Microlite Number Density Research Articles

Andesite magma genesis, conduit dynamics and variable decompression from shallow reservoirs drive contrasting PDC events at Volcán de Colima, Mexico

Volcán de Colima, one of the Earth's most active and hazardous andesitic stratovolcanoes, experienced its last major explosive eruption on July 2015 with two distinct events. The July-10 event comprised the collapse of a large summit dome, forming block-and-ash flow deposits of dense dome lava clasts. The July-11 event produced pyroclastic density currents (PDCs) that flowed to maximum 10.5 km-distance from the crater, burned the vegetation, and formed valley and overbank deposits of up to 30 vol% vesicular scoria clasts. Whereas the July-10 event fits well within the last 20 years' typical activity of Volcán de Colima, the July-11 PDCs were unprecedented. We present a close examination of the July-11 deposits via combined field, microtextural, and chemical analyses, including electron microscopy and X-ray microtomography. Results show that andesite magma (58–59 wt% SiO2) at 1021 °C and having 2 wt% H2O degassed and crystallized (102–105 mm−3 vesicles and 102–103 mm−3 microlite number densities) while ascending to the surface from ∼2 km-depth reservoirs, driven by decompression rates of 0.4–1.7 MPa s−1. These variable rates reflected heterogenous andesite magma rheology produced by different stages of melt genesis, mobility, stalling, crystallization and vesiculation. Prior to experiencing fragmentation, the andesite magma mingled with rhyolite melts produced from partial melting of silicic mush stored at depths from ∼2 to 5 km. Magmas fragmented at maximum strain rates of 10−3 s−1, powering the July-11 energetic (106–107 kg s−1) and pulsating PDCs that released 102–103 m3 s−1 on surface. The rapid <20 h transition from the July-10 to the July-11 events suggests only hours timescales from dome collapse to magma decompression and explosive eruption.

Construction of obsidian during explosive-effusive eruptions: insights from microlite crystals in obsidian pyroclasts

Obsidian pyroclasts are common in deposits from silicic sub-Plinian eruptions and can record pre- and syn-eruptive processes in the volcanic conduit. Previous work focusing on dissolved volatiles and vesicle textures has been useful in extracting timescales of sintering, diffusion, and vesicle relaxation recorded by obsidian pyroclasts. Here we focus on microlite crystals (&amp;lt;100 µm in size) to augment previous work because they form at different rates than vesicles or than rates of volatile degassing. Hence, they have the potential to disclose additional information about processes occurring in an explosive conduit. We examine microlites in 72 samples from tephra deposits of the 1340 C.E. North Mono eruption, California, U.S.A., and complement these measurements with hydrothermal experiments at 800°C, 10–50 MPa, and durations from 1 to 7 h. Three observations of the natural obsidians further elucidate their formation. First, microlite number densities (MND) increased as the eruption progressed. Second, multiple microlite morphologies occur for feldspars (blocky, swallowtail, tabular, cluster, skeletal) and pyroxenes (individual rods or clusters of acicular crystals) in each obsidian, regardless of any other characteristic. Third, microlite orientations correlate with the dominant morphology of vesicles, being generally well aligned in samples with ellipsoid vesicles, generally poorly aligned in samples with spherical vesicles, and either unaligned or aligned into planes in samples with distorted vesicles. In hydrothermal experiments, MND increase with time, microlites display only one morphology, and microlites are randomly oriented at any given pressure or temperature. When compared to natural obsidians, our experiments suggest most of the microlites could have grown in ≤∼7 h. The variety of microlite morphologies and orientations argue for repeated in-conduit fragmentation and sintering, consistent with the idea that each individual obsidian pyroclast is the product of ash sintering at multiple depths in the conduit prior to finally being erupted. During most of the eruption, obsidian pyroclasts were extracted from many depths in the conduit, preserving an array of volatile contents and microlite textures. Near the end of the explosive phase, however, higher MND record longer periods of stalling while dissolved volatile contents record vapor-melt equilibration at shallow depths in the conduit.

Dynamics of the Young Merapi (

We studied nine pumice fall deposits of the Young Merapi stage (<2.2 ka – 1,788 CE) observed in the western and southern flanks of Merapi volcano. All deposits include a wide variation of lithics (10–42% Clithic), with thicker deposits (i.e., more voluminous eruption) being more lithic-rich and vice versa. Two different magma types (hereafter referred to as type I and type II) were identified based on petrography, bulk-rock, glass, and feldspar microlite compositions. Type I magma has abundant amphibole and pyroxene, is rich in calcium (>9 wt% CaObulk), poor in both silica (50.8–53.7 wt% SiO2bulkand 62.3–66.6 wt% SiO2glass) and strontium (<580 ppm, bulk-rock), and has more calcic feldspar microlites (An38–79). Type II magma also contains abundant amphibole, but has less pyroxene and is poorer in calcium (<9 wt% CaObulk), higher in both silica (53.2–54.5 wt% SiO2bulk and 63.3–70.8 wt% SiO2glass) and strontium (>580 ppm), with less calcic feldspar microlites (An31–77). These two magma types alternately fed the explosive eruptions during the Young Merapi stage; however, their juvenile products are distinctive in terms of syn-eruptive microtextures (i.e., matrix-vesicles and microlites). Pumices from type II magma have a higher matrix-vesicle number density (MVND) and microlite number density (MND) values than those of pumices from type I magma (1.0–6.5 × 1015 and 1.8–7.4 × 1015 m−3, and 0.6–2.3 × 1015 and 0.7–1.8 × 1015 m−3, respectively). A positive correlation between MVND with SiO2 and MND suggests that a colder (i.e., less calcic feldspar microlites indicate lower temperature and vice versa) and more evolved (higher SiO2) magma facilitates more extensive matrix-bubble nucleation and deeper microlite crystallization than hotter magmas, allowing type II magma to erupt more explosively than type I magma.

