Magma Density Research Articles

Igneous differentiation by deformation

In a paper published in 1920, Bowen conceived of a situation where forces acting on a crystalline mesh could extract the liquid phase from the solid, and in doing so cause variations in chemistry distinct from the purely gravitational effects of fractional crystallisation. His paper was a call-to-arms to explore the role of deformation as a cause of variation in igneous rocks, but was never followed-up in a rigorous way. Inspired by this, we have developed a quantitative model showing how shear deformation of a crystallised dense magma (ϕ > 70%) with poro-elastic properties is analogous to a granular material. The critical link between the mechanics and associated compositional changes of the melt is the degree to which the crystallising magma undergoes dilation (volume increase) during shear. It is important to note that the effect can only take place after the initial loose solid material has undergone mechanical compaction such that the grains comprising the rigid skeleton are in permanent contact. Under these conditions, the key material parameters governing the dilatancy effect are the physical permeability, mush strength, the shear modulus and the contact mechanics and geometry of the granular assemblage. Calculations show that dilation reduces the interstitial fluid (melt) pressure causing, in Bowen’s words, “the separation of crystals and mother liquor” via a suction effect. At shear strain rates in excess of the tectonic background, deformation-induced melt flow can redistribute chemical components and heat between regions of crystallising magma with contrasting rheological properties, at velocities far in excess of diffusion or buoyancy forces, the latter of course the driving force behind fractional crystallisation and viscous compaction. Influx of hotter, less evolved melt drawn internally from the same magma body into regions where crystallisation is more advanced (auto-intrusion), may result in reverse zoning and/or resorption of crystals. Because dilatancy is primarily a mechanical effect independent of melt composition, evolved, chemically distinct melt fractions removed at this late stage may explain miarolitic alkaline rocks, intrusive granophyres in basaltic systems and late stage aplites and pegmatites in granites (discontinuous variations), as proposed by Bowen. Post-failure instabilities include hydraulic rupture of the mush along shear zones governed by the angles of dilation and internal friction. On the macro-scale, a combination of dilatancy and fracturing may provide a means to extract large volumes of chemically evolved melt from mush columns on short (< 1000 year) geological timescales.

Magma Oscillations in a Conduit‐Reservoir System, Application to Very Long Period (VLP) Seismicity at Basaltic Volcanoes: 2. Data Inversion and Interpretation at Kīlauea Volcano

AbstractVery long period (VLP) seismic events (with dominant periods of 15 to 40 s), observed from 2007 to 2018 at the summit of Kīlauea Volcano, Hawai‘i, arise from resonant oscillations in the shallow magma plumbing system. Utilizing an oscillation model developed in the companion paper (Liang et al., 2020), we perform Bayesian inversions on seismic data from four representative VLP events separately for the parameters of the shallow conduit‐reservoir system, exploring both sphere and crack reservoir geometries. Both sphere and crack geometries are preferentially located ∼1–2 km beneath the northeast edge of Halema‘uma‘u crater and produce similar fits to the data. Considering a reasonable range for reservoir storativity, magma density, and density contrast between the top and bottom of the conduit, we favor a spherical reservoir with a radius of 0.8 to 1.2 km and a short conduit of less than a few hundred meters. For this geometry, buoyancy from density stratification in the conduit provides the dominant restoring force for the VLP oscillation. Viscosity is constrained within an order of magnitude for each event (e.g., approximately 2 to 23 Pa s for one event versus 27 to 513 Pa s for another). Changes in VLP period T and quality factor Q can be explained by changes in viscosity, density stratification, and/or conduit/reservoir geometry. In particular, observed fluctuations in Q over short time intervals (e.g., hours) with minimal changes in T apparently require rapid changes of magma viscosity by over an order of magnitude, assuming geometry remains unchanged, possibly reflecting changes in volatile content, bubble concentration, or conduit flow regime.

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.

