In situ observation of nanolite growth in volcanic melt: A driving force for explosive eruptions

In Fig. 1, we present STEM images documenting previously unidentified examples of 20- to 50-nm nanolites in the glass from two low-viscosity Plinian eruptions of Mt. Etna 122 BCE (Italy) (15), Tambora 1815 (Indonesia) (25), and a sub-Plinian event that occurred at Colli Albani volcano located in the ultrapotassic Roman Province (Italy) (42). Also represented are experimentally synthesized samples all of which are described in the Supplementary Materials. Raman spectra for the nanolite-bearing samples show the established “nanolite” peak (33) at ~670 cm⁻¹ between the wider bands centered at ~500 and ~950 cm⁻¹, which arise from vibrations related to the amorphous structure of the silicate glass. As these nano-sized particles are clearly crystalline (see Materials and Methods), they can correctly be referred to as nanolites [or the ultrananolites of (31)]. Also included in Fig. 1 are some corresponding Raman spectra for natural and experimental glasses that appear to be free of nanolites, at least at the resolution of the STEM. The factors controlling the sharpness and height of the nanolite signature are complex and discussed in the Supplementary Materials.

Preliminary runs in the wire furnace and other published data (33, 43) suggest that an undercooling ranging between 50° and 200°C could be the decisive factor triggering the formation of iron-bearing nanolites. These observations directed us to target a few critical conditions for the in situ x-ray studies.

The initial in situ analytical technique used to test our question involved high-energy XRD (Fig. 2), collected on beamline I15 at Diamond Light Source (DLS; see Materials and Methods). Figure S1 illustrates the in situ XRD scattering intensity for the pure Mt. Etna basalt melt in air at 1300°C [above the liquidus estimated with rhyolite-MELTS (44), which is 1260°C at NNO + 2] and the glass when quenched to 25° from 1300°C at the maximum possible quench rate of ~10° to 20°C s⁻¹, achieved by cutting power to the furnace. Notably, this glass provided the first sign of the small-angle peak (see fig. S1 and also Fig. 2B), which was not present in the melt. Determining the structure factors at larger angles combined with this sharp very-small angle scattering peak at ~0.3° in fig. S1 is consistent with the coexistence of a melt or glass matrix and a “separated phase” (see Materials and Methods) on the order of nanometers in size (45). The experiment proved that nanoscale phase separation was not present in the melt at 1300°C but formed with a rapid increase of undercooling. This led to a series of experiments at different cooling rates and, thereby, melt undercooling.

Figure 2A illustrates the in situ XRD intensities collected at different temperatures during slow, 1°C min⁻¹ (0.017°C s⁻¹) cooling of the Mt. Etna basalt from 1300°C under oxidizing conditions (i.e., in air). The appearance of clear diffraction peaks in the XRD pattern (e.g., 4.4, 5.1, and 5.4 Å⁻¹) shows the onset of crystallization of the melt between ~1224° and ~1208°C. A structural refinement of these peaks indicates that the unit cell of first precipitating phases is probably magnetite. The extent of crystallization increases substantially at ~1187°C, where many sharper diffraction peaks are observed, which can be ascribed to the expected mineral crystallization sequence as predicted by rhyolite-MELTS (44) and previous experiments (46) (i.e., plagioclase then pyroxene with magnetite appearing dependant on the ƒO₂). During this slow quench (over 1 to 4 hours), we observed no evidence for a peak at 0.3° (0.2 Å⁻¹) that would imply nanoparticle formation. This is significant as it suggests that our observations (Fig. 2A) do not simply record the progression from small, nano-sized crystal nuclei to a crystal size of >100 nm.

As nanoparticles are not formed at slow cooling rates of 0.017°C s⁻¹ (Fig. 2A) but are observed at the faster (~10° to 20°C s⁻¹) quench rate after a drop to 25°C (fig. S1), we explored the effect of faster cooling rates at temperatures more representative of those in a volcanic conduit. For some experiments, we also imposed a more reduced oxygen fugacity by using a pure Ar atmosphere. This ƒO₂ is closer to natural magmatic conditions where silicate melt is stored at depth or during transport to Earth’s surface under conditions that are more reduced.

