Textural heterogeneities in pumices from the climactic eruption of Mount Pinatubo, 15 June 1991, and implications for magma ascent dynamics

The climactic event of Mount Pinatubo represents one of the most thoroughly studied eruptions of the century and has provided important insights into the dynamics of explosive volcanism. We have performed detailed textural analyses of the white and gray pumices of the plinian and pyroclastic flow deposits, and found that differences in color and clast density reflect different crystal and vesicle amounts and size distributions. White pumice has higher vesicularity, deformed and highly coalesced vesicles with thin walls, euhedral phenocrysts and microlite-free groundmass. Gray pumice shows lower vesicularity, wider ranges in vesicle number density, limited coalescence, vesicles with thick walls that are less deformed, phenocrysts and microphenocrysts with abundant solution pitting, and groundmass containing ubiquitous microlites and crystal fragments. The presence of white and gray pumice varieties and the broad range in vesicularity and vesicle number density that characterizes both of them appear to record the complexities of conduit processes such as magma vesiculation and fragmentation and the development of conduit regions marked by different rheological behaviors. In particular, the results of this study suggest the likely importance of intense shear and viscous dissipation at the conduit walls, a mechanism that may be responsible for the creation and discharge of the gray pumice of this eruption along with the dominant white variety.     

Heterogeneous pumice populations in the 2.08-Ma Cerro Gal��n Ignimbrite: implications for magma recharge and ascent preceding a large-volume silicic eruption

Triggering mechanisms of large silicic eruptions remain a critical unsolved problem. We address this question for the ~2.08-Ma caldera-forming eruption of Cerro Galán volcano, Argentina, which produced distinct pumice populations of two colors: grey (5%) and white (95%) that we believe may hold clues to the onset of eruptive activity. We demonstrate that the color variations correspond to both textural and compositional variations between the clast types. Both pumice types have bulk compositions of high-K, high-silica dacite to low-silica rhyolite, but there are sufficient compositional differences (e.g., ~150?ppm lower Ba at equivalent SiO₂ content and 0.03?wt.% higher TiO₂ in white pumice than grey) to suggest that the two pumice populations are not related by simple fractionation. Trace element concentrations in crystals mimic bulk variations between clast types, with grey pumice containing elevated Ba, Cu, Pb, and Zn concentrations in both bulk samples (average Cu, Pb, and Zn concentrations are 27, 35, and 82 in grey pumice vs. 11, 19, and 60 in white pumice) and biotite phenocrysts and white pumice showing elevated Li concentrations in biotite and plagioclase phenocrysts. White and grey clasts are also texturally distinct: White pumice clasts contain abundant phenocrysts (44?C57%), lack microlites, and have highly evolved groundmass glass compositions (76.4?C79.6?wt.% SiO₂), whereas grey pumice clasts contain a lower percentage of phenocrysts/microphenocrysts (35?C49%), have abundant microlites, and have less evolved groundmass glass compositions (69.4?C73.8?wt.% SiO₂). There is also evidence for crystal transfer between magma producing white and grey pumice. Thin highly evolved melt rims surround some fragmental crystals in grey pumice clasts and appear to have come from magma that produced white pumice. Furthermore, based on crystal compositions, white bands within banded pumice contain crystals originating in grey magma. Finally, only grey pumice clasts form breadcrusted surface textures. We interpret these compositional and textural variations to indicate distinct magma batches, where grey pumice originated from an originally deeper, more volatile-rich dacite recharge magma that ascended through and mingled with the volumetrically dominant, more highly crystalline chamber that produced white pumice. Shortly before eruption, the grey pumice magma stalled within shallow fractures, forming a vanguard magma phase whose ascent may have provided a trigger for eruption of the highly crystalline rhyodacite magma. We suggest that in the case of the Cerro Galán eruption, grey pumice provides evidence not only for cryptic silicic recharge in a large caldera system but also a probable trigger for the eruption.     

Complex changes in eruption dynamics during the 79 AD eruption of Vesuvius

The 79 AD eruption of Vesuvius included 8 eruption units (EU1–8) and several complex transitions in eruptive style. This study focuses on two important transitions: (1) the abrupt change from white to gray pumice during the Plinian phase of the eruption (EU2 to EU3) and (2) the shift from sustained Plinian activity to the onset of caldera collapse (EU3 to EU4). Quantification of the textural features within individual pumice clasts reveals important changes in both the vesicles and groundmass crystals across each transition boundary. Clasts from the white Plinian fall deposit (EU2) present a simple story of decompression-driven crystallization followed by continuous bubble nucleation, growth and coalescence in the eruptive conduit. In contrast, pumices from the overlying gray Plinian fall deposit (EU3) are heterogeneous and show a wide range in both bubble and crystal textures. Extensive bubble growth, coalescence, and the onset of bubble collapse in pumices at the base of EU3 suggest that the early EU3 magma experienced protracted vesiculation that began during eruption of the EU2 phase and was modified by the physical effects of syn-eruptive mingling-mixing. Pumice clasts from higher in EU3 show higher bubble and crystal number densities and less evidence of bubble collapse, textural features that are interpreted to reflect more thorough mixing of two magmas by this stage of the eruption, with consequent increases in both vesiculation and crystallization. Pumice clasts from a short-lived, high column at the onset of caldera collapse (EU4) continue the trend of increasing crystallization (enhanced by mixing) but, unexpectedly, the melt in these clasts is more vesicular than in EU3 and, in the extreme, can be classified as reticulite. We suggest that the high melt vesicularity of EU4 reflects strong decompression following the partial collapse of the magma chamber.Editorial responsibility: D.B. Dingwell     

