From melt to crystals: The effects of cooling on Fe[sbnd]Ti oxide nanolites crystallisation and melt polymerisation at oxidising conditions

The state and properties of magma are crucial controls on the eruptive behaviour of volcanic systems. Nowhere is the importance of the link between the state and properties more dramatically expressed than in the rheology of multiphase magmatic systems, and it is magma rheology in turn that exerts a primary control on eruption style. Magmas ascending to shallow levels are subjected to decompression that leads to volatile loss and melt viscosity increase as well as the nucleation and growth of microlites and nanolites. Yet the effects of nanolites on magma rheology have only been investigated to date in a reconnaissance fashion. In order to better constrain the influence of cooling on Fe[sbnd]Ti oxide nanolite crystallisation and silicate melt structure, we conducted magma cooling experiments at controlled cooling rates of 0.1 − 50 K min⁻¹. All experiments were run in air at 1 bar on a Fe-rich rhyolitic magma at superliquidus starting conditions. We analysed the resultant glasses via micro-Raman spectrometry to monitor the structural changes in the melt induced during the transition from a crystal-free melt to a nanolite-bearing magma, as well as the process of nanolite crystallisation itself. The Raman spectral data indicate that nanolite formation together with a concomitant increase in melt polymerisation occur at cooling rates of 0.5 K min⁻¹ or less. The timescales for nanolite formation are estimated to be on the order of 10⁴ s for both dynamic crystallisation and isothermal crystallisation. Our experimental results, obtained at oxidising conditions and slow cooling rates, provide insights into the formation of Fe[sbnd]Ti oxide nanolites and structural changes of silicate melts that can also be observed and are expected in equivalent natural volcanic systems. The higher degree of melt polymerisation and the higher load of crystals both due to the formation of nanolites in Fe-rich rhyolites are likely to cause increases in the magma viscosity. In addition the nanolites likely provide sites for heterogeneous bubble nucleation in these degassing magmas. Taken together these effects may have the potential to shift shallow magmas from an effusive eruption style into conditions favourable for an explosive eruption.