Figure 2B shows typical XRD spectra taken at the end of a series of relatively fast (~10° to 20°C s⁻¹) single cooling steps from 1475°C down to four different target temperatures above and below the 1180°C liquidus, at 1200°, 1100°, 980°, and 890°C. Also shown for comparison is the pattern for the glass, after rapidly quenching from 1475° to 25°C under the same fO₂ conditions. The diffuse XRD signal measured at 1475°C (red curve in Fig. 2B) is the characteristic of a nanolite-free melt. During the fast (single step) cooling to 1200°C, the small peak appearing at 0.2 Å⁻¹ (* in Fig. 2B) indicates nanoparticle formation at slightly above the expected liquidus. As the target temperature for the single step is reduced to 1100°, 980°, and 890°C, the height of the small-angle peak at 0.2 Å⁻¹ undergoes a systematic and substantial increase (* in Fig. 2B) revealing that nanophase separation is facilitated by larger undercooling (supersaturation) in the melt. The intensity of the small-angle peak in the 890°C undercooling measurement is very similar to that obtained for the glass rapidly quenched to room temperature. This also confirms that nanophase separation is clearly occurring above the onset of the glass transition temperature of the basaltic melt, which has been determined to be ~640°C for a slower cooling rate of 20°C min⁻¹ (47). Although the data in Fig. 2B are focused on the smaller angle region, it is still clear that the Bragg peaks characteristic of Fig. 2A are absent, implying that the microlites do not have time to nucleate and grow in the few seconds (<30 s) covering cooling and acquisition.

To gain better access to the small-angle peak region, hidden by the beamstop in Fig. 2 and fig. S1, SAXS-WAXS measurement was made on beamline I22 at DLS (UK). In fast acquisition mode, these techniques provide real-time analyses, on a time scale of seconds, rapidly probing the transient evolution of these small particles during cooling and crystallization. We show one example where we have determined the size and time scale of nanoparticle formation and growth in a sample rapidly cooled in a single step from well above the liquidus temperature (1600°C) and then held at 950°C while continuously collecting in situ SAXS-WAXS measurements at an extremely fast acquisition rate (every 0.5 s). The initial SAXS-WAXS patterns collected at 1600°C (Fig. 3, A and B) illustrates that the sample was fully molten and free of nanoparticles as demonstrated by the absence of both a SAXS signal and Bragg peaks in the WAXS region (Fig. 3B). During the first 15 s of in situ isothermal measurements at 950°C, there is the clear development of a symmetric band centered at 0.064 Å⁻¹ in the SAXS region (Fig. 3A), while the WAXS spectra (Fig. 3B) are yet to show any clearly resolved Bragg peaks that would indicate sizable, well-formed crystals.

This local increase in Fe─O bonds might explain the low broad peak observed in some Raman spectra (Fig. 1). Modeling of the SAXS pattern (Fig. 3C) suggested that, during the first 20 s of experiment, the melt evolved into a suspension carrying spherical entities ranging between 2- and 15-nm radius. In addition, SAXS patterns collected within the first 10 s are characterized by a rapid increase in intensity at low q (q < 0.22 nm⁻¹, Fig. 3A). SAXS patterns collected during further 10 s show prominent local maximum developed at q ~ 0.25 nm⁻¹ with continued increase in the intensity at lower q < 0.12 nm⁻¹. The rapid increase in the intensity at lower q likely originates from the formation of particle aggregates in the melt described by a dense-packed mass fractal dimension (48). The subsequent SAXS profiles demonstrate continuous growth of particles up to 50 nm before size information is lost as it increases outside the experimental q minimum range (Fig. 3, A and C). After 30 s into the isothermal crystallization, the second population of spherical polydisperse particles emerges in the melt. They measure ~8 nm by the end of the cycle. Simultaneously, we observed (Fig. 3B) the appearance of broad diffraction peaks in the WAXS pattern at 2.485 and 4.255 Å⁻¹ appearing after 15 s of holding the temperature at 950°C. (Fig. 3B). This is also consistent with the lack of Bragg peaks in the first 10 s of cooling in Fig. 2B and the agglomeration seen in the natural Mt. Etna sample in Fig. 1 (E and F). The emerging peaks are identified as magnetite and hematite Bragg reflections (the peak at 3.173 A⁻¹ could not be assigned unambiguously). The emergence of hematite crystallites is the probable source of the second population of particles seen in the SAXS profiles and may represent oxidation at the sample-air interface. Overall, the peak intensity increases with time, whereas the peak width decreases suggesting an increase in the degree of crystallinity in the melt. The modeling of the SAXS pattern together with the observation of crystallinity from the WAXS signal suggested that we were observing the growth of magnetite nanolites with a maximum size of 50 nm, achieved after ~60 s of dwell time at 950°C (Fig. 3). This is consistent with the images for natural samples, including Mt. Etna in Fig. 1E.