The color of pumice: case study on a trachytic fall deposit, Meidob volcanic field, Sudan

 This paper examines the cause of color variations of trachytic pumices which are essentially uniform in chemical composition and proposes a geological model for their formation. A pyroclastic sequence of distinct subunits with brown, buff, and black pumices was deposited during the 5000-B.P. eruption of a tuff ring in the central Meidob volcanic field (Sudan). Subunits of buff pumices locally contain minor amounts of streaky pumice with pale-gray and dark-gray domains. The combined evidence of petrographic studies, chemical analyses of whole pumices and groundmass separates, electron microprobe analyses, optical spectroscopy, transmission electron microscopy, and magnetic susceptibility measurements show that color variations of the pumice clasts are related to the size and distribution of Fe³⁺-rich oxide microcrysts. Buff pumice and light-gray domains of streaky pumice have a colorless, transparent groundmass with very few microcrysts. Dark-gray domains of streaky pumice contain abundant hematite and/or magnetite microcrysts visible in thin section within a transparent, colorless glass groundmass. The groundmass of the black pumice clasts is brown in thin section which is most likely caused by submicroscopic magnetite microcrysts. Brown pumice clasts have a mixed groundmass consisting of brown domains and domains with opaque microcrysts in transparent glass. Variations in the eruption dynamics have been inferred from lithological observations. Subunits of black pumices are related to eruption pulses with low magma discharge and high water/magma mass ratio, whereas subunits of buff pumice were deposited during eruption pulses with high magma discharge and low water/magma mass ratio. Brown pumices represent the top part of the magma body, and the initial stage of the eruption probably had a low magma discharge. Streaky pumices are interpreted as the product of syn-eruptive mixing of Fe³⁺-rich oxide microcryst-bearing magma and microcryst-free magma. Received: 3 February 1997 / Accepted: 28 July 1997     

The Plinian phase of the Campanian Ignimbrite eruption (Phlegrean Fields,Italy): evidence from density measurements and textural characterization of pumice

Textural characterization of pumice clasts from explosive volcanic eruptions provides constraints on magmatic processes through the quantification of crystal and vesicle content, size, shape, vesicle wall thickness and the degree of interconnectivity. The Plinian fallout deposit directly underlying the Campanian Ignimbrite (CI) eruption represents a suitable case to investigate pumice products with different textural characteristics and to link the findings to processes accompanying conduit magma ascent to the crater. The deposit consists of a lower (LFU) and upper (UFU) pumice lapilli bed generated by the sub-steady eruption of trachytic magma with <5 vol%. crystals and a peak discharge rate of 3.2×10 ⁸ kg/s. Density measurements were performed on samples collected from different stratigraphic intervals at the Voscone-type outcrop, and their textural characteristics were investigated at different magnifications through image analysis techniques. According to clast densities, morphologies and vesicle textures pumice clasts were classified into microvesicular (heterogeneous vesicles), tube (elongated/deformed vesicles) and expanded (coalesced/inflated vesicles).The combination of density data and textural investigations allowed us to characterize both representative areas and textural extremes of pumice products. Bulk vesicularity spans a broad interval varying from 0.46 to >0.90, with vesicle number density ranging from 10 ⁷–10 ⁸ cm ^-3. The degree of vesicle coalescence is high for all pumice types, with interconnected vesicles generally representing more than 90% of the bulk vesicle population. The results show a high degree of heterogeneous textures among pumice clasts from both phases of the eruption and within each eruption phase, the different pumice types and also within each single pumice type fragment. The origin of pumice clasts with different textural characteristics is ascribed to the development of conduit regions marked by different rheological behavior. The conclusions of this study are that vesicle deformation, degree of coalescence and intense shear at the conduit walls play a major role on the degassing process, hence affecting the entire conduit dynamics.     

Changes in eruptive style during the A.D. 1538 Monte Nuovo eruption (Phlegrean Fields, Italy): the role of syn-eruptive crystallization

