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Abstract High‐temperature Raman spectroscopy offers a cost‐effective alternative to extensive infrastructure and sensitive instrumentation for investigating nanolite crystallization in undercooled volcanic melts, a key area of interest in volcanology. This study examined nanolite formation in anhydrous andesite melts in situ at high temperatures, identifying distinct Raman peaks at 310 and 670 cm⁻¹appearing above the glass transition temperature. The initial amorphous glass remained stable up to 655°C, beyond which Fe‐Ti‐oxide nanolites progressively formed at higher temperatures, as also confirmed by complementary XRD analysis. The evolution of the 310 cm⁻¹peak depends only on the magnitude of nanolite crystallization, while the intensity of the 670 cm⁻¹peak is temperature‐dependent and challenging to observe above 500°C. Complementary low‐temperature rock‐magnetic analyses confirmed Fe‐Ti‐oxide nanocrystallization with nanolites around 20 nm in diameter. The study tested lasers of different wavelengths (from 355 to 514 nm) and found the green laser to be the most effective for collecting spectra at both room and high temperature. However, above 720°C, black body radiation significantly hinders Raman observation with the green laser when using a non‐confocal setup and analyzing poorly transparent samples. If higher temperature measurements are desired, switching to a confocal setup and using lower wavelength lasers should be considered. This research offers a protocol for studying nanolite formation and melt dynamics at high temperatures, providing a foundation for future studies of volcanic processes. 

Understanding the viscosity of mantle-derived magmas is needed to model their migration mechanisms and ascent rate from the source rock to the surface. High pressure–temperature experimental data are now available on the viscosity of synthetic melts, pure carbonatitic to carbonate–silicate compositions, anhydrous basalts, dacites and rhyolites. However, the viscosity of volatile-bearing melilititic melts, among the most plausible carriers of deep carbon, has not been investigated. In this study, we experimentally determined the viscosity of synthetic liquids with ~31 and ~39 wt% SiO2, 1.60 and 1.42 wt% CO2 and 5.7 and 1 wt% H2O, respectively, at pressures from 1 to 4.7 GPa and temperatures between 1265 and 1755 °C, using the falling-sphere technique combined with in situ X-ray radiography. Our results show viscosities between 0.1044 and 2.1221 Pa·s, with a clear dependence on temperature and SiO2 content. The atomic structure of both melt compositions was also determined at high pressure and temperature, using in situ multi-angle energy-dispersive X-ray diffraction supported by ex situ microFTIR and microRaman spectroscopic measurements. Our results yield evidence that the T–T and T–O (T = Si,Al) interatomic distances of ultrabasic melts are higher than those for basaltic melts known from similar recent studies. Based on our experimental data, melilititic melts are expected to migrate at a rate ~from 2 to 57 km·yr−1 in the present-day or the Archaean mantle, respectively.