Magma Decompression Research Articles

The initiation and development of a caldera-forming Plinian eruption (172 ka Lower Pumice 2 eruption, Santorini, Greece)

The rhyodacitic 172ka Lower Pumice 2 (LP2) eruption terminated the first magmatic cycle at Santorini (Greece), producing a proximal <50m thick succession of pyroclastic fall deposits, diffusely-stratified to massive ignimbrites and multiple lithic breccias. The eruption commenced with the development of a short-lived precursory eruption column, depositing a <15cm blanket of 1–2cm sized pumice fragments at near vent localities (LP2-A1). The precursor deposits are conformably overlain by a <30m thick sequence of reversely-graded/ungraded pumice fall deposits that reflect opening and widening of a point-source vent, increasing mass discharge rates up to 108kgs−1, and the development of a 36km high Plinian eruption column (LP2-A2, A3). The progressive increase in maximum vesicle number density (NVF) in rhyodacitic pumice, from 3.2×109cm−3 in the basal fall unit of LP2-A2-1 to 9.2×109cm−3 in LP2-A3, translates to an increase in magma decompression rate from 18 to 29MPas−1 over the course of the initial Plinian phase. This is interpreted to be a consequence of progressive vent widening and a deepening of the fragmentation surface. Such interpretations are supported by the increase in lithic clast abundance vertically through LP2-A, and the occurrence of basement-derived (deep) lithic components in LP2-A3. The increasing lithic clast content and the inability to effectively entrain air into the eruption column, due to vent widening, resulted in column collapse and the development of pyroclastic density currents (PDCs; LP2-B). A major vent excavation event or the opening of new vents, possibly associated with incipient caldera collapse, facilitated the ingress of water into the magmatic system, the development of widespread PDCs and the deposition of a <20m thick massive phreatomagmatic tuff (LP2-C). The eruption cumulated in catastrophic caldera collapse, the enlargement of a pre-existing flooded caldera and the discharge of lithic-rich PDCs, depositing proximal <9m thick lithic lag breccias (LP2-D).

High magma decompression rates at the peak of a violent caldera-forming eruption (Lower Pumice 1 eruption, Santorini, Greece)

We use the deposit sequence resulting from the first catastrophic caldera collapse event recorded at Santorini (associated with 184 ka Lower Pumice 1 eruption), to study the shallow conduit dynamics at the peak of caldera collapse. The main phase of the Lower Pumice 1 eruption commenced with the development of a sustained buoyant eruption column, producing a clast-supported framework of rhyodacitic white pumice (LP1-A). The clasts have densities of 310–740 kg m−3, large coalesced vesicles that define unimodal size distributions and moderate to high vesicle number densities (1.2 × 109–1.7 × 109 cm−3). Eruption column collapse, possibly associated with incipient caldera collapse, resulted in the development of pyroclastic flows (LP1-B). The resulting ignimbrite is characterised by rhyodacitic white pumice with a narrow density range (250–620 kg m−3) and moderate to high vesicle number densities (1.3 × 109–2.1 × 109 cm−3), comparable to clasts from LP1-A. An absence of deep, basement-derived lithic clast assemblages, together with the occurrence of large vesicles and relatively high vesicle number densities in pumice from the fallout and pyroclastic flow phases, suggests shallow fragmentation depths, a prolonged period of bubble nucleation and growth, and moderate rates of decompression prior to fragmentation (7–11 MPa s−1). Evacuation of magma during the pyroclastic flow phase led to under-pressurisation of the magma reservoir, the propagation of faults (associated with the main phase of caldera collapse) and the formation of 20 m thick lithic lag breccias (LP1-C). Rhyodacitic pumices from the base of the proximal lithic lag breccias show a broader range of density (330–990 kg m−3), higher vesicle number densities (4.5 × 109–1.1 × 1010 cm−3) and higher calculated magma decompression rates of 15–28 MPa s−1 than pyroclasts from the pre-collapse eruptive phases. In addition, the abundance of lithic clasts, including deeper, basement-derived lithic assemblages, records the opening of new vents and a deepening of the fragmentation surface. These data support numerical simulations which predict rapid increases in magma decompression and mass discharge rates at the onset of caldera collapse.