The Monte Nuovo eruption is the most recent event that occurred at Phlegrean Fields (Italy) and lasted from 29 September to 6 October 1538. It was characterized by 2 days of quasi-sustained phreatomagmatic activity generating pumice-bearing pyroclastic density currents and forming a 130-m-high tuff cone (Lower Member deposits). The activity resumed after a pause of 2 days with two discrete Vulcanian explosions that emplaced radially distributed, scoria-bearing pyroclastic flows (Upper Member deposits). The juvenile products of Lower and Upper Members are, respectively, phenocryst-poor, light-coloured pumice and dark scoria fragments with K-phonolitic bulk compositions, identical in terms of both major and trace elements. Groundmass is formed by variable proportions of K-feldspar and glass, along with minor sodalite and Fe-Ti oxide present in the most crystallized samples. Investigations of groundmass compositions and textures were performed to assess the mechanisms of magma ascent, degassing and fragmentation along the conduit and implications for the eruptive dynamics. In pumice of the Lower Member groundmass crystal content increases from 13 to 28 vol% from the base to the top of the sequence. Products of the Upper Member consist of clasts with a groundmass crystal content between 30 and 40 vol% and of totally crystallized fragments. Crystal size distributions of groundmass feldspars shift from a single population at the base of the Lower Member to a double population in the remaining part of the sequence. The average size of both populations regularly increases from the Lower to the Upper Member. Crystal number density increases by two orders of magnitude from the Lower to the Upper Member, suggesting that nucleation dominated during the second phase of the eruption. The overall morphological, compositional and textural data suggest that the juvenile components of the Monte Nuovo eruption are likely to record variations of the magma properties within the conduit. The different textures of pumice clasts from the Lower Member possibly reflect horizontal gradients of the physical properties (P, T) of the ascending magma column, while scoriae from the second phase are thought to result from the disruption of a slowly rising plug crystallizing in response to degassing. In particular, crystal size distribution data point to syn-eruptive degassing-induced crystallization as responsible for the transition in eruptive style from the first to the second phase of the eruption. This mechanism not only has been proved to profoundly affect the dynamics of dome-forming calc-alkaline eruptions, but may also have a strong influence in driving the eruption dynamics of alkaline magmas of intermediate to evolved compositions.Editorial responsibility: J. Donnelly-Nolan     

Crystallization of microlites during magma ascent: the fluid mechanics of 1980–1986 eruptions at Mount St Helens

Eruptions of Mount St Helens (Washington, USA) decreased in intensity and explosivity after the main May 18, 1980 eruption. As the post-May 18 eruptions progressed, albitic plagioclase microlites began to appear in the matrix glass, although the bulk composition of erupted products, the phenocryst compositions and magmatic temperatures remained fairly constant. Equilibrium experiments on a Mount St Helens white pumice show that at 160 MPa water pressure and 900°C, conditions deduced for the 8 km deep magma storage zone, the stable plagioclase is An₄₇. The microlites in the natural samples, which are more albitic, had to grow at lower water pressures during ascent. Isothermal decompression experiments reported here demonstrate that a decrease in water pressure from 160 to 2 MPa over four to eight days is capable of producing the albitic groundmass plagioclase and evolved melt compositions observed in post-May 18 1980 dacites. Because groundmass crystallization occurs over a period of days during and after decreases in pressure, microlite crystallization in the Mount St Helens dacites must have occurred during the ascent of each magma batch from a deep reservoir rather than continuously in a shallow holding chamber. This is consistent with data on the kinetics of amphibole breakdown, which require that a significant portion of magma vented in each eruption ascended from a depth of at least 6.5 km (160 MPa water pressure) in a matter of days. The size and shape of the microlite population have not been studied because of the small size of the experimental samples; it is possible that the texture continues to mature long after chemical equilibrium is approached. As the temperature, composition, crystal content and water content of magma in the deep reservoir remained approximately constant from May 1980 to at least March 1982, the spectacular decrease in eruption intensity during this period cannot be attributed to changes in viscosity or density of the magma. Simple fluld mechanical considerations indicate, however, that the observed changes in mass flux of magma can be modelled by a five-fold decrease in conduit radius from 35 to 7 m, produced perhaps by plating of magma along the conduit walls. The decreased ascent rates which accompanied the decrease in conduit radius can explain the change from closed-system to open-system degassing and the shift from explosive to effusive eruptions during 1980.     

A COMPARATIVE STUDY ON THE CHARACTERISTICS OF TWO STAGES OF FALLOUT PUMICES DEPOSITS FROM THE MILLENNIUM ERUPTION OF TIANCHI VOLCANO IN CHANGBAISHAN AREA