To quantify the potential physical effect of such nanolites in magmas, we performed viscosity measurements using 15-nm spherical silica nanoparticles dispersed in silicon oil as a magma analog. Four samples were prepared, characterized by a volume percent (ϕ) of nanoparticles of 0.3, 0.6, 2.4, and 3.7 volume %. This low volume % range is relevant because the amount of iron in the low-viscosity magmas such as Mt. Etna basalt and Colli Albani phonotephrite is ~10 weight % (wt %) (15, 42) and, assuming the complete extraction of iron from the melt to form single crystal magnetite nanolites, the maximum volume percent that could be formed would be ~7 volume %, given the higher density of magnetite (5.2 g cm⁻³) than that of a basalt (~3.0 g cm⁻³). To explore the possibility of non-Newtonian behavior, the nanolite suspensions were measured at steady shear rates in the volcanologically relevant range of 1 to 100 s⁻¹. As illustrated in Fig. 4A, the relative viscosity (η_r = η_suspension/η_oil) of the suspension with the highest volume percent of nanoparticles (still only 3.7 volume %) was ~400×, ~150×, and 15× higher than the viscosity of the silicon oil measured at shear rates of 1.0, 3.5, and 100 s⁻¹, respectively (Fig. 4A). As shown in Fig. 4B and on the basis of the detailed modeling calculations presented in the Supplementary Materials, for a 1.0 s⁻¹ shear rate, ~65 volume % of microparticles (in orange) is required to reach the maximum packing density (ϕ_m) and lock up the suspension. The same effect on viscosity is produced by just ~8 volume % of nanoparticles. Even at the lowest volume percent of particles (0.3 volume %), the viscosity of our nanosuspensions was still found to display non-Newtonian behavior as the viscosity decreased with increasing shear rate $\stackrel{.}{\mathrm{\gamma }}$ from 1 to 100 s⁻¹, whereas a microsuspension with 0.3 volume % would behave as a Newtonian fluid (26). In Fig. 4B, the relative viscosities reported in Fig. 4A have been translated to absolute viscosities of a nanolite-bearing basalt by calculating the viscosity of the liquid phase on a chemical basis (49) at eruptive conditions [<1070°C (41, 46)] and considering the physical effect of nanolites measured at different shear rates (Fig. 4A). The result of the modeling suggests that the magma approaches the fragmentation threshold (41) of ~10⁶ Pa s at 8, 11, and 32 volume % of particles at shear rates of 1.0, 3.5, and 100 s⁻¹, respectively.

Although we have demonstrated that nanolites can form in response to undercooling by a simple drop in temperature, in the case of an ascending magma, undercooling is more likely to be related to degassing and the resultant increase in the liquidus temperature. Related to this, it has been suggested (30) (and references therein) that nanolites play a role in the heterogeneous nucleation of bubbles with important implications for the dynamics of volcanic eruptions and their style. In particular, the formation of nanolites can induce the nucleation of bubbles and cause them to remain coupled to the rising magma. Such distinctive vesicle patterns could be used to fingerprint the influence of syn-eruptive nanolite formation (50), even if the nanolites are transient and subsequently reabsorbed or “ripened” to give microlites. The in situ XRD techniques used in this study are difficult to adapt to a pressurized system that would allow controlled undercooling by degassing. We therefore explored this mechanism by using a hydrous version of the same Mt. Etna melt and combining a DSC-TGA with TEM and Raman spectroscopy analyses.

