Excess explosivity driven by melt inclusions during the 946 CE Plinian eruption of Baekdusan

This study reports a unique pumice texture generated by the instantaneous bursting of melt inclusions in alkali feldspars during the 946 CE Plinian eruption at Baekdusan. The burst produced super-vesicular (80−90 vol.% voids), subspherical (i.e., non-sheared), and subcentimeter-sized “bubble pockets” indicating their formation at the moment of magma fragmentation. Their volume fraction (6–24 vol.%) suggests that the melt inclusions acted as an additional volcanic gas source and increased signi�cantly the volume of the erupting magma at the moment of magma fragmentation. The “excess explosivity” induced by the bursting melt inclusions should be taken into account when modeling eruption dynamics and assessing volcanic hazards, therefore having broad volcanological implications. Two distinctive feldspar–glass assemblages of either sanidine-bearing trachyte or anorthoclase-bearing rhyolite in the bubble pockets also indicate that the chemical bimodality of the hemisphere-scale, 946 CE Baekdusan tephra resulted from cryptic mixing of two magmas.


Introduction
The growth of magmatic gas bubbles increases the volume of the ascending magma, thereby accelerating the ascent of magmatic foams through conduits to drive explosive volcanic eruptions (Schmincke, 2004).In ascending magma, small volumes of the melt may be trapped within the crystals as inclusions.The host melt is spontaneously saturated with volatiles, exsolves gases, and then nucleates bubbles as decompression reaches a critical level (Hurwitz and Navon, 1994;Cluzel et al., 2008).However, melt inclusions retain gases for longer periods, unless crystal seals are broken via the process so-called decrepitation (Tait, 1992).As melt inclusions are mechanically separated from the host melt by crystalline solids, their chemistry and the timing of volatile exsolution cannot but be different from those of the host melt, causing bubble formation in multiple stages.However, the behavior and role of melt inclusions as a volcanic gas source in erupting magma have been poorly explored, despite the potential signi cance of this process in modulating eruption intensities (Tait, 1992;Belkin and De Vivo, 1993; Blundy and Cashman, 2005;Maclennan, 2017).
Baekdusan (or Changbaishan in Chinese) Volcano is a Quaternary intraplate stratovolcano situated on the border between North Korea and China (128°05′E, 42°01′N).This volcano has produced a series of explosive eruptions culminating in one of its most explosive (VEI of ~ 7) known events in 946 CE, known as the Millennium Eruption (Oppenheimer et al., 2017).The Millennium Eruption produced the B-Tm (Baekdusan-Tomakomai) tephra, which is a hemispheric-scale tephra marker readily identi able by its unique trachytic-rhyolitic compositional heterogeneity in glass shards (Machida, 1999;Sun et al., 2014;McLean et al., 2016).The heterogeneous glass shards in the tephra have been attributed to two consecutive eruptions of rhyolitic and trachytic magmas during the Millennium Eruption (Chen et al., 2016;Pan et al., 2017;Nara et al., 2021).However, the roles and interplay of the two magmas in driving the 946 CE eruptions at Baekdusan remain poorly understood.
In this study, we document a unique chemo-textural zonation structure, here named "bubble pocket", found in the proximal B-Tm gray pumice.Based on the bubble pocket structure, we restored the two-step bubble formation during the 946 CE Plinian eruption at Baekdusan and reassessed the signi cant but overlooked role of melt inclusions as an additional volcanic gas source potentially enhancing volcanic explosivity.

