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The molar abundance ratio of H2 Om /OH of rhyolitic porous pyroclasts, obsidian pyroclasts, and flow obsidians from the 1060CE Glass Mountain eruption at Medicine Lake Volcano (USA) were obtained by Diffuse Reflectance Infrared Fourier Transform spectroscopy. Samples were also analyzed for their total water content, [H2 Ot ], and isotopic composition, 𝜔D, via Temperature Conversion Elemental Analysis (TC/EA). Porous clasts exhibit a H2 Om /OH ratio of 0–4.63 (and a [H2 Ot ] measured by Giachetti et al. (2020) of 0.34–1.2 wt%) that is positively correlated with their porosity and [H2 Ot ], contrary to the low [H2 Ot ] and low H2 Om /OH ratios of both obsidian pyroclasts (0.32–0.72 wt.%, 0.14–0.63) and flow obsidians (0.04–0.07 wt.%, 0.24–0.28). These results confirm the interpretations of Giachetti et al. (2020) that porous clasts were rehydrated for about 1000 years after the eruption via diffusion of overwhelmingly molecular, meteoric water in the matrix-glass, whereas obsidian pyroclasts and flow obsidians are essentially dense and thus rehydrate poorly. Analyses were also conducted on two size fractions of flow obsidian (∱ 63 εm and 63–250 εm) that were ground either (1) immediately before analysis or, (2) about nine years prior to analysis and kept in closed containers ever since. Results show that the ∱ 63 εm ground samples gained 0.06–0.24 wt% of meteoric water in just nine years, with a H2 Om /OH ratio increasing from 0.32–0.54 when measured immediately after crushing, to 3.35–4.64 when measured nine years later. For these smaller particles, even 24 h of heating at 130 ⋛ C under vacuum before analysis is insufficient to remove all the water gained by rehydration. We thus recommend the use of coarser powders (∲ 63 εm) and longer pre-analysis heating time under vacuum (∲2 days) for more reliable [H2 Ot ], H2 Om /OH, and 𝜔D measurements on obsidian samples. Given the thinness of the glass in between vesicles in porous pyroclasts and thus their ability to quickly rehydrate, total water content obtained via analysis of the bulk material (e.g., by TC/EA, Loss on Ignition, Karl Fischer titration) must be interpreted together with 𝜔D and/or H2 Om /OH data to evaluate the extent of rehydration, even for relatively young samples. 

Abstract Determining whether fresh magma has reached the surface during a volcanic eruption can provide important information for forecasts of future activity, especially in the early stages of an eruption. However, identifying fresh, juvenile pyroclasts in tephra fall deposits can be challenging and inconclusive. We studied the products of explosions at Poás Volcano, Costa Rica, in 2016–2019, a period during which the volcano transitioned from a pressurized, hydrothermally sealed state to an open conduit with increased degassing to the atmosphere. The activity consisted of semi-continuous explosions producing < 500-m-high plumes, with the exception of explosions on April 14 and 22, 2017, that produced 4-km-high plumes. We analyzed the grain size distribution, componentry, and particle density of the products of twenty explosions, and collected groundmass glass composition on juvenile particles for three of them. Our work demonstrates varying degrees of magma involvement with the hydrothermal system through time, with juvenile material representing a wide range of abundance (~ 10–70 vol.%) in deposits of individual explosions. Before early April 2017, we infer that small phreatomagmatic explosions were triggered by contact between magmatic fluids and/or magma and the hydrothermal system, based on the presence of abundant hydrothermal fragments and minor juvenile magma. Concurrent with decreasing hydrothermal component in the deposits, explosions in April–May 2017 eroded the walls of the shallow plumbing system, evidenced by an increase in wall-rock lithics in the deposits. These changes coincided with drying of the crater lake, leading to several magmatic explosions in April-September 2017, whose juvenile-rich deposits are consistent with primary fragmentation of fresh magma in the conduit. The eruptive activity changed after this magmatic phase, and in 2019, small explosions mostly recycled the heterogeneous deposits of previous events, producing fine material with high proportions of recycled particles. All explosions from 2016 to 2019 remobilized already-emplaced magma from the 1953–1955 eruptive period, although distinct glass compositions between explosions suggest difference in crystallization or they tapped different portions of this magma body. Our work sheds light on the eruption dynamics and shallow plumbing system of this persistently active volcano and provides a case study for understanding the variable efficiency of phreatomagmatic fragmentation. 