In experiment 1, we subjected a hydrous glass [H₂O = 1.48 wt % (47)] to a constant heating of 30 K min⁻¹ in a controlled atmosphere (N₂) to avoid the oxidation of the iron dissolved in the melt. During heating, we observed (Fig. 5A) the onset of the glass transition region at 516°C, which indicated the beginning of the transition from glassy to liquid state of our sample. Subsequently, at 590°C, we observed a weak exothermic event that we associated with the incipient formation of nanolites (see experiment 2). Following this exothermic event, a more important new exothermic event appeared at 633°C, which we associate with the growth and possibly agglomeration of nanolites. This event was followed by a vigorous endothermic event and a rapid loss of weight from the sample. The endothermic event marked the melt degassing and basaltic pumice formation (Fig. 5, A and B, and fig. S8, A and B).

The collected Raman spectrum (fig. S8C) is compatible with a glass matrix characterized by the presence of FeO-bearing nanolites. The pumice was subjected to a high-angle annular dark-field (HAADF)–STEM analysis (Fig. 5, C and D), and the results confirmed the presence of agglomerates of FeO-bearing nanolites across the sample having a shape and size (r ~ 40 nm) similar to those of the products erupted during the 122 BCE Plinian eruption of Mt. Etna (Fig. 1E). To the best of our knowledge, this is the first experimental evidence of the formation of a nanolite-bearing basaltic pumice.

In experiment 2, we carried out a second DSC-TGA experiment where the heating of the hydrous sample was stopped at 620°C, which corresponds to a temperature higher than that of the incipient formation of nanolites (590°C in Fig. 5A) but lower than that of their growth/agglomeration. Results showed (Fig. 5, F and G) the presence of more dispersed FeO-bearing nanolites across the sample with a size (r ~ 15 nm) lower than observed in experiment 1. The sample recovered at the end of the experiment was visually unaffected with respect to its initial state (i.e., a dense black glass). The Raman spectrum (fig. S9C) for this black glass is substantially different from that of the pumice, and the low nanolite content reduces the 670 cm⁻¹ peak to resemble a typical nanolite-free basaltic glass (30).

The TEM foils (of approximate dimensions of 20 μm by 12 μm by 0.1 μm) were extracted using a combined SEM/focused ion beam instrument (FEI Helios Nanolab 600i, University of Bristol). A platinum deposit was made at each extraction site for protection before milling and thinning. The foils were subsequently loaded onto holey carbon films by ex situ liftout using an optical microscope and micromanipulator before analysis by TEM. Bright-field and HAADF-STEM images were collected at the David Cockayne Centre for Electron Microscopy (Material Sciences, Oxford University) using a Jeol ARF-200F operating at 200 kV with a cold field emission source and featuring 100-mm² Centurion EDX detector and Gatan GIF Quantum 965 ER featuring dual electron energy-loss spectroscopy and energy-filtered transmission electron microscopy (EFTEM) capabilities.

The analysis of the experimental samples was carried out using scanning TEM (FEI Titan G2 80-200 S/TEM, Bayerisches Geoinstitut, University of Bayreuth) equipped with EDS consisting of four-channel silicon drift dectector (Bruker, QUANTAX EDS) and operated at 200 kV.

Raman spectra were acquired using a Thermo Scientific DXR3xi Raman Imaging Microscope at the University of Bristol, School of Earth Sciences, using a 532-nm (green) doubled Nd:YVO4 DPSS excitation laser and a 900 lines mm⁻¹ grating. Raman spectra were acquired between 200 and 1500 cm⁻¹ with a 100× objective, 25-μm confocal pinhole, and a laser power of 3 mW. The Raman scattering was acquired for 5 s and averaged over 10 scans.