Application of time–temperature–transformation and time–pressure–transformation diagrams for cooling- and decompression-induced crystallization to estimate the critical cooling rate required to form glassy obsidian

The formation of glass from silicic magmas during eruption is an enigmatic process. To understand the formation of obsidian, we used time–temperature–transformation (TTT) and time–pressure–transformation (TPT) diagrams to estimate the critical cooling and decompression rates needed to form glassy obsidian. The TTT diagram is a contour map of the crystallized volume fraction as a function of the crystallization temperature and time. A contour line for a given crystal fraction in a time versus temperature diagram has a cone shape, with the “nose” of the curve corresponding to the minimum time and corresponding temperature required to generate a specific crystal fraction. We also present a TPT diagram, based on the TTT diagram, and liquidus curve for decompression-induced crystallization. We applied the TTT diagram to the Tokachi–Ishizawa obsidian from Shirataki, Hokkaido, Japan, and used it to estimate the critical cooling rate that would have been required to form its glassy texture by cooling-induced crystallization during eruption. For decompression-induced crystallization, we used the TPT diagram to estimate the critical decompression rate needed to form glassy obsidian. Based on our calculation results, the critical cooling and decompression rates are highly dependent on the interfacial energy between crystals and melt. We compared the estimated critical cooling and decompression rates with those estimated from microlite number density and a one-dimensional solution of the heat conduction equation. Our calculation results reveal that glassy obsidian can be formed during an eruption, particularly under conditions of high interfacial energy.

Decompression and degassing, repressurization, and regassing during cyclic eruptions at Guagua Pichincha volcano, Ecuador, 1999–2001

In 1999–2001, Guagua Pichincha volcano, Ecuador, produced a series of cyclic explosive and effusive eruptions. Rock samples, including dense blocks and pumiceous clasts collected during the eruption sequence, and ballistic bombs later collected from the crater floor, provide information about magma storage, ascent, decompression, degassing, repressurization, and regassing prior to eruption. Pairs of Fe-Ti oxides indicate equilibrium within 1.2–1.5 log units above the NNO oxidation buffer and equilibrium temperatures from 805 to 905 °C. Melt inclusions record H2O contents of 2.7–4.6 wt% and CO2 contents (uncorrected for CO2 segregation into bubbles) from 19 to 310 ppm. Minimum melt inclusion saturation pressures fall between 69 and 168 MPa, or equilibration depths of 2.8 and 6.8 km, the lower end of which is coincident with the maximum inferred equilibration depths for the most vesicular breadcrust bombs sampled. Amphibole phenocrysts lack breakdown rims (except for one sample) and plagioclase phenocrysts have abundant oscillatory compositional zones. Plagioclase areal microlite number densities (Na) range over less than one order of magnitude (8.9×103–8.7×104 mm-2) among all samples, with the exception of a dense, low crystallinity sample (Na = 3.0×103 mm−2) and a pumiceous sample erupted on 17 December 1999 (Na = 1.7×103 mm−2). Plagioclase microlite shapes include tabular, hopper, and swallowtail forms. Taken together, the relatively high plagioclase microlite number densities, the high number of oscillatory zones in plagioclase phenocrysts, the presence of CO2 in groundmass glass, seismicity, and time-varying tilt cycles provide a picture of sudden evacuation of magma residing at different levels in the shallow conduit. Explosive eruptions punctuate inter-eruptive repose periods marked by time-varying rates of degassing (volatile fluxing) and re-pressurization. Shallow residence time in the conduit was sufficient to allow precipitation of silica-phase in the groundmass, but insufficient to allow breakdown of hornblende phenocrysts, with the one exception of the final dome sample from 2000, which has the longest preceding repose time. These results support a model of cyclic pressure cycling, volatile exsolution and regassing, and magma decompression decoupled from ascent.

Eruptive dynamics and fragmentation mechanisms during cyclic Vulcanian activity at Sakurajima volcano (Japan): Insights from ash texture analysis

