Crystal-rich Magma Research Articles

Magmatic architecture of dome-building eruptions at Volcán de Colima, Mexico

Changes in the physical, chemical and rheological properties of ascending magma regulate the style of volcanic eruptions. Volcan de Colima’s eruptive cycles of lava dome growth and explosions have been thoroughly monitored during the period 1998–2010 and provide a remarkable opportunity for deepening our understanding of the underlying processes responsible for the evolution of magma properties. Here, we integrate direct observation with analytical and experimental data to: (1) constrain the configuration of the shallow plumbing system and its influence on eruptive activity, (2) describe the rheological behaviour of the magma and (3) assess the conditions that lead to fragmentation and, ultimately, to explosive eruptions. The configuration of the shallow plumbing system was inferred from direct observation of extrusion sites and porosity of the erupted products. During the ongoing eruptive phase, magma was never extruded from a central vent: Both explosive and effusive activities were restricted to discrete vents inside the crater. Extensive field-based density measurements on 500 blocks in pyroclastic flow deposits reveal a bimodality of porosity at values of 12 and 26 vol.%. The least porous rocks tend to be altered, whereas the more porous rocks are pristine. This bimodal distribution, combined with the lack of a central vent, suggests the presence of a central, dense, altered plug, the fragments of which are entrained during explosive eruptions. During effusive periods, the plug appears to deflect the ascent of magma at a shallow depth and, consequently, the site of lava extrusion. The rheological properties and deformation-induced seismogenic behaviour of the magmas were investigated using a uniaxial deformation apparatus instrumented with acoustic sensors. The homogeneity in the physicochemical properties of the erupted magma permits the description of a flow law at eruptive temperature and strain rate conditions. The crystal-rich magma of Volcan de Colima exhibits a shear thinning rheology and becomes increasingly brittle at higher strain rates. Complete failure of magma can be predicted using the material failure forecast method, which integrates the acceleration of released acoustic energy throughout the deformation. Rapid decompression experiments of samples pressurised with argon were performed to assess the fragmentation conditions under which explosive eruptions progress. In the absence of gas loss due to permeable flow, the pore pressure required to fragment volcanic products is inversely proportional to the porosity. At Volcan de Colima, a rapid decompression of >6 MPa is required to fragment magma averaging 26 vol.% pores and to thereby instigate an explosive eruption. Analysis of ballistic impacts (4–6 km away from the vent) from recent explosive eruptions further requires decompression as great as 20 MPa, which is sufficient to disrupt the more porous material as well as a fraction of the denser plug. The continuing growth of a lava dome and of a larger, more impermeable plug could have consequences for the stability of the edifice and development of stronger and more erratic explosive activity at Volcan de Colima.

Porosity redistribution enhanced by strain localization in crystal-rich magmas

Magma degassing, characterized by changes in permeability and porosity distribution, has a crucial control on the style of eruption. During ascent, magma might develop large porosities and crystallize while it is subjected to shear. Shear, in turn, enhances complex fabrics that result from the reorganization of the different phases (crystals, gas, melt). Such fabrics have not yet been evaluated experimentally on a three-phase system. We performed torsion experiments on a synthetic crystal-rich hydrous magma at subsolidus conditions with 11 vol% porosity to establish a link between strain partitioning and porosity redistribution. Crystals induce non-Newtonian deformation, resulting in localization of the shear strain. Three-dimensional microtomography and two-dimensional scanning electron microscope imaging show gas accumulation in local microstructures caused by shear-induced crystal fabric. Our data show that strain localization is a mechanism that could enable magma degassing at very low vesicularity.