Tianchi volcano in Changbaishan area is located at the border between China and Democratic People's Republic of Korea, and is one of the most dangerous volcanoes in China. It has experienced several explosive eruptions in late Pleistocene and Holocene, i.e. 50000aBP eruption, 946 AD eruption, 1668 AD eruption, 1702 AD eruption, 1903 AD eruption. Especially, the 946 AD eruption(also known as "Millennium eruption")of this volcano is considered to be one of the largest volcanic eruptions in the world in the past 2000a. The eruption history and strata sequence of Tianchi volcano have long been the focus of attention. The stratigraphic unit division of fallout deposits in the past millennium is controversial, especially for the heterogeneous trachytic pumices(erupted from the Yuanchi stage)above the off-white pumices(erupted from the Chifeng stage). In this paper, through the detailed field exploration and strata comparation, it was found that there was no depositional interval between the two stage eruptions, or the interval was not long, and thus, it is believed that two stages of fallout pumice should be classified into the Millennium eruption. The off-white fallout pumices in Chifeng stage are relatively homogeneous, with angular shape, normal grading and good sorting. The median size(Md_Φ)and the sorting coefficient(σ_Φ)of Chifeng pumice are in the range of -4.25~-1.3 and 0.93~1.53, respectively. The eruption of Yuanchi stage is in pulsing pattern, and the strata show interbedding of rich khaki pumice layer and rich black pumice layer. The pumices with angular shape show inconspicuous grain grading and good sorting. The median size(Md_Φ)and the sorting coefficient(σ_Φ)of Yuanchi pumice are in the range of -2.55~-0.6 and 1~1.68, respectively. Both the granularities of the pumice particles from two stages are normally distributed and fall into the air-fall field in the median diameter versus sorting diagram. The pumices from 50000aBP and pyroclastic flow of Millennium eruption were also shown in the diagram. Phenocrysts in pumices are mainly feldspar and pyroxene, but the phenocrysts with obvious resorbed characteristic in Yuanchi black pumice are bigger, and the phenocryst contents are a little higher than those in others. Feldspar content in off-white pumice in Chifeng stage was 0.24%~1.77%, that in khaki pumice in Yuanchi stage was 0.2%~7.5%, and that in black pumice in Yuanchi stage was 3.02%~8.0%. The phenocrysts in Chifeng pumice are broken, which represents more violent explosion. The vesicles inside the pumice also reflect the intensity of the eruption. The Chifeng pumices have large, continuous vesicles and thin vesicle walls. The Yuanchi khaki pumices have continuous vesicles but thicker vesicle wall than the Chifeng pumices. The vesicularity is the lowest and the vesicle walls are the thickest in the black pumices in Yuanchi stage, indicating the eruption strength become weaker from Chifeng stage to Yuanchi stage. The Chifeng pumices with SiO₂ content of 69.12~72.71wt%, K₂O content of 4.33~4.52wt%, Na₂O content of 5.26~5.39wt%, Al₂O₃ content of 10.32~11.99wt%, CaO content of 0.29~0.95wt%, MgO content of 0.11~0.51wt%, TiO₂ content of 0.23~0.43wt% are comendite in composition. The pumices from 50000aBP eruption are comendite in composition, and their SiO₂ content(65.56~68.28wt%)is slightly lower than Chifeng pumices. The Yuanchi khaki pumices with SiO₂ content of 62.14~63.29wt%, K₂O content of 5.35~5.7wt%, Na₂O content of 5.35~5.62wt%, Al₂O₃ content of 15.00~15.59wt%, CaO content of 1.06~1.61wt%, MgO content of 0.25~0.57wt%, TiO₂ content of 0.4~0.64wt% belong to trachyte in composition, and are close to the composition of the black pumices on the Tianwen Peak. The Yuanchi black pumices are also trachyte in composition, but have obviously lower SiO₂(59.51~60.59wt%), K₂O(4.39~4.84wt%), and Na₂O(4.94~5.08wt%)content, and higher Al₂O₃(15.81~16.42wt%), CaO(2.78~3.66wt%), MgO(1.43~1.9wt%), TiO₂(1.04~1.4wt%)content than the khaki pumices. The above results show that the eruptive intensity of the Yuanchi stage is weaker than that of the Chifeng stage and the several magmatic compositions of pumices from the Millennium eruption reveal a complex magma system under the Tianchi volcano. The magma layers with different compositions may exist in the magma chamber contemporaneously. At Chifeng stage, only the upper comendite magma erupted, but the magma below erupted in the pulsing pattern at the Yuanchi stage.     

Modeling of the ascent of magma during the plinian eruption of Vesuvius in A.D. 79

The ascent of magma during the A.D. 79 eruption of Vesuvius was studied by a steady-state, one-dimensional, and nonequilibrium two-phase flow model. The gas exsolution process was modeled by assuming a chemical equilibrium between the exsolved and dissolved gas, whereas the magma density and viscosity were modeled by accounting for the crystal content in magma. The exsolution, density, and viscosity models consider the effect of different compositions of the white and gray magmas. By specifying the conduit geometry and magma composition, and employing the model to search for the maximum discharge rate of magma which is consistent with the specified geometry and magma composition, the model was then used to establish the two-phase flow parameters along the conduit. It was found that for all considered conditions the magma pressure in the conduit decreases below the lithostatic pressure near the magma fragmentation level, and that in the deep regions of the conduit the white magma pressure is larger and the gray magma pressure is lower than the lithostatic one. The exsolution and fragmentation levels were found to be deeper for the white than for the gray magma, and the changing composition during the eruption causes an increase of the exit pressure and decrease of the exit gas volumetric fraction. The model also predicted a minimum conduit diameter which is consistent with the white and gray magma compositions and mass flow-rates. The predictions of the model were shown to be consistent with column collapses during the gray eruption phase, large presence of carbonate lithics in the gray pumice fall deposit, and magma-water interaction during a late stage of the eruption.     

Transitions between fall phases and pyroclastic density currents during the AD 79 eruption at Vesuvius: building a transient conduit model from the textural and volatile record

The magmatic phase of the AD 79 eruption of Vesuvius produced alternations of fall and pyroclastic density current (PDC) deposits. A previous investigation demonstrated that the formation of several PDCs was linked with abrupt increases in the proportion of denser juvenile clasts within the eruptive column. Under the premise that juvenile clast density is controlled by vesiculation processes within the conduit, we investigate the processes responsible for these variations at or close to fragmentation levels. Pumice textures (vesicle sizes, numbers, and connectivity combined with crystal textures) from the AD 79 PDC deposits are compared to those from interbedded fall samples. Both PDC and fall deposits preserve textures that represent a full spectrum of degassing and outgassing processes, from bubble nucleation to collapse. Combining the textural and volatile (groundmass H₂O) data, we derive a conduit model that satisfies all the textural and physical observations made for this phase of the eruption: lateral vesicularity/density stratifications are produced by maturing of bubble textures with superimposed localized shearing of bubble-rich magmas, which enhance outgassing of H₂O. The incorporation of denser slower-moving magma from the conduit margins (??lateral magma density gradient??) is likely to be responsible for the higher abundances of dense juvenile pumice that triggered partial column collapses. We also illustrate how variations in the fragmentation depth (tapping a ??vertical magma density gradient??) can be responsible for variations in erupted clast density distributions, and potentially in the extent of degassing/outgassing.     