Quantitative morpho-textural analysis of volcanic ash is one of the most effective tools to characterize the style of ash-dominated volcanic activity, and to investigate the complex interplay between conduit processes and the associated eruptive dynamics. In this framework, many questions still remain unanswered about the role of conduit processes, particularly of the magma fragmentation processes, in controlling the eruptive dynamics of the unsteady and highly-transient Vulcanian explosions. For this reason, we analyzed ash samples collected during a five days-long eruptive sequence at Sakurajima volcano (Japan), to derive information about ash morphometry and textural features over the entire sequence. During the observed sequence, eruptive activity showed high unsteadiness in the modalities of ash emission, which included all the main different eruptive styles typical of the recent period of Sakurajima activity. Three main intra-eruptive phases (Phase1, Phase2 and Phase3) were recognized based on visual observations and thermal data. Quantitative information about external ash morphometry (i.e., shape) and internal textures were measured for the particles associated with the different phases and discussed in terms of the observed eruptive variability. We quantified crystallinity and vesicularity of ash grains, the crystal size distribution (CSD) of microlites and the microlite number density (MND) of the groundmass for a representative set of ash particles. We discussed the links between the eruptive dynamics and the dominant processes of magma fragmentation, as shown by the combination of the morpho-textural features of ash throughout the whole eruptive sequence and the observed variations of the eruptive phases. All the evidence presented in this work confirm the constant presence at Sakurajima of a highly stratified and degassed magma within the conduit, suggesting that the transient dynamics of the eruption were strongly controlled by variations in the process of magma fragmentation driving the eruptions during the different phases. In particular, the morpho-textural characteristics of ash suggest that Vulcanian eruptions at Sakurajima can be controlled by the progressive pressurization of the upper portion of the magma conduit (between 10 and 50 m in depth). Moreover, using crystal textures, we inferred that the time needed for conduit refilling during intra-eruptive stages is comprised between 1 and 10 month. The resulting information on the eruptive dynamics of Sakurajima is of primary importance for a more exhaustive comprehension of the low-to-mid intensity, ash-dominated explosive activities.

Syn-Eruptive Processes During the January–February 2019 Ash-Rich Emissions Cycle at Mt. Etna (Italy): Implications for Petrological Monitoring of Volcanic Ash

Low-intensity emission of volcanic ash represents the most frequent eruptive activity worldwide, spanning the whole range of magma compositions, from basalts to rhyolites. The associated ash component is typically characterized by heterogeneous texture and chemical composition, leading to misinterpretation of the role of syn-eruptive processes, such as cooling and degassing during magma ascent or even magma fragmentation. Despite their low intensity, the ash emission eruptions can be continuous for enough time to create problems to health and life networks of the communities all around the volcano. The lack of geophysical and/or geochemical precursor signals makes the petrological monitoring of the emitted ash the only instrument we have to understand the leading mechanisms and their evolution. Formation of low-level plumes related to ash-rich emissions has increasingly become a common eruptive scenario at Mt. Etna (Italy). In January–February 2019, an eruptive cycle of ash-rich emissions started. The onset of this activity was preceded on 24 December 2018 by a powerful Strombolian-like eruption from a fissure opened at the base of the New Southeast Crater. A lava flow from the same fissure and an ash-rich plume, 8–9 km high a.s.l., from the crater Bocca Nuova occurred concurrently. After about 4 weeks of intra-crater strombolian-like activity and strong vent degassing at summit craters, starting from 23 January 2019, at least four episodes of ash-rich emissions were recorded, mainly issued from the Northeast Crater. The episodes were spaced in time every 4–13 days, each lasting about 3–4 days, with the most intense phases of few hours. They formed weak plumes, up to 1 km high above the crater, that were rapidly dispersed toward different directions by dominant winds and recorded up to a distance of 30 km from the vent. By combining observations on the deposits with data on textural and chemical features of the ash components, we were able to discriminate between clasts originated from different crater sources and suggest an interpretive model for syn-eruptive processes and their evolution. Data indicate the occurrence of scarce (&amp;lt;10 vol.%) fresh juvenile material, including at least four groups of clasts with marked differences in microlite content and number density, and matrix glasses and minerals composition. Moreover, a large amount of non-juvenile clasts has been recognized, particularly abundant at the beginning of each episode. We propose that the low amount of juvenile ash results from episodic fast ascent of small magma batches from shallow reservoirs, traveling within a slow rising magma column subjected to cooling, degassing, and crystallization. The large number of non-juvenile clasts deriving from the thick crater infill of variably sealed or thermally altered material at the top of the magma column is suggested to contribute to the ash generation. The presence of a massive, granular crater infilling accumulating in the vent area may contribute to buffer the different geophysical signals associated with the active magma fragmentation process during the low-energy ash eruptions, as already evidenced at other volcanoes.

Mafic explosive volcanism at Llaima Volcano: 3D x-ray microtomography reconstruction of pyroclasts to constrain shallow conduit processes

