Solidification behaviour of natural silicate melts and volcanological implications

Abstract

The thermodynamic and physical properties of magmas have been extensively investigated as a function of T, P, fO2 and composition allowing the development of accurate phase stability, viscosity, and diffusion models. However, how the silicate melt properties are influenced by kinetic effects is still an open question. The most important transformation of a magma is its solidification due to cooling, i.e. the transition from a silicate melt to a rock. Solidified magmas may be crystalline, vitreous, or a mixture of glass and crystals. If the cooling rate is larger enough to prevent crystallization, a magma can encompass the supercooling region without crystallisation. The smallest cooling rate that suppresses or strongly limited the nucleation of crystals is the critical cooling rate Rc. Melts with low Rc persist in a metastable liquid state and have a good glass forming ability (GFA). GFA and Rc of melts can be quantitatively estimated using (1) the reduced glass transition parameter Trg = Tg/Tm (Tg, temperature of glass transition; Tm, temperature of melting), and (2) the viscosity fragility concept. As stated by the theory, strong liquids with high Trg values have good GFA and low Rc, whereas fragile liquids with low Trg have a poor GFA and high Rc. Using available experimental data and theoretical models, we analyze the kinetic effects in dry magmas of different composition. The obtained results are relevant for the formation of lava flows and domes. In sub-alkaline magmas, Trg linearly increases and Rc decreases as the Si and Al content increases. Rc of basalts range between 101 and 103 K/s. In dacitic and rhyolitic melts, Rc is between 10-3 and 10-5 K/s. Alkaline melts have Trg values lower than those of sub-alkaline compositions. Results are consistent with the available experimental data. The sluggish kinetics of nucleation determined by using the relation Rc vs Trg is also in agreement with the experimental and theoretical data for synthetic silicate melts. The outlined solidification behaviour of magmatic melts has a profound influence on the viscosity paths of magmas. Depending on the Trg and Rc values, less evolved magmas may have a viscosity larger than that of more evolved magmas due to the rapid crystallization induced by the cooling during their flowing on the Earth. The glassy portion of poorly evolved magmas is indicative of rapid cooling, whereas the glassy fraction of evolved magmas is not unequivocally indicative of rapid cooling being their typical Rc values low. Basaltic lavas may flow on the Earth surface for long times only if they have a temperature close to Tm, whereas more evolved lavas can flow for longer periods with temperatures well below Tm. Fully glassy lavas like obsidians have invariably rhyolitic or trachytic compositions.