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- All Subjects: Petrology
- Creators: Amanda, Clarke B
- Creators: Clarke, Amanda B
- Member of: Theses and Dissertations
Description
Fluorine (F) is a volatile constituent of magmas and hydrous mantle minerals. Compared to other volatile species, F is highly soluble in silicate melts, allowing F to remain in the melt during magma differentiation and rendering F less subject to disturbance during degassing upon magma ascent. Hence, the association between fluorine in basalts and fluorine in the mantle source region is more robust than for other volatile species. The ionic radius of F- is similar to that of OH- and O2-, and F may substitute for hydroxyl and oxygen in silicate minerals and melt. Fluorine is also incorporated at trace levels within nominally anhydrous minerals (NAMs) such as olivine, clinopyroxene, and plagioclase. Investigating the geochemical behavior of F in NAMs provides a means to estimate the pre-eruptive F contents of degassed magmas and to better understand the degassing behavior of H. The partition coefficients of F were determined for clinopyroxene, olivine, plagioclase, and hornblende within melts of olivine-minette, augite-minette, basaltic andesite, and latite compositions. The samples analyzed were run products from previously-published phase-equilibria experiments. Fluorine was measured by secondary ion mass spectrometry (SIMS) using an 16O- primary beam and detection of negative secondary ions (19F-, 18O-, 28Si-). SIMS ion intensities are converted to concentrations by analyzing matrix-matched microanalytical reference materials and constructing calibration curves. For robust F calibration standards, five basaltic glasses (termed Fba glasses) were synthesized in-house using a natural tholeiite mixed with variable amounts of CaF2. The Fba glasses were characterized for F content and homogeneity, using both SIMS and electron-probe microanalysis (EPMA), and used as F standards. The partition coefficients for clinopyroxene (0.04-028) and olivine (0.01-0.16) varied with melt composition such that DF (olivine-minette) < DF (augite-minette) < DF (basaltic andesite) < DF (latite). Crystal chemical controls were found to influence the incorporation of F into clinopyroxene, but none were found that affected olivine. Fluorine partitioning was compared with that of OH within clinopyroxenes, and the alumina content of clinopyroxene was shown to be a strong influence on the incorporation of both anions. Fluorine substitution into both olivine and clinopyroxene was found to be strongly controlled by melt viscosity and degree of melt polymerization.
ContributorsGuggino, Steve (Author) / Hervig, Richard L (Thesis advisor) / Donald, Burt M (Committee member) / Amanda, Clarke B (Committee member) / Lynda, Williams B (Committee member) / Stanley, Williams N (Committee member) / Arizona State University (Publisher)
Created2012
Description
Volcanoes can experience multiple eruption styles throughout their eruptive histories. Among the most complex and most common are eruptions of intermediate explosivity, such as Vulcanian and sub-Plinian eruptions. Vulcanian eruptions are characterized by small-scale, short-lived, ash-rich eruptions initiated by the failure of a dense magma plug or overlying dome that had sealed an overpressured conduit. Sub-Plinian eruptions are characterized by sustained columns that reach tens of kilometers in height.
Multiple eruption styles can be observed in a single eruptive sequence. In recent decades, transitions in eruption style during well-documented eruptions have been described in detail, with some workers proposing explanatory mechanisms for the transitions. These proposed mechanisms may be broadly classified into processes at depth, processes in the conduit, or some combination of both.
The present study is focused on the Pietre Cotte sequence because it may have encompassed up to three different eruptive cycles, each representing different degrees of explosivity. The first deposits are composed of repeated layers of fine ash and lapilli composed of latite and rhyolite endmembers, efficiently mixed at sub-cm scales. The thin layers and bubble/crystal textures indicate that the magma underwent numerous decompressions and open-system degassing, and that the eruptions waned with time. The second phase of the sequence appears to have been initiated by cm-scale mixing between a volatile-rich, mafic magma from deeper in the system and a shallow silicic body. Textures indicate that the magma ascended rapidly and experienced little to no open-system degassing. The final phase of the sequence again produced repeated layers of fine ash and lapilli, of uniform trachyte composition, and waned with time. The first and last phases were likely produced in Vulcanian eruptions, while the pumice-rich layers were likely produced in Vulcanian to sub-Plinian eruptions.
In summary, the Pietre Cotte sequence is characterized by up to three magma recharge events in ~200 years. The differences in eruptive style appear to have been controlled by variations in the volatile content of the recharge magma, as well as the efficiency and scale of magma mixing and resulting overpressures.
Multiple eruption styles can be observed in a single eruptive sequence. In recent decades, transitions in eruption style during well-documented eruptions have been described in detail, with some workers proposing explanatory mechanisms for the transitions. These proposed mechanisms may be broadly classified into processes at depth, processes in the conduit, or some combination of both.
The present study is focused on the Pietre Cotte sequence because it may have encompassed up to three different eruptive cycles, each representing different degrees of explosivity. The first deposits are composed of repeated layers of fine ash and lapilli composed of latite and rhyolite endmembers, efficiently mixed at sub-cm scales. The thin layers and bubble/crystal textures indicate that the magma underwent numerous decompressions and open-system degassing, and that the eruptions waned with time. The second phase of the sequence appears to have been initiated by cm-scale mixing between a volatile-rich, mafic magma from deeper in the system and a shallow silicic body. Textures indicate that the magma ascended rapidly and experienced little to no open-system degassing. The final phase of the sequence again produced repeated layers of fine ash and lapilli, of uniform trachyte composition, and waned with time. The first and last phases were likely produced in Vulcanian eruptions, while the pumice-rich layers were likely produced in Vulcanian to sub-Plinian eruptions.
In summary, the Pietre Cotte sequence is characterized by up to three magma recharge events in ~200 years. The differences in eruptive style appear to have been controlled by variations in the volatile content of the recharge magma, as well as the efficiency and scale of magma mixing and resulting overpressures.
ContributorsKim, Jisoo (Author) / Clarke, Amanda B (Thesis advisor) / Roggensack, Kurt (Thesis advisor) / Barboni, Melanie (Committee member) / Arizona State University (Publisher)
Created2020