Mafic volcanic activity is dominated by effusive to mildly explosive eruptions. Plinian and ignimbrite-forming mafic eruptions, while rare, are also possible; however, the conditions that promote such explosivity are still being explored. Eruption style is determined by the ability of gas to escape as magma ascends, which tends to be easier in low-viscosity, mafic magmas. If magma permeability is sufficiently high to reduce bubble overpressure during ascent, volatiles may escape from the magma, inhibiting violent explosive activity. In contrast, if the permeability is sufficiently low to retain the gas phase within the magma during ascent, bubble overpressure may drive magma fragmentation. Rapid ascent may induce disequilibrium crystallization, increasing viscosity and affecting the bubble network with consequences for permeability, and hence, explosivity. To explore the conditions that promote strongly explosive mafic volcanism, we combine microlite textural analyses with synchrotron x-ray computed microtomography of 10 pyroclasts from the 12.6 ka mafic Curacautín Ignimbrite (Llaima Volcano, Chile). We quantify microlite crystal size distributions (CSD), microlite number densities, porosity, bubble interconnectivity, bubble number density, and geometrical properties of the porous media to investigate the role of magma degassing processes at mafic explosive eruptions. We use an analytical technique to estimate permeability and tortuosity by combing the Kozeny-Carman relationship, tortuosity factor, and pyroclast vesicle textures. The groundmass of our samples is composed of up to 44% plagioclase microlites, > 85% of which are < 10 µm in length. In addition, we identify two populations of vesicles in our samples: (1) a convoluted interconnected vesicle network produced by extensive coalescence of smaller vesicles (> 99% of pore volume), and (2) a population of very small and completely isolated vesicles (< 1% of porosity). Computed permeability ranges from 3.0 × 10−13 to 6.3 × 10−12 m2, which are lower than the similarly explosive mafic eruptions of Tarawera (1886; New Zealand) and Etna (112 BC; Italy). The combination of our CSDs, microlite number densities, and 3D vesicle textures evidence rapid ascent that induced high disequilibrium conditions, promoting rapid syn-eruptive crystallization of microlites within the shallow conduit. We interpret that microlite crystallization increased viscosity while simultaneously forcing bubbles to deform as they grew together, resulting in the permeable by highly tortuous network of vesicles. Using the bubble number densities for the isolated vesicles (0.1-3−3 × 104 bubbles per mm3), we obtain a minimum average decompression rate of 1.4 MPa/s. Despite the textural evidence that the Curacautín magma reached the percolation threshold, we propose that rapid ascent suppressed outgassing and increased bubble overpressures, leading to explosive fragmentation. Further, using the porosity and permeability of our samples, we estimated that a bubble overpressure > 5 MPa could have been sufficient to fragment the Curacautín magma. Other mafic explosive eruptions report similar disequilibrium conditions induced by rapid ascent rate, implying that syn-eruptive disequilibrium conditions may control the explosivity of mafic eruptions more generally.

Magma chamber stratification of the 1815 Tambora caldera-forming eruption

The eruption of the Tambora volcano in 1815 was initiated by two precursory Plinian falls and formed two generations of pyroclastic density current (PDC) deposits. In this study, we found slight changes in phenocrysts (modal mineralogy, content, and size), bulk-rock and feldspar microlite chemical compositions, and bubble and microlite number densities through the stratigraphic position. Plinian fall units are characterized by a lower phenocryst abundance (avg. of 5.1%), smaller phenocryst size (avg. of 0.06 mm2), and higher silica content (bulk-rock, 58–58.5 wt.%). The PDC deposits are characterized by a relatively higher crystal abundance (avg. of 12.1%), larger phenocryst sizes (avg. of 0.13 mm2), and lower silica content (bulk-rock, 56.7–57.9 wt.%). Therefore, the deposit stratigraphy and analyses suggest that phenocryst stratification in the magma chamber was established prior to the 1815 eruption and was thus responsible for yielding a slight contrast in bulk compositions. Feldspar microlite moves toward slightly more albitic compositions from Plinian falls to the PDCs, suggesting a slight decrease in the initial melt temperature from the upper to the lower magma chamber portion. Because the Plinian eruptions extracted the hottest magma, the degree of supercooling became low, and consequently yielded microlite-poor juveniles. By contrast, the PDCs experienced a larger degree of supercooling because the temperature was relatively low, thus yielding microlite-rich juveniles. Moreover, such temperature stratification coupled with the evidence of homogeneous melt composition (58.5–58.9 wt.% SiO2) and the minor evidence of crystal mush (at most 27%) might suggest that the Tambora case is still in the early stage of magmatic evolution under cooling from the surrounding rocks.

Eruption of compositionally heterogeneous andesites from a complex storage region during the 2006 eruption of Augustine Volcano

Despite the common occurrence of heterogeneous andesitic eruptions, few studies have investigated the compositional effects on microlite crystallization and vesiculation in co-erupted natural samples. In 2006, Augustine Volcano erupted compositionally heterogeneous andesites that range from 56.4 to 63.3 wt% SiO2 and include two endmember lithologic groups: low-silica andesite (LSA) and high-silica andesite (HSA). Textural and compositional differences between LSA and HSA end members are explored for two discrete, sequential vulcanian explosions from January 17 (event 9) and 27 (event 10), 2006. Groundmass glass compositions of pyroclasts within LSA and HSA compositional suites are not colinear and do not correlate with plagioclase microlite crystallinities, indicating eruption from multiple isolated shallow magma reservoirs with various pressure-temperature pathways. HSA pyroclasts have lower crystallinities, 0.02–0.24, than most LSA pyroclasts, 0.16–0.39, demonstrating the influence of composition on crystallinity. Additionally, microlite textural and groundmass glass compositional differences exist between consecutive vulcanian explosions. The event 9 deposits have a typical bimodal density distribution and groundmass glass compositions range from 65 to 78 wt% SiO2. Plagioclase microlite number densities (MNV) are 104.6–6.4 mm−3 and crystallinities are 0.02 to 0.28, similar to products from other andesitic vulcanian eruptions. Deposits from the early phase of event 10 have a bimodal density distribution and contain a high proportion of LSA pyroclasts, similar to event 9. Groundmass glass compositions range from 72 to 79 wt% SiO2 and plagioclase MNV are 105.9–6.3 mm−3, forming narrower ranges compared to event 9. A transition occurred during the later phase of event 10 to a unimodal density distribution, a more homogeneous groundmass glass composition, 75–78 wt% SiO2, a higher proportion of HSA pyroclasts, and the highest MNV of 105.9–6.7 mm−3. We interpret this shift to reflect eruption from reservoir depths around 4–6 km and the cessation of pre-eruptive magma staging in the upper conduit, transitioning the eruption to continuous and effusive phases. Attention to heterogeneous andesitic erupted products reveals additional details about heterogeneous shallow magma storage, variable upper conduit magma staging, and a range of pressure-temperature paths prior to fragmentation.

