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We document the presence, composition, and number density (TND) of titanomagnetite nanolites and ultra‐nanolites in aphyric rhyolitic pumice, obsidian, and vesicular obsidian from the 1060 CE Glass Mountain volcanic eruption of Medicine Lake Volcano, California, using magnetic methods. Curie temperatures indicate compositions of Fe2.40Ti0.60O4 to Fe3O4. Rock‐magnetic parameters sensitive to domain state, which is dependent on grain volume, indicate a range of particle sizes spanning superparamagnetic (<50–80 nm) to multidomain (>10 μm) particles. Cylindrical cores drilled from the centers of individual pumice clasts display anisotropy of magnetic susceptibility with prolate fabrics, with the highest degree of anisotropy coinciding with the highest vesicularity. Fabrics within a pumice clast require particle alignment within a fluid, and are interpreted to result from the upward transport of magma driven by vesiculation, ensuing bubble growth, and shearing in the conduit. Titanomagnetite number density (TND) is calculated from titanomagnetite volume fraction, which is determined from ferromagnetic susceptibility. TND estimates for monospecific assemblages of 1,000 nm–10 nm cubes predict 1012 to 1020 m−3 of solid material, respectively. TND estimates derived using a power law distribution of grain sizes predict 1018 to 1019 m−3. These ranges agree well with TND determinations of 1018 to 1020 m−3 made by McCartney et al. (2024), and are several orders of magnitude larger than the number density of bubbles in these materials. These observations are consistent with the hypothesis that titanomagnetite crystals already existed in extremely high number‐abundance at the time of magma ascent and bubble nucleation. Plain Language Summary We use magnetism experiments to prove that nanometer‐sized magnetic particles are present in volcanic rocks with low iron content and few visible crystals. Nanolites (particles between 30 and 1,000 nm) and ultra‐nanolites (particles smaller than 30 nm) are extremely difficult to detect in volcanic rocks composed mainly of glass using conventional methods such as optical and electron microscopy. Titanomagnetite nano‐particles may play a role in controlling the explosiveness of volcanic eruptions. The magnetic signatures of minerals can be used to determine their chemical composition, particle size range, and particle abundance. Pumice and obsidian contain the mineral titanomagnetite, with no evidence of prolonged crystallization at high oxygen levels at the Earth's surface. Observed magnetic behaviors are very similar to those of previously published studies of titanomagnetite in the 10–1,000 nm size range, and similar to mathematical models that simulate this size range. We find that pumice clasts have a magnetic fabric, suggesting that the nanolites and ultra‐nanolites were aligned in spatial patterns before the magma solidified, with stronger alignment coinciding with high degrees of vesicularity. Our results indicate that titanomagnetite crystals are highly abundant, and had crystallized in the magma chamber before the eruption. Key Points Magnetic methods document titanomagnetite nanolites in rhyolitic materials from Glass Mountain, Medicine Lake Volcano, California Titanomagnetite number densities for pumice, obsidian, and vesicular obsidian span 1012 to 1020 m−3 of solid material Titanomagnetite crystals already existed in extremely high number‐abundance at the time of magma ascent and bubble nucleation

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−1 appearing 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−1 peak depends only on the magnitude of nanolite crystallization, while the intensity of the 670 cm−1 peak 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. Plain Language Summary Understanding how tiny crystals, known as nanolites, form in volcanic melts is crucial for studying volcanic processes and predicting eruptions. However, observing these crystals at high temperatures typically requires sophisticated and expensive equipment. Our research demonstrates that Raman spectroscopy, a relatively accessible technique, can effectively study nanolite formation in volcanic melts, specifically in anhydrous andesite. In our experiments, we heated andesite samples to temperatures above the glass transition, where the material begins to soften. As the temperature increased beyond 655°C, we observed the formation of nanolites through specific Raman signals. These observations show that nanolites continue to develop both as temperatures rise and as the duration of exposure to heat increases. This study presents a reliable methodology for investigating high‐temperature nanolite formation, offering valuable insights into volcanic melt dynamics and providing a foundation for future research in volcanology. Key Points Nanocrystallization in Fe‐bearing volcanic melts has been observed for the first time using in situ Raman spectroscopy at high temperatures Fe‐Ti‐oxide nanolites form and progressively grow with increasing temperature and time, reaching 2.6 vol% and 20 nm in mean diameter The 310 cm−1 peak solely depends on the degree of nanolite crystallization, while the 670 cm−1 peak is strongly temperature‐dependent