P32 富士火山山頂噴火におけるマグマ減圧過程の時間変化 : 湯船第二スコリアからの制約(ポスターセッション)

P32 富士火山山頂噴火におけるマグマ減圧過程の時間変化 : 湯船第二スコリアからの制約(ポスターセッション)

Magma decompression rates during explosive eruptions of Kīlauea volcano, Hawaii, recorded by melt embayments

The decompression rate of magma as it ascends during volcanic eruptions is an important but poorly constrained parameter that controls many of the processes that influence eruptive behavior. In this study, we quantify decompression rates for basaltic magmas using volatile diffusion in olivine-hosted melt tubes (embayments) for three contrasting eruptions of Kīlauea volcano, Hawaii. Incomplete exsolution of H2O, CO2, and S from the embayment melts during eruptive ascent creates diffusion profiles that can be measured using microanalytical techniques, and then modeled to infer the average decompression rate. We obtain average rates of ~0.05–0.45 MPa s−1 for eruptions ranging from Hawaiian style fountains to basaltic subplinian, with the more intense eruptions having higher rates. The ascent timescales for these magmas vary from around ~5 to ~36 min from depths of ~2 to ~4 km, respectively. Decompression-exsolution models based on the embayment data also allow for an estimate of the mass fraction of pre-existing exsolved volatiles within the magma body. In the eruptions studied, this varies from 0.1 to 3.2 wt% but does not appear to be the key control on eruptive intensity. Our results do not support a direct link between the concentration of pre-eruptive volatiles and eruptive intensity; rather, they suggest that for these eruptions, decompression rates are proportional to independent estimates of mass discharge rate. Although the intensity of eruptions is defined by the discharge rate, based on the currently available dataset of embayment analyses, it does not appear to scale linearly with average decompression rate. This study demonstrates the utility of the embayment method for providing quantitative constraints on magma ascent during explosive basaltic eruptions.

Experimental Constraints on Plagioclase Crystallization during H2O- and H2O–CO2-Saturated Magma Decompression

Experiments simulating magma decompression allow the textures of volcanic rocks to be calibrated against known eruptive conditions. Interpretation of natural samples may be complicated, however, by both the decompression path and the composition of exsolving volatiles, which affect the time evolution of crystal textures. Here we present the results of decompression experiments at elevated temperature and pressure designed to assess the effects of degassing path on crystallization of Mount St. Helens rhyodacite. Three families of experiments were employed to simulate varied PH2O–t trajectories: single-step, H2O-saturated decompression (SSD); continuous, H2O-saturated decompression (CD); continuous, H2O–CO2-saturated decompression. Quantitative textural data (crystal abundance, number density, and size) are used to calculate plagioclase nucleation and growth rates and assess deviations from equilibrium in run products. These are the first experiments to quantify feldspar nucleation and growth rates during H2O–CO2-saturated decompression. We find that reducing the initial melt water content through addition of CO2 increases nucleation rates relative to the pure water case, an effect most pronounced at low dP/dt. Moreover, these early formed textural distinctions persist at the lowest pressures examined, suggesting that deep H2O– CO2 fluids could leave a lasting textural ‘fingerprint’ on erupted magmas. Crystals formed prior to decompression during the annealing process also modulate sample textures, and growth on pre-existing crystals contributes significantly to added crystallinity at a wide range of experimental conditions. The phase assemblage itself is a dynamic variable that can be used in conjunction with textural data to infer conditions of magma ascent and eruption. Finally, quantitative textural data from experimental samples are compared with those of natural pyroclasts erupted during the summer 1980 explosive–effusive transition at Mount St. Helens. This comparison supports a model of magma ejected from multiple storage regions present in the upper crust following the May 18 Plinian eruption, such that subsequent eruptions tapped magmas that experienced varied decompression and degassing histories.

Magma Fragmentation

Magma fragmentation is the breakup of a continuous volume of molten rock into discrete pieces, called pyroclasts. Because magma contains bubbles of compressible magmatic volatiles, decompression of low-viscosity magma leads to rapid expansion. The magma is torn into fragments, as it is stretched into hydrodynamically unstable sheets and filaments. If the magma is highly viscous, resistance to bubble growth will instead lead to excess gas pressure and the magma will deform viscoelastically by fracturing like a glassy solid, resulting in the formation of a violently expanding gas-pyroclast mixture. In either case, fragmentation represents the conversion of potential energy into the surface energy of the newly created fragments and the kinetic energy of the expanding gas-pyroclast mixture. If magma comes into contact with external water, the conversion of thermal energy will vaporize water and quench magma at the melt-water interface, thus creating dynamic stresses that cause fragmentation and the release of kinetic energy. Lastly, shear deformation of highly viscous magma may cause brittle fractures and release seismic energy.