Controls on eruption style at Rabaul, Papua New Guinea – Insights from microlites, porosity and permeability measurements

Rabaul in Papua-New-Guinea is an extremely active andesitic caldera complex that displays a large spectrum of eruption styles. Since 1878, four sub-plinian (VEI-4) and ten VEI 1–3 (effusive, strombolian, vulcanian) eruptions occurred from Tavurvur and Vulcan, the two main active vents. We study pumiceous tephra, ballistic bombs and a lava flow from five of these eruptions to investigate (1) magma ascent rates and (2) volatile escapement processes during the ascent.We measured total and connected porosities, permeability, and connectivity and related these results with measurements of crystallinity, Microlite Number Density (MND) and Microlite Size Distribution (MSD) of plagioclases and pyroxenes. From the application of existing percolation models, we find that explosive products yield a percolation threshold comprised between 50 and 60 vol% total porosity, while petrophysical parameters of the lava flow and some of the ballistic bombs are explained by bubble collapse driven by surface tension. Sub-plinian products show low phenocryst contents (5–15 vol%), microlites generated by nucleation-driven crystallization or glassy textures due to kinetic crystallization lags. Vulcanian, strombolian and effusive products on the other hand, show medium to high phenocryst contents (15–40 vol%) and microlites that crystallised by growth-dominated processes. MSDs in sub-plinian pumiceous tephra constitute a partial record of drastic magma acceleration. In comparison, MSDs in vulcanian and strombolian ballistic bombs and lava flow show a classic pattern of crystallization at a steady-state due to gradual magma ascent.We find that magma feeding sub-plinian eruptions ascends 2–3 orders of magnitude faster than magma feeding vulcanian/effusive eruptions (≥ 1–100 m/s vs 0.1 m/s). In the case of sub-plinian products, due to the important kinetic crystallization lag, these speeds cannot be estimated using microlite crystallization triggered by decompression as a proxy. Combining petrophysical and textural measurements we suggest that slight changes in initial conditions in the reservoir such as crystallinity or the presence of exsolved volatiles, can have a profound impact on the ascent rate and generate positive or negative feedback effects leading to powerful sub-plinian activity or intermittent vulcanian/ passive effusive activity respectively.

Theoretical Models of Decompression-Induced Plagioclase Nucleation and Growth in Hydrated Silica-Rich Melts

Microlites crystallization changes the flow properties of ascending magmas, with major implications on eruption dynamics. In order to predict microlite number density and size, we present a theoretical modelling of plagioclase nucleation and growth in rhyolitic melts, aimed at reproducing the decompression-induced isothermal crystallization experiments of Mollard et al. (2012). Thus, the modelling is valid for plagioclase crystallization in haplotonalitic/rhyolitic melts decompressed at 875 °C from 200 MPa to final pressures of 50, 75, or 100 MPa, which represents effective undercooling (Teff) of 110, 80 and 55 °C, respectively. The nucleation-rate calculation using the Classical Nucleation Theory (CNT; case of homogeneous nucleation) and values of crystal-melt interfacial tensions () either from literature or empirically calculated strongly disagree with the experimental nucleation rates. Inverting the CNT calculation by using the experimentally-determined nucleation rates suggests plagioclase-liquid from 0.041 to 0.059 J.m-2 with Teff increasing from 55 to 110 °C, which is about 2-3 times lower than  determined empirically and macroscopically. Further refining  by considering a non-smooth and non-spherical plagioclase nucleus leads to an elongated and rough morphology with a shape parameter () of 500-1350 and a rugosity parameter (ds) of 2.3. The experimental plagioclase growth rates have been modelled by atom diffusion in melt, following Fick’s second law adapted to multicomponent systems. The component with the smallest flux (i.e. limiting growth) is calcium, as determined from the analytical profiles of the experimental glasses. The crystal-growth model considers a crystal-melt interface advancing over time and a chemically-closed finite reservoir. The overall good agreement between the model and the experimental growth laws validates diffusion as the main process controlling isothermal decompression-induced plagioclase growth under moderate Teff <110 °C. The calculated CaO diffusion coefficients in silicic melt at 875 °C and water saturation pressures from 50 to 100 MPa ranges from 10-14 to 10-15 m2/s. The simulations may be relevant to predict nucleation and growth of plagioclase microlites in phenocryst-poor rhyolitic magmas that rapidly ascend from a storage zone and crystallize at shallower. This may be relevant to eruption dynamics such as dome-related blasts.

