Buoyant Magma Research Articles

New evidence of syn-eruptive magma-carbonate interaction: the case study of the Pomici di Avellino eruption at Somma-Vesuvius (Italy)

Calcareous lithics are commonly found within the products of some explosive eruptions of Somma-Vesuvius. The pumice fragments from the final phase of the Plinian fallout event of the Pomici di Avellino eruption contain abundant calcareous xenoliths. Previous work on that eruption, including numerical simulations, suggested that the release of CO2 from the entrapment of carbonates may have prolonged the magmatic phase of the eruption by maintaining sufficient driving pressure in the feeding dike. The texture and thermo-metamorphic reactions of carbonate xenolith-bearing pumice fragments of the Pomici di Avellino eruption are analyzed through petrography, scanning electron microscope images, energy dispersive spectrometer analyses, and micro-computed X-ray tomography to deduce the behavior of short-term carbonate-magma interaction and its contribution to the eruption dynamics. Results show that calcareous xenoliths experienced short-term magma-carbonate interaction, which took place in three steps: (i) entrainment, i.e., the mechanical process of carbonate xenoliths entrapment into a magma; (ii) decarbonation, related to high-temperature decomposition reaction of the xenoliths; and (iii) digestion or dissolution of the incorporated calcareous xenoliths into the melt with diffusion of Ca and Mg. The CO2 released during the syn-eruptive decarbonation process thus provided extra volatiles to the rising magma, which may have maintained magma buoyancy longer than expected if only magmatic volatiles were involved in the eruption.

Formation of a Large Cold Groundwater Mantle Helium Anomaly and High Temperature Geothermal Resources in Response to Bimodal Magmatism Near Roosevelt Hot Springs and Utah FORGE, Milford Valley, Southwest Utah

AbstractA large mantle helium anomaly and separate domains of high heat flow are the predominant manifestations of bimodal magmatic activity in the Milford valley. The mantle helium anomaly (1.9–2.6 R/Ra) covers 270 km2 and is subdivided into two separated domains: a cold shallow groundwater regime and high temperature hydrothermal activity. The zone of anomalous heat flow covers >100 km2 and is also subdivided into two adjacent domains, comprising hydrothermal activity at Roosevelt Hot Springs (RHS) (3–7 W/m2) and conductive heat flow (100–180 mW/m2). While the transfer of heat and mantle helium at RHS are coupled, heat and helium transfer are decoupled in the adjacent cold groundwater regime to the west. Both the mantle helium and geothermal anomalies are attributed to recent mafic‐felsic magmatic intrusions of >400 km3, however, the absence of volcanic eruptions <500,000 years indicates magmas stall before rising to shallow crustal level <10 km depth. Deep level magmatism produces a felsic composition melt, which is inferred to be responsible for the widespread and near uniform range of diluted mantle helium values. A thick and impermeable mass of crystalline granitic basement rock at the mid‐crustal level divides the ascent of mantle helium into separate flow paths. It may also impede the rise of buoyant magma trapping thermal energy that facilitates partial melting, slow cooling, and development of a thick thermal aureole. Partitioning of convective and conductive thermal regimes and independent flow paths supplying deeply derived helium characterize the development of a large long‐lived magma‐related geothermal system.

Insights Into Venus' Crustal Plateaus From Dyke Trajectories Below Craters

AbstractOn Venus, radar observations of the surface have highlighted two categories of craters: bright‐floored, interpreted as pristine, and dark‐floored, interpreted as being partially filled by lava. While volcanic resurfacing occurs within and outside craters in the plains, it seems mainly concentrated within the interior of dark‐floored craters in the crustal plateaus, suggesting that the magma is negatively buoyant there. Indeed, crater unloading may facilitate vertical ascent of a negatively buoyant magma by decompressing the underlying crust. However, the crater topography also generates a shear stress which would tend to horizontalize the vertical propagation of a dyke. We use numerical simulations of magma ascent in an axisymmetric crater stress field to demonstrate that, depending on the crust thickness and the magma‐crust density contrast, a negatively buoyant magma can indeed erupt only in the crater interior while remaining stored in the crust elsewhere. In particular, we identify four different behaviors depending on if and where a magma‐filled crack ascending below a crater reaches the surface. We draw a regime diagram as a function of two characteristic dimensionless numbers. For eruption to occur only in the crater interior requires a crust thinner than 45 km and a limited range of magma‐crust density contrasts, between 40 and 280 kg m−3 for crust thicknesses between 20 and 45 km, the permissible range decreasing for increasing crustal thicknesses. These results suggest that the crustal plateaus may not be particularly thick and could be slightly differentiated, but probably not very felsic.

