Growth rate of alkali feldspars in decompression-induced crystallization experiments in a trachytic melt of the Phlegraean Fields (Napoli, Italy)

Abstract: We have performed decompression experiments on a Monte Nuovo trachytic composition, at pressure between 30 and 200 MPa, temperature between 750 C and 850 C, time between 7200 and 50,400 (200) seconds and fO2¼ NNOþ 0.8 from which the growth rate of alkali feldspar in trachytic melts were obtained. The crystal growth rate (G) varies from 1.39108 to 1.65107 cm/s and is higher in experiments with shorter duration (7200 s), while it tends to decrease with increasing duration (21,600 and 50,400 s). The values of G presented here are similar (108 cm/s) or higher by almost an order of magnitude (107 cm/s) when compared with the growth rates obtained from an earlier study of Monte Nuovo eruptive dynamics. Therefore, the magma ascent times calculated are similar (1–2 days) or slightly lower (several hours) relative to those estimated in that study. Key-words: crystallization kinetics, decompression, trachytic melt, alkali feldspars, Phlegraean Fields.