Optimal depth of subvolcanic magma chamber growth controlled by volatiles and crust rheology

Storage pressures of magma chambers influence the style, frequency and magnitude of volcanic eruptions. Neutral buoyancy or rheological transitions are commonly assumed to control where magmas accumulate and form such chambers. However, the density of volatile-rich silicic magmas is typically lower than that of the surrounding crust, and the rheology of the crust alone does not define the depth of the brittle–ductile transition around a magma chamber. Yet, typical storage pressures inferred from geophysical inversions or petrological methods seem to cluster around 2 ± 0.5 kbar in all tectonic settings and crustal compositions. Here, we use thermomechanical modelling to show that storage pressure is controlled by volatile exsolution and crustal rheology. At pressures $$\lesssim$$ 1.5 kbar, and for geologically realistic water contents, chamber volumes and recharge rates, the presence of an exsolved magmatic volatile phase hinders chamber growth because eruptive volumes are typically larger than recharges feeding the system during periods of dormancy. At pressures $$>rsim$$ 2.5 kbar, the viscosity of the crust in long-lived magmatic provinces is sufficiently low to inhibit most eruptions. Sustainable eruptible magma reservoirs are able to develop only within a relatively narrow range of pressures around 2 ± 0.5 kbar, where the amount of exsolved volatiles fosters growth while the high viscosity of the crust promotes the necessary overpressurization for eruption. Volatile exsolution and crustal viscosity dictate that the optimum pressure for the growth of an eruptible magma reservoir is 2 kbar in all tectonic settings and crustal compositions, according to thermomechanical modelling.

Investigations into ice scaling during eutectic freeze crystallization of brine streams at low scraper speeds and high supersaturation

Eutectic Freeze Crystallization (EFC) is a potentially cost-effective method to treat reverse osmosis (RO) brines, through the simultaneous crystallization of ice and salt. A major operational challenge, limiting the implementation of EFC in industry, is the formation of an ice scale layer on heat exchanger surfaces. This research determined the influence of the heat transfer driving force, hydrodynamics and brine composition on the severity of ice scaling during continuous EFC of a Na2SO4-H2O system. Experiments were conducted in a hybrid crystallizer-separator with a scraped 2 L crystallization zone. The research focussed on high heat transfer driving forces and slow scraping rates. A unique induction time for scale formation was observed for each set of process conditions. The induction time increased with the increase in scraper speed and decreased as the heat transfer driving force increased. The increase in induction time at higher rotational Reynolds number was attributed to faster distribution of supersaturation, as the flow regime became more turbulent, and more efficient scraping of ice off the wall. The scaling potential decreased as the magma density increased until a certain threshold was reached where it increased again due to the solids impeding effective mixing and heat transfer. Scaling tendencies were reduced during crystallization of multi-component brine streams as compared to binary streams.

Conduit processes during the February 11, 2010 Vulcanian eruption of Soufrière Hills, Montserrat

We analyzed pumice from the February 11, 2010 Vulcanian explosion that immediately followed a large dome collapse at Soufrière Hills volcano. We obtained pre-explosive values of porosity, pressure, and depth by combining textural analyses and glass water content determinations. Our data suggest that the February 2010 explosion evacuated the upper 3 km of the conduit from the dense magma (≤10 vol% porosity) it contained. The low porosity distribution in the volcanic conduit implies that the magma rising from the reservoir had time to extensively degas during emplacement. We use a conduit flow model to characterize the effects of permeability on ascent conditions. Model input parameters were fitted to match our pre-explosive porosity data, which yielded first-order constraints on conduit radius, mass flux, outgassing efficiency, and permeability. This parametric study suggests that efficient lateral gas escape is necessary to explain the low pre-explosive porosities. Steady-state solutions fitting the observed range of dome extrusion rate in the month preceding the February 11 event suggest permeabilities < 10−13 m2 deeper than 500 m, which are values typical of crack-supported permeability. Conversely, solutions with parameters consistent with bubble-supported permeability imply transient flow conditions prior to the February 11 event. The transient conditions imply that our data represent a snapshot of the porosity distribution within the conduit that does not preclude the temporary presence of much higher porosities in the conduit. Such unsteady conduit flow conditions are consistent with the irregular but active dome growth in the month prior to the February 11 event.