Nucleation of H2O vapor bubbles in magma requires surpassing a chemical supersaturation threshold via decompression. The threshold is minimized in the presence of a nucleation substrate (heterogeneous nucleation, <50 MPa), and maximized when no nucleation substrate is present (homogeneous nucleation, >100 MPa). The existence of explosively erupted aphyric rhyolite magma staged from shallow (<100 MPa) depths represents an apparent paradox that hints at the presence of a cryptic nucleation substrate. In a pair of studies focusing on Glass Mountain eruptive units from Medicine Lake, California, we characterize titanomagnetite nanolites and ultrananolites in pumice, obsidian, and vesicular obsidian (Brachfeld et al., 2024, https://doi.org/10.1029/2023GC011336), calculate titanomagnetite crystal number densities, and compare titanomagnetite abundance with the physical properties of pumice to evaluate hypotheses on the timing of titanomagnetite crystallization. Titanomagnetite crystals with grain sizes of approximately 3–33 nm are identified in pumice samples from the thermal unblocking of low‐temperature thermoremanent magnetization. The titanomagnetite number densities for pumice are 1018 to 1020 m−3, comparable to number densities in pumice and obsidian obtained from room temperature methods (Brachfeld et al., 2024, https://doi.org/10.1029/2023GC011336). This range exceeds reported bubble number densities (BND) within the pumice from the same eruptive units (average BND ∼4 × 1014 m−3). The similar abundances of nm‐scale titanomagnetite crystals in the effusive and explosive products of the same eruption, together with the lack of correlation between pumice permeability and titanomagnetite content, are consistent with titanomagnetite formation having preceded the bubble formation. Results suggest sub‐micron titanomagnetite crystals are responsible for heterogeneous bubble nucleation in this nominally aphyric rhyolite magma. Key Points Aphyric rhyolite eruptions staged from shallow magma reservoirs lack the overpressure needed for homogeneous bubble nucleation Heterogeneous bubble nucleation may occur on sub‐µm titanomagnetite crystals, which are undetectable using standard analytical techniques Sub‐µm titanomagnetite crystals can be detected and quantified with low temperature magnetic analyses

Water degassing plays a major role in magma transport and eruption by increasing liquidus temperatures, bubble and crystal volume fractions, and strongly affecting the viscosity of bulk magma. High spatial resolution textural analysis detailing the dynamics of bubble and crystal growth is key to unravelling the swift changes in magma crystallinity and gas content that affect the conditions of magma flow, fragmentation, and eruption. Ex situ observation of samples from a previous experimental study of magma degassing reveals that vesicles are surrounded by chemically heterogeneous residual glass that may be produced by newly formed minerals that are not observable at the microscale. Here, we present new in situ high-temperature (500–1100 °C), time-elapsed (every ~ 20 min at 200–800 °C, ~ 10 min at 900–1000 °C, and ~ 5 min at 1100 °C) observations of degassing of synthesised, hydrous (4.2 wt.% H 2 O) dacite glasses using scanning transmission electron microscopy at 0.4 nm resolution. The experiments reproduce degassing of a silicic melt by high-temperature heated stage mounted in the analytical instrument. We monitor the dynamics of nucleation and growth of nanobubbles that experience coalescence and formation of microbubbles and trigger the nucleation and growth of nanolites of plagioclase, clinopyroxene, Fe-Ti oxides, and quartz, at the expense of the residual melt. The ability to image degassing and crystallisation at nanoscale reveals a sequence of complex physical and chemical changes of the residual melt and shows that the kinetics of crystallisation in silicic melts is modulated by the melt’s ability to exsolve fluids that help form mineral nuclei and nanolites. Finally, we highlight that the competition between gas retention and crystallisation is initiated at the nanoscale and may anticipate the role of microlites in controlling rates of magma ascent in a volcanic conduit and modulating the style of the consequent volcanic eruption.

Melt viscosity is one of the most critical physical properties controlling magma transport dynamics and eruptive style. Although viscosity measurements are widely used to study and model the flow behavior of magmas, recent research has revealed that nanocrystallization of Fe–Ti-oxides can compromise the reliability of viscosity data. This phenomenon can occur during laboratory measurements around the glass transition temperature ( T g ) and lead to the depletion of iron and titanium in the residual melt phase, with a significant increase in viscosity. Accurate viscosity measurements play a crucial role in determining the reliability of empirical models for magma viscosity, which are used to evaluate eruptive scenarios in hazardous areas. Here, we quantify the reliability of empirical models by elaborating a new viscosity model of Stromboli basalt that relies exclusively on viscosity data obtained from nanocrystal-free samples. We show that empirical models so far used to estimate melt viscosity at eruptive conditions overestimate Stromboli viscosity by a factor ranging between 2 and 5. In the context of numerical modelling of magmatic processes at Stromboli volcano, we analyse and interpret this finding. Based on our findings, we draw the conclusion that Stromboli basalt is anticipated to ascend from the storage area to the vent at a faster rate than previously hypothesized.

