Unlike their silicic counterparts, mafic eruptions are known for being on the low-end of the explosivity spectrum with eruption styles commonly ranging from effusive to Hawaiian fire fountaining. However, there are increasing discoveries of large mafic Plinian eruptions, sometimes generating ignimbrites, suggesting that this phenomenon might not be so uncommon. So, what processes lead a mafic magma to fragment violently enough to generate extensive ignimbrites? We sampled pumices from ignimbrites and PDCs with a compositional range from basaltic-andesite (Curacautín ignimbrite, Volcàn Llaima, Chile), andesite (Marapi, Indonesia) to trachyte (Gunungkawi ignimbrite, Batur, Indonesia). We use SEM imagery and X-ray Microtomography on pyroclasts from these deposits to characterize phenocryst, microlite and vesicle textures. From vesicle number densities we estimate fragmentation decompression rates in the range of 0.4–1.6 MPa/s for the three deposits. With a combination of EPMA and SIMS analyses we characterise pre-eruptive storage conditions. Based on the bulk and groundmass compositions, the storage temperature (1,050–1,100°C), pressure (50–100 MPa) and phenocryst content (1.0–2.5 vol%), we conclude that the basaltic-andesitic Curacautín magma was at sub-liquidus conditions, which allowed fast and widespread disequilibrium matrix crystallization (0–80 vol%) during ascent to the surface. Combined with the important decompression rate, this intense crystallization led to a magma bulk viscosity jump from 10 3 up to >10 7 Pa s and allowed it to fragment brittlely. Conversely, for the Marapi PDC and Gunungkawi ignimbrite, similar decompression rates coupled with larger initial bulk viscosities of 10 5 –10 6 Pa s were sufficient to fragment the magma brittlely. The fragmentation processes for these latter two deposits were slightly different however, with the Marapi PDC fragmentation being mostly driven by vesicle overpressure, while a combination of bubble overpressure and intense strain-rate were the cause of fragmentation for the Gunungkawi ignimbrite. We conclude that mafic ignimbrites can form due to a combination of peculiar storage conditions that lead to strongly non-linear feedback processes in the conduit, particularly intense microlite crystallization on very short timescales coupled with intense decompression rates. Conversely, the high viscosity determined by pre-eruptive storage conditions, including temperature and volatile-content, are key in controlling the formation of more evolved magmas PDCs'.
<p>Details on the methods and all fitted diffusion profiles, and a dataset containing the apatite single points and traverses data, water content values, and diffusion modelling parameters. </p>
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 VEI1–3 (effusive, strombolian, vulcanian) eruptions occurred from Tavurvur and Vulcan, the two main active vents. We study the lava flows, bombs and pumices from five of these eruptions to investigate magma ascent rates and volatile outgassing during ascent. We measured total and connected porosities, permeability, and bubble connectivity that we relate with crystallinity, MND (Microlite Number Density) and MSD (Microlite Size Distribution) of plagioclases and orthopyroxenes. From the application of existing percolation models, we find that explosive products yield a percolation threshold between 50-60 vol% total porosity, while petrophysical parameters of effusive products and some of the bombs can be explained by bubble collapse driven by surface tension. Permeabilities range from 10 -10 m for pumices and bomb cores, while they range from 10 -10 m for bomb rinds and the lava flow. Sub-plinian products show low phenocryst contents (5-15 vol%), microlites generated by nucleation-driven crystallization, or glassy textures due to a lag in kinetic crystallization. Bombs and the lava flow on the other hand, show medium to high phenocryst contents (15-40 vol%) and microlites that crystallized by growth-dominated processes. Sub-plinian MSDs can be interpreted as a partial record of drastic magma acceleration, while bombs and lava flows show slower, more gradual ascent patterns. We estimate that magma feeding sub-plinian eruptions ascends 2-3 orders of magnitude faster than magma feeding vulcanian/effusive eruptions (~1 versus 10 MPa/s). The faster rising speeds cannot be estimated using MNDs due to the important lag in kinetic crystallization, but are instead estimated by the average dimension of pyroclasts and their characteristic timescale of permeable gas escape. Combining petrophysical and textural measurements we suggest that slight changes of initial conditions in the reservoir, such as phenocryst content, can have profound impacts on the ascent rate and generate positive or negative feedback reactions leading to powerful sub-plinian activity or intermittent vulcanian/ quiet effusive activity respectively.