Maximum horizontal range of volcanic ballistic projectiles ejected during explosive eruptions at Santorini caldera

This study investigates the hazard posed by Volcanic Ballistic Projectiles (VBPs) to the Santorini islands considering eruption scenarios that include low (VEI=2–3) and higher energy (VEI >3) eruptions. A model that describes rapid decompression of pressurized magma below a caprock along with its fragmentation and acceleration of particles is utilized for estimating initial velocities during vulcanian-style eruptions. These initial velocities are inserted into the ballistic equations assuming that VBPs have a cube-like shape, are subjected to gravity/drag forces and are launched into a zone of reduced drag. Four different diameters of VBPs are considered (0.35m, 1.0m, 2.0m, 3.0m) and also different values of gas fractions and extent of the reduced drag zone are investigated. The results of these calculations show that an area of 1–2km width along the western coast of Thera will be within the maximum range of VBPs, provided that the eruptive vent will develop either on Nea Kameni or between Nea Kameni and Thera. Initial velocities for higher energy eruptions are estimated by considering the conversion efficiency of thermal to kinetic energy. For the case of an eruption with VEI=4 and a number of vents centered between Nea and Palea Kameni, calculations show that the coastal areas of Thera and Therasia are within the maximum horizontal range of VBPs with diameter larger than 0.35m. As the exact position of the eruptive vent seems to be of crucial importance for determining the areas at risk, continuous seismic and geodetic monitoring of the caldera is needed in order to decipher its likely location.

Prevalence of growth twins among anhedral plagioclase microlites

Crystal textures of volcanic rocks record the processes involved in magma storage and eruptive ascent. Syn-eruptive crystallization, in which groundmass crystals form and grow according to environmental factors such as thermodynamic undercooling, strongly influences the texture of erupted magma. This stage is difficult to isolate for study in natural rocks, but well-suited for laboratory experiments because the chemical compositions and crystallization timescales of eruptive processes can be emulated. This study examines the incipient stages of plagioclase crystallization in hydrous rhyodacite magma undergoing decompression-driven degassing. Experimental samples in which crystal growth at both near-equilibrium and far-from equilibrium conditions were examined using electron backscatter diffraction (EBSD) analysis to ascertain crystallographic lattice orientations of individual crystals. The crystal orientation investigation affirms a common assumption invoked in textural studies of crystal number density: contiguous crystals with parallel faces are crystallographically continuous, whereas contiguous crystals with non-parallel faces have unrelated crystal lattice orientations and as such, represent separate crystals. In the highly undercooled sample, twinning is identified in ~87% of the crystals examined; in the near-equilibrium sample, 38% of the crystals are twinned. We find that the observed twinning is unlikely to be the result of deformation, transformation, or synneusis, but rather a result of growth defects introduced during the incipient stages of crystallization. We suggest internal structural defects (twins) control macroscopic morphological defects (embayments, swallowtails, and melt inclusions) as a result of the high energy of the twin plane boundary. Formation of twins during the incipient stages of plagioclase crystallization is the single most important factor contributing to anhedral morphologies of feldspar microlites growing during magma decompression, and plays a role in the development of some plagioclase-hosted melt inclusions.

Revisiting the last major eruptions at Stromboli volcano: inferences on the role of volatiles during magma storage and decompression