A Model of Keyboard-Like Nonuniform Exhumation as a Possible Cause Zoning of Metallogenic Belts in Folded Areas (Eastern Transbaikalia and Southern Primorye)

Abstract —Zoning of metallogenic belts is observed in folded areas of all metallogenic epochs from Precambrian to Cenozoic. The changes observed across the strike of folded areas on geological mapping are usually treated as geodynamic, petrometallogenic, or temporal specialization of corresponding belts (terranes) and zones. However, there are serious petrological reasons to expect a significant downward change in the basicity of magmatites and the composition of related mineralization in folded areas. The change may be caused both by relationships between the densities of magmas and country rocks and by relationships between the solidus temperatures of different magmas and the thermal gradient pattern on the magma ascent path. More basic magmatites and related mineralization should be detained at the deeper levels of the lithosphere because of the higher density and higher solidus temperature of magmas. During orogeny, the vertical differential movements of several adjacent terranes located between deep-seated faults can create keyboard-like structures composed of parallel belts variously uplifted and subsided, similarly to white and black keys of a piano. In belts and blocks of such structures more uplifted and then more subjected to denudation, the ore-magmatic complexes earlier located at deeper levels can expose at the surface. As a result, zoning of metallogenic belts can form, which is totally or only partly related to the metallogenic specialization of structure-facies zones and terranes. The possibility of this origin of zoning of metallogenic belts is shown with the use of geophysical data for two important ore-producing regions of Russia — eastern Transbaikalia and southeastern Primorye. The above model is proposed as one of the possible ways of metallogenic-belt formation in some other folded areas. The author hopes that this work will stimulate a more profound analysis of the nature of metallogenic-belt zoning in the considered and other world regions with the use of modern geophysical methods. The use of the described regularities of the storied arrangement of magmatites of different basicity and oxidation states and genetically related hydrothermal ores may facilitate prospecting for ores of different metals and their 3D forecasting in keyboard-like structures of Phanerozoic orogens and ancient platforms.

Magma chamber growth models in the upper crust: A review of the hydraulic and inertial constraints

Finite volumes of magma moving in confinement, store hydraulic potential energy for the generation, control and transmission of power. The Pascal's principle in a hydraulic jack arrangement is used to model the vertical and lateral growth of sills. The small input piston of the hydraulic jack is equivalent to the feeder dike, the upper large expansible piston equivalent to the magmatic chamber and the inertial force of the magma in the dike is the input force. This arrangement is particularly relevant to the case of sills expanding with blunt tips, for which rapid fracture propagation is inhibited. Hydraulic models concur with experimental data that show that lateral expansion of magma into a sill is promoted when the vertical ascent of magma through a feeder dike reaches the bottom contact with an overlying, flat rigid-layer. At this point, the magma is forced to decelerate, triggering a pressure wave through the conduit caused by the continued ascent of magma further down (fluid-hammer effect). This pressure wave can provide overpressure enough to trigger the initial hydraulic lateral expansion of magma into an incipient sill, and still have enough input inertial force left to continue feeding the hydraulic system. The lateral expansion underneath the strong impeding layer, causes an area increase and thus, further hydraulic amplification of the input inertial force on the sides and roof of the incipient sill, triggering further expansion in a self-reinforcing process. Initially, the lateral pressure increase is larger than that in the roof allowing the sill to expand. However, expansion eventually increases the total integrated force on the roof allowing its uplift into either a laccolith, if the roof preserves continuity, or into a piston bounded by a circular set of fractures. Hydraulic models for shallow magmatic chambers, also suggest that laccolith-like intrusions require the existence of a self-supported chamber roof. In contrast, if the roof of magmatic chambers loses the self-supporting capacity, lopoliths and calderas should be expected for more or less dense magmas, respectively, owing to the growing influence of the density contrast between the host rock and the magma.