Numerical modelling of magma transport in dykes

The rheology and dynamics of an ascending pure melt in a dyke have been extensively studied in the past. From field observations, it is apparent that most dykes actually contain a crystalline load. The presence of a crystalline load modifies the effective rheology of such a system and thus the flow behaviour. Indeed, the higher density and viscosity of each crystal, compared to the melt, cause a decrease of the ascent velocity and modify the shape of the velocity profile, from a typical Poiseuille flow, to a Bingham-type flow. A common feature observed in the field is the arrangement of crystals parallel or at a very low angle to the edge of the dyke. Such a structural arrangement is often interpreted as the result of magma flow, which caused the crystals to rotate and align within the flow direction, but this process remains unclear. Another issue related to the introduction of a crystalline load concerns the possibility for crystals to be segregated from a viscous granitic melt phase during magma ascent. The implications of such a process on magmatic differentiation have not previously been considered, nor has such a process been previously investigated via numerical models. In this study, we examine the flow dynamics of a crystal bearing granitic melt ascending in a dyke via numerical models. In our models, both the crystal and melt phases are represented as highly viscous fluids in a Stokes regime. Our results reveal that the presence of crystals in the melt modifies the magma velocity profile across the dyke. Furthermore, we observe that whilst crystals continually rotate in the shear flow, over one period of revolution, their major axis has a high probability to be aligned parallel to the flow direction. Moreover, some experiments showed that the melt phase can effectively be squeezed out from a crystal-rich magma when subjected to a given pressure gradient range. This demonstrates that crystal-melt segregation in dykes during granitic magma ascent constitutes a viable mechanism for magmatic differentiation.

Thermo-mechanical reactivation of locked crystal mushes: Melting-induced internal fracturing and assimilation processes in magmas

Thermal reactivation of locked crystal mushes in the upper crust is a fundamental step towards volcanic eruptions of crystal-rich magmas. Models of such reactivation events indicate that partial melting of the crystalline framework is energetically costly and lead to average crystallinities that are lower than those observed in many erupted crystal mushes. Here, we show that internal overpressurization of the mush induced by small amounts of melting (10–20 %) breaks the crystalline framework by microfracturing and allows for efficient unlocking of the mush. Hence, this melting-induced overpressurization, enhanced by addition of gas in wet magmatic systems, plays an important role in generating volcanic deposits with crystal contents close to the rheological lock-up (∼ 50 vol % crystals) by accelerating the incorporation of highly crystalline parts of the magma chamber (self-assimilation). It can also participate in disintegrating pieces of country rock that are commonly scavenged in magmas, leading to bulk assimilation of crustal lithologies in shallow reservoirs.

Episodic intrusion, internal differentiation, and hydrothermal alteration of the Miocene Tatoosh intrusive suite south of Mount Rainier, Washington

The Miocene Tatoosh intrusive suite south of Mount Rainier is composed of three broadly granodioritic plutons that are manifestations of ancestral Cascades arc magmatism. Tatoosh intrusive suite plutons have individually diagnostic characteristics, including texture, mineralogy, and geochemistry, and apparently lack internal contacts. New ion-microprobe U-Pb zircon ages indicate crystallization of the Stevens pluton ca. 19.2 Ma, Reflection-Pyramid pluton ca. 18.5 Ma, and Nisqually pluton ca. 17.5 Ma. The Stevens pluton includes rare, statistically distinct ca. 20.1 Ma zircon antecrysts. Wide-ranging zircon rare earth element (REE), Hf, U, and Th concentrations suggest late crystallization from variably evolved residual liquids. Zircon Eu/Eu*–Hf covariation is distinct for each of the Reflection-Pyramid, Nisqually, and Stevens plutons. Although most Tatoosh intrusive suite rocks have been affected by weak hydrothermal alteration, and sparse mineralized veins cut some of these rocks, significant base or precious metal mineralization is absent. At the time of shallow emplacement, each of these magma bodies was largely homogeneous in bulk composition and petrographic features, but, prior to final solidification, each of the Tatoosh intrusive suite plutons developed internal compositional variation. Geochemical and petrographic trends within each pluton are most consistent with differential loss of residual melt, possibly represented by late aplite dikes or erupted as rhyolite, from crystal-rich magma. Crystal-rich magma that formed each pluton evidently accumulated in reservoirs below the present level of exposure and then intruded to a shallow depth. Assembled by episodic intrusion, the Tatoosh intrusive suite may be representative of midsized composite plutonic complexes beneath arc volcanoes.