Abstract Stromboli is a unique open conduit volcano and a natural laboratory for investigating how volatiles migrate and concentrate under dynamic conditions. Fluid phases are involved in magma decompression and pressurization, modulate Strombolian activity and govern magma rise and fragmentation processes. Here, we have revisited the available data on the last two major eruptions at Stromboli volcano and concentrated our analysis on the 2007 eruption. First, we analysed petrological-geochemical data to assess equilibrium conditions by using standard thermobarometry; we then used a grid of selected reactions which involve solid-melt-fluid equilibria to better constrain the P – T regimes that adequately describe our system. Primitive hydrous basaltic melts, reported in literature and preserved as melt inclusions in olivine (with 2.3–3.8 wt% of H 2 O and 890–1590 ppm CO 2 ), are in equilibrium with forsteritic olivine and a diopsidic clinopyroxene at average pressures of 260 (±47) MPa for temperatures approaching 1170 (±17) °C and calculated (mole fraction of CO 2 within the melt) in the range 0.60–0.76. Ca-rich or ultracalcic melts are regarded as the result of decompression along a steep adiabatic and/or isothermal curve. During this process the magma will cross-cut the stability field of diopside and enter the liquidus field. The earlier crystallized diopside is destabilized and reacts with the coexisting liquid phase leading to the formation of ultracalcic melts. Ejected golden pumices (with 2–3 wt% H 2 O) are in equilibrium with Ca-pyroxene, forsteritic olivine and anorthitic plagioclase at 150–220 MPa and temperatures of 1120–1150 °C. Evolved melt inclusions (substantially degassed) in less magnesian olivine ( c. Fo 70 ) of the scorias show average equilibration pressures of 78 (±20) MPa and temperatures of 1138 (±14) °C. In summary, the higher P–T regimes associated with the origin of primitive melt inclusions are representative of the base of the chamber, where the ferromagnesian phases may crystallize and cumulate. The magma with a bulk composition typical of the pumices is stored in the middle and main part of the chamber (likely its axial sector) and these materials are erupted during paroxysmal and, more rarely, major explosions. Finally, more evolved melt inclusions found in the olivine of the scorias are indicative of crystallization within the conduit or its root zone connected to the upper part of the chamber. Pure extensional regimes and recent geophysical data suggest the existence of a prolate ellipsoidal magma chamber below Stromboli. To constrain its volume we estimated the magma volumes associated with SO 2 degassing (during the 2007 major eruption) by applying a refined petrological model that allowed us to estimate the magma fluxes in the subvolcanic region (i.e. the magma flux entering the degassing zone). The long-term trend of this magma flux follows an overall exponential decay, typical of pressurized magmatic systems, and indicates that magma rise was accompanied and followed by slow decompression. This trend was shown to be consistent with release of elastic strain accumulated either by pressurization of the rocks surrounding the magma reservoir, by pressurization of the magma itself or both. By analysing the reservoir elastic response during magma decompression, we found that the current Stromboli magma chamber volume may be adequately constrained to 1–2 km 3 .

A nanolite record of eruption style transition

Microlite characteristics have been successfully used as indicators of magma decompression rate. However, microlites do not record the explosive-effusive transitions in sub-Plinian eruptions, when such transitions are governed by the decrease of exsolved volatile pressure in the shallow conduit rather than by the magma ascent rate. To overcome this limitation, we studied the nanolites in the quenched products of the 2011 Shinmoedake (Japan) eruption. Nanolites are nanometer-scale components of the groundmass minerals and exhibit a steeper slope of crystal size distribution than that of microlites. In the 2011 eruption, the style of activity had undergone transformation from sub-Plinian eruption to Vulcanian explosion and intermittent effusion of lava. We found that, although the products formed by different eruptive activities have similar microlite characteristics, such products can be distinguished clearly by their mineral assemblage of nanolites. The samples of pumice of sub-Plinian eruptions and Vulcanian explosions and the dense juvenile fragments of lava (in descending order of explosivity) contained, respectively, nanolites of low-Ca pyroxene, low-Ca pyroxene + plagioclase, and low-Ca pyroxene + plagioclase + Fe-Ti oxides. Nanolites crystallize when undercooling of the magma due primarily to dehydration increases rapidly near the surface. The water contents of the interstitial glass indicate that the quenched depths did not differ greatly between eruption styles. Hence, the different nanolite assemblages of each eruption style are assumed to have resulted from differences in magma residence time near the surface. Therefore, we propose that nanolites have the potential to indicate the physicochemical conditions of magma at the transition points of eruption styles.

Concealed Basalt-Matrix Diatremes with Cu-Au-Ag-(Mo)-Mineralized Xenoliths, Santa Cruz Porphyry Cu-(Mo) System, Pinal County, Arizona