The nanolite peak (see Fig. 1A) represents the vibration of Fe─O bonds, but the position and sharpness are consistent with those bonds being in the FeO oxide mineral (possibly magnetite), suggesting that the nanoparticles are “crystalline” rather than features of the melt structure [e.g., (66)]. This crystallinity has also been confirmed by lattice images observed during HAADF imaging and by selected area electron diffraction during our STEM imaging. For some experimental hydrous Colli Albani samples that appear to have nanolites in the TEM image, the corresponding Raman spectrum characterized by a small broad feature slightly offset from 670 cm⁻¹ rather than a clear sharp peak. This illustrates a point that the nanolites may have not developed any substantial macrocrystalline structure, even if the particles have formed sizeable nanoagglomerates. There will be several stages to their formation with initial meso-range order in the melt before nuclei form and enough molecules gather to be considered a crystal lattice.

The starting glassy material used for our XRD experiments is the composition reported and used in Polacci et al. (70) and obtained by melting volcanic products retrieved from the lower vents of the 2001 Mt. Etna eruption. The natural samples were crushed in a jaw crusher, powdered in a carbide ring mill, and melted in a thin-walled Pt crucible in a box furnace at 1400°C for 4 hours. Afterward, the melt was poured, in air, onto a steel plate and rapidly quenched to glass. This procedure was repeated twice to ensure sample homogeneity. For the experiments, the homogeneous glass was finely powdered using an agate sphere mortar grinder.

High-temperature in situ measurements were made using a microheating cell (69) consisting of a 1-mm-diameter Pt-Ir 10% wire. A flattened region was formed at the midpoint of the wire, within which a ~1-mm bore was made to form the sample chamber. Each experiment has a new wire, which is calibrated so the temperature for a given power setting is known within ±5°C. For details on the temperature calibration procedure, see the study by Neuville et al. (69). To load the sample, a finely ground, powdered sample was loaded in the sample chamber and rapidly heated to 1600°C to obtain a pure, crystal-free melt. The sample was held at high temperature for a minimum of 30 min before running an experiment, to ensure equilibration at the correct oxygen fugacity and the removal of any crystalline features (nano or micro) that might have been present in the starting glass powder.

Movement of the wire due to heating is considerable. For slow cooling experiments, there is time to realign the synchrotron beam for each temperature. For fast cooling and acquisition, the position is precalibrated before the loaded sample comes into alignment at the temperature required for measurements. Only an approximate maximum cooling rate can be reported of ~16°C s⁻¹. It depends on the starting point and range of temperatures and is unlikely to be strictly constant. However, the time taken for the wire hole to align and let the beam through confirms this cooling rate. Presaturation of the Pt-Ir wire with Fe-bearing melt (remove with HF) at appropriate conditions ensures minimal Fe-loss (or gain) during an experiment. For controlled-atmosphere experiments, the wire heater is enclosed by a chamber with Kapton windows to allow unimpeded access to the sample for the x-ray beam. Reduced conditions were achieved by passing pure Ar gas through the cell with a flow rate of ~1 liter min⁻¹.

Angle-dispersive XRD measurements were made for the high-temperature basalt melt at beamline I15 at the DLS, UK, at an energy of at 72.0 keV with an incident x-ray beam of wavelength λ = 0.1722 Å and collimated using a 70-μm W-pinhole. 2D XRD patterns of the high-temperature melts were measured using a PerkinElmer image plate detector. The in situ slow cooling rate measurements were performed by reducing the temperature by 5°C every 5 min. The XRD diffraction intensity, I(2θ), was collected as a function of scattering angle 2θ for 60 s at each time step. The in situ fast cooling rate measurements were performed using 1-s acquisitions. At the end of each run, the sample was remelted at 1600°C, and the pre- and postexperiment melt diffraction patterns were compared to ensure sample composition stability.