Subsidence-induced early doming at a large ignimbrite caldera

Uplift at many well-documented resurgent calderas started only after completion of the associated ignimbrite eruption, but arching of the large Bachelor caldera in the Southern Rocky Mountain volcanic field, Colorado (USA), began during the eruption. A well-defined arched or domical structure, initiated within thickly accumulating ignimbrite as the caldera subsided, is documented by growth of keystone faults as the caldera filled, rheomorphism and local diapiric mobilization of early-erupted tuff, decreased dips in upper welding zones, and wedging of a late-erupted dacitic phase against flanks of the growing dome. Early subsidence-induced doming may have been triggered by preferential magma draw-down along ring-fault vents, relatively impermeable vesiculation and differential magma buoyancy centrally within the caldera, peripheral loading at caldera margins by landslides and talus from caldera walls, or some combination of factors. Early inception of caldera doming has implications for models of magma withdrawal and residual compositional gradients in non-erupted magma.

Hot plutonism in a cold accretionary wedge: What terminated the Cadomian orogeny along the northern periphery of Gondwana?

The late Ediacaran to Cambrian end of the Cadomian accretionary orogeny was likely a complex cascade of events that culminated in the opening of the Rheic Ocean and formation of a passive margin along the northern Gondwana periphery. However, the geodynamic causes and kinematics of this protracted transition remain elusive. To address this issue, we examine the emplacement and tectonic setting of the ca. 524–523 Ma, shallow-level Kdyně gabbro to diorite pluton in the southwestern part of the Bohemian Massif as it represents the earliest event following the end of the Cadomian trench-sediment accretion at around 527 Ma. Our structural analysis combined with investigation of the anisotropy of magnetic susceptibility (AMS) suggests that the pluton was emplaced via magma wedging, some minor lateral expansion, and magmatic stoping, when the ascending magma utilized a network of orthogonal extension fractures (∼NNE–SSW and ∼WNW–ESE). Furthermore, the pluton is interpreted as an apical part of a dome located in an extensional stepover between two dextral transtensional zones, forming a large-scale structure resembling a metamorphic core complex. Nevertheless, our data imply that final pluton emplacement was controlled by magma buoyancy and less so by regional tectonic deformation. We suggest that these structural features together with the remarkably short time span between the end of Cadomian accretion and the earliest plutonism in the overall cold forearc region are compatible with slab break-off as the potential geodynamic cause of the necessary rapid heat input into the base of the host accretionary wedge. Taken together, the high-temperature events following Cadomian accretion, represented by the Kdyně pluton and other Cambrian plutons as young as ca. 505 Ma, suggest that magmatism may have been an important geodynamic driver of the Cambro–Ordovician rifting of northern Gondwana and that the rifting mode was active during at least the initial stages before being overridden by the slab pull force of the subducting Iapetus Ocean.

Early Precursory Changes in the 3He/4He Ratio Prior to the 2021 Tajogaite Eruption at Cumbre Vieja Volcano, La Palma, Canary Islands

AbstractWe report significant changes in the 3He/4He ratio measured at the Dos Aguas CO2‐rich cold mineral spring (La Palma, Canary Islands) that represent early precursory signals of the 2021 Tajogaite eruption at Cumbre Vieja volcano. Air‐corrected 3He/4He ratio has an average value of 9.80 RA in the last 30 years (1991–2021). Helium isotope ratios higher than the average value were observed during the period 2008–2013 suggesting an input of 3He‐rich less‐degassed magma beneath La Palma lithosphere. Magma ascended in 2017–2018 toward another reservoir located beneath Cumbre Vieja at 10–15 km depth, causing detectable seismic swarms located between 15 and 25 km, followed in 2020 by additional inputs of 3He‐rich less‐degassed magma that intruded at depth. Finally, in 2021 the buoyancy of magma and the pressure of the exsolved gases overcame the lithospheric resistance and a volcanic eruption took place between 19 September and 13 December 2021.

