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 FeTi 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 FeTi 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.

岩浆的状态和性质是控制火山系统喷发行为的关键控制因素。在多相岩浆系统的流变学中，状态和性质之间的联系的重要性无处可表达，而岩浆流变学又对喷发类型起主要控制作用。上升到浅层的岩浆会经历减压，这会导致挥发损失和熔体粘度增加以及微晶石和纳米晶的成核和生长。迄今为止，仅以侦察方式研究了纳米岩对岩浆流变学的影响。为了更好地限制冷却对铁的影响氧化钛纳米晶的结晶和硅酸盐熔体的结构，我们进行了岩浆冷却实验，控制冷却速率为0.1-50 K min ^-1。所有实验均在超液相线起始条件下，在富含铁的流纹岩浆中于1 bar的空气中进行。我们通过微拉曼光谱法分析了所得玻璃，以监测从无晶体熔体到含纳米晶的岩浆转变过程中诱导的熔体的结构变化，以及纳米晶结晶过程本身。拉曼光谱数据表明，在冷却速率为0.5 K min ^-1时，会形成纳米晶并伴随熔融聚合增加或更少。对于动态结晶和等温结晶而言，纳米晶形成的时间尺度估计约为10 ⁴ s。我们的实验结果是在氧化条件下和缓慢的冷却速率下获得的，可以洞悉铁的形成钛氧化物纳米岩和硅酸盐熔体的结构变化在等效的天然火山系统中也可以观察到和预期到。均由于富含铁的流纹岩中纳米晶的形成而导致的较高的熔体聚合度和较高的晶体负载很可能导致岩浆粘度增加。另外，在这些脱气岩浆中，纳米岩可能会提供异质气泡成核的位置。这些影响加在一起可能会使浅层岩浆从喷发型转变成有利于爆发性喷发的条件。