The effects of undercooling and deformation rates on the crystallization kinetics of Stromboli and Etna basalts |
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| Authors: | A Vona C Romano |
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| Institution: | 1. Dipartimento di Scienze, Università degli Studi Roma Tre, L.go San Leonardo Murialdo 1, 00146, Rome, Italy
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| Abstract: | We have investigated the effect of undercooling and deformation on the evolution of the texture and the crystallization kinetics of remelted basaltic material from Stromboli (pumice from the March 15, 2007 paroxysmal eruption) and Etna (1992 lava flow). Isothermal crystallization experiments were conducted at different degrees of undercooling and different applied strain rate (T = 1,157–1,187 °C and $ \dot{\gamma }_{i} $ γ · i = 4.26 s?1 for Stromboli; T = 1,131–1,182 °C and $ \dot{\gamma }_{i} $ γ · i = 0.53 s?1 for Etna). Melt viscosity increased due to the decrease in temperature and the increase in crystal content. The mineralogical assemblage comprises Sp + Plg (dominant) ± Cpx with an overall crystal fraction (?) between 0.06 and 0.27, increasing with undercooling and flow conditions. Both degree of undercooling and deformation rate deeply affect the kinetics of the crystallization process. Plagioclase nucleation incubation time strongly decreases with increasing ΔT and flow, while slow diffusion-limited growth characterizes low ΔT—low deformation rate experiments. Both Stromboli (high strain rate) and Etna (low strain rate) plagioclase growth rates (G) display relative small variations with Stromboli showing higher values (4.8 ± 1.9 × 10?9 m s?1) compared to Etna (2.1 ± 1.6 × 10?9 m s?1). Plagioclase average nucleation rates J continuously increase with undercooling from 1.4 × 106 to 6.7 × 106 m?3 s?1 for Stromboli and from 3.6 × 104 to 4.0 × 106 m?3 s?1 for Etna. The extremely low value of 3.6 × 104 m?3 s?1 recorded at the lowest undercooling experiment for Etna (ΔT = 20 °C) indicates that the crystallization process is growth-dominated and that possible effects of textural coarsening occur. G values obtained in this paper are generally one or two orders of magnitude higher compared to those obtained in the literature for equivalent undercooling conditions. Stirring of the melt, simulating magma flow or convective conditions, facilitates nucleation and growth of crystals via mechanical transportation of matter, resulting in the higher J and G observed. Any modeling pertaining to magma dynamics in the conduit (e.g., ascent rate) and lava flow emplacement (e.g., flow rate, pāhoehoe–‘a‘ā transition) should therefore take the effects of dynamic crystallization into account. |
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