Supersaturation Nucleation and Growth of Plagioclase: a numerical model of decompression-induced crystallization

Supersaturation Nucleation and Growth of Plagioclase (SNGPlag) is a numerical model that predicts the nucleation and growth of plagioclase crystals in a decompressing magma as a function of time. The model is written in Matlab, but is available as a standalone compiled program. SNGPlag uses the MELTS webservice to determine equilibrium plagioclase mode, for a user-defined magma composition, as a function of pressure and temperature. User inputs include decompression path, the presence and size distributions of antecrysts and phenocrysts, and crystal shape. At each time step, the model evaluates the difference between the calculated crystallinity and equilibrium crystallinity for a given pressure and temperature to determine the degree of supersaturation, which then sets plagioclase nucleation and growth rates. Growth rates are used to grow the existing crystals whereas nucleation adds new crystals. SNGPlag produces results that can be compared to quantitative textures in natural volcanic rocks, including total crystallinity, microlite number density, microlite crystal size distribution, the characteristic size of microlite crystals, as well as a time series of crystallinity. Model results are consistent with the established crystallization theory. As expected, microlite crystallinity increases as decompression rate slows. Decompression path greatly affects microlite textures. For the same average decompression rate, single-step paths have higher crystallinities and microlite number densities than multi-step decompressions, which are in turn more crystalline than continuous paths. Pre-existing crystals damp microlite crystallization, as these crystals provide a substrate to accommodate crystal growth and thus reduce supersaturation. The size distribution and volume fraction of these pre-existing crystals determines the magnitude of the damping. SNGPlag predicts that melt composition and temperature also exert important controls. Higher temperatures and higher silica contents both reduce microlite crystallization. In comparison with the previous studies of decompression rate based on microlite crystallization experiments, SNGPlag generally predicts minimum decompression rates that are up to three-to-four times slower. The difference is likely because those studies applied single- or multi-step decompression experiments to simulate natural magma ascent, which may be better represented by continuous decompression pathways or series of continuous decompression intervals punctuated with pauses. Previous studies also fail to account for the effects of phenocrysts or antecrysts on microlite nucleation and growth.

Fast ascent rate during the 2017–2018 Plinian eruption of Ambae (Aoba) volcano: a petrological investigation

In September 2017, after more than a hundred years of quiescence, Ambae (Aoba), Vanuatu’s largest volcano, entered a new phase of eruptive activity, triggering the evacuation of the island’s 11,000 inhabitants resulting in the largest volcanic disaster in the country’s history. Three subsequent eruptive phases in November 2017, March 2018, and July 2018 expelled some of the largest tropospheric and stratospheric SO2 clouds observed in the last decade. Here, we investigate the mechanisms and dynamics of this eruption. We use major elements, trace elements, and volatiles in olivine and clinopyroxene hosted melt inclusions, embayments, crystals, and matrix glasses together with clinopyroxene geobarometry and olivine, plagioclase, and clinopyroxene geothermometry to reconstruct the physical and chemical evolution of the magma, as it ascends to the surface. Volatile elements in melt inclusions and geobarometry data suggest that the magma originated from depth of ~ 14 km before residing at shallow (~ 0.5 to 3 km) levels. Magma ascent to the surface was likely facilitated by shallow phreatic eruptions that opened a pathway for magma to ascend. Succeeding eruptive phases are characterised by increasingly primitive compositions with evidence of small amounts of mixing having taken place. Mg–Fe exchange diffusion modelling yields olivine residence times in the magma chamber ranging from a few days to a year prior to eruption. Diffusion modelling of volatiles along embayments (melt channels) from the first two phases of activity and microlite number density suggests rapid magma ascent in the range of 15–270 km/h, 4–75 m/s (decompression rates of 0.1 to ~ 2 MPa/s) corresponding to a short travel time between the top of the shallow reservoir and the surface of less than 2 min.

Using microlites to gain insights into ascent conditions of differing styles of volcanism at Soufrière Hills Volcano

Microlite textural characteristics are analyzed in the products of Vulcanian explosions, ash venting occurring synchronously with lava effusion (syn-extrusive ash venting), a precursory explosion, and lava dome effusion associated with Phases 3, and 5 of the current Soufrière Hills Volcano (SHV) eruption. 2D microlite population statistics and crystal size distributions are used to infer decompression pathways for each style of volcanism. In addition, magma ascent rates for each eruptive style were calculated using microlite number density (Toramaru et al. 2008).Strong differences between pyroclastic and effusive microlites are observed in 2D population statistics and crystal size distributions, indicating higher amounts of smaller microlites (<0.0054 mm) in the lava dome compared with pyroclastic products. Crystal size distributions show that the lava dome and Vulcanian explosion samples have similar amounts of large microlites (>0.0084 to 0.012 mm). However, syn-extrusive ash venting samples have comparatively greater amounts of large microlites. The similarity between lava dome and Vulcanian explosions at larger microlite sizes suggests comparable crystallization conditions in the deeper conduit system (>2 km depth). The contrast in eruption style despite similarity at depth suggests a shallow control to eruption processes at SHV. The greater amounts of large microlites in syn-extrusive ash venting compared to the lava dome indicates two distinct decompression pathways within a single conduit, inferred to result from differences in ascent rate. Sluggish conduit margin ascent leading to lower decompression rates is inferred to produce the increased degree of crystal growth observed in syn-extrusive ash venting compared with the lava dome.Calculated magma ascent rates for Vulcanian explosions and syn-extrusive ash venting have similar ranges, while lava effusion is up to a factor of 10 faster. Differences in magma ascent rate are inferred to result from variations in the depth at which ascent rate is calculated for each eruptive style. Ascent rate is calculated at microlite nucleation depth using the method of Toramaru et al. (2008). However, the average microlite nucleation depth will vary because of differences in microlite nucleation rate during ascent. More small microlites in the lava dome suggest greater nucleation rates in the shallow conduit, recording an on average shallower magma ascent rate (<1 km depth). Rapid decompression during pyroclastic eruptions restricts shallow crystallization, suggesting the microlite population is on average recording the deeper conduit system (>2 km depth).