Role of volatiles in highly explosive basaltic eruptions

Water and carbon dioxide are the most abundant volatile components in terrestrial magmas. As they exsolve into magmatic vapour, they promote magma buoyancy, accelerating ascent and modulating eruptive dynamics. It is commonly thought that an increase in pre-eruptive volatile content produces an increase in eruption intensity. Using a conduit model for basaltic eruptions, covering the upper 6 km of conduit, we show that for the same chamber conditions mass eruption rate is not affected by CO2 content, whereas an increase in H2O up to 10 wt.% produces an increase in eruption rate of an order of magnitude. It is only when CO2 is injected in the magma reservoir from an external source that the resulting pressurisation will generate a strong increase in eruption rate. Results also show that ascent velocity and fragmentation depth are strongly affected by pre-eruptive volatile contents demonstrating a link between volatile content and eruptive style.

Disrupt the upper or the lower conduit? The dual role of gas exsolution in the conduits of persistently active volcanoes

Many volcanoes emit a significant portion of the gas they transport to the atmosphere during continual passive degassing rather than during eruptions. To maintain a high gas and thermal flux without erupting magma, the flow field in the volcanic conduit must be approximately balanced with gas-rich, buoyant magma ascending and degassed, heavy magma descending. In vertical conduits, this exchange flow takes the form of core–annular flow, where the gas-rich magma forms a core enclosed by an annulus of degassed magma. The flow dynamics of core–annular flow have been studied extensively in fluid dynamics, but mostly for constant material properties. Our study aims to advance our understanding of how core–annular flow responds to volatile exsolution – a simple, yet ubiquitous disruption in volcanic conduits, which alters both the density and the viscosity of the core fluid. By deriving an evolution equation for the core–annular interface based on a generalized exchange-flow condition using a lubrication approximation, we find that the response of the system to volatile exsolution depends on the conduit flow regime. The same nucleation event can lead to a flow adjustment only in the upper, only in the lower or in both portions of the volcanic conduit. Our results emphasize that the thermodynamic evolution of magma properties and volcanic conduit flow are intricately linked, which may help understand the observed variability of eruptive behaviour at persistently degassing volcanoes.

Magma buoyancy drives rhyolitic eruptions: A tale from the VEI 5 2008-2009 Chaitén eruption (Chile) from seismological and geodetic data

Magma buoyancy is a mechanism usually neglected with respect to magma overpressure in eruption models constrained with geophysical data. In this study we provide evidence for a buoyancy-driven mechanism of magma ascent during the effusive phase of the 2008-2009 Chaitén rhyolitic eruption. We also constrain the volcano plumbing system with source models from InSAR and teleseismic data. InSAR data did not detect unambiguous evidence of pre-eruptive ground deformation up to 1 year before the eruption. Teleseismic data indicate that a dike opened twice during the first two days of the eruption, while InSAR data show that three sources deflated during the eruption. These include one dike and a sill during the onset of the eruption, and a spherical source halfway through it, all located at depths between 3 and 12 km. 75% of the total ground deformation of ∼0.5 m occurred during the first three weeks of the eruption. A Pléiades DEM show that the dome has a bulk volume of 1 km3, and was extruded with an exponential trend, but the lack of deformation during most of the effusive phase of the eruption implies that the dome extrusion did not result in depressurization of a magma reservoir. Instead, we show that the time series of extruded volume can be explained by magma ascending due its buoyancy instead of its overpressure. Further, the end of the first pulse of dome effusion in September 2008 can be explained by an increase in the dome surface load that equals the effect of the magma buoyancy. These results imply that in some cases ground deformation data alone cannot be used to forecast the temporal evolution of an eruption. They also call for the acquisition of denser time series of extruded volume, as a way to better constrain their evolution through time.