The Santa Cruz porphyry Cu-(Mo) system near Casa Grande, Arizona, includes the Sacaton mine deposits and at least five other concealed, mineralized fault blocks with an estimated minimum resource of 1.5 Gt @ 0.6% Cu. The Late Cretaceous-Paleocene system has been dismembered and rotated by Tertiary extension, partially eroded, and covered by Tertiary-Quaternary basin-fill deposits. The mine and mineralized fault blocks, which form an 11 km (~7 miles) by 1.6 km (~1 mile) NE-SW–trending alignment, represent either pieces of one large deposit, several deposits, or pieces of several deposits. The southwestern part of the known system is penetrated by three or more diatremes that consist of heterolithic breccia pipes with basalt and clastic matrices, and subannular tuff ring and maar-fill sedimentary deposits associated with vents. The tephra and maar-fill deposits, which are covered by ~485 to 910 m (~1,600–3,000 ft) of basin fill, lie on a mid-Tertiary erosion surface of Middle Proterozoic granite and Late Cretaceous porphyry, which compose most xenoliths in pipes and are the host rocks of the system. Some igneous xenoliths in the pipes contain bornite-chalcopyrite-covellite assemblages with hypogene grades >1 wt % Cu, 0.01 ounces per ton (oz/t) Au, 0.5 oz/t Ag, and small amounts of Mo (<0.01 wt %). These xenoliths were derived from mineralized rocks that have not been encountered in drill holes, and attest to additional, possibly higher-grade deposits within or subjacent to the known system. The geometry, stratigraphy, and temporal relationships of pipes and tephras, interpreted from drill hole spacing and intercepts, multigenerational breccias and matrices, reequilibrated and partially decomposed sulfide-oxide mineral assemblages, melted xenoliths, and breccia matrix compositions show that the diatremes formed in repeated stages. Initial pulses of basalt magma fractured granite, porphyry, and other crustal rocks during intrusion, transported multi-sized fragments of these rocks upward, and partially melted small fragments. Rapid decompression of magma induced catastrophic devolatilization that ruptured overlying rocks to the surface, and generated fragment-volatile suspensions that abraded conduits into near-vertical cylindrical structures. Fragments entrained in suspensions were milled and sorted, and ejected as basal surge, pyroclastic deposits, and airfall tephra that built tuff rings around vents and filled vent depressions. Comminuted m- to mm-sized fragments of wall rocks in magma and suspensions that remained in conduits solidified as heterolithic breccias. Subsequent pulses of basalt magma ascended through the same conduits, brecciated older heterolithic breccias, devolatilized, and quenched, leaving two or more generations of nested and mingled heterolithic breccias and internal zones of fluidized fragments. Tephra and maar-fill deposits from later eruptions are composed of more hydrous and oxidized minerals than earlier tephras, reflecting a higher proportion of water in transport fluid which, based on fluid inclusion populations in mineralized xenoliths, was saline water and CO2. The large vertical extent (~600 m; ~2,000 ft) of basalt matrix in pipes, near-paleosurface matrix vesiculation, and plastically deformed basalt lapilli indicates that diatreme eruptions were predominantly phreatic. Diatreme xenoliths represent crustal stratigraphy and, as in the Santa Cruz system, provide evidence of concealed mineral resources that can guide exploration drilling through cover. Vectors to the source of bornite-dominant xenoliths containing >1% Cu and significant Au and Ag could be determined by refinement of breccia pipe geometries, by reassembly of mineralized fault blocks using modal, chemical, and temporal characteristics of hydrothermal mineral assemblages and fluid inclusions, and by paleodrainage analysis.

Generation of CO2-rich melts during basalt magma ascent and degassing

To test mechanisms of basaltic magma degassing, continuous decompressions of volatile-bearing (2.7–3.8 wt% H2O, 600–1,300 ppm CO2) Stromboli melts were performed from 250–200 to 50–25 MPa at 1,180–1,140 °C. Ascent rates were varied from 0.25 to ~1.5 m/s. Glasses after decompression show a wide range of textures, from totally bubble-free to bubble-rich, the latter with bubble number densities from 104 to 106 cm−3, similar to Stromboli pumices. Vesicularities range from 0 to ~20 vol%. Final melt H2O concentrations are homogeneous and always close to solubilities. In contrast, the rate of vesiculation controls the final melt CO2 concentration. High vesicularity charges have glass CO2 concentrations that follow theoretical equilibrium degassing paths, whereas glasses from low vesicularity charges show marked deviations from equilibrium, with CO2 concentrations up to one order of magnitude higher than solubilities. FTIR profiles and maps reveal glass CO2 concentration gradients near the gas–melt interface. Our results stress the importance of bubble nucleation and growth, and of volatile diffusivities, for basaltic melt degassing. Two characteristic distances, the gas interface distance (distance either between bubbles or to gas–melt interfaces) and the volatile diffusion distance, control the degassing process. Melts containing numerous and large bubbles have gas interface distances shorter than volatile diffusion distances, and degassing proceeds by equilibrium partitioning of CO2 and H2O between melt and gas bubbles. For melts where either bubble nucleation is inhibited or bubble growth is limited, gas interface distances are longer than volatile diffusion distances. Degassing proceeds by diffusive volatile transfer at the gas–melt interface and is kinetically limited by the diffusivities of volatiles in the melt. Our experiments show that CO2-oversaturated melts can be generated as a result of magma decompression. They provide a new explanation for the occurrence of CO2-rich natural basaltic glasses and open new perspectives for understanding explosive basaltic volcanism.