Multiple levels of magma storage during the 1980 summer eruptions of Mount St. Helens, WA

Transitions in eruptive style—explosive to effusive, sustained to pulsatory—are a common aspect of volcanic activity and present a major challenge to volcano monitoring efforts. A classic example of such transitions is provided by the activity of Mount St. Helens, WA, during 1980, where a climactic Plinian event on May 18 was followed by subplinian and vulcanian eruptions that became increasing pulsatory with time throughout the summer, finally progressing to episodic growth of a lava dome. Here we use variations in the textures, glass compositions and volatile contents of melt inclusions preserved in pyroclasts produced by the summer 1980 eruptions to determine conditions of magma ascent and storage that may have led to observed changes in eruptive activity. Five different pyroclast types identified in pyroclastic flow and fall deposits produced by eruptions in June 12, July 22 and August 7, 1980, provide evidence for multiple levels of magma storage prior to each event. Highly vesicular clasts have H₂O-rich (4.5–5.5 wt%) melt inclusions and lack groundmass microlites or hornblende reaction rims, characteristics that require magma storage at P≥160 MPa until shortly prior to eruption. All other clast types have groundmass microlites; P_H20 estimated from both H₂O-bearing melt inclusions and textural constraints provided by decompression experiments suggest pre-eruptive storage pressures of ∼75, 40, and 10 MPa. The distribution of pyroclast types within and between eruptive deposits can be used to place important constraints on eruption mechanisms. Fall and flow deposits from June 12, 1980, lack highly vesicular, microlite-free pyroclasts. This eruption was also preceded by a shallow intrusion on June 3, as evidenced by a seismic crisis and enhanced SO₂ emissions. Our constraints suggest that magma intruded to a depth of ≤4 km beneath the crater floor fed the June eruption. In contrast, eruptions of July and August, although shorter in duration and smaller in volume, erupted deep volatile-rich magma. If modeled as a simple cylinder, these data require a step-wise decrease in effective conduit diameter from 40–50 m in May and June to 8–12 m in July and August. The abundance of vesicular (intermediate to deep) clast types in July and August further suggests that this change was effected by narrowing the shallower part of the conduit, perhaps in response to solidification of intruded magma remaining in the shallow system after the June eruption. Eruptions from July to October were distinctly pulsatory, transitioning between subplinian and vulcanian in character. As originally suggested by Scandone and Malone (1985), a growing mismatch between the rate of magma ascent and magma disruption explains the increasingly pulsatory nature of the eruptions through time. Recent fragmentation experiments Spieler et al. (2004) suggest this mismatch may have been aided by the multiple levels at which magma was stored (and degassed) prior to these events.Editorial responsibility: J Stix     

Sr–Nd isotopic and chemical characteristics of the silicic magma reservoir of the Aira pyroclastic eruption, southern Kyushu, Japan

Sr and Nd isotope and geochemical investigations were performed on a remarkably homogeneous, high-silica rhyolite magma reservoir of the Aira pyroclastic eruption (22,000 years ago), southern Kyushu, Japan. The Aira caldera was formed by this eruption with four flow units (Osumi pumice fall, Tsumaya pryoclastic flow, Kamewarizaka breccia and Ito pyroclastic flow). Quite narrow chemical compositions (e.g., 74.0–76.5 wt% of SiO₂) and Sr and Nd isotopic values (⁸⁷Sr/⁸⁶Sr=0.70584–0.70599 and _Nd=−5.62 to −4.10) were detected for silicic pumices from the four units, with the exception of minor amounts of dark pumices in the units. The high Sr isotope ratios (0.7065–0.7076) for the dark pumices clearly suggest a different origin from the silicic pumices. Andesite to basalt lavas in pre-caldera (0.37–0.93 Ma) and post-caldera (historical) eruptions show lower ⁸⁷Sr/⁸⁶Sr (0.70465–0.70540) and higher _Nd (−1.03 to +0.96) values than those of the Aira silicic and dark pumices. Both andesites of pre- and post-caldera stages are very similar in major- and trace-element characteristics and isotope ratios, suggesting that the both andesites had a same source and experienced the same process of magma generation (magma mixing between basaltic and dacitic magmas). Elemental and isotopic signatures deny direct genetic relationships between the Aira pumices and pre- and post-caldera lavas. Relatively upper levels of crust (middle–upper crust) are assumed to have been involved for magma generation for the Aira silicic and dark pumices. The Aira silicic magma was derived by partial melting of a separate crust which had homogeneous chemistry and limited isotope compositions, while the magma for the Aira dark pumice was generated by AFC mixing process between the basement sedimentary rocks and basaltic parental magma, or by partial melting of crustal materials which underlay the basement sediments. The silicic magma did not occupy an upper part of a large magma body with strong compositional zonation, but formed an independent magma body within the crust. The input and mixing of the magma for dark pumices to the base of the Aira silicic magma reservoir might trigger the eruptions in the upper part of the magma body and could produce a slight Sr isotope gradient in the reservoir. An extremely high thermal structure within the crust, which was caused by the uprise and accumulation of the basaltic magma, is presumed to have formed the large volume of silicic magma of the Aira stage.     