Magma buoyancy drives rhyolitic eruptions: a tale from the VEI 5 2008-2009 Chaiten eruption from seismological and geodetic data

Magma buoyancy drives rhyolitic eruptions: a tale from the VEI 5 2008-2009 Chaiten eruption from seismological and geodetic data

Corrigendum to 'Deep entrapment of buoyant magmas by orogenic tectonic stress: Its role in producing continental crust, adakites, and porphyry copper deposits' [Earth Science Reviews, 220 (2021) 103744

Corrigendum to 'Deep entrapment of buoyant magmas by orogenic tectonic stress: Its role in producing continental crust, adakites, and porphyry copper deposits' [Earth Science Reviews, 220 (2021) 103744

Interactions Between Gas Slug Ascent and Exchange Flow in the Conduit of Persistently Active Volcanoes

AbstractMany volcanoes around the world are persistently active with continuous degassing for years or even centuries, sometimes exceeding historic records. Such long‐term stability contrasts with short‐term instability, reflected in eruptive episodes that punctuate passive degassing. These two aspects of persistent activity, long‐term stability as opposed to short‐term instability, are often conceptualized through two distinct model frameworks: Exchange‐flow in volcanic conduits is commonly invoked to explain the long‐term thermal balance and sustained passive degassing, while the ascent of large gas slugs is called upon to understand explosive eruptions. While typically considered separately, we propose here that both flow processes could occur jointly in the conduits of persistently active volcanoes and in transient connections between subvolcanic melt lenses. To understand the dynamic interplay between exchange flow and slug ascent, we link analogue laboratory experiments with direct numerical simulations. We find that the two flows superimpose without creating major disruptions when only considering the ascent of a single gas slug. However, the sequential ascent of multiple gas slugs is disruptive to the ambient exchange flow, because it may entail continual buildup of buoyant magma at depth. While our study focuses on the laboratory scale, we propose that the dependence of exchange‐flow stability on sequential slug ascent is relevant for understanding why explosive sequences are sometimes followed by effusive eruptions. Taken together, our work suggests that integrating exchange flow and slug ascent could provide a more complete understanding of persistently active volcanoes than either model framework offers in isolation.

Apatite Crystals Reveal Melt Volatile Budgets and Magma Storage Depths at Merapi Volcano, Indonesia

Abstract Magma volatile budgets and storage depths play a key role in controlling the eruptive styles of volcanoes. Volatile concentrations in the melt can be inferred from analyses of glass inclusions, which however may not be present in the investigated rocks or may have experienced post-entrapment processes that modify their volatile records. Apatite is becoming an alternative robust tool for unraveling the information of magmatic volatiles. Here we report a comprehensive dataset for the concentrations of volatiles and major elements in apatite crystals in the rocks from two eruptions with contrasting eruptive styles: the 2006 (dome-forming) and 2010 (explosive) eruptive events at Merapi volcano (Java, Indonesia). We obtained two-dimensional compositional distributions and in situ concentrations of H2O, CO2, F, Cl and S in 50 apatite crystals occurring at various textural positions. The CO2 concentrations we report are probably the first ones from natural volcanic apatite. Using the volatile concentrations in apatite and existing thermodynamic models and geothermobarometers, we have calculated the volatile abundances of the pre-eruptive melts of the two eruptions. We find that the apatite from the 2006 and 2010 deposits have a similar compositional range of volatiles, with a bimodal distribution of F–H2O–CO2 contents. The apatite included in amphibole has higher H2O (0·9–1·0 wt %) and CO2 (≥2400 ppm), but lower F (0·9–1·4 wt %), compared to crystals included in plagioclase, clinopyroxene, or in the groundmass (H2O: 0·4–0·7 wt %; CO2: 40–900 ppm; F: 1·7–2·3 wt %). Using these volatile concentrations and apatite-melt exchange coefficients we obtained two distinct ranges of H2O–CO2–S–F–Cl concentrations in the melt. Melts in equilibrium with apatite included in amphibole had 3–8 wt % H2O, ≥8000 ppm CO2, 340–2000 ppm S, whereas melts in equilibrium with apatite included in anhydrous minerals and in the groundmass had lower H2O (1·5–4 wt %), CO2 (60–2500 ppm), and S (10–130 ppm). We calculated the melt H2O–CO2 saturation pressures and found that they correspond to two main magma storage depths. The shallow reservoir with melts stored at ≤10 km below the crater agrees with the depths constrained by melt inclusions, as well as the geodetic, geophysical and seismic tomography studies from the literature. We have also found a significantly deeper melt storage zone at ≥25–30 km recorded by the C- and H2O-rich apatite in amphibole and barometry calculations using amphibole and high-Al clinopyroxene, which matches with the depths reported in seismic tomography studies. The high CO2/H2O and CO2/SO2 concentrations of the deep melt can help to explain the sharp increase in these ratios in fumarolic gas that were sampled just before the eruption in 2010. Supply of deep volatiles to the shallower magma column before the eruption in 2010 could have increased the magma buoyancy, and thus led to higher magma ascent rates and associated eruption explosivity. Evidence for the faster pre-eruptive magma ascent in 2010 than 2006 is also found on the diffusion distance of Cl in apatite microlites.