Relating vesicle shapes in pyroclasts to eruption styles

Vesicles in pyroclasts provide a direct record of conduit conditions during explosive volcanic eruptions. Although their numbers and sizes are used routinely to infer aspects of eruption dynamics, vesicle shape remains an underutilized parameter. We have quantified vesicle shapes in pyroclasts from fall deposits of seven explosive eruptions of different styles, using the dimensionless shape factor $\Omega $ , a measure of the degree of complexity of the bounding surface of an object. For each of the seven eruptions, we have also estimated the capillary number, Ca, from the magma expansion velocity through coupled diffusive bubble growth and conduit flow modeling. We find that $\Omega $ is smaller for eruptions with ${\rm{Ca}} \ll 1$ than for eruptions with Ca $\gg 1$ . Consistent with previous studies, we interpret these results as an expression of the relative importance of structural changes during magma decompression and bubble growth, such as coalescence and shape relaxation of bubbles by capillary stresses. Among the samples analyzed, Strombolian and Hawaiian fire-fountain eruptions have ${\rm{Ca}} \ll 1$ , in contrast to Vulcanian, Plinian, and ultraplinian eruptions. Interestingly, the basaltic Plinian eruptions of Tarawera volcano, New Zealand in 1886 and Mt. Etna, Italy in 122 BC, for which the cause of intense explosive activity has been controversial, are also characterized by ${\rm{Ca}} \gg 1$ and larger values of $\Omega $ than Strombolian and Hawaiian style (fire fountain) eruptions. We interpret this to be the consequence of syn-eruptive magma crystallization, resulting in high magma viscosity and reduced rates of bubble growth. Our model results indicate that during these basaltic Plinian eruptions, buildup of bubble overpressure resulted in brittle magma fragmentation.

Evidence for a recent change in the shallow plumbing system of Mt. Etna (Italy): Gas geochemistry and structural data during 2001–2005

We analyzed crater SO2 fluxes from Mt Etna, together with soil CO2 effluxes from the volcano's flanks, in the period from 2001 to 2005. Between the 2001 and 2002–2003 eruptions, persistently low values of both parameters suggest that no new gas-rich magma was accumulating at shallow depth (<5km) within Etna's central conduit, whereas very high SO2 sin-eruptive fluxes during the two eruptions indicated sudden decompression of an un-degassed magma rising along newly-formed eccentric conduits. In November 2003, soil CO2 data indicate migration of gas-rich magma from deep (>10km) to shallow (<5km) portions of the feeding conduits, preceded by an increase in crater SO2 fluxes. A similar behavior was observed also during and after the following 2004–2005 eruption. This degassing style matches a period of increased structural instability of the volcanic edifice caused by acceleration of spreading that affected both its eastern and southern flanks. Spreading could have triggered progressively deeper depressurization in the central conduit, inducing release of the more soluble gas (SO2) first, and then of CO2, contrary to what was observed before the 2001 eruption. This suggests that the edifice has depressurized, promoting ascent of fresh-magma and increasing permeability favouring release of CO2 flux. By integrating geochemical and structural data, previous degassing models developed at Mt. Etna have been updated to advance the understanding of eruptive events that occurred in recent years.