Crystal nucleation and growth produced by continuous decompression of Pinatubo magma

High-temperature decompression experiments demonstrate that crystal textures preserve a record of the style and rate of magmatic ascent. To reinforce this link, we performed a suite of isothermal decompression experiments using starting material from the climactic 1991 Pinatubo eruption. We decompressed experiments from 220 MPa to final, quench pressures of 75 or 30 MPa using continuous decompression rates of 100, 30, 10, 3, 1, and 0.3 MPa h−1. Amphibole, clinopyroxene, and plagioclase crystallized during the experiments, with plagioclase microlites dominating the assemblage. Total microlite number densities range from 107.6±0.4 up to 108.2±0.2 cm−3, with plagioclase accounting for up to 65% of the total number. Plagioclase microlite area increased systematically from 19 ± 8 to 937 ± 487 µm2 with increasing experiment duration. Our textures provide time-integrated records of crystal kinetics. Average nucleation and areal growth rates of plagioclase are highest in the fastest decompressions (~ 107.5 cm−3 h−1 and 10.1 ± 4.1 µm2 h−1, respectively) and more than an order of magnitude lower in the slowest experiments (~ 105.5 cm−3 h−1 and 0.8 ± 0.2 µm2 h−1, respectively). Both nucleation and growth rates are highest at high degrees of disequilibrium. We find that peak supersaturation-dependent instantaneous rates are generally more than an order of magnitude faster than average rates. We use those instantaneous nucleation and growth rates to introduce an iterative model to evaluate the effects of different decompression rates, decompression paths (continuous, single-step or multistep), and the presence of phenocrysts on final crystallinity and microlite size distribution.

Conduit processes during the climactic phase of the Shinmoe-dake 2011 eruption (Japan): Insights into intermittent explosive activity and transition in eruption style of andesitic magma

The climactic phase of the Shinmoe-dake 2011 eruption was characterized by sub-Plinian events (January 26p.m., 27a.m., and 27p.m.) and lava accumulation in the crater (late on January 27 to January 29), all of which were accompanied by Vulcanian events. The magma discharge rate for lava accumulation was close to that of the sub-Plinian events. We discuss evolution of syneruptive magma ascent through the climactic phase on the basis of decompression-induced crystallization of groundmass microlites, in order to reveal how the final eruption style was determined at the explosive–effusive transition. We examined pumice and lava-like pyroclasts from three sub-Plinian events, the January 28 Vulcanian event, and a lava block ballistically ejected during the February 1 Vulcanian event. The samples exhibit variable groundmass textures due to variable syneruptive ascent conditions, not due to pre-eruptive inhomogeneity of the andesitic magma and different degrees of syneruptive volatile exsolution preceding quenching.Eruptive units were first characterized according to bulk density, assuming that slower ascent resulted in higher density. A decrease in the magma ascent rate led to a gradual shift to an effusive eruption style. There was little variation in ascent rate among magmas erupted simultaneously during the first and second sub-Plinian events (bulk density 0.8–2.1g/cm3), but differences increased in the third sub-Plinian and January 28 Vulcanian eruptions with minor appearance of slow magma (0.9–2.8g/cm3). Only slow magma occupied the shallow conduit at the completion of lava accumulation (February 1 lava; 2.1g/cm3). Among the first and second sub-Plinian units, the early stage of the second sub-Plinian event had the greatest variation in ascent rate, with extension to a slightly higher bulk density (1.0–2.1g/cm3) than the other events (0.8–1.7g/cm3). Ascent-rate variations in the early stage of the second sub-Plinian event, the third sub-Plinian event, and the January 28 Vulcanian event are all due to the preceding calm phase in the intermittent explosive activity. The moving magma column, which showed a vertical zonation in the degree of degassing, formed in the shallow conduit during the calm phase due to a decrease in the discharge rate, and this resulted in the co-ejection of magmas with variable degassing upon the start of explosive events.The crystal size distribution (CSD) of plagioclase microlites constrains the syneruptive branching (i.e., a point of divergence) in ascent rate and eruption style, which occurred after the start of microlite crystallization. The CSDs of the different samples show differences only in small crystal sizes; slope at these size fractions is smallest for low-density (<2.0g/cm3) pyroclasts from all eruptive phases, medium for high-density pyroclasts and lava (>2.0g/cm3) of the January 28 and February 1 events, and greatest for high-density pyroclasts (>2.0g/cm3) of the third sub-Plinian event. We propose, based on the similarity in CSD, that the shallow magma-feeding system was common for the two craters active during the January 28 explosive–effusive hybrid activity, where the Vulcanian event and emplacement of lava of the later February 1 event occurred in different parts of the original crater. The samples have variable plagioclase microlite number densities (1.4×105–3.5×106mm−3), based on variations in small crystal sizes among different CSDs. This variability resulted not only from the usual increasing trend in number density with increasing degree of undercooling, but also the decreasing trend associated with the strong undercooling during explosive eruptions.

