Mechanical Spectroscopy on Volcanic Glasses

Mechanical relaxation behaviour of various natural volcanic glasses have been investigated in the temperature range RT − 1000C using special low frequency flexure (f≈0.63Hz) pendulum experiments. The rheological properties complex Young’s modulus M⋆(ω, τ) and internal friction Q−1(ω, τ) have been studied from a pure elastic solid at room temperature to pure viscous melt at log(η[Pas]) = 8. The Young’s modulus at room temperature MRT = (70 ± 10)GPa is nearly constant. There is a positive correlation with the water content and a weak negative correlation with the cooling rate. Several relaxation processes are assumed to act: the primary α-relaxation (viscoelastic process, Ea = (344...554)kJ/mol) above the glass transition temperature Tg = (935...1105)K and secondary anelastic β , β and γ-relaxation processes below Tg. The dynamic glass transition, i.e. the viscoelastic α-transition, can be characterized with hierarchically coupled relaxation processes which lead to an equivalent distribution of relaxation times nearly independent of the fragility for all examined glasses. The observed secondary relaxation processes can be explained with different mechanisms: (γ, β) cooperative movement of alkali ions in the vitreous state. (β) cooperative movement of alkaline earth ions and non bridging oxygen’s near the glass transition range. Here the influence of water must be taken into account as well as alteration effects due to structural α-relaxation. With a simple fractional Maxwell model with asymmetrical relaxation time distribution, H(τ), phenomenological the mechanical relaxation behaviour, is described. This establish a basis of realistic concepts for modelling of volcanic or magmatic processes. ⋆ [email protected] Preprint submitted to Elsevier Science 24 March 2008