Abstract Bagana is a persistently active stratovolcano located on Bougainville Island, Papua New Guinea. Characteristic activity consists of prolonged lava effusion over months to years, with occasional shifts to explosive vulcanian or sub Plinian eruptions that threaten surrounding communities. Satellite observations have shown that Bagana is a major SO2 emitter, particularly during eruptive intervals. Despite persistent and potentially hazardous activity, no previous geophysical, petrological, or geochemical studies have constrained the magma storage conditions and reservoir processes at Bagana. To address this knowledge gap, we present new bulk rock major, trace element, and radiogenic isotope data, plus mineral phase major element compositions, for Bagana lavas erupted in 2005 and 2012 and ash erupted in 2016. We use our new data to understand the magmatic processes controlling the typical effusive activity and provide the first estimates of magma storage conditions beneath Bagana. The basaltic andesite bulk rock compositions (56–58 wt% SiO2) of our Bagana lavas reflect accumulation of a plagioclase + clinopyroxene + amphibole + magnetite + orthopyroxene crystal cargo by andesitic-dacitic (57–66 wt% SiO2) carrier melts. Constraints from clinopyroxene and amphibole thermobarometry, amphibole hygrometry, and experimental petrology suggest that the high-An plagioclase + clinopyroxene + amphibole + magnetite assemblage crystallizes from basaltic-basaltic andesite parental magmas with >4 wt% H2O, over a temperature interval of ~1100–900°C, at pressures of ~130–570 MPa, corresponding to ~5–21 km depth. Continued crystallization in the magma storage region at ~5–21 km depth produces andesitic to dacitic residual melts, which segregate and ascend towards the surface. These ascending melts entrain a diverse crystal cargo through interaction with melt-rich and mushy magma bodies. Degassing of carrier melts during ascent results in crystallization of low-An plagioclase and the formation of amphibole breakdown rims. The radiogenic isotope and trace element compositions of Bagana lavas suggest that parental magmas feeding the system derive from an enriched mantle source modified by both slab fluids and subducted sediments. Our findings suggest that the prolonged lava effusion and persistently high gas emissions that characterise Bagana’s activity in recent decades are sustained by a steady state regime of near-continuous ascent and degassing of magmas from the crustal plumbing system. Our characterisation of the Bagana magmatic plumbing system during effusive activity provides a valuable framework for interpreting ongoing monitoring data, and for identifying any differences in magmatic processes during any future shift to explosive activity.
Water-rich silicic magmas are capable of erupting effusively and explosively, and this drastic change in eruptive styles, termed effusive-explosive transition, has important implications in managing volcanic hazards. Some volcanoes exhibit effusive-explosive transitions during the same eruptive event, while others show this behavior between different eruptions. In the latter case, magma chamber processes induce physical-chemical changes in the magma, which can favor either effusivity or explosivity. This is the case for the Nisyros-Yali volcanic center, from the South Aegean Sea. In the recent stages of activity (past 120 ky), the volcanic area generated eight rhyolitic effusive and explosive events (five on the island of Nisyros and three on the island of Yali), including two caldera-forming eruptions. Changes of water content, temperature and pre-eruptive water-saturation between effusive and explosive deposits point to a potential time-dependency between the two eruptive styles. We investigate this time-dependency by applying UTh disequilibrium dating to zircon crystals. Our eruptive age estimates of the investigated units range from 118.7 ± 10 ka to 19.9 ± 1.5 ka for Nisyros, and from 40 ± 5.2 ka to 22.7 ± 1.6 ka for Yali. Yali volcano has developed after the two caldera-forming events on Nisyros, which occurred at 63.1 ± 4.7 ka and 58.4 ± 2.7 ka. Yali marks the transition to a more geometrically complex system, where the upper-crustal silicic mush hosts at least two eruptible magma chambers (one under Yali, and one under Nisyros). The eruptive styles at both volcanoes seem to be correlated with the length of the repose periods. Effusive events occur after longer periods of volcanic quiescence, while explosive events are generated after shorter periods of repose of ~5–10 ky, which can be extended based on eruption age uncertainty to <18 ky for Nisyros and <12 ky for Yali. This observation is explained by the physical state of the volatiles in the magma chamber, with longer repose periods favoring volatile build-up. This can lead to water-supersaturation at storage pressures which was shown to favor effusivity. Based on this interpretation, both Nisyros and Yali volcanoes are presently in the effusive time window, which makes it probable for the next eruptions to be non-explosive.
