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Determining magmatic volatile contributions to the Campanian Ignimbrite, Italy

Murray, Allyson (2021) Determining magmatic volatile contributions to the Campanian Ignimbrite, Italy. Masters thesis, Northern Arizona University.

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The trachytic Campanian Ignimbrite was emplaced at 39.8 ka near Naples, Italy, in the largest European eruption of the past 200 ka. Many factors, including the far-reaching extent of deposits, with co-ignimbrite fallout found as far away as Russia, indicate emplacement via a dilute (gas-rich and solids-poor) pyroclastic density current. Elevated concentrations of magmatic volatiles (i.e., H2O, Cl, and S) may represent a mechanism for dilution of the pyroclastic density current. Trace- and major-element and volatile concentrations of clinopyroxene-hosted melt inclusions were analyzed via Secondary Ion Mass Spectrometry and Electron Microprobe analysis in both the proximal and distal deposits of the Campanian Ignimbrite to quantify the magmatic volatile contributions.Trace elements and volatiles are similarly enriched in melt as they are both excluded from growing crystalline phases. Unlike trace elements, volatiles will reach a point in which exsolution via second boiling occurs. Very incompatible trace elements will continue to enrich in the melt until a mineral phase that accepts them begins to form, while volatiles create a separate vapor phase rather than concentrate in the melt (i.e., saturation). Melt inclusions in the Campanian Ignimbrite show increasing incompatible trace elements (e.g., zirconium), while water concentrations stay consistent at ~2 wt%. The uniformity of measured water concentrations with highly variable incompatible trace elements suggests that these melt inclusions were volatile saturated prior to eruption. It is unlikely that the melt inclusions were all trapped at the same time and record a similarly undersaturated ~2 wt% H2O magma, especially because the melt inclusions have trace element concentrations that vary over a factor of ~6 implying different degrees of evolution. It is proposed that magmas feeding the Campanian Ignimbrite eruption underwent shallow storage (~1.5 km), with H2O contents re-equilibrating to their saturation limit at this storage depth prior to eruption. Higher water concentrations (~3.4 wt%) and lower trace element concentrations found in some inclusions likely correspond to eruption of magmas from deeper portions of the magma system. This can be envisaged as discrete sills at differing depths that supplied magmas for different portions of the Campanian Ignimbrite eruption. Preliminary hydrogen diffusion modeling suggests that re-equilibration at shallow depths could occur within ~2 hours to 2 weeks prior to eruption. Therefore, melt inclusions do not record the full magmatic volatile history of the Campanian Ignimbrite, but, rather, melt inclusions record magmatic volatile conditions during a short-lived, shallow storage event during which re-equilibration allowed H+ to escape from the melt inclusions while water exsolved from the magma. This re-equilibration may have provided a large amount of free gas phase just at the time of the eruption, with little time before the eruption for it to escape the system.

Item Type: Thesis (Masters)
Publisher’s Statement: © Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords: Campanian Ignimbrite; Melt Inclusions; Re-equilibration ; Trachyte; Volatiles; Italy; Volcanic eruptions
Subjects: Q Science > QC Physics
NAU Depositing Author Academic Status: Student
Department/Unit: Graduate College > Theses and Dissertations
College of the Environment, Forestry, and Natural Sciences > School of Earth Sciences and Environmental Sustainability
Date Deposited: 11 Feb 2022 20:34
Last Modified: 11 Feb 2022 20:34
URI: https://openknowledge.nau.edu/id/eprint/5686

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