Kinetics of crystal evolution as a probe to magmatism at Stromboli (Aeolian Archipelago, Italy)

Abstract

The kinetics of hydrated basaltic melts erupted during the present activity at Stromboli have been studied to estimate the growth and dissolution rate of plagioclase. Specifically, a high-K basalt composition (PST-9) has been studied to investigate magma and eruption dynamics at Stromboli volcano by combining crystallization kinetics of plagioclase and CSD measurements on natural samples from literature (Armienti et al., 2007; Fornaciai et al., 2009). A series of water-saturated decompression experiments over a range of final water pressure (Pf = 75–5 MPa) at constant temperature (1075 C) show that plagioclase is systematically present from 50 to 5 MPa at water saturated conditions. Moreover, these experiments show that anorthite (An) content decreases with decreasing PH2O, reaching the same composition as the natural plagioclase in Stromboli scoria at pressure below 20 MPa and that the plagioclase crystal fraction increases as the experimental conditions tend to lower final pressure. Plagioclase growth rate (GL) is observed to increase with undercooling for the Pf investigated during decompression experiments, except for the 75 MPa Pf serie that only has two samples with the presence of plagioclase crystals. The values of GL vary from 10 7 to 10 8 cm/s for Pf from 75 to 25 MPa, while at Pf from 10 to 5 MPa growth rates are approximately of 10 6 cm/s. A series of dissolution experiments at atmospheric pressure and over a range of temperature has been done for plagioclase (T range of 1220–1240 C). Dissolution rate (G-) for plagioclase (10 7 cm/s) 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 h). These trends could be related to development of a diffusion-limited boundary layer adjacent to the dissolving crystal. By comparison of the experimental data on plagioclase composition, growth rates and dissolution in Stromboli basalt, it is possible to place the reservoir of the crystal-rich magma in the upper part (from 400 m to the surface) of the volcanic conduit. Kinetic data of the plagioclase, the most important phase of the shallow magmatic system of Stromboli, show that the magmatic processes are quite dynamic and in a relatively short time (hours or several days) the system can change considerably. Furthermore, the results from this work combined with observations on natural samples help to improve our knowledge of the magma plumbing system, of interactions between resident magma and new magmas, the dynamics of volcanic activity of Stromboli, and the time scales of magmatic processes that change in a few hours to 1 month.