A non-horizontal floor during accumulation of the Bushveld Complex – Evidence and implications

Many models of igneous layered complexes assume that the instantaneous boundary between accumulated grains and overlying magma was horizontal. In the case of the Bushveld Complex, several km of hot, dense magma intruded into low-density sandstones and shales. This unstable density profile and the effect of heating of the floor rocks produced diapirism, features that have been well-documented around the intrusion. As a consequence, the grain - magma interface was probably not horizontal. The Al2O3 content of orthopyroxene is very sensitive to pressure (0.2% increase per km depth). It is shown here that the orthopyroxene compositions in the Upper Group 2 chromitite unit from nine detailed profiles change from 1.5 to 1.1% Al2O3 (at the same Mg/Mg + Fe value) along a strike length of 160 km in the western Bushveld. Such changes indicate that there was topographic relief possibly >1.5 km to the grain - magma interface. Such low Al2O3 concentrations and the lateral variations challenge models that envisage injections of pyroxene slurries either from depth or from a laterally displaced source. Basal, dense additions of magma at such intervals as the UG2 chromitite and Merensky units would need to have been >1.5 km in vertical extent in order to flood over such topographic variations. The proposed model to explain such pyroxene compositions involves two-phase convection with initiation of crystallization close to the top of the magma chamber and continuing grain growth and new grains nucleating as the package of grains plus magma descended to the uneven floor.

Fate of water transported into the deep mantle by slab subduction

The roles of water in the mantle transition zone, lower mantle, and the core-mantle boundary are investigated. The evidence for a wet mantle transition zone has been suggested based on hydrous mineral inclusions in diamond. Seismic wave velocity and electrical conductivity profiles together with mineral physics data are consistent with existence of stagnant slabs in a wet mantle transition zone. The transition zone may contain continental crustal components in these stagnant slabs. Dense hydrous magmas may exist at the base of the upper mantle. Fluids or volatile-rich magmas may also exist at the top of the lower mantle due to the large contrast in water contents between the mineral assemblages in the mantle transition zone and the lower mantle, and the crossing of the convective descent of the cold hydrated materials. Dense magmas are not likely to be formed at the top of the lower mantle and hydrous magmas generated in this region move upwards and metasomatize the overlying mantle transition zone. Water can be transported deeper into the lower mantle by gravitational collapse of the stagnant slabs, which supply water into the lower mantle, including the core-mantle boundary. Hydrous δ-H solid solution may be the most important hydrous phase in lower mantle, and existence of this phase reduces the aluminum content in coexisting bridgmanite and post-perovskite, and thus modifies the physical properties of the lower mantle. Hydrous δ-H solid solution can accumulate at the base of the lower mantle. The iron-water reaction at the core-mantle boundary can also create pyrite-type FeOOH which can be a potential candidate material for the ultralow velocity zone (ULVZ).

Density, Viscosity and Velocity (Ascent Rate) of Alkaline Magmas

ABSTRACT Three distinct alkaline magmas, represented by shonkinite, lamprophyre and alkali basalt dykes, characterize a significant magmatic expression of rift-related mantle-derived igneous activity in the Mesoproterozoic Prakasam Alkaline Province, SE India. In the present study we have estimated emplacement velocities (ascent rates) for these three varied alkaline magmas and compared with other silicate magmas to explore composition control on the ascent rates. The alkaline dykes have variable widths and lengths with none of the dykes wider than 1 m. The shonkinites are fine- to medium-grained rocks with clinopyroxene, phologopite, amphibole, K-feldspar perthite and nepheline as essential minerals. They exhibit equigranular hypidiomorphic to foliated textures. Lamprophyres and alkali basalts characteristically show porphyritic textures. Olivine, clinopyroxene, amphibole and biotite are distinct phenocrysts in lamprophyres whereas olivine, clinopyroxene and plagioclase form the phenocrystic mineralogy in the alkali basalts. The calculated densities [2.54-2.71 g/cc for shonkinite; 2.61-2.78 g/cc for lamprophyre; 2.66-2.74 g/cc for alkali basalt] and viscosities [3.11-3.39 Pa s for shonkinite; 3.01-3.28 Pa s for lamprophyre; 2.72-3.09 Pa s for alkali basalt] are utilized to compute velocities (ascent rates) of the three alkaline magmas. Since the lamprophyres and alkali basalts are crystal-laden, we have also calculated effective viscosities to infer crystal control on the velocities. Twenty percent of crystals in the magma increase the viscosity by 2.7 times consequently decrease ascent rate by 2.7 times compared to the crystal-free magmas. The computed ascent rates range from 0.11-2.13 m/sec, 0.23-2.77 m/sec and 1.16-2.89 m/sec for shonkinite, lamprophyre and alkali basalt magmas respectively. Ascent rates increase with the width of the dykes and density difference, and decrease with magma viscosity and proportion of crystals. If a constant width of 1 m is assumed in the magma-filled dyke propagation model, then the sequence of emplacement velocities in the decreasing order is alkaline magmas (4.68-15.31 m/sec) &amp;gt; ultramafic-mafic magmas (3.81-4.30 m/sec) &amp;gt; intermediate-felsic magmas (1.76-2.56 m/sec). We propose that SiO2 content in the terrestrial magmas can be modeled as a semi-quantitative “geospeedometer” of the magma ascent rates.