Samples And Methods
On the summit caldera of Baekdusan, the proximal air-fall pumice generated from the Millennium Eruption shows a sharp vertical transition in color and bulk rock chemistry from the climactic phase-1 light-gray rhyolitic pumice, here referred to as the B-Tm gray pumice, to the less-explosive phase-2 black trachytic pumice (Pan et al., 2017) (Extended Data 1).The pumice samples analyzed for this study are the rhyolitic B-Tm gray pumice collected from three proximal air-fall deposit sites on the rim of the summit caldera and from medial to distal PDC deposits north of Baekdusan (Extended Data 1).
The micro-texture of the B-Tm pumice was investigated using computerized X-ray micro-tomography (CT) (Shimadzu SMX-225CT) for the hand specimens and back-scattered electron (BSE) imaging for the polished sections.The vesicularity and volume fraction of bubbles on the CT and BSE images of ve randomly selected pumice samples were calculated using the open-source image-analysis program IMAGEJ™ (https://imagej.net)(Extended Data 2 and 3).The chemical compositions of the glass and feldspar phenocrysts were determined at Gyeongsang National University using an electron probe microanalysis (EPMA, JEOL JXA-8530F) equipped with ve wavelength-dispersive X-ray spectrometers (Extended Data 4).

Bubble pocket texture
The B-Tm gray pumice contains an unusual vesicle structure referred to as bubble pockets.The bubble pockets are near-spherical clusters of bubbles ranging in diameter from a few millimeters to more than one centimeter.A reticular network of extremely thin (< 10 µm) glass lms lls the interior of bubble pockets and surrounds fractured alkali feldspar phenocrysts (Fig. 1).On three-dimensional X-ray tomographic sections, the bubble pockets appear as spherical voids (high X-ray penetrance owing to extremely high vesicularity) containing high-brightness (low X-ray penetrance) crystal fragments (Fig. 1b-c).In contrast, the domain outside the pockets shows medium brightness (moderate X-ray penetrance) and tubular-stretched vesicle textures (Fig. 1c).The volume fraction of the bubble pockets exceeded 20% for three of the ve pumice samples and was up to 24% for one sample (Fig. 2a).
Individual bubbles inside the pockets are sub-spherical, with diameters > 100 µm and up to > 1000 µm in some cases (Fig. 1d).In contrast, bubbles in the host domain are elliptical or elongated, with thicker and often composite bubble walls.All individual bubble pockets contain at least one phenocryst or glomerocryst, mainly sanidine or anorthoclase.These crystals are variably fragmented into pieces with jigsaw-t margins (Fig. 1d).Feldspar fragments have numerous cavities lled with vesiculated glass inclusions (Fig. 1f).These cavities commonly have fracture-controlled openings as inclusion glass expands into foam, a typical feature of ruptured melt inclusions (Belkin and De Vivo, 1993).The vesicularity, calculated from the cross-sectional area of vesicles, ranged between 82.8% and 90.6% inside the bubble pockets and between 59.4% and 65.8% outside the bubble pockets (Fig. 2b).

Petrochemical characteristics of the B-Tm gray pumice
The B-Tm gray pumice is characterized by a whole-rock composition of sodic rhyolite (Pan et al., 2017) but shows considerable heterogeneity in glass and feldspar compositions at the microscopic level (Fig. 3).The EPMA point analysis revealed that bubble pockets in B-Tm gray pumice have two distinctive glass-alkali feldspar assemblages (Fig. 3).The two distinctive types of bubble pockets, namely type-A and type-B, and the host domain surrounding bubble pockets make a unique triple chemo-textural zonation pattern of the B-Tm gray pumice.
The type-A bubble pockets (red domain in Fig. 3a) comprise trachytic (65-70 in SiO 2 wt.%) bubble-wall and remnant (but vesiculated) inclusion glasses (Fig. 3b), and fractured sanidine phenocrysts (Fig. 3c).In contrast, the type-B bubble pockets (yellow domain in Fig. 3a) contain a rhyolitic (73-75 in SiO 2 wt.%) bubble-wall and remnant inclusion glasses (Fig. 3b) and anorthoclase phenocrysts (Fig. 3c).The host domain (orange domain in Fig. 3a), lacking in feldspar phenocryst, has generally rhyolitic glass compositions, but it further covers intermediate realm toward trachyte (70-75 in SiO 2 wt.%) particularly near the type-A bubble pockets (Fig. 3b).The trachytic type-A bubble pockets are commonly larger in pocket diameter (mostly > 5 mm and occasionally > 10 mm) than the rhyolitic type-B bubble pockets (mostly < 5 mm in diameter) and contain larger phenocrysts.The textural and petrochemical evidence, such as fragmented alkali feldspars and ruptured inclusion glasses therein, suggests that the bubble pockets in the B-Tm pumice were produced by the burst of melt inclusions formerly trapped in alkali feldspar phenocrysts in an already highly vesiculated and shear-strained host magma.The tubular-stretched vesicle texture outside the bubble pockets (host domain) indicates the extreme viscous shear deformation of bubble-rich magma ascending through a conduit during the climactic Plinian eruptive phase (Manga et al., 1998).In contrast, the fragmented sanidine/anorthoclase phenocrysts inside each bubble pocket (Figs.1b and 1c) and the nearspherical bubble pockets indicate a single isotropic dilation of the bursting melt inclusions without further shear deformation.The lack of shear deformation implies that the burst of melt inclusions occurred when their host magma reached the fragmentation level, immediately before or more possibly at the moment of fragmentation.Rapid chilling preserved the ne wall textures within the bubble pockets.Post-eruptive rupture of the melt inclusions is unlikely, as bubble growth in such high-viscosity rhyolitic magmas is limited after eruption (Klug and Cashman, 1991; Gardner, 1995).