Numerical Modeling of Dike Propagation Out of Continuously and Episodically Growing Midcrustal Magma Chambers

AbstractIn the continental crust, the probability of dike propagation out of magma chambers is governed by thermal, rheological, and pressure conditions of magma chamber‐wall rock systems. Incremental injection of melt into an average‐size, laccolith‐shaped, midcrustal magma chamber produces a volume of mobile magma at the bottom of the chamber that has the potential to escape as dikes through the upper, immobile portion of the chamber and the roof. Here we numerically model the conditions needed for dike propagation out of a magma chamber during continuous and episodic injections of melt into the chamber. The roles of magma buoyancy and overpressure from melt injections in generating dikes are explored within 1.78 × 104 to 1.78 × 108 Pa·s range of magma viscosities (μmag), 10 to 40 GPa range of elastic moduli (E) of the immobile top portion of the magma chamber, and 10 and 20 kyr durations of chamber growth. During episodic, high‐flux melt injections (tens of km3/yr), magma overpressure can reach >100 MPa and initiate dike propagation even when μmag and E are near the high ends of the examined ranges. The probability of generating dikes diminishes when the injection flux is lower. Continuous low‐flux injections favor magma accumulation because injection overpressure never exceeds 20 MPa. During either continuous or episodic growth of magma chamber, there is never a sufficient amount of mobile magma in the chamber for dikes to be induced by magma buoyancy alone.

Magmatic overpressures, volatile exsolution and potential explosivity of fissure eruptions inferred via dike aspect ratios

Buoyant magmas abundant in exsolved volatiles (bubbles) drive the rapid upward-propagation of feeder dikes from magma chambers. The consequence of a feeder dike reaching the surface can result in an explosive volcanic eruption depending, partly, on the retention of volatiles. Therefore, timely detection of the vesicularity and overpressure of the magma during feeder dike ascent is critical for the prediction of the explosivity of any future eruption. In this study, we evaluated the explosivity of eruptions based on field investigations of the erupted products and the overpressure of magma in the conduit based on the dimensions of exposed feeder dikes. We found a positive correlation between the explosivity of eruptions and the magma overpressure generated in the conduit during recent fissure eruptions of Miyakejima volcano. Because the buoyancy of low-density magma produces positive overpressure at the dike’s top, feeder dikes with highly-vesiculated magmas possess high amounts of overpressure. An enlargement of the opening width of a feeder dike by magmatic overpressure results in a higher flux of vesiculated magma, which causes vigorous explosive activity. Our results suggest the possibility of forecasting the explosivity of an impending eruption if the width (or opening) of an ascending feeder dike is monitored in real-time through measurements of ground deformation and seismicity induced by the dike.