Supersaturation of H2O during magma ascent leads to degassing of melt by formation and growth of vesicles that may power explosive volcanic eruptions. Here, we present experiments to study the effect of initially dissolved H2O concentration (cH2Oini) on vesicle formation, growth, and coalescence in phonolitic melt. Vesuvius phonolitic melts with cH2Oini ranging between 3.3 and 6.3 wt% were decompressed at rates of 1.7 and 0.17 MPa·s−1 and at temperatures ≥ 1323 K. Decompression started from 270 and 200 MPa to final pressures of 150–20 MPa, where samples were quenched isobarically. Optical microscopy and Raman spectroscopic measurements confirm that the glasses obtained were free of microcrystals and Fe-oxide nanolites, implying that the experiments were superliquidus and phase separation of the hydrous melt was homogeneous. A minimum number of the initially formed vesicles, defined by the number density normalized to vesicle-free glass volume (VND), is observed at ~ 5 wt% cH2Oini with a logVND of ~ 5 (in mm−3). The logVND increases strongly towards lower and higher cH2Oini by one order of magnitude. Furthermore, an important transition in evolution of vesiculation occurs at ~ 5.6 wt% cH2Oini. At lower cH2Oini, the initial VND is preserved during further decompression up to melt porosities of 30–50%. At higher cH2Oini, the initial vesicle population is erased at low melt porosities of 15–21% during further decompression. This observation is attributed to vesicle coalescence favored by low melt viscosity. In conclusion, cH2Oini determines the VND of initial phase separation and the evolution of vesiculation during decompression that controls the style of volcanic eruptions.

Crystallization of groundmass minerals may record the physicochemical conditions of magmatic processes upon eruption and is thus a topic of interdisciplinary research in the disciplines of mineralogy, petrology, and volcanology. Recent studies have reported that the groundmass crystals of some volcanic rocks exhibit a break in their crystal size distribution (CSD) slopes that range from a few micrometers to hundreds of nanometers. The crystals consisting of the finer parts of the break were defined as nanolites. In this study, we report the presence of nanometer-scale crystals down to 1 nm in the pyroclasts of the 2011 eruption of Shinmoedake, the Kirishima volcano group, based on field emission-scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). We discovered a gap (hiatus) from ∼100 to ∼30 nm in the size distribution of pyroxene in a dense juvenile fragment of a vulcanian explosion. The pyroxene crystals ∼20-30 nm on a diameter were ferroaugite (C2/c), while those a few hundred nanometers in width had a composite structure consisting of the domains of orthopyroxene (Pbca), augite (C2/c), and sub-calcic augite (C2/c). In high-angle annular dark-field scanning TEM images of the same sample, bright spots ∼1-2 nm in diameter were recognized with a gap in size from ∼10-20 nm titanomagnetite (Fd,3m). They are presumed to have Fe-rich compositions, although their phases were too small to be determined. In addition, we found that crystals smaller than a few tens of nanometers for pyroxene and 100 nm for plagioclase did not exist or their number densities were too low for accurate determination. This indicates that there are practical minimum sizes of the crystals. These observations show that nucleation of the nanoscale crystals almost paused (froze) in the late stage of groundmass crystallization, possibly due to a decrease in undercooling, increase in interfacial free energy, and decrease in diffusivity in a dehydrated melt, whereas crystal growth was mostly continuous. In this paper, we introduce the novel term \"ultrananolite,\" to refer to crystals smaller than 30 nm in diameter, and redefine \"nanolite\" simply as those 30 nm to 1 µm in width, complementing the size interval of crystals in volcanic groundmass smaller than microlites (1-30 µm). In the transient nucleation process, the presence of subcritical size clusters is required. The observed ultrananolite-sized particles might partly include subcritical clusters. The difference in the slope of CSDs, presence of gaps in size distribution, and minimum crystal size among the eruption styles of the 2011 Shinmoedake eruption may be interpreted by considering the difference in magma residence time and fragmentation pressure in the shallow conduit, and possibly the rewelding process in the crater.