Degassing behavior and eruptive dynamics in rhyolitic melts: geochemical, textural and numerical analyses of hybrid eruptions
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Abstract
Degassing, the exsolution of volatiles—with water being the predominant component—from magma into a co-existing vapor phase, is a crucial mechanism, controlling behavior and evolution of volcanic eruptions. Accordingly, water concentrations and hydrogen isotopic signatures (expressed as δD) are frequently applied to trace degassing histories to detect the progress of eruptions. Specifically silicic melts can produce highly explosive eruptions, venting vast quantities of gas, ash, and pyroclastic material, with profound impacts on the ecosystem, environment and life. This thesis particularly focuses on studying the degassing behavior of rhyolitic melts and investigates pre-historic eruptions at Big Glass Mountain (BGM), California, USA, and three eruptions at Lipari, Sicily, Italy. Through field studies, texture and geochemical analyses, and degassing modeling, this thesis provides new insights into eruption dynamics. Moreover, the boron isotope system was explored as a potential additional degassing tracer.
A vital outcome of this work is the development of “VolcDeGas”, a user-friendly program which is designed to iteratively model degassing systems for the hydrogen and boron isotopic systems, which also enables best-fit modeling for natural data. The accuracy of the program was confirmed by verification against data and models of previous studies. Results reveal that batched-degassing models most accurately reproduce observed eruption trends. This batched-system can mimic both closed- and open-degassing system behaviors by varying the step size during degassing. It is also linked to hybrid activity—characterized by simultaneous explosive and effusive phases—a pattern which was recently observed in eruptions such as Chaitén (2008) and Cordón Caulle (2011) in Chile. A batched system with gradually decreasing step sizes explains explosive impulses and their declining intensities, and both are linked to the successive dominance of effusive activity over the course of the hybrid sequence. These findings furthermore support the recently proposed “cryptic fragmentation” hypothesis in which fragmentation and subsequent sintering of clastic material in tuffisites or conduits could initiate batched degassing conditions fostering hybrid eruption behavior.
Field studies, textural analyses, H2O and hydrogen isotopic measurements at BGM and Lipari reveal evidence for hybrid activity. Both sites reveal that a batched-system with variable step size represents the degassing history best, and both exhibit textures indicative of tuffisites and sintering processes. Additionally, deposits show similarities to the hybrid deposits of Chaitén and Cordón Caulle. At Lipari, the δD-H2O degassing histories of all three eruptions establish a singular coherent trend, and their textures and eruption chronology exhibit strong similarities, indicating a co-eruption. Moreover, two outlier samples with elevated deuterium concentrations were identified at Lipari. While the dataset is limited, these anomalies may be the result of regassing that could enrich the sample in deuterium, possibly occurring during the hybrid activity.
Boron isotopic signatures and concentrations were measured in BGM samples and compared to their corresponding δD and H2O contents. Furthermore, theoretical models were generated by “VolcDeGas” and best-fitted to published boron data. The goal was to assess whether the boron isotopic system can serve as a tracer for degassing. Comparisons between the boron and hydrogen system in the same BGM samples showed that, while the hydrogen system tracks an apparent degassing history, boron data appears scattered and random. While theoretically feasible, the fractionation and partitioning effects associated with boron during degassing appear to be inhibited by slow diffusion rates, making them too subtle to be detected in natural systems as they are concealed by other geochemical processes. Consequently, boron can be employed as a tracer for such processes without the need to account for degassing variations.
The discoveries of this thesis provide significant insights into degassing mechanisms and eruption dynamics. By linking field observations, geochemical analyses, and modeling, this work establishes guidelines for identifying hybrid eruptions in pre-historic volcanoes. The VolcDeGas program introduces a robust tool for future studies, enabling rapid and precise modeling of degassing histories. Furthermore, these findings raise important questions about processes such as cryptic fragmentation, regassing, and hybrid eruption mechanisms, which all demand further investigations. They also contribute to improved hazard assessments and more effective volcanic risk management.