Magmatic control of the 946 CE Plinian eruption at Baekdusan
The two different types of bubble pockets coexisting within the chemically intermediate host domain of the B-Tm gray pumice imply that the Millennium Eruption was driven by two different parental magmas that underwent cryptic mixing.Here, we suggest a four-step schematic model of the inceptive cryptic magma mixing and subsequent simultaneous burst of melt inclusions that formed the unique triple chemo-textural domains within the B-Tm gray pumice (Fig. 4).
The volcano-stratigraphic studies of the Baekdusan Volcano have commonly reported a rhyolitic Plinian event at c. 50 ka BP (known as the TWF eruption) that deposited a thick pile of yellowish air-fall pumice right below the B-Tm gray pumice (Pan et al., 2017 and references therein).Given this stratigraphic relationship, we infer that there existed a resident anorthoclase-bearing rhyolite chamber when a sanidinebearing trachyte magma batch newly arrived.An injection of the crystal-rich tip of the trachyte magma might have developed overpressure (through an increase of magmatic volume and gas pressure; Sparks As the chamber magma began to rise through volcanic conduit, the initially separated trachyte and rhyolite bodies began to be mechanically mixed by shear.The dominance of rhyolitic host glass relative to trachytic grass in the B-Tm pumice (Fig. 3b) implies that the incorporated trachyte magma was volumetrically smaller than the rhyolite magma.In this stage, sanidine phenocrysts containing abundant trachytic melt inclusions were drawn into the anorthoclase-bearing rhyolite magma body.As decompression reached a critical level, saturated volatiles (mostly H 2 O) began to spontaneously exsolve from the host melt and nucleate gas bubbles (Step 2).
Along with the continuous magma ascent, gas bubbles in the host melt progressively increased in size and population, and they attained tubular-stretched forms by shear.The increasing volume fraction of gas bubbles made the rising magma body more viscous but mechanically fragile and thus ultimately prone to fragmentation (Schmincke, 2004).As the magma approached the fragmentation level, both sanidine and anorthoclase phenocrysts began to rupture simultaneously owing to extreme decompression, shock wave, chocked ow, and fragmentation stresses (Bindeman, 2005;Miwa and Geshi, 2012;Maclennan, 2017).The simultaneous rupture of alkali-feldspars allowed the volatilesupersaturated melt inclusions to suddenly reach the near-atmospheric pressure and initiate the secondstage bubble formation (Step 3).
The melt inclusions, preserving the original volatile contents, then violently burst out of fractured crystal seals and instantaneously expanded to form spherical domains (bubble pockets) approximately 20% more vesicular than the host domain (Step 4).We deduce that the catastrophic burst of melt inclusions resulted in additional pressurization, expansion, and structural instability of the fragmenting magma body, ultimately enhancing the e ciency of magma fragmentation and the explosivity of the 946 CE Plinian eruption at Baekdusan.The highly variable volume fraction (6-24%) of the bubble pockets among the B-Tm pumice samples re ects incomplete cryptic magma mixing.Despite this heterogeneity, the impact of second-stage bubble formation on the erupting magma body appears to have been signi cant.