Understanding the conditions and timescales of storage and remobilization of magma bodies in the upper crust is key to interpreting the signals of potential reawakening at active volcanoes. In this paper, we provide the first volcanological and petrochronological characterization of the Singkut caldera, a young volcanic system located in northern Sumatra (Indonesia), in close proximity to Medan, one of the country's most popoluous cities. Singkut formed at ~44 ka during a VEI 6 explosive eruption that deposited at least ~26 km3 of tephra (dense rock equivalent, DRE). The cataclysmic eruption was preceded by >200 ky of mostly effusive pre-caldera activity and followed by effusive to mildly explosive post-caldera activity. The lavas and pumices have high crystallinity (up to 62% crystals) with andesitic to dacitic bulk-rock composition and rhyolitic glass. Mineral textures and matrix glass compositions indicate resorption of quartz, plagioclase and zircon. Zircon crystallization ages show a complete overlap with the eruption ages in pre-caldera lavas, while a time gap in zircon crystallization (>50 ky) is identified in the caldera-forming tuff and post-caldera lavas. Ti-in-zircon thermometry shows that the Singkut magma body experienced a temperature increase starting approximately upon eruption of the pre-caldera lavas (~254 ka). Such thermal perturbation determined progressive melting of mineral phases in the cumulate crystal mush, caused the resorption of the youngest zircon domains before the caldera-forming eruption, and hampered zircon crystallization between the caldera-forming eruption and the effusion of the post-caldera lavas (~16 ka). Our data demonstrate how cumulate melting processes played a key role in leading the volcanic system towards a caldera-forming eruption and controlled the transitions in eruptive style between the effusive phases and the explosive climactic eruption.
Abstract One of the biggest challenges in volcanology is assessing the role of magma properties (volatile budgets, storage depths, and ascent rates) in controlling eruption explosivity. We use a new approach based on apatite to estimate volatile contents and magma ascent rates from a sequence of sub-Plinian, effusive, and Vulcanian eruption deposits at Rabaul caldera (Papua New Guinea) emplaced in 2006 CE to probe the mechanisms responsible for the sudden transitions in eruption styles. Our findings show that all magmas were originally stored at similar conditions (2–4 km depth and 1.8–2.5 wt% H2O in the melt); only the magma that formed the lava flow stalled and degassed at a shallower level (0.2–1.5 km) for several months. A more energetic batch of magma rose from depth, bypassed the transient reservoir, and ascended within ≤8 h to Earth's surface (mean velocity ≥0.2 m/s), yielding the initial sub-Plinian phase of the eruption. The shallowly degassed magma was then able to reach the surface as a lava flow, likely through the path opened by the sub-Plinian magma. The magma of the last Vulcanian phase ascended without storage at a shallow depth, albeit more slowly (ascent rate 0.03–0.1 m/s) than the sub-Plinian magma. Our study illustrates how the complexity of plumbing systems may affect eruption styles, including at other volcanic systems, and have implications for interpreting volcano monitoring data.