The data were calibrated using the diffraction pattern obtained for a LaB₆ standard and reduced to 1D profiles accounting for geometrical effects and incident beam polarization using the Data Analysis WorkbeNch (DAWN) software suite. Structural refinement of the crystalline Bragg diffraction peaks was carried out using the Le Bail method in the program General Structure Analysis System (GSAS) (71). The liquid and glass total structure factors S(q) were obtained by normalization to the q-dependent self-scattering and Compton scattering components, as described by Drewitt et al. (72), where the scattering vector $q=\frac{4\mathrm{\pi }}{\mathrm{\lambda }}\text{sin}\mathrm{\theta }$. The corresponding G(r) functions were obtained from the Fourier transform relation $(r)=\frac{1}{2{\mathrm{\pi }}^{2}r\mathrm{\rho }}{\int }_0^{q_{\text{max}}}q[S(Q)-1]\text{sin}(\mathrm{qr})\mathrm{d}q$, where r is a distance in real space, ρ is the atomic number density, and q_max is the high-q truncation limit. Reproducibility of results was checked by repeating the fast cooling rate experiments two to three times, observing the same formation and/or resorption of nanolites in all cases.

The crystal nucleation process in the melt was studied in situ by a synchrotron-based time-resolved simultaneous SAXS/WAXS technique at beamline I22 at the DLS, UK. High-temperature experiments were made using the same microheating cell (69) and methodology described in the XRD paragraph.

A monochromatic x-ray beam at 12.4 keV was used to obtain 2D scattered intensities, which were collected with Dectris Pilatus P3-2M large-area pixel array detectors at SAXS and WAXS geometries. The x-ray beam was focused down to 0.180 mm in the vertical and 0.480 mm in the horizontal dimensions. Data were acquired as series of 200-ms frames with a 500-ms dead time in between frames. The series were spaced in time by 1 s. Transmission was measured by a photodiode installed into the beamstop of the SAXS detector. A sample-to-SAXS detector distance was calibrated against a 100-nm periodicity grating, and a sample-to-WAXS detector distance was calibrated with a National Institute of Standards and Technology (NIST) silicon powder standard. The setup allowed for a usable q range of 0.03 < q < 31 nm⁻¹ provided by SAXS and WAXS ranges overlap. The scattered intensity was calibrated to absolute units against a NIST glassy carbon standard. Different backgrounds were acquired from the empty wire heated to the temperature of interest before each experiment.

The initial SAXS data processing and reduction including masking, normalizations and correction for transmission, background subtraction, and data integration of the collected 2D data to 1D were performed with the DAWN software (73). Model fitting and validation were performed with use of the Small Angle Scattering Analysis Software Package SasView (74). An isometric (octahedral) crystal shape characterizes magnetite, and therefore, for modeling SAXS data, we approximated the shape of magnetite nanolites as spheres. SAXS patterns were modeled assuming a unimodal distribution of nanolites with the 1D scattering intensity calculated in the following way (74) where scale is a volume fraction, V is the volume of the scatterer, r is the radius of the sphere, background is the background level, and ∆ρ is the difference of the scattering length densities of the scatterer and the solvent (∆ρ = 40.7 × 10⁻⁶ was used for magnetite nanolites and ∆ρ = 25.6 × 10⁻⁶ for the melt).

To characterize better the rheological effects of nanoparticles in magmas, we extended the range of previous investigations on analog microsuspensions (26, 64) to a smaller particle size range by using SiO₂ nanoparticles in silicon oil. This Newtonian fluid is commonly used to characterize the effect of microlites in magmas in terms of changes to the base fluid viscosity, volume percent, size, and shape of microparticles. Previous experiments have demonstrated how the increasing crystal content of a magma eventually causes the magma to “lock” as the viscosity increases exponentially to the fragmentation threshold. For microparticles, the critical volume percent ranges from 35 to 70 volume % depending on the crystal shape (26). This is referred to as the maximum packing density. As the volume fraction approaches the maximum packing fraction, the magma behaves as an increasingly non-Newtonian suspension, eventually fragmenting under applied deformation rates (11). For silica-rich magmas, the pure melt itself may begin with a viscosity of 10⁶ to 10⁷ Pa s, and so, fragmentation occurs (11, 13) even before crystal growth. However, for low-silica, basaltic magma, the melt viscosity is more often closer to 10¹ to 10³ Pa s, and so, a large volume percent of crystals would be required to elevate the bulk viscosity by four to six orders of magnitude (or an unreasonably high strain rate of 10,000 s⁻¹) to reach explosive conditions.