Glass shard heterogeneity in the B-Tm tephra
The B-Tm tephra, widespread over the northern hemisphere, shows a wide-range chemical heterogeneity in the glass shard composition between trachyte and rhyolite (Sun et al., 2014;Chen et al., 2016).The heterogeneity was attributed to the mixing of different magmas, which erupted in two different phases (rhyolitic phase-1 and trachytic phase-2) of the Millennium Eruption (Chen et al., 2016;Pan et al., 2017;Nara et al., 2021).However, the two-phase hypothesis appears unnecessary to account for the glass shard heterogeneity, given the pervasive chemo-textural heterogeneity (the trachytic type-A and rhyolitic type-B bubble pockets with the intermediate host domain) found in almost every clast of the phase-1 B-Tm gray pumice.
The phase-2 black trachytic pumice deposit is locally found on the summit caldera rim of Baekdusan and is absent in the medial and distal PDC deposits of the Millennium Eruption (Extended data 1; Wei et al., 2013;Pan et al., 2017).This infers that the 2nd phase of the Millennium Eruption was signi cantly less explosive than the 1st climactic Plinian phase, thus unlikely to have contributed to the formation of the distal B-Tm tephra.In conclusion, we suggest that the B-Tm tephra does not represent the whole eruptive phases of the Millennium Eruption but is only a product of the earlier climactic Plinian phase fed by a cryptically mixed source magma.

Conclusions
We have investigated the chemo-textural character of the proximal B-Tm pumice and drawn the following conclusions.
1.The earlier phase of the 946 CE Plinian eruption at Baekdusan was driven by cryptic magma mixing and simultaneous burst of melt inclusions as evidenced by the three distinctive chemo-textural domains (bubble pockets type-A and type-B and the host domain) within the B-Tm gray pumice.2. The simultaneous bursting of melt inclusions resulted in additional pressurization and structural instability of the magma and increased its volume by about 6-24% near the magma fragmentation level.The volume change of the magma probably contributed to increasing the eruption rate and exit velocity of tephra from the vent, thereby enhancing the overall explosivity of the 946 CE Plinian eruption at Baekdusan.
3. The heterogeneous glass shard chemistry of the distal B-Tm tephra is not a consequence of two eruptive phases of different magmas but of a single earlier climactic-phase Plinian eruption fed by a cryptically mixed source magma.
The bubble pocket texture reported here may be a common but overlooked feature of pumice generated by explosive silicic volcanism.The excess explosivity induced by the burst of melt inclusions at the nal moment of magma fragmentation seems to be an important process that needs to be taken into account when modelling the eruption dynamics and assessing volcanic hazards.Further investigation into the diversity of bubble pocket textures from multiple volcanoes may help understand the variations in eruption styles and intensities of silicic volcanoes.
et al., 1977; Cas and Wright, 2012) and readily cracked open the roof of the subvolcanic chamber that consequently initiated magma ascent for the 946 CE Plinian eruption (Step 1).
-scale cross-sectional images of the B-Tm gray pumice.(a) Photograph of a pumice hand specimen.(b) and (c) Computerized tomographic sections normal and parallel to the elongation directions of vesicles, showing the 3D shape and population of bubble pockets and host foam in the B-Tm gray pumice.(d) Merged SEM-BSE image showing details of bubbles, bubble walls and crystal fragments inand outside of a bubble pocket.SEM-BSE images showing textural details of (e) gas bubbles near the boundary of the bubble pocket, and (f) vesiculated melt inclusions bursting out from cavities in fragmented K-feldspar phenocryst.

Figure 2 Cross
Figure 2