Transient uprise of gas and gas-rich magma batches fed the pulsating behavior of the 2006 eruptive episodes at Mt. Etna volcano

Volcanic rock samples were systematically collected during the 2006 eruptive episodes at Mt. Etna. Major and trace elements, together with Sr–Pb isotope ratios, were then obtained on the whole rocks, for different times of emission and vent locations. On the same samples, selected plagioclase crystals were investigated through high-contrast BSE images and core-to-rim compositional profiles for An% and FeO wt.% were obtained. The major element variation through time suggests the involvement of a more basic magma, because MgO/FeOtot and CaO/Al2O3 ratios tend to increase from the beginning to the end of the eruption. This more basic magma was characterized by a peculiar geochemical and isotopic signature. This is testified by the negative trend of LREEs, some HFSEs (e.g., Nb, Zr, Y) and lead isotopes, together with increase of some incompatible element ratios (e.g., Rb/La, Zr/Nb) with respect to products erupted before. Concerning the plagioclase crystals, four main texture types were recognized at the core and rim, which indicate either near-equilibrium crystallization or disequilibrium accompanied by strong destabilization. These textures were attributed to: i) crystallization at equilibrium conditions at depth or within the open-conduit (two types of oscillatory zoning), ii) magma decompression at variable ascent rates (coarsely sieve-textures and dissolution at the core), iii) inputs of more basic magma (sieve-textured rims). All the available data, coupled with CO2/SO2 molar ratio measured in the gas plume obtained from literature, support the idea that the pulsating regime observed for the volcanic phenomena at the surface during the period July–December 2006 reflects the transient, rapid ascent of several CO2 bubble fluxes and/or CO2-rich magma batches from the deep to the shallow levels of the plumbing system. This integrated study has also led to the identification of the replenishment event during the eruption that allowed the ingress of the magma with distinct geochemical/isotopic signature.

Features of the dynamics of the heavy bubble magma state in explosive volcanic eruptions

The explosive eruption of a volcano that occurs when a volcanic canal is suddenly depressurized is one of the mysterious phenomena of nature, the mechanism of which is associated with the sudden decompression of a high-pressure magma containing dissolved gas and microcrystallites and with its transition to a multiphase state. With the obvious lack of the possibility to directly investigate these processes, the problem of experimental and numerical modeling, and creation of mathematical models that would allow describing the state of magma during various stages of eruption with a certain degree of adequacy are becoming topical. A model combining systems of equations of the mechanics of multiphase media is proposed. The model is formulated on the basis of the modified Iordansky–Kogarko–van Wijngaarden model with the Navier-Stokes equation for the variable viscosity, and the full range of kinetic relationships, including spontaneous nucleation and saturation dynamics of magma with cavitation nuclei. An analysis of a pre-explosive state of a number of volcanoes was carried out, which showed that the structural features of explosive volcanoes are analogous to the schemes of hydrodynamic shock tubes (UT), such as Glass-Heuckroth. Experimental modeling of the flow structure demonstrates the probability of the formation of the projectile regime as a consequence of coalescence of bubbles in the flow. A physical model of combined explosive volcanic eruptions is proposed; the process of formation in the magma flux of an independent flow of crystalline clusters (magmatic ”bombs“) moving with respect to the flow of cavitating magma at a much higher rate is experimentally studied.

Permeability controls on expansion and size distributions of pyroclasts

[1] We synthesize field and experimental data to evaluate relationships between the textures and sizes of pyroclasts and the processes that shape them, with emphasis on the role of permeability. In (mafic) scoria, the important competition that determines preserved vesicularity is between gas escape and magma expansion by bubble growth; post-fragmentation expansion occurs rapidly until the permeability increases substantially. In (silicic) pumice, the competition is between gas escape and fragmentation by bubble overpressure; sufficiently permeable regions allow gas escape although high viscosity hinders further expansion. Thus the preserved vesicularity of both pumice and scoria is controlled by the permeability threshold, the vesicularity at which there is an abrupt increase in permeability over a small increase in vesicularity, which appears to be ∼70–80%. High permeability thresholds may also explain the high fine ash content of silicic Plinian eruptions. At the local scale, magma ruptures because of stresses in viscous melt around expanding isolated bubbles. Local control is illustrated by the correspondence between the size distributions of the ash and of bubbles in individual pumice clasts of the 1980 Mount St. Helens Plinian eruption: the medians and modes for grain and bubble sizes are of order tens of microns, and fractal dimensions are similar (3.1–3.2 for grains; 3.4 for bubbles). Fractal dimensions for total grain size distributions and bubble size distributions in pumice from other silicic eruptions are similar (3.0–3.2, 2.9–3.9, respectively) and larger than generated by crushing and grinding rocks. This suggests that fragmentation efficiency depends on the balance between rates of magma decompression (overpressurization) and gas escape, which explains relationships between ash content and eruption rate. In summary, the vesicularity distribution of pyroclasts places important constraints on the permeability threshold of expanding magma, whereas the whole deposit grain size distribution of pyroclastic deposits places critical limits on the permeability structure of the magma at the point of fragmentation.