Lunar floor-fractured craters: Modes of dike and sill emplacement and implications of gas production and intrusion cooling on surface morphology and structure

Lunar floor-fractured craters (FFCs) represent the surface manifestation of a class of shallow crustal intrusions in which magma-filled cracks (dikes) rising to the surface from great depth encounter contrasts in host rock lithology (breccia lens, rigid solidified melt sheet) and intrude laterally to form a sill, laccolith or bysmalith, thereby uplifting and deforming the crater floor. Recent developments in the knowledge of lunar crustal thickness and density structure have enabled important revisions to models of the generation, ascent and eruption of magma, and new knowledge about the presence and behavior of magmatic volatiles has provided additional perspectives on shallow intrusion processes in FFCs. We use these new data to assess the processes that occur during dike and sill emplacement with particular emphasis on tracking the fate and migration of volatiles and their relation to candidate venting processes. FFCs result when dikes are capable of intruding close to the surface, but fail to erupt because of the substructure of their host impact craters, and instead intrude laterally after encountering a boundary where an increase in ductility (base of breccia lens) or rigidity (base of solidified melt sheet) occurs. Magma in dikes approaching the lunar surface experiences increasingly lower overburden pressures: this enhances CO gas formation and brings the magma into the realm of the low pressure release of H2O and sulfur compounds, both factors adding volatiles to those already collected in the rising low-pressure part of the dike tip. High magma rise velocity is driven by the positive buoyancy of the magma in the part of the dike remaining in the mantle. The dike tip overshoots the interface and the consequent excess pressure at the interface drives the horizontal flow of magma to form the intrusion and raise the crater floor. If sill intrusion were controlled by the physical properties at the base of the melt sheet, dikes would be required to approach to within ∼300 m of the surface, and thus eruptions, rather than intrusions, would be very likely to occur; instead, dynamical considerations strongly favor the sub-crustal breccia lens as the location of the physical property contrast localizing lateral intrusion, at a depth of several kilometers. The end of lateral and vertical sill growth occurs when the internal magma pressure equals the external pressure (the intrusion just supports the weight of the overlying crust). Dynamical considerations lead to the conclusion that dike magma volumes are up to ∼1100 km3, and are generally insufficient to form FFCs on the lunar farside; the estimated magma volumes available for injection into sills on the lunar nearside (up to ∼800 km3) are comparable to the observed floor uplift in many smaller FFCs, and thus consistent with these FFCs forming from a single dike emplacement event. In contrast, the thickest intrusions in the largest craters imply volumes requiring multiple dike contributions; these are likely to be events well-separated in time, rather than injection of new magma into a recently-formed and still-cooling intrusion.We present a temporal sequence of 1) dike emplacement, 2) sill formation and surface deformation, 3) bubble rise, foam layer formation and collapse, 4) intrusion cooling, and a synthesis of predicted deformation sequence and eruption styles. Initial lateral injection of the sill at a depth well below the upper dike tip initiates upbowing of the overburden, leveraging deformation of the crater floor melt sheet above. This is followed by lateral spreading of the sill toward the edges of the crater floor, where crater wall and rim deposit overburden inhibit further lateral growth, and the sill grows vertically into a laccolith or bysmalith, uplifting the entire floor above the intrusion. Subsidiary dikes can be emplaced in the fractures at the uplift margins and will rise to the isostatic level of the initial dike tip; if these contain sufficient volatiles to decrease magma density, eruptions can also occur. This initial phase of intrusion, sill lateral spreading and floor uplift occurs within a few hours after initial dike emplacement. During the subsequent cooling of the sill, bubbles can rise hundreds of meters to the top of the intrusion to create a foam layer; when drainage of gas bubble wall magma occurs in the foam layer, a continuous gas layer forms above the foam. Gas formation and upward migration produces an increase in sill thickness, while subsequent cooling and solidification cause a thickness decreases and subsidence. The total topographic evolution history, following an initial 2 km thick sill intrusion and floor uplift (hours), includes further floor uplift by gas formation and migration (decades; ∼30 m), followed by cooling, solidification and subsidence (∼a century; ∼350 m). An initial 2 km thick sill is predicted to have a final thickness of ∼1.7 km. This predicted sequence of events can be compared with the sequence of floor deformation and volcanism in FFCs in order to test and refine this model.