The magma melt phase was represented by silicon oil, a Newtonian fluid with a viscosity of 0.136 Pa s (η_oil) at 20°C. This is lower than the viscosity of pure basaltic melt at eruptive conditions (~200 Pa s) but allows a wider range of the volcanologically relevant parameter space to be investigated within the technical abilities of the rheometer. One advantage of using these analog mixtures is that the volume percent of particles can be very accurately controlled (at a constant temperature), and thereby, the effect of such particles on the viscosity can be accurately parameterized. As discussed in the section, Viscosity of magma analogs with nanoparticles, assuming the complete extraction of iron from the melt to form single crystal FeO nanolites, the maximum volume percent that could be formed would be ~7 volume %. We thus prepared nanoparticle-bearing suspensions containing ~few volume % of nanolites, as expected in nature. The shear rates used are those relevant to volcanological flows. The silicon oil was mixed with fumed SiO₂ spherical nanoparticles with a particle average size of ~14 nm to obtain a suspension with a known volume percent of nanoparticles. The initial suspension was shaken and mixed horizontally overnight using an electronic roller mixer. Afterward, we centrifuged the mixture (silicon oil + nanoparticles) between 5 and 30 min at 4000 to 5000 rpm. This allowed the homogenization of the suspension and the removal of bubbles potentially entrapped during the preparation of the mixture. The suspensions were then subjected to ultrasonification ultrasound treatment to both maximize the uniform dispersion of nanoparticles and minimize particle aggregation in the base fluid. All suspensions displayed homogeneous colors, suggesting that samples were homogenized successfully after centrifuge and ultrasonification. However, to ensure complete homogenization, we gently stirred each sample before loading the suspension into the crucible for viscosity measurement. We performed the measurement of the density of the standard oil and suspensions at 25°C to retrieve the actual density of SiO₂ nanoparticles and reveal any potential anomaly in the measure of the sample density due to the entrapment of air in the suspension. The calculated density of the fumed SiO₂ particles was 1.9 ± 1 g cm⁻³ that agrees with the density of standard fumed SiO₂. The density measurements were repeated weekly for 1 month, and no change in density with time was observed.

We used a MARS III (Modular Advanced Rheometer System) device equipped with a concentric cylinder geometry to measure the viscosity of suspensions. We loaded ~16 ml of sample into the crucible before each measurement. The viscosity was measured at 20°C according to the two-stage protocol. We applied a preshear treatment to the sample that involved shearing the suspension from 0 to 100 s⁻¹ and then back again. This ensured removal of potential transient effects on the rheology of our sample due to particle reorientation and organization. Afterward, we performed viscosity measurement that consisted of three ramps where the shear rate was increased from 0 to 100 s⁻¹ (upward ramp), kept constant at 100 s⁻¹ for a minimum of 30 s (constant ramp), and then decreased from 100 to 0 s⁻¹ (downward ramp). We stepped the shear rate value by ~3.3 s⁻¹ through the upward and downward ramps to obtain 30 viscosity measurements during each segment. During each measurement, the stress was recorded until it reached a constant value. The viscosity measurements of our suspensions were repeated 2 weeks later, and no change in viscosity with time was observed.

The degassing and pumice formation experiments were performed using a simultaneous thermogravimetric analyzer-differential scanning calorimetry (TGA-DSC 3+, Mettler-Toledo, Switzerland) equipped with a water cooling device. The measurements were carried out under N₂ atmosphere (60 ml min⁻¹ flow rate) using a PtRh20 crucible.