Crystallization kinetics in Stromboli magma

Shoshonitic basalt composition (PST9) has been studied to investigate the kinetics of crystallisation and implications for magma dynamics at Stromboli volcano. A series of decompression experiments over a range of pressure (100 - 5 MPa) at constant temperature (1075°C) show that plagioclase is stable at 50 MPa or less under water saturated conditions. Moreover, these experiments show that plagioclase An content decreases with decreasing P(H2O) reaching the same composition, as the natural plagioclase in Sromboli scoria and that the bulk crystal fraction increases as the experimental conditions tend to lower pressure, associated with water exsolution. Nucleation density increase for lower undercooling whereas for higher decompression the nucleation is almost constant. Growth rate (G+) is observed to increase with undercooling, although at long time (16 hrs), growth rates approach constant values. The values of G+ vary from 10-9 to 10-10 cm/sec for pressure from 50 to 25 MPa, while at pressure from 10 to 5 MPa growth rates are from 10-6 to 10-7 cm/sec (slower in long duration experiments). A series of dissolution experiments at atmospheric pressure and over a range of temperature has been done both for plagioclase (plg) and clinopyroxene (cpx) (temperature range of 1220 - 1240°C and 1140 - 1240°C respectively for plg and cpx). Dissolution rate (G-) for plagioclase (10-7 cm/sec) tends to be slightly higher at higher temperature in the range of 1220 - 1240°C and appears to be time independent for the experimental durations investigated (10 - 30 hrs). For clinopyroxene, we have observed higher dissolution rates (G-) (5 - 6*10-6 cm/sec) at the highest temperatures, whereas the dissolution rates decrease to 1 - 2*10 -6 for longer experimental times. These trends could be related to development of a diffusion limited boundary layer adjacent to the dissolution crystal. By comparison of the experimental data on plagioclase composition, growth rates, nucleation density (Nv), nucleation rate (Jm) and dissolution in Stromboli shoshonitic basalt, it is possible to place the scoria reservoirs in the upper part (from 500 m. to the surface) of volcanic conduit. Furthermore, The results from this work combined with observations on natural samples will help to improve our knowledge of the magma plumbing system and of interactions between resident magma and new magma and in general the dynamics of volcanic activity of Stromboli and the time scale of magmatic processes.