Density of hydrous magma

Magmas in the interior of the Earth have been geophysically observed. The generation of magmas is closely related to H2O because the water-induced decrease in the melting temperature of mantle rocks is an essential process, which forms hydrous magmas due to H2O incorporation into the molten rocks. Knowledge on physical properties of hydrous magmas, such as density, can give us better understanding of the magma behavior inside the Earth. The density of hydrous magma can be controlled by the partial molar volume of H2O (V¯H2O) and its pressure- and temperature-dependences. The new compression behavior of V¯H2O using Vinet equation of state is proposed based on compiled data. Since H2O is one of the most compressible oxides in the magma at lower pressure, a rapid densification of hydrous magma can be observed at shallower mantle. The dense hydrous magma can be gravitational stable above and below mantle transition zone.

Constraining the Magmatic System at Mount St. Helens (2004–2008) Using Bayesian Inversion With Physics‐Based Models Including Gas Escape and Crystallization

AbstractPhysics‐based models of volcanic eruptions track conduit processes as functions of depth and time. When used in inversions, these models permit integration of diverse geological and geophysical data sets to constrain important parameters of magmatic systems. We develop a 1‐D steady state conduit model for effusive eruptions including equilibrium crystallization and gas transport through the conduit and compare with the quasi‐steady dome growth phase of Mount St. Helens in 2005. Viscosity increase resulting from pressure‐dependent crystallization leads to a natural transition from viscous flow to frictional sliding on the conduit margin. Erupted mass flux depends strongly on wall rock and magma permeabilities due to their impact on magma density. Including both lateral and vertical gas transport reveals competing effects that produce nonmonotonic behavior in the mass flux when increasing magma permeability. Using this physics‐based model in a Bayesian inversion, we link data sets from Mount St. Helens such as extrusion flux and earthquake depths with petrological data to estimate unknown model parameters, including magma chamber pressure and water content, magma permeability constants, conduit radius, and friction along the conduit walls. Even with this relatively simple model and limited data, we obtain improved constraints on important model parameters. We find that the magma chamber had low (&lt;5 wt %) total volatiles and that the magma permeability scale is well constrained at ∼10−11.4m2 to reproduce observed dome rock porosities. Compared with previous results, higher magma overpressure and lower wall friction are required to compensate for increased viscous resistance while keeping extrusion rate at the observed value.

The Ferrar Large Igneous Province: field and geochemical constraints on supra-crustal (high-level) emplacement of the magmatic system

Abstract The Ferrar Large Igneous Province forms a linear outcrop belt for 3250 km across Antarctica, which then diverges into SE Australia and New Zealand. The province comprises numerous sills, a layered mafic intrusion, remnants of extensive lava fields and minor pyroclastic deposits. High-precision zircon geochronology demonstrates a restricted emplacement duration (&lt;0.4 myr) at c. 182.7 Ma, and geochemistry demonstrates marked coherence for most of the Ferrar province. Dyke swarms forming magma feeders have not been recognized, but locally have been inferred geophysically. The emplacement order of the various components of the magmatic system at supra-crustal levels has been inferred to be from the top-down lavas first, followed by progressively deeper emplacement of sills. This order was primarily controlled by magma density, and the emptying of large differentiated magma bodies from depth. An alternative proposal is that the magma transport paths were through sills, with magmas moving upwards to eventually reach the surface to be erupted as extrusive rocks. These two hypotheses are evaluated in terms of field relationships and geochemistry in the five regional areas where both lavas and sills crop out. Either scenario is possible in one or more instances, but neither hypothesis applies on a province-wide basis.

