the lens of crystals and their inclusions, which through their volatile, major, trace, and isotopic signatures yield vital insights about magma sources, evolution, storage, and ascent. Petrologists have long used the chemistry of minerals and inclusions to understand magmatism, but some regions are more studied than others. In my dissertation, I focus on Nyiragongo and Nyamulagira volcanoes, located in the Western Branch of the East African rift in a region called the Virunga Volcanic Province. Situated near the border of Democratic Republic of the Congo and Rwanda, these volcanoes are highly hazardous, posing significant danger to growing populations in the region. By measuring the major, trace, and volatile elements, and oxidation states in olivine-hosted melt inclusions, I constrained primitive magma compositions, volatile budgets, and degassing processes in magmas at both volcanoes. Coupling these results with geochemical models, I show how magmas formed by melting of metasomatized mantle sources began crystallizing at depths between 10 and 18 km, and degassed during ascent and eruption. Using the common phenocryst mineral olivine, I applied diffusion chronometry to estimate the time elapsed between protracted magma storage and eruption, showing that eruptions at flank cones occur months to weeks after magma mixing episodes. Combining the knowledge of volatile contents, storage depths, and mineral chemistry, I used measurements of H+ in olivine to estimate magma ascent rates in the minutes and seconds prior to eruption, showing that rapid decompression rates of (~0.5±0.2 MPa/s) facilitate explosive eruptions. This dissertation includes both published (currently in review) and unpublished co-authored material.