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

The aims of this work are to estimate timescales of phlegraean eruptions and the eruptive dynamics of Campi Flegrei volcanoes through crystallization kinetic experiments. Cooling and decompression experiments have been carried out on trachytic composition melt in order to investigate crystallization kinetics of alkali feldspar and the implication for magma dynamics during the ascent towards the surface. Experiments have been conducted using Cold Seal Pressure Vessel apparatus, at pressure between 30 and 200 MPa, temperature between 750 and 860 °C, time between 7,200 and 57,600 seconds and fO2 NNO +0.8, thereby reproducing syn-eruptive conditions of the Campi Flegrei volcanoes. Alkali feldspar is the main phase present in this trachyte and its abundance can strongly vary with small changes in pressure, temperature and water content in the melt, implying appreciable variations in the textures and in the eruptive style. Texture analysis allow us to quantify the real sizes and number of alkali feldspars for each sample and consequently to calculate nucleation density, volume fraction, nucleation and growth rate of alkali feldspar. Results obtained show that these variables are strictly related to degree of undercooling(DeltaT = Tliquidus - Texperimental), time, water content in the melt, viscosity and element diffusivity. DeltaT is the driving force for crystallization and it has a strong infuence on nucleation and growth processes. In fact, generally at small DeltaT growth process dominates crystallization, whereas at large DeltaT nucleation dominates crystallization. Time also is important variable during crystallization process, because long experiment durations involve more nucleation events of alkali feldspar than short experiment durations. This is an important aspect to understand magma evolution in the magma chamber and in the conduit and to estimate growth rate (YL) and nucleation rate (Im). YL and Im of alkali feldspar are significant variables to assess the timescales of phlegraean eruptions. In fact, the magma ascent time from the magma chamber to the surface was constrained for phlegraean volcanoes, combining the growth rates (YL) estimated for alkali feldspars and the crystal size distribution (CSD) theory. The order of magnitude of alkali feldspar growth rates obtained in this study vary from 10-7 to 10-8 cm/s, implying changes in residence time into the conduit before the eruption from several hours to around two week, respectively. Water content in the melt is another important variable to study crystallization kinetics of alkali feldspar in trachytic melt. It can enhance or inhibit the nucleation and growth processes of alkali feldspar. Generally, the results obtained in this work show that low water content and high polymerization degree facilitate nucleation, whereas higher water content and lower polymerization degree enhance growth. This behavior is not appliable to all melts, indeed it is only relative to alkali feldspars and trachytic melts. It actually means that a mineral phase may have different crystallization kinetics as a function of the melt bulk composition. Partition coefficients are an important tool to understand magma evolution in a volcanic system. Therefore, in order to better understand major and trace elements partition coefficient of alkali feldspar during growth experiments, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) measurements have been carried out. Barium and strontium are strongly compatible elements in alkali feldspar and their partition coefficients are influenced by potassium content, because Ba2+ and Sr2+ can enter in the K+ site. Furthermore, combining textural data and trace elements analysis obtained in this work and other studies, it is possible to conclude that crystallization processes strongly depend on starting melt composition.