Pressure evolution in shallow magma chambers upon buoyancy‐driven replenishment

AbstractThe invasion of active magma chambers by primitive magma of deeper provenance is a frequent occurrence in volcanic systems, and it is commonly associated with pressurization. Chamber replenishment is driven by pressure and buoyancy forces that cause magma ascent towards shallow depths. We examine the end‐member case of pure buoyancy‐driven (natural) convection in crustal reservoirs deriving from the presence of degassed, dense magma at shallow level, that can originate a gravitational instability. Space‐time‐dependent numerical simulations of magma dynamics in composite underground systems reveal highly nonlinear pressure evolution dominated by decompression at shallow depths. This counterintuitive result originates from the compressible nature of multiphase magmas and their complex convection and mixing dynamics. Shallow magma chamber decompression on replenishment is favored by large volatile contents of the uprising magma, resulting in large density contrasts among the resident and the incoming components. These results show that the intuitive concept of magma chamber pressurization upon replenishment may not always hold in real situations dominated by buoyancy, and provide new perspectives for the interpretation of geophysical records at active volcanoes.

Climate response to the Samalas volcanic eruption in 1257 revealed by proxy records

The eruption of Samalas in Indonesia in 1257 ranks among the largest sulfur-rich eruptions of the Common Era with sulfur deposition in ice cores reaching twice the volume of the Tambora eruption in 1815. Sedimentological analyses of deposits confirm the exceptional size of the event, which had both an eruption magnitude and a volcanic explosivity index of 7. During the Samalas eruption, more than 40 km3 of dense magma was expelled and the eruption column is estimated to have reached altitudes of 43 km. However, the climatic response to the Samalas event is debated since climate model simulations generally predict a stronger and more prolonged surface air cooling of Northern Hemisphere summers than inferred from tree-ring-based temperature reconstructions. Here, we draw on historical archives, ice-core data and tree-ring records to reconstruct the spatial and temporal climate response to the Samalas eruption. We find that 1258 and 1259 experienced some of the coldest Northern Hemisphere summers of the past millennium. However, cooling across the Northern Hemisphere was spatially heterogeneous. Western Europe, Siberia and Japan experienced strong cooling, coinciding with warmer-than-average conditions over Alaska and northern Canada. We suggest that in North America, volcanic radiative forcing was modulated by a positive phase of the El Nino–Southern Oscillation. Contemporary records attest to severe famines in England and Japan, but these began prior to the eruption. We conclude that the Samalas eruption aggravated existing crises, but did not trigger the famines.

Secondary nucleation studies on alpha lactose monohydrate under stirred conditions

Secondary nucleation is the dominant source of nuclei at the industrial scale. Traditionally, secondary nucleation has been expressed empirically as a function of stirrer speed and magma density with little or no understanding about the role of individual mechanisms (attrition and contact). This study considers the various mechanisms of secondary nucleation of alpha-lactose monohydrate crystals and link them to either crystal–crystal or crystal-impeller collisions. An agitated system was used to conduct trials at various levels of stirring speed (400 and 550 rpm), seed size (150, 250, 357 and 502 μm) and seed loading (2%, 5% and 10%, v/v) at a constant supersaturation. Crystal-impeller contacts are proposed to be the major source of secondary nucleation. It was found that the kinetic energy and not the collision frequency govern the rate of secondary nucleation with a threshold kinetic energy value below which no secondary nucleation occurs.