Pressure waves in a supersaturated bubbly magma

SUMMARY We study the interaction of acoustic pressure waves with an expanding bubbly magma. The expansion of magma is the result of bubble growth during or following magma decompression and leads to two competing processes that affect pressure waves. On the one hand, growth in vesicularity leads to increased damping and decreased wave amplitudes, and on the other hand, a decrease in the effective bulk modulus of the bubbly mixture reduces wave velocity, which in turn, reduces damping and may lead to wave amplification. The additional acoustic energy originates from the chemical energy released during bubble growth. We examine this phenomenon analytically to identify conditions under which amplification of pressure waves is possible. These conditions are further examined numerically to shed light on the frequency and phase dependencies in relation to the interaction of waves and growing bubbles. Amplification is possible at low frequencies and when the growth rate of bubbles reaches an optimum value for which the wave velocity decreases sufficiently to overcome the increased damping of the vesicular material. We examine two amplification phase-dependent effects: (1) a tensile-phase effect in which the inserted wave adds to the process of bubble growth, utilizing the energy associated with the gas overpressure in the bubble and therefore converting a large proportion of this energy into additional acoustic energy, and (2) a compressive-phase effect in which the pressure wave works against the growing bubbles and a large amount of its acoustic energy is dissipated during the first cycle, but later enough energy is gained to amplify the second cycle. These two effects provide additional new possible mechanisms for the amplification phase seen in Long-Period (LP) and Very-Long-Period (VLP) seismic signals originating in magma-filled cracks.

A new type of high-pressure low-flow metering valve for continuous decompression: First experimental results on degassing of rhyodacitic melts

A novel type of high-pressure low-flow metering valve has been designed for experiments at continuous decompression in internally heated pressure vessels (IHPV) at pressures up to 500 MPa and temperatures up to 1500 °C. It consists of a modified high-pressure valve coupled with a piezoelectric nanopositioning system. The piezoelectric actuator (1) positions the needle of the valve statically with very high precision on a nanometer scale for infinitesimally slow decompression and, alternatively, (2) opens and closes the valve completely, within milliseconds, to achieve a fast pressure drop only limited by the diameter of the high-pressure tubing and the entire volume of the autoclave system. The valve connected to an IHPV has been successfully tested to simulate continuous decompression of volatile-bearing magma. Water saturated rhyodacitic melt was synthesized at 1050 °C and 300 MPa and subsequently decompressed to 50 MPa at an integrated decompression rate of 0.28 MPa/s using continuous decompression as well as single-step and multi-step decompression techniques. The experimental results of the three methods show significant differences, having important implications for the interpretation of textures observed in natural volcanic rocks. Bubble number density (BND) values increase from continuous to multi-step and to single-step decompression by two orders of magnitude. Bubble size distribution (BSD) also differs significantly. The BSD curve of the single-step decompression shows largest variation both in size and in population number density, whereas the BSD trend for the sample of multi-step decompression shows the smallest variation between size and population density of bubbles. The BSD of the continuously decompressed sample indicates similar proportions of bubbles with different size. Channel formation in the sample of single-step decompression may indicate fluid escape from over-pressurized bubbles through the melt. Single-step and continuous decompression style represent two extreme cases of magma degassing in which bubble nucleation and bubble growth are predominant processes, respectively, whereas multi-step decompression resembles the intermediate case.

Ascending seismic source during an explosive eruption at Tungurahua volcano, Ecuador

[1] Application of seismic techniques to an explosion event at Tungurahua volcano, Ecuador, provided clear images to elucidate its source process. A source location method using high-frequency seismic amplitudes with an S-wave velocity of 2000 m/s indicates that the event was triggered at a depth of 6 km below the summit, and the source ascended toward the summit at a speed of about 1600 m/s. Waveform inversion of low-frequency signals at the event onset points to an isotropic mechanism with initial deflation at a similar depth of 6 km. The ascending source suggests that a pressure wave propagated along the magma conduit, triggering fragmentation of magma at shallow depths. Rapid decompression of magma in a shock tube has been considered to be an important mechanism for explosive eruptions triggered by ruptures at the magma surface. However, our study suggests that explosive eruptions are triggered by pressure disturbances in magma at depth.