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

We have performed decompression experiments on a Monte Nuovo trachytic composition, at pressure between 30 and 200 MPa, temperature between 750 degrees C and 850 degrees C, time between 7200 and 50,400 (2X00) seconds and fO (sub 2) =NNO+0.8 from which the growth rate of alkali feldspar in trachytic melts were obtained. The crystal growth rate (G) varies from 1.39X10 (super -8) to 1.65X10 (super -7) 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 (10 (super -8) cm/s) or higher by almost an order of magnitude (10 (super -7) 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.