Ascent and emplacement dynamics of obsidian lavas inferred from microlite textures

To assess the eruption and emplacement of volumetrically diverse rhyolite lavas, we measured microlite number densities and orientations from samples collected from nine lavas in Yellowstone Caldera and two from Mono Craters, USA. Microlite populations are composed of Fe-Ti oxides ± alkali feldspar ± clinopyroxene. Number densities range from 108.11 ± 0.03 to 109.45 ± 0.15 cm−3 and do not correlate with distance from the vent across individual flows and are remarkably similar between large- and small-volume lavas. Together, those observations suggest that number densities are unmodified during emplacement and that ascent rates in the conduit are similar between small domes and large lava flows. Microtextures produced by continuous decompression experiments best replicate natural textures at decompression rates of 1–2 MPa hr−1. Acicular microlites have a preferred orientation in all natural samples. Because the standard deviation of microlite orientation does not become better aligned with distance travelled, we conclude that microlites exit the conduit aligned and that strain during subaerial flow was insufficient to further align microlites. The orientations of microlite trend and plunge in near-vent samples indicate that pure shear was the dominant style of deformation in the conduit. We speculate that collapsing permeable foam(s) provides a mechanism to concurrently allow microlite formation and alignment in response to the combination of degassing and flattening by pure shear.

Rates of water exsolution and magma ascent inferred from microstructures and chemical analyses of the Tokachi–Ishizawa obsidian lava, Shirataki, northern Hokkaido, Japan

Very few quantitative textural data exist for viscous obsidian lava eruptions, and it is still unclear from the mechanical behavior of ascending magmas if outgassing is controlled dominantly by brittle or ductile deformation. In order to obtain insights into how degassing and ascent proceed in such highly viscous magmas, we conducted textural and chemical analyses of the Tokachi–Ishizawa (TI) obsidian lava, in the Shirataki rhyolite volcanic area, northern Hokkaido, Japan, and estimated the water exsolution rate and ascent rate. The storage conditions of the TI lava are estimated from the Rhyolite-MELTS program as T=840–860°C and P=50MPa using the mineral assemblages and the chemical compositions of plagioclase phenocrysts and glass. To estimate the magma ascent rate, we measured the length, width, and number of oxide microlites using three-dimensional techniques. Textural analysis indicates that the microlite number densities (Nv [number/m3]) of oxide microlites in TI lava samples are 2.1×1013 to 1.4×1014, which correspond to water exsolution rates of 3.5×10−9 to 1.7×10−8wt.%/s and ascent rates of 1.7×10−6 to 1.1×10−5m/s. Together with an estimate of viscosity, the inferred ascent velocities allow us to examine the mechanical behavior of the magma in the conduit. We conclude that the development of permeability leading to outgassing is controlled by ductile deformation rather than brittle fracturing.

Pre-eruptive storage conditions and eruption dynamics of a small rhyolite dome: Douglas Knob, Yellowstone volcanic field, USA

The properties and processes that control the size, duration, and style of eruption of rhyolite magma are poorly constrained because of a paucity of direct observations. Here, we investigate the small-volume, nonexplosive end-member. In particular, we determine the pre-eruptive storage conditions and eruption dynamics of Douglas Knob, a 0.011-km3 obsidian dome that erupted from a 500-m-long fissure in the Yellowstone volcanic system. To determine pre-eruptive storage conditions, we analyzed compositions of phenocrysts, matrix glass, and quartz-hosted glass inclusions by electron microprobe and Fourier-transform infrared analyses. The pre-eruptive melt is a high-silica rhyolite (∼75 wt.% SiO2) and was stored at 760 ± 30 °C and 50 ± 25 MPa prior to eruption, assuming vapor saturation at depth. To investigate emplacement dynamics and kinematics, we measured number densities and orientations of microlites at various locations across the lava dome. Microlites in samples closest to the inferred fissure vent are the most aligned. Alignment does not increase with distance traveled away from the vent, suggesting microlites record conduit processes. Strains of <5 accumulated in the conduit during ascent after microlite formation, imparted by a combination of pure and simple shear. Average microlite number density in samples varies from 104.9 to 105.7 mm−3. Using the magma ascent model of Toramaru et al. (J Volcanol Geotherm Res 175:156–157, 2008), microlite number densities imply decompression rates ranging from 0.03 to 0.11 MPa h−1 (∼0.4–1.3 mm s−1 ascent rates). Such slow ascent would allow time for passive degassing at depth in the conduit, thus resulting in an effusive eruption. Using calculated melt viscosity, we infer that the dike that fed the eruption was 4–8 m in width. Magma flux through this dike, assuming fissure dimensions at the surface represent its geometry at depth, implies an eruption duration of 17–210 days. That duration is also consistent with the shape of the dome if produced by gravitational spreading, as well as the ascent time of magma from its storage depth.