Zeolitization and feldspathization processes of the Campanian Ignimbrite, southern Italy

Laplant, Paige Elizabeth (2021) Zeolitization and feldspathization processes of the Campanian Ignimbrite, southern Italy. Masters thesis, Northern Arizona University.

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Abstract

The Campanian Ignimbrite formed the Campi Flegrei caldera, located to the west of Naples, Italy, at about 39 ka. The dilute pyroclastic density current, which had an emplacement temperature above 580 ºC, surmounted high topography (over 1400 m) and traveled over water, depositing a trachytic to phonolitic tuff. What would have supplied the additional gas needed for this dilute current, however, is still unknown. Today, there is a hydrothermal system at Campi Flegrei and if one were present 39 ka, this could add the additional gas required to sustain such a diluted ignimbrite. Campi Flegrei’s proximity to the coastline makes seawater (or a mixture of seawater with another source) a reasonable source for a hydrothermal system. The alteration mineral assemblage of two distal deposits, the Welded Gray Ignimbrite and the Lithified Yellow Tuff, contain alteration minerals whose chemistry reflect that of the fluid. This study analyzed chemical compositions of chabazite, analcime, phillipsite, and feldspars to see if there was an enrichment of Na, K, Mg, Sr, etc. that may be found in specific water sources (i.e. seawater, meteoric, or magmatic). XRPD, SEM/EDS, WDS, and thermodynamic modeling methods were used to analyze the Campanian Ignimbrite. These methods revealed an abundance of Na-rich and K-rich minerals. Basal units contain more Na-rich minerals and calcite, likely precipitated from dissolution of carbonaceous clasts that could have then neutralized the fluids in conjunction with the dissolution of trachytic glass, allowing minor zeolitization to later occur. The Lithified Yellow Tuff contains mostly K-rich zeolites and evidence for vapor-phase alteration in fast-growth, spherulite crystals and the potential presence of marialite. These findings indicate a hydrothermal system existed within Campi Flegrei at the time of the Campanian Ignimbrite, but I propose that this system did not become incorporated into the deposits. Rather, devitrification processes and interstitial pore fluid would have been sufficient to generate the mineral assemblage observed in the Welded Gray Ignimbrite and part of the Lithified Yellow Tuff. Further zeolitization occurred in the Lithified Yellow Tuff by meteoric water, once the deposit cooled enough and became favorable for zeolitization. Na-rich zeolites found in this uppermost unit are likely detrital material. They would have come from the Campi Flegrei hydrothermal system, forming prior to eruption in marine-like fluids, and later becoming incorporated into the Breccia Museo and some of the distal deposit.

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:	Alteration; Feldspars; Hydrothermal; Italy; Vapor-phase ; Zeolites; Campi Flegrei caldera
Subjects:	Q Science > QE Geology
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:	09 Feb 2022 17:41
Last Modified:	09 Feb 2022 17:41
URI:	https://openknowledge.nau.edu/id/eprint/5679

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