<p><span><span>Despite the largest explosive eruptions posing significant potential hazards, the recurrence rate of these so called &#8216;super-eruptions&#8217; remains poorly constrained. The younger portion of the Yellowstone-Snake River Plain province is well-known for large-scale explosive volcanism; however, the older history within the Snake River Plain remains poorly-known and partially obscured by later basaltic volcanism. To address this, we characterised the mineral cargo of four widely spaced rhyolitic ignimbrites found at the margins of the Snake River Plain that reveal a strong compositional similarity in bulk geochemistry, major crystal phases (e.g. pyroxene and ilmenite), and radiogenic isotopes. To test whether these four compositionally similar units may have had a common origin we used a tandem in-situ and isotope dilution method for U/Pb geochronology of zircon crystals. The youngest populations of zircons from all four samples are equivalent in age, and together define a pooled weighted mean <sup>238</sup>U/<sup>206</sup>Pb age of 11.030 &#177; 0.006 (MSWD = 1.44, n=24). These results reveal an event with a conservatively estimated erupted volume ~1,470 km<sup>3</sup>, of similar magnitude to the largest Yellowstone eruptions. Numerous widely dispersed tephra deposits found across the western portions of North America with geochemical affinities to the Snake River Plain province hint at the existence of other such voluminous ignimbrites. The improved ability to correlate deposits of an individual eruption shown by this and other recent studies implies that &#8216;super&#8217; eruptive events are more common than previously thought. </span></span></p>
Abstract The origins of felsic low-δ18O melts (< +5.5‰) are usually attributed to assimilation of high-temperature hydrothermally altered (HTHA) rocks. Very few alkaline (silica-undersaturated and/or peralkaline) examples are known. Here, we classify the Miocene Fataga Group in Gran Canaria, a silica-undersaturated to mildly saturated alkaline volcanic sequence consisting of trachytic to phonolitic extra-caldera ignimbrites and lavas, as a new low-δ18O felsic locality. We provide new mineral, glass, and bulk geochemical data linked to a well-constrained stratigraphy to assess the processes involved in the magma reservoir that fed the Fataga eruptions. New high-precision single crystal feldspar 40Ar/39Ar ages of the study area span 13.931 ± 0.034 Ma to 10.288 ± 0.016 Ma. Fractional crystallization at shallow depths of sanidine/anorthoclase, biotite, augite/diopside, titanite, ilmenite, and titanomagnetite is the main driving process to produce phonolitic magmas from trachytic melts. Evidence of mafic hotter recharge is not found in the field, but some units exhibit trachytic compositions characterized by positive Eu/Eu* anomalies and high Ba contents, interpreted as melts of feldspar-dominated cumulates, the solid remnants of fractional crystallization. Hence, recharge magmas halted in the crystal mush and provided the heat needed to sustain cumulate melting and volcanic activity. This cumulate signature might be lost if fractional crystallization continues before the eruption. The interplay among meteoric water, the caldera-fault system, intra-caldera ignimbrites (Mogán Group), and the Fataga magma reservoir favoured assimilation of up to ca. 30% of HTHA rocks. Such assimilation is variable through time and recorded by δ18Omelt values down to +4.73‰. We did not find any direct relation between assimilation and silica saturation of the Fataga volcanic deposits.
Basaltic volcanoes can remain active for tens to thousands of years with the continual presence of magma, requiring storage and transport conditions that can sustain persistently eruptible melt. Magma storage conditions beneath these volcanoes may significantly change with time, leading to sudden and dramatic changes in explosivity. Determining the rates and causes of these changes and how they modulate eruptive style over societally relevant timescales is of paramount importance for evaluating potential hazards. In June-August 2019, one major explosion and two paroxysms occurred at Stromboli volcano (Southern Italy) within only 64 days offering a unique opportunity to study the short-term variations in a basaltic plumbing system that can lead to paroxysmal events.Stromboli is an active open conduit basaltic volcano well-known for its persistent mild (normal) Strombolian activity occasionally interrupted by sudden, short-lived events ranging in size and intensity from major (violent Strombolian) to paroxysmal explosions. Strombolian activity, effusive eruptions and major explosions, all involve a degassed, highly porphyritic (hp) magma from a shallow reservoir. Deep-seated more mafic and, volatile-rich low-porphyritic (lp) magma is erupted, alongside hp-magma, during paroxysms, and in smaller quantities during some of the major explosions. Both lp- and hp-magmas were erupted during the 3 July and 28 August 2019 paroxysms, whereas only hp-magma was erupted during the major explosion on 25 June 2019.Via a multifaceted approach using clinopyroxene from the summer 2019 paroxysms, we reveal a key role for batches of volatile-rich lp-magma recharge arriving in the shallow reservoir up to a few days before these events. Our data indicate a rejuvenated Stromboli plumbing system where the extant crystal mush is efficiently permeated by recharge lp-magma with minimum remobilisation promoting a direct linkage between the deeper (lp) and shallow (hp) reservoirs. This sustains the current variability of eruptive styles with near immediate eruptive response to mafic magma recharge. The remarkable agreement between our calculated recharge timescales and the observed variation in time of various monitoring signals strongly supports such a model.Our approach provides vital insights into magma dynamics and their effects on monitoring signals demonstrating that detailed petrological studies integrated with volcano monitoring signals are fundamental for a fast response during a volcanic unrest phase or crisis.This work has been published in Nature Communication: Petrone, C.M., Mollo, S., Gertisser, R. et al. Magma recharge and mush rejuvenation drive paroxysmal activity at Stromboli volcano. Nat Commun 13, 7717 (2022). https://doi.org/10.1038/s41467-022-35405-z.