Mechanisms for transition in eruptive style at a monogenetic scoria cone revealed by microtextural analyses (Lathrop Wells volcano, Nevada, U.S.A.)

Explosive activity at Lathrop Wells volcano, Nevada, U.S.A. originated with weak Strombolian (WS) eruptions along a short fissure, and transitioned to violent Strombolian (VS) activity from a central vent, with lava effusion during both stages. The cause for this transition is unknown; it does not reflect a compositional change, as evidenced by the consistent bulk geochemistry of all the eruptive products. However, comparison of agglutinate samples from the early, WS events with samples of scoria from the later, VS events reveal differences in the abundance and morphology of groundmass phases and variable textures in the rims of olivine phenocrysts. Scanning electron microscope (SEM) examination of thin sections from the WS samples show euhedral magnetite microlites in the groundmass glass and olivine phenocrysts show symplectite lamellae in their rims. Secondary ion mass spectrometry (SIMS) depth profiles of these symplectites indicate they are diffusion-controlled. The calculated D_Fe-Mg allows an estimation of the oxygen fugacity (fO₂) and indicates an increased fO₂ during eruption of the WS products. Conversely, the VS samples show virtually no magnetite microlites in the groundmass glass, a lack of symplectites in the olivines, and a lower calculated fO₂. These microtextural features suggest that the Lathrop Wells trachybasalt experienced increased oxidation during WS activity. As magma ascended through the original fissure, exsolved bubbles were concentrated in the wider part(s) (the protoconduit) and this bubble flux drove convective circulation that oxidized the magma through exposure to atmosphere and recirculation. This oxidation resulted in groundmass crystallization of magnetite within the melt and formation of symplectites within the olivine phenocrysts. Bubble-driven convection mixed magma vertically within the protoconduit, keeping it fluid and driving Strombolian bursts, while microlite crystallization in narrower parts of the fissure helped to focus flow. Development of a central conduit increased the magma ascent velocity (due to a greater product volume in the later eruptive stages) and caused the shift in eruption intensity. Consequently, variations in microtextures of the Lathrop Wells products reveal how a combination of fluid dynamic and crystallization processes in the ascending magma resulted in different styles of activity while the products maintained a consistent bulk composition.     

Textural and geochemical constraints on eruptive style of the 79 <Emphasis Type="SmallCaps">ad</Emphasis> eruption at Vesuvius

The 79 ad Plinian eruption of Vesuvius produced first a white pumice fallout from a high steady eruptive column, and then a grey pumice fallout originating from an oscillatory eruptive column with several partial column collapse events after which there was a total column collapse. This first total collapse was followed by renewed Plinian activity and produced the last grey pumice (GP) fallout deposit of the eruption. Textural characteristics (vesicularity and microcrystallinity) of a complete sequence of the pumice fallout deposits are presented along with the major element compositions and residual volatile contents (H₂O, Cl) to constrain the degassing processes and the eruptive dynamics. Large variations in residual volatile contents exist between the different eruptive units. Textural features also strongly differ between white and grey pumices, but also within the grey pumices. The degassing processes were thus highly heterogeneous. We propose a new model of the 79 ad eruption in which pre-eruptive conditions (H₂O saturation, magma temperature and viscosity) are the critical controls on the diversity of the syn-eruptive degassing processes and hence the eruptive dynamics. Cl contents measured in melt inclusions show that only the white pumice and the upper part of the grey pumice magma were H₂O saturated prior to eruption. The white pumice eruptive units represent a typical closed-system degassing evolution, whereas the first grey pumice one, stored under similar pre-eruptive saturation conditions, follows a particular open-system degassing evolution. We suggest that the oscillatory regime that dominated the grey pumice eruptive phase is linked to pre-eruptive water undersaturation of most of the grey magma, and the associated time delays necessary for H₂O exsolution. We also suggest that the high residual H₂O content of the last grey pumice, deposited after the renewal of Plinian activity following the first total column collapse event, is due to syn-eruptive saturation of GP magma and reduced H₂O exsolution efficiency resulting from speciation of dissolved H₂O in the melt.     