Evolution of magma chambers generating the phonolitic Cão Grande Formation on Santo Antão, Cape Verde Archipelago

The Cão Grande Formation (CGF) on the western plateau of Santo Antão is a sequence of four phonolitic tephras (Canudo Tephra, Cão Grande I Tephra, Cão Grande II Tephra and Furninha Tephra) produced by highly explosive eruptions that alternatingly originated from a basanitic - phonolitic and a nephelinitic - phonolitic magmatic system. Detailed stratigraphy and petrological investigations of each unit are used to demonstrate the unusual situation that two distinct highly evolved magmas differentiated contemporaneously in separate magmatic systems. Chemical thermobarometry suggests that both magmatic systems not only temporally co-existed, but also that their magma chambers resided close to each other at 7 to 16km depth, beneath the western plateau of Santo Antão. However, the distinct melt and magma compositions indicate that both systems evolved independently. The only interaction between both magmatic systems was an injection of magma from the nephelinitic - phonolitic magmatic system into the Cão Grande II Tephra (CG II) phonolitic reservoir, which is associated to the basanitic - phonolitic magmatic system. Compositional zonations in the tephra deposits indicate that the eruptions of the CGF tapped stratified magma reservoirs that mainly resulted from crystal accumulation generating downward increasing magma density. However, the CG II tephras also show a significant gradient in melt (glass) compositions. Magmas of the Canudo Tephra (CT) and the Cão Grande I Tephra (CG I) were H2O-saturated and their eruptions were probably triggered by fluid overpressure in the magma chamber. On the other hand, the CG II magma was H2O-undersaturated; we therefore assume that the injection of the hot nephelinitic - phonolitic magma system-type melt/magma triggered the eruption. The zoned deposit of the Furninha Tephra (FT) indicates mafic magma replenishment into a phonolitic reservoir directly prior to the eruption, thus providing a probable triggering mechanism. The new magma chamber models and thermobarometric results for the four CGF units provide constraints for hazard assessments, because similar events may occur in the future considering the longevity of the CGF magma systems.

Conduit Dynamics in Transitional Rhyolitic Activity Recorded by Tuffisite Vein Textures from the 2008–2009 Chaitén Eruption

The mechanisms of hazardous silicic eruptions are controlled by complex, poorly-understood conduit processes. Observations of recent Chilean rhyolite eruptions have revealed the importance of hybrid activity, involving simultaneous explosive and effusive emissions from a common vent. Such behaviour hinges upon the ability of gas to decouple from magma in the shallow conduit. Tuffisite veins are increasingly suspected to be a key facilitator of outgassing, as they repeatedly provide a transient permeable escape route for volcanic gases. Intersection of foam domains by tuffisite veins appears critical to efficient outgassing. However, knowledge is currently lacking into textural heterogeneities within shallow conduits, their relationship with tuffisite vein propagation, and the implications for fragmentation and degassing processes. Similarly, the magmatic vesiculation response to upper conduit pressure perturbations, such as those related to the slip of dense magma plugs, remains largely undefined. Here we provide a detailed characterization of an exceptionally large tuffisite vein within a rhyolitic obsidian bomb ejected during transitional explosive-effusive activity at Chaiten, Chile in May 2008. Vein textures and chemistry provide a time-integrated record of the invasion of a dense upper conduit plug by deeper fragmented magma. Quantitative textural analysis reveals diverse vesiculation histories of various juvenile clast types. Using vesicle size distributions, bubble number densities, zones of diffusive water depletion, and glass H2O concentrations, we propose a multi-step degassing/fragmentation history, spanning deep degassing to explosive bomb ejection. Rapid decompression events of ~3-4 MPa are associated with fragmentation of foam and dense magma at ~200-350 metres depth in the conduit, permitting vertical gas and pyroclast mobility over hundreds of metres. Permeable pathway occlusion in the dense conduit plug by pyroclast accumulation and sintering preceded ultimate bomb ejection, which then triggered a final bubble nucleation event. Our results highlight how the vesiculation response of magma to decompression events is highly sensitive to the local melt volatile concentration, which is strongly spatially heterogeneous. Repeated opening of pervasive tuffisite vein networks promotes this heterogeneity, allowing juxtaposition of variably volatile-rich magma fragments that are derived from a wide range of depths in the conduit. This process enables efficient but explosive removal of gas from rhyolitic