<p>Understanding the conditions and timescales of storage and remobilization of magma bodies in the upper crust is key to interpret the signals of potential reawakening of the volcanic activity at active volcanic systems.&#160;In this study we provide the first volcanological and petrological characterization of the Singkut volcanic system located in northern Sumatra, ~35 km N of the Toba caldera and ~40 km SW of the major city of Medan.&#160;Singkut is a ~9 km diameter caldera delimited by ~300 m-high rims where pre-caldera lavas are exposed. The inner part of the collapsed structure is occupied by three post-caldera volcanoes and currently hosts an active geothermal field. We utilize field observations and correlation with a distal marine tephra layer to map the extension and thickness of the tuff erupted during the caldera-forming eruption and use these data to estimate the erupted magma volume. We use major and trace element data of bulk-rock, matrix glasses and minerals to characterize the pre-eruptive conditions of pre- and post-caldera lavas and caldera-forming tuff and<sup> 14</sup>C and U/Th-He zircon dating to determine the eruption ages. In addition, a combination of U/Th and U/Pb in-situ zircon dating and zircon trace element geochemistry provides insights into the mechanisms and timescales that led to the Singkut caldera-forming eruption and those that controlled the post-caldera activity.&#160;Our data show that Singkut caldera formed ~50 ka during a large explosive eruption that deposited ~60 km<sup>3</sup> of pyroclastic material. The cataclysmic eruption was preceded by at least 200 ky of mostly effusive pre-caldera activity and followed by effusive and mildly explosive post-caldera activity, with the last eruption reported at 1881 AD. The lavas and pumices have high crystallinity (24-62% crystals) and contain pl+amph+bt+opx+Fe-Ti ox+ap+zr&#177;qtz. Notably, large and strongly resorbed quartz crystals are abundant in the pre-caldera lavas and scarce or absent in the caldera-forming tuff and post-caldera lavas. Bulk-rock composition of pumices and lavas varies from andesitic to dacitic, while the matrix glass in the pumices is rhyolitic. Trace element composition of glass (e.g., positive Eu anomalies) indicate resorption of feldspars. Crystallization ages of the youngest zircons in pre-caldera lavas overlap with eruption ages (~250 ka) while crystallization ages of the youngest zircons in the caldera-forming tuff and post-caldera lavas are significantly older (~100 ka) than the eruption ages (~50 and ~16 ka, respectively). Ti-in-zircon thermometry combined with zircon geochronology show that the Singkut magma body experienced a heating phase which started approximately upon eruption of the pre-caldera lavas and continued at least until the eruption of the post-caldera lavas. Such prolonged heating event determined progressive melting of the least refractory mineral phases (mostly quartz and feldspars) and hampered zircon crystallization for ~50 ky before the caldera-forming eruption and ~80 ky before the effusion of the post-caldera lavas. Heating was likely due to an increase of the recharge flux in the magma reservoir which reduced the crystallinity of the crystal mush and promoted remobilization and eruption of the Singkut magma body.</p>
Abstract Apatite has been recognized as a robust tool for the study of magmatic volatiles in terrestrial and extraterrestrial systems due to its ability to incorporate various volatile components and its common occurrence in igneous rocks. Most previous studies have utilized apatite to study individual magmatic systems or regions. However, volatile systematics in terrestrial magmatic apatite formed under different geological environments has been poorly understood. In this study, we filtered a large compilation of data for apatite in terrestrial igneous rocks ( n > 20,000), categorized the data according to tectonic settings, rock types, and bulk-rock compositions, and conducted statistical analyses of the F–Cl–OH–S–CO 2 contents (~ 11,000 data for halogen and less for other volatiles). We find that apatite from volcanic arcs preserves a high Cl signature in comparison to other tectonic settings and the median Cl contents differ between arcs. Apatite in various types and compositions of igneous rocks shows overlapping F–Cl–OH compositions and features in some rock groups. Specifically, apatite in kimberlite is characterized as Cl-poor, whereas apatite in plutonic rocks can contain higher F and lower Cl contents than the volcanic counterparts. Calculation using existing partitioning models indicates that apatite with a high OH (or F) content does not necessarily indicate a H 2 O-rich (or H 2 O-poor) liquid because it could be a result of high (or low) magma temperature. Our work may provide a new perspective on the use of apatite to investigate volatile behavior in magma genesis and evolution across tectonic settings, volatile recycling at subduction zones, and the volcanic-plutonic connection.
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