Dynamic cooling experiments of latite and trachyte rocks from Campi Flegrei (Naples, Italy)

The silicate melt-magmatic rock phase change is one of the most important transitions occurring on the Earth. Even if this process is qualitatively well know, the physics of solidification and crystallization of natural glasses is still poorly known and constrained. A series of dynamic cooling experiments, at ambient pressure and oxygen fugacity, were run on two alkaline intermediate silicate rock compositions (latites and trachites) from Campi Flegrei (Naples, Italy) with the aim to shed light on this relevant process. Experiments were carried out using (a) five different cooling rates at a fixed thermal range of solidification (1300-800°C), and (b) different quenching temperatures at a fixed cooling rate (0.5°C/min). The results from 21 experiments, mainly analysed by SEM, EPMA and XRPD, show that the trachytic melt does not significantly crystallise for any imposed cooling rate, and only few tiny pyroxene microlites (< 1% by volume) sometimes form at lower cooling rates. On the opposite, the latitic melt starts to crystallise for cooling rates equal or slower than 3°C/min. At higher cooling rates (25 and 12.5°C/min), the run products were totally glassy. Furthermore, the second set of experiments clearly show that crystals are lacking above 900°C. Oxides and pyroxenes start to crystallise at 900°C and cooling rate=0.5°C/min. In the range between 900°C and 800°C, the latitic composition completely crystallises (> 90% by volume).