长白山天池火山千年大喷发不同颜色浮岩的岩石化学特征

长白山天池火山大约 1 000年前的大喷发,形成了巨厚的火山碎屑流堆积层,其主要组成是浮岩与火山灰。以往的研究普遍认为其中的浮岩为灰白色,属流纹质。笔者在考察中发现了不少黑色及少量其它颜色的浮岩,系统地采集了各色样品作浮岩化学成分分析,结果表明,灰白色浮岩与黑色浮岩分别为流纹质和粗面质,灰色浮岩属于粗面质但靠近流纹质端元。它们都来源于地壳岩浆房,是岩浆房内不同分异演化阶段的产物,它们同时喷出说明岩浆房内具有分带性及不同性质岩浆的混合     

Permeability and pore-connectivity variation of pumices from a single pyroclastic flow eruption: Implications for partial fragmentation

The relationship between permeability and vesicularity in volcanic rocks has been used to infer the degassing behavior of hydrous magma. Recent data on natural samples from various eruptions show a wide variation, fitting a power–law relationship of the percolation models with low (< 30%) critical vesicularity (Ф_C). In this study, we present data on permeability and pore-connectivity of juvenile rhyolitic pumice clasts in a pyroclastic flow around Onikobe volcano, NE Japan, and investigate their relationship with vesicularity developed in a single eruption event. The permeability of the pumices having a relatively low abundance of microlites and microphenocrysts shows a trend increasing by 4 orders of magnitude (from 10^− 13.8 to 10^− 10.1 m²) in a high and narrow vesicularity range (from 72 to 80%). This trend intersects at a high angle with the fit to the permeability–vesicularity data in the previous studies that has a low Ф_C, and is located on the extension of the trend for the products of isotropic decompression experiments. The two-dimensional (2D) connectivities of pores for the pumices were also measured from thin sections. From the point of view of percolation theory, connectivity provides information about the probability of percolation. They showed a steep increase from ca. 0 to 0.7 in an almost similar vesicularity range, as compared to their permeabilities. We attribute the increase in 2D connectivity to the increasing amount of ruptured bubble walls, which might have provided less-tortuous paths through larger apertures for gas flow. This, in turn, would cause an effective increase in the permeability. Aggregates of bubble-wall-shaped glass shards were found in the pumices, and their amount and degree of welding are higher in the pumices that have a higher abundance of microlites and microphenocrysts. These pumices have relatively high permeability and 2D connectivity at low vesicularity, which is accounted for by the existence of large irregularly shaped pores. These textural characteristics suggest that a series of partial fragmentation processes, including local rupturing of bubble walls and subsequent foam-collapse with permeable gas flow, might have occurred before the ultimate bulk fragmentation, thus resulting in the increase in permeability. We suggest that the 2D connectivity of pores is a useful parameter to quantify the degree of fragmentation of bubble walls and has the potential for use to assess their permeability.     

Magmatic processes revealed by textural and compositional trends in Merapi dome lavas

Syn-eruptive degassing of volcanoes may lead to syn-eruptive crystallization of groundmass phases. We have investigated this process using textural and compositional analysis of dome material from Merapi volcano, Central Java, Indonesia. Samples included dome lavas from the 1986–88, 1992–93, 1994 and 1995 effusive periods as well as pyroclastic material deposited by the November 1994 dome collapse. With total crystallinities commonly in excess of 70% (phenocrysts+microlites), the liquids present in Merapi andesites are highly evolved (rhyolitic) at the time of eruption. Feldspar microlites in dome rocks consist of plagioclase cores (Ab₆₃An₂₉Or₈) surrounded by alkali feldspar rims (Ab₅₃An₅Or₄₂), compositional pairs which are not in equilibrium. A change in the phase relations of the ternary feldspar system caused by degassing best explains the observed transition in feldspar composition. A small proportion of highly vesicular airfall tephra grains from the 1994 collapse have less evolved glass compositions than typical dome material and contain rimless plagioclase microlites, suggesting that the 1994 collapse event incorporated less-degassed, partially liquid magma in addition to fully solidified dome rock.As decompression drives volatile exsolution, rates of degassing and resultant microlite crystallization may be governed by magma ascent rate. Microlite crystallinity is nearly identical among the 1995 dome samples, an indication that similar microlite growth conditions (P_H2O and temperature) were achieved throughout this extrusive period. However, microlite number density varied by more than a factor of four in these samples, and generally increased with distance from the vent. Low vent-ward microlite number densities and greater microlite concentrations down-flow probably reflect progressively decreasing rates of undercooling at the time of crystal nucleation during extrusion of the 1995 dome. Comparison between dome extrusion episodes indicates a correlation between lava effusion rate and microlite number density, suggesting that extrusion slowed during 1995. Crystal textures and compositions in the 1992–93 and 1994 domes share the range exhibited by the 1995 dome, suggesting that transitions in crystallization conditions (i.e., rates of undercooling determined by effusion rate) are cyclic.     