Magnetic Fabric and Geochronology of a Cambrian “Isotropic” Pluton in the Neoproterozoic Araçuaí Orogen

AbstractThe Araçuaí Orogen (AO) has been interpreted as a Neoproterozoic example of a large, “hot” orogen, based on a broad zone (250 km) of midcrustal metamorphic assemblages with a long, 70‐Myr history of crustal melting and episodic magmatism throughout the late Neoproterozoic and earliest Paleozoic. Here, we present results of U‐Pb sensitive high‐resoution ion microprobe (SHRIMP) zircon dating and detailed anisotropy of magnetic susceptibility (AMS) study on a late Cambrian, bimodal pluton related to final period of collapse of the AO. New U‐Pb zircon ages constrain the crystallization age of different suites within the Santa Angélica Pluton, 506 ± 3 Ma for the early felsic phase and 498 ± 5 Ma for the mafic core. New AMS data indicate that the emplacement of the Santa Angélica Pluton corresponds to two coupled plutons with concentric structures arrayed about a twin, bull's eye pattern. During the final stages of intrusion, upward relative movement of the northeastern lobe exposed the deeper levels of the intrusion, relative to the more shallowly eroded southwestern lobe. These observations suggest that magma emplacement was controlled by magma buoyancy forces, with little influence of regional tectonic stress. This behavior contrasts with the well‐defined, tectonic‐controlled fabrics of coeval plutons occurring to the north which was still hot, therefore highlighting the contrasting thermal evolution between different sectors of the orogen during its final stages.

Unexpected large eruptions from buoyant magma bodies within viscoelastic crust

Large volume effusive eruptions with relatively minor observed precursory signals are at odds with widely used models to interpret volcano deformation. Here we propose a new modelling framework that resolves this discrepancy by accounting for magma buoyancy, viscoelastic crustal properties, and sustained magma channels. At low magma accumulation rates, the stability of deep magma bodies is governed by the magma-host rock density contrast and the magma body thickness. During eruptions, inelastic processes including magma mush erosion and thermal effects, can form a sustained channel that supports magma flow, driven by the pressure difference between the magma body and surface vents. At failure onset, it may be difficult to forecast the final eruption volume; pressure in a magma body may drop well below the lithostatic load, create under-pressure and initiate a caldera collapse, despite only modest precursors.

A Preliminary Model for the Migration of Sulfide Droplets in a Magmatic Conduit and the Significance of Volatiles

Abstract A close relationship between Ni–Cu–(PGE) sulfide deposits and magmatic conduit systems has been widely accepted, but our present understanding still rests on empirical inductions that sulfide liquids are entrained during magma ascent and aggregated at hydrodynamic traps such as the opening of a conduit into a larger magma body. In this contribution, a preliminary quantitative model for the dynamics of mm-scale sulfide droplets in a vertical magmatic conduit is developed, examining such limiting parameters as the size, transport velocity and the magmas’ maximum carrying capacity for sulfide droplets. Addition of numerous dense sulfide droplets significantly reduces magma buoyancy and rapidly increases the bulk viscosity, and the resulting pressure gradient in the propagating conduit dyke restricts the maximum volume fraction of droplets that can be carried by ascending magma. For sulfide droplets alone, the maximum carrying capacity is low, but it will be improved dramatically by the addition of volatiles which reduces the density and viscosity of silicate melt. Potential volatile degassing during decompression further facilitates sulfide entrainment by reducing bulk magma density, and the formation of buoyant compound vapour-sulfide liquid bubble drops also greatly enhances the carrying capacity. The breakdown of compound drops by detachment of parts of the vapour bubble or sulfide droplet may occur at low pressure, which liberates sulfide liquids from rising compound drops, potentially to collect in traps in the conduit system. When sulfide-laden magma flows through a widening conduit, many droplets can be captured by the re-circulation flow just downstream of the expanding section, followed by sulfide liquid accumulation and enhanced chemical interaction via diffusive exchange with the recirculating magma, potentially resulting in an economic, high-tonnage ore body. We apply our models to the emplacement of sulfide-rich magmatic suspensions at Noril’sk and show that the disseminated mineralization in intrusions could have formed when magmas carrying re-suspended sulfide liquid entrained from pre-existing sulfide accumulations in the conduit system reached their limiting sulfide carrying capacity as dictated by buoyancy and were deflected into blind sills flanking the principal conduit for flood basalt volcanism.

