Seminars Archive

Experimental study of the crystallization kinetics and eruption dynamics in Campi Flegrei trachytic melts.

Abstract
The Phlegraean area, with more than a million inhabitants, is one of the areas with the highest volcanic hazard in the world. This is due to the numerous explosive eruptions that have occurred in this area in the last 60 Ka (Pappalardo et al., 1999) and to fumarolic emissions, hydrothermalism, ground deformations, resurgence and seismicity (Orsi et al., 1999). The research project proposed for this work involves collaboration with Osservatorio Vesuviano – National Institute of Geophysics and Volcanology (INGV), ETH of Zurich (University of Zurich) and experimental laboratory of Geochemistry in Camerino. The aim of this work is to study the crystallization kinetics of alkali feldspar, the most abundant crystalline phase in phlegraean trachytic magmas. This involves high pressure (P) and temperature (T) experiments reproducing the pre-eruptive conditions in the laboratory (850°C, up to 200 MPa) and a combined study of textural and mineralogical features of synthetic samples and natural samples, trying to define magma behavior during the ascent towards the surface, that is during the eruption. Although the results are generally applicable to all trachytic magmas in the phlegraean area, the most recent eruption at Monte Nuovo in 1538 is of particular interest because it is used as a model of the must likely style of future eruptions in the phlegraean area. Available experimental data and thermodynamic models (e.g. Ghiorso, 1997) allow calculation of equilibrium phase estimation of physical properties for many magma compositions over a range of T and P and oxidation state fO2. However, at the moment there exist no suitable models to calculate the kinetics of magmatic processes. In particular, this study will be useful to improve our knowledge concerning nucleation and crystal growth kinetics and to quantify timescales of crystal growth in trachytic melts and consequent implication for eruption dynamic. This information is important because crystallization textures develop during cooling or decompression and these are time dependent phenomena. Also, with textural determination (crystals size distribution) it is possible to constrain residence time of magma in magma-chamber and ascent times in the conduits that feed volcanic eruptions. The ability of a crystal to grow depends on it reaching a critical size, which is dependent upon the amount of undercooling, the degree of undercooling ∆T is defined through follow equation: ∆T = T liquidus - T experimental it can be considered the driving force of crystallization. The critical size of nucleous varies from an infinite size above the liquidus temperature to smaller sizes at greater ∆T. Another factor in the nucleation of crystals from a melt is the mobility of atoms and molecules in the melt as measured by the diffusion coefficient (Swanson, 1977). As the temperature drops below the liquidus, nucleation will increase from zero to a maximum at a given undercooling. Diffusion rates are then very low and nucleation decreases with further decrease in temperature. This explains the pattern of nucleation in liquids because the liquid diffusivity decreases with decreasing temperature and it is consistent with the results observed in this study. Crystal growth from a melt proceeds by attachment of chemical species from the liquid to the crystal nuclei. Rate of growth is thus a function of the rate of attachment at crystal-melt interface and the mobility of crystal-forming species within the melt, which can be related to melt viscosity. Decreasing the viscosity of the melt, the diffusivity of elements can increase favoring nucleation and growth of crystals. The crystals present in a volcanic rock and the observed variations in their dimensions and compositions reflect the integrated pressure (P)- temperature (T) - oxygen fugacity fO 2 -composition (X) - time (t) history of the sample. Because crystals take a finite time to nucleate and grow (rates depending on ∆T, composition and percentage of H 2 O dissolved in the melt, which affects viscosity and diffusivity), variations in their sizes, size distributions and compositions can provide insights into magmatic processes and their time-scales. In order to use observed variations in the textures of volcanic rocks to gain quantitative insights into the timescales of magmatic processes, we require quantitative information on the rates of nucleation (I) and growth (Y) as a function of undercooling (∆T) for the crystals and melts of interest. This study is useful to improve the knowledge crystallization kinetics of silicate melts through textural analysis of volcanic rocks and will allow to increase the specific knowledge of pre- and syn-eruptive processes of the Campi Flegrei, with the aim of monitoring and forecasting the kind and the evolution of future eruptions, estimating the time scales of eruptions (magma rising time during the eruption) of phlegraean volcanoes, that by our results can change between a few hours to several days (data for Monte Nuovo eruption, 1538).