Fault textures in volcanic conduits: evidence for seismic trigger mechanisms during silicic eruptions

It is proposed that fault textures in two dissected rhyolitic conduits in Iceland preserve evidence for shallow seismogenic faulting within rising magma during the emplacement of highly viscous lava flows. Detailed field and petrographic analysis of such textures may shed light on the origin of long-period and hybrid volcanic earthquakes at active volcanoes. There is evidence at each conduit investigated for multiple seismogenic cycles, each of which involved four distinct evolutionary phases. In phase 1, shear fracture of unrelaxed magma was triggered by shear stress accumulation during viscous flow, forming the angular fracture networks that initiated faulting cycles. Transient pressure gradients were generated as the fractures opened, which led to fluidisation and clastic deposition of fine-grained particles that were derived from the fracture walls by abrasion. Fracture networks then progressively coalesced and rotated during subsequent slip (phase 2), developing into cataclasite zones with evidence for multiple localised slip events, fluidisation and grain size reduction. Phase 2 textures closely resemble those formed on seismogenic tectonic faults characterised by friction-controlled stick-slip behaviour. Increasing cohesion of cataclasites then led to aseismic, distributed ductile deformation (phase 3) and generated deformed cataclasite zones, which are enriched in metallic oxide microlites and resemble glassy pseudotachylite. Continued annealing and deformation eventually erased all structures in the cataclasite and formed microlite-rich flow bands in obsidian (phase 4). Overall, the mixed brittle–ductile textures formed in the magma appear similar to those formed in lower crustal rocks close to the brittle–ductile transition, with the rheological response mediated by strain-rate variations and frictional heating. Fault processes in highly viscous magma are compared with those elsewhere in the crust, and this comparison is used to appraise existing models of volcano seismic activity. Based on the textures observed, it is suggested that patterns of long-period and hybrid earthquakes at silicic lava domes reflect friction-controlled stick-slip movement and eventual healing of fault zones in magma, which are an accelerated and smaller-scale analogue of tectonic faults.Editorial responsibility: J. Stix     

Constraints on cooling rates and permeabilities of pumice in an explosive eruption jet from colour and magnetic mineralogy

In explosive volcanic eruptions, vesicular magma droplets, produced by fragmentation, are propelled into the atmosphere where they are chilled to form pumices. The thermal history of droplets and the permeability of their internal bubble networks determine how much they are deformed in the eruption jet, and hence what information pumices record about the state of the magma at fragmentation. We study these aspects of the `Minoan' plinian eruption of Santorini Volcano by quantifying the rate of oxidation reactions that took place when air entered the hot magma fragments. In our experiments white Minoan pumices were heated for minutes to hours between 600 and 850°C, either in air, or in an atmosphere with an oxygen fugacity at the Ni–NiO buffer. Pumices were unchanged by heating at Ni–NiO. Those heated in air often became pink to dark pink, depending on heating time, and their Curie temperatures, as determined by magnetic susceptibility measurements, increased. We use oxidation rates deduced from these experiments, in conjunction with calculations of the rate of conductive cooling and of the rate at which air can enter a pumice, to constrain the conditions experienced by pumices during the eruption. Natural Minoan pumices less than about 5 cm in radius are white, whereas larger ones often have white rims and pink interiors with Curie temperatures higher than those of white material. We infer that small pumices were cooled before being oxidized, and that oxidation of the interiors of large clasts mostly took place during flight, at temperatures within a few tens of degrees of magmatic values. White rims of large pumices, despite being permeable, were cooled before oxidation could occur. Permeability developed in the liquid state, but did not develop early enough, with respect to cooling, or was not large enough to allow extreme oxidation. We give measurements of pumice permeabilities that should be close to magmatic values.     

Characterization of juvenile pyroclasts from the Kos Plateau Tuff (Aegean Arc): insights into the eruptive dynamics of a large rhyolitic eruption

Silicic pumices formed during explosive volcanic eruptions are faithful recorders of the state of the magma in the conduit, close to or at the fragmentation level. We have characterized four types of pumices from the non-welded rhyolitic Kos Plateau Tuff, which erupted 161,000 years ago in the East Aegean Arc, Greece. The dominant type of pumice (>90 vol.%) shows highly elongated tubular vesicles. These tube pumices occur throughout the eruption. Less common pumice types include: (1) “frothy” pumice (highly porous with large, sub-rounded vesicles), which form 5–10 vol.% of the coarsest pyroclastic flow deposits, (2) dominantly “microvesicular” and systematically crystal-poor pumices, which are found in early erupted, fine-grained pyroclastic flow units, and are characterized by many small (<50 μm in diameter) vesicles and few mm-sized, irregular voids, (3) grey or banded pumices, indicating the interaction between the rhyolite and a more mafic magma, which are found throughout the eruption sequence and display highly irregular bubble shapes. Except for the grey-banded pumices, all three other types are compositionally identical and were generated synchronously as they are found in the same pyroclastic units. They, therefore, record different conditions in the volcanic conduit leading to variable bubble nucleation, growth and coalescence. A total of 74 pumice samples have been characterized using thin section observation, SEM imagery, porosimetry, and permeametry. We show that the four pumice types have distinct total and connected porosity, tortuosity and permeability. Grey-banded pumices show large variations in petrophysical characteristics as a response to mingling of two different magmas. The microvesicular, crystal-poor, pumices have a bimodal bubble size distribution, interpreted as reflecting an early heterogeneous bubble nucleation event followed by homogeneous bubble nucleation close to fragmentation. Finally, the significant differences in porosity, tortuosity and permeability in compositionally identical tube and frothy pumices are the result of variable shear rates in different parts of the conduit. Differential shear rates may be the result of either: (1) pure shear, inducing a vertical progression from frothy to tube and implying a relatively thick fragmentation zone to produce both types of pumices at the same time or (2) localized simple shear, inducing strongly tubular vesicles along the wall and near-spherical bubbles in the centre of the conduit and not necessarily requiring a thick fragmentation zone.