Magmatic differentiation and convective stirring of the mantle in early planets 2: Effects of the properties of mantle materials

Summary Magmatism and mantle convection closely couple with each other, when the mantle is hot and magmatism is extensive. Here, I explore dynamics of the coupled system for a planet of the Earth's size based on a two-dimensional numerical model. Magmatism occurs as a permeable flow of basaltic magma generated by decompression melting through matrix, while mantle convection is driven by thermal, compositional, and melt-buoyancy as well as the volume change of matrix caused by magma-migration. The viscosity of matrix strongly depends on temperature. Basaltic solid materials are denser than the average mantle materials except at the top of the lower mantle and in the crust. A positive feedback between magmatism and mantle convection caused by melt-buoyancy (the MMUb feedback) dominates mantle dynamics, when the Rayleigh number Ra is higher than a threshold RMMUb, around 107: buoyancy of magma generated by mantle upwelling flow drives the upwelling flow itself. The feedback reinforces convective stirring of the mantle and also allows the buoyancy of basaltic materials at the top of the lower mantle to strongly impede mass exchange between the upper and lower mantles. As a consequence, an extensive magmatism can only moderately differentiate the mantle at Ra > RMMUb. At Ra < RMMUb, another type of positive feedback is important (the MMUc feedback): volume change of matrix caused by magma-migration drives the mantle upwelling flow that generates the magma. This feedback does not noticeably enhance convective stirring and hence enhances differentiation of the mantle by magmatism. I also calculated several cases where melt is denser than matrix at the base of the upper mantle and found that a compositional discontinuity develops along the top of the lower mantle to make the mantle layered, when Ra < RMMUb. In the early Earth where magmatism was probably extensive, the MMUb feedback is most likely to have operated to keep the mantle rather homogeneous despite differentiation by the magmatism.

Message in a bottle: Spontaneous phase separation of hydrous Vesuvius melt even at low decompression rates

Violent explosive volcanic eruptions are destructive and threaten millions of people and infrastructure. Ejected ash, pumice and gases are end-products of volcanic factories sourced deep within magma chambers and conduits. The different production stages are not directly observable. However, experimental simulation reveals different stages of dynamic volcanic processes. The starting point of explosive volcanic eruptions is determined by the phase separation of an H2O fluid from a supersaturated hydrous silicate melt. The number of formed H2O fluid vesicles per unit volume of silicate melt (VND) is a basic property that controls the efficiency of fluid-melt separation, ascent velocity and finally explosive volcanism. We performed decompression experiments at superliquidus temperatures to simulate phase separation of a single phase hydrous silicate melt during ascent with AD79 Vesuvius white pumice composition using decompression rates of 0.024–1.7 MPa⋅s−1. The white pumice buried Herculaneum and Pompeii and is representative of other catastrophic phonolitic and trachytic explosive eruptions like the violent 39 ka Campi Flegrei and the 1815 AD Tambora eruption. Here we report a high log⁡VND of 5.2 (in mm−3) that is independent from decompression rate within the investigated range. Even at a decompression rate of 0.024 MPa⋅s−1 the formation of a high VND inevitably causes rapid degassing due to short H2O diffusion distances from the melt into fluid vesicles. A decompression rate meter based on nucleation theory, which is commonly used to estimate magma ascent velocity during volcanic eruptions using VND of volcanic ejecta, cannot be adapted to explain our experimentally determined decompression rate independent VND. Alternatively, decompression induced H2O–silicate melt phase separation may be described by diffusion controlled spinodal decomposition where maximum supersaturation is reached. This process occurs spontaneously and free of activation energy if hydrous melt is driven into thermodynamic instability where the second derivative of free energy of mixing to the H2O content is ≤0. However, the decompression rate independent VND has profound consequences for the dynamics of natural polyphase hydrous magma phase separation. Even at low ascent rates spontaneous hydrous melt phase separation facilitates rapid density decrease accompanied by sudden increase of magma buoyancy triggering explosive eruptions.