Open-conduit basaltic volcanoes can be characterised by sudden large explosive events (paroxysms) that interrupt normal effusive and mild explosive activity. In June-August 2019, one major explosion and two paroxysms occurred at Stromboli volcano (Italy) within only 64 days. Here, via a multifaceted approach using clinopyroxene, we show arrival of mafic recharges up to a few days before the onset of these events and their effects on the eruption pattern at Stromboli, as a prime example of a persistently active, open-conduit basaltic volcano. Our data indicate a rejuvenated Stromboli plumbing system where the extant crystal mush is efficiently permeated by recharge magmas with minimum remobilisation promoting a direct linkage between the deeper and the shallow reservoirs that sustains the currently observed larger variability of eruptive behaviour. Our approach provides vital insights into magma dynamics and their effects on monitoring signals demonstrating the power of petrological studies in interpreting patterns of surficial activity.
<p>Most volcanoes, across tectonic settings, can show both explosive and effusive eruptions, either as separated eruptive events or within the same eruptive episode. Such differences in eruptive style have significant implications for depositional morphology and hazards associated with the eruptions. In many cases, chemically nearly identical magmas may produce either explosive or effusive events. In contrast, we find the intercalated ignimbrites and lavas of trachytic to phonolitic compositions of the Fataga Group, Gran Canaria, differ markedly in several aspects. First, field observations revealed that the lavas pinch out toward the caldera rim and are therefore likely originated from extra-caldera sources. Second, while the explosive deposits vary in crystallinity (here referring to crystals with a long axis > 250 &#181;m) from a few percent in crystal-poor portions to 10 &#8211; 20% in crystal-rich clasts, the lavas are almost aphyric with crystallinities < 1%. Further, while the ignimbrites have a mineral assemblage containing alkali-feldspar, biotite, pyroxene, amphibole, titanite, and Fe-Ti oxides, the lavas mostly contain alkali feldspar as a nearly unique mineral phase. Third, major elemental compositions show that while ignimbrites and lavas may overlap within the trachyte field on a TAS plot, only the lavas have compositions that extend to phonolite. Trace elemental compositions of lavas suggest extensive fractionation with compatible elements (e.g. Ba, max 227 ppm, avg. 30 ppm) depleted and incompatibles (e.g. Zr, Hf, Ce, Rb) enriched. While crystal-poor juveniles from the ignimbrites may have compositions approaching those of the lavas, crystal-rich juvenile clasts are markedly enriched in feldspar-phyric elements (e.g. Ba, max 1892 ppm, avg. 1496 ppm) suggesting involvement with a feldspar-dominated cumulate pile. Furthermore, CIPW norm calculations show that the lavas trend towards nepheline-normative compositions. The normative prediction of nepheline occurrence in the lavas is confirmed by petrographic observation of nepheline both as groundmass constituent and rarely as phenocrysts and thus suggests a somewhat different petrogenetic history for the lavas. Fourth, on a crystal scale, the feldspar phenocrysts in the lavas have relatively restricted, and low Ba contents (20 - 500 ppm), while ignimbrites&#160;have extremely Ba-enriched feldspars (up to 18,000 ppm Ba) in their crystal-rich portions.</p><p>Deposit geometry, petrography, and geochemical data lead us to the conclusion that the Fataga lavas must have followed a different petrogenetic path than their explosive counterparts. We, therefore, suggest the magmas feeding the effusive eruptions may have bypassed the main caldera system and thus provide a different window into the Miocene magmatism of Gran Canaria.</p>
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