Abstract Nanometer‐scale titanomagnetite crystals have been detected in nominally aphyric rhyolite pumice, but whether they are numerous enough to impact bubble nucleation in explosive silicic volcanism was unresolved. This study examines sub‐micron crystals using rock magnetic techniques, Rhyolite‐MELTS modeling, and physical characterization. We analyzed pumice from four eruptions spanning wide ranges in intensity, storage depth, and bubble number density (10¹⁶to 10¹³ m⁻³liquid): 1060 CE Glass Mountain, 1912 CE Novarupta, 232 CE Taupo, and 0.45 Ma Pudahuel. Calculations assuming monospecific assemblages of 10 and 1,000 nm cubic particles yield titanomagnetite number densities of 10²¹to 10¹³m⁻³dense rock equivalent, respectively. In all cases, titanomagnetite is thermodynamically stable at pre‐eruptive storage conditions and magnetic susceptibility (χ_LF) is independent of vesicularity and permeability, indicating that crystals likely formed prior to vesiculation. The existence of nm‐scale Fe‐Ti oxides in four diverse cases suggests that heterogeneous bubble nucleation is a general feature of explosive rhyolite volcanism. 

This dataset archived with the Earthref Magnetics Information Consortium contains low-temperature remanent magnetization data generated at the Institute for Rock Magnetism, University of Minnesota. This dataset accompanies the publication McCartney, K., Hammer, J.E., Shea, T., Brachfeld, S., Giachetti, T., 2024. Investigating the role of nanoscale titanomagnetite in bubble nucleation via novel applications of magnetic analyses (Dataset), Magnetics Information Consortium (MagIC), doi:10.7288/V4/MAGIC/20019. 

This dataset archived with the Magnetics Information Consortium contains rock-magnetic data for rhyolitic pumice and obsidian from Glass Mountain, Medicine Lake, California, USA. Data were generated at Montclair State University and include magnetic susceptibility measured at 976Hz and 3904Hz, magnetic susceptibility vs. temperature, anhysteretic remanent magnetization (ARM), and magnetic hysteresis measurements. This dataset accompanies the publication Brachfeld, S., McCartney, K., Hammer, J.E., Shea, T., Giachetti, T., Evaluating the role of titanomagnetite in bubble nucleation: Rock magnetic detection and characterization of nanolites and ultra-nanolites in rhyolite pumice and obsidian from Glass Mountain, California, Geochemistry Geophysics Geosystems, https://doi.org/10.1029/2023GC011336. 

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 10^18 to 10^20 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'>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 × 10^14 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. 

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 10^12 to 10^20 m^−3 of solid material, respectively. TND estimates derived using a power law distribution of grain sizes predict 10^18 to 10^19  m^−3. These ranges agree well with TND determinations of 10^18 to 10^20  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. 

Abstract Pyroclastic density currents (PDCs) are the most lethal volcanic process on Earth. Forecasting their inundation area is essential to mitigate their risk, but existing models are limited by our poor understanding of their dynamics. Here, we explore the role of evolving grain-size distribution in controlling the runout of the most common PDCs, known as block-and-ash flows (BAFs). Through a combination of theory, analysis of deposits and experiments of natural mixtures, we show that rapid changes of the grain-size distribution transported in BAFs result in the reduction of pore volume (compaction) within the first kilometres of their runout. We then use a multiphase flow model to show how the compressibility of granular mixtures leads to fragmentation-induced fluidisation (FIF) and excess pore-fluid pressure in BAFs. This process dominates the first ~2 km of their runout, where the effective friction coefficient is progressively reduced. Beyond that distance, transport is modulated by diffusion of the excess pore pressure. Fragmentation-induced fluidisation provides a physical basis to explain the decades-long use of low effective friction coefficients used in depth-averaged simulations required to match observed flow inundation.