PETROLOGY, GEOCHEMISTRY, AND STRUCTURE OF THE CHUGWATER ANORTHOSITE, LARAMIE ANORTHOSITE COMPLEX, SOUTHEASTERN WYOMING

The Chugwater Anorthosite is one of several 1.43 Ga intrusions that make up the Laramie Anorthosite Complex in the Laramie Mountains, Wyoming, USA. The southwestern portion of the Chugwater Anorthosite has a mappable magmatic stratigraphy totaling at least 8000 meters, and probably 10,000 m or more. It consists of three major units, each of which comprises a lower, dominantly anorthosite portion (>90 vol.% plagioclase) and an upper, dominantly gabbroic anorthosite section (80–90 vol.% plagioclase). Each anorthosite portion contains layers of gabbroic anorthosite and vice versa , on a variety of scales ranging down to centimeters. Plagioclase is dominantly An 50–55 , although higher and lower values are also found. This “main series” of the Chugwater Anorthosite mainly lacks modal and normative olivine. Oxygen fugacity ranged from FMQ to FMQ + 0.5. We interpret that the “main series” was produced by at least three major injections of mildly hyperfeldspathic magma containing approximately 40% entrained plagioclase megacrysts (tabular crystals at least 5 cm across). The Ti contents of the megacrysts (0.3–0.4 wt% TiO 2 ) suggest that these had crystallized at pressures near 10 kbar, consistent with initial formation in a magma chamber at or near the base of the crust. Emplacement pressure is poorly constrained, but appears to have been near 3.5–4 kbar. However, before the main series had completely solidified, it was repeatedly intruded by at least two (and probably more) leucotroctolitic magmas. Contacts of leucotroctolite are sharp against anorthosite, but commonly are diffuse against gabbroic anorthosite, suggesting that sufficient residual melt remained in the latter to permit local mixing with leucotroctolite. We estimate that 80–85% of the Chugwater Anorthosite is main-series, 10–15% is mixed rock, and 5% is leucotroctolite. Prior to final solidification, the entire Chugwater Anorthosite was domed, probably as a result of gravitational instability of the relatively buoyant plagioclase-rich material. The emplacement of the Chugwater Anorthosite as a series of crystal-rich magmas, followed by continued fractionation in a magma chamber at a mid-crustal level and subsequent doming, are characteristics that make the Chugwater Anorthosite intermediate between the Poe Mountain Anorthosite to the north ( in situ fractionation in a magma chamber) and the classic diapiric emplacement of a crystal-rich mush.

Gas accumulation in particle-rich suspensions and implications for bubble populations in crystal-rich magma

Gas mobility plays an important role in driving volcanic eruptions and controlling eruption style. The explosivity of an eruption depends, among other factors, on how easily gas can escape from the magma. Many magmatic systems have high concentrations of suspended crystals that inhibit gas migration through the melt. We use suspensions of plastic beads in corn syrup to investigate interactions between rising bubbles and particles. We observe different interaction styles as the ratio ψ of bubble to particle size is varied. Large bubbles (ψ > 1) deform and sometimes break up as they move around particles. Small bubbles (ψ < 1) are frequently trapped within the suspension, increasing the concentration of gas held within the system. We compare our experiments to bubble populations in tephra from Stromboli volcano, Italy. We show that these samples typically have bubbles and crystals of similar sizes and suggest that crystals might play a role in controlling bubble size in this natural system as well as in our experiments. Because small bubbles (ψ < 1) get trapped within the suspension, and can be formed by breakup of larger bubbles, we expect that an increase in gas flux will result in an increase in the population of small bubbles. Changes in bubble number density and vesicularity in tephra erupted during periods of different eruptive intensity may thus provide a way of tracking changes in gas flux through the magma prior to eruption.

Rheological control on the dynamics of explosive activity in the 2000 summit eruption of Mt. Etna

Abstract. In the period from January to June 2000 Mt. Etna exhibited an exceptional explosive activity characterized by a succession of 64 Strombolian and fire-fountaining episodes from the summit South-East Crater. Textural analysis of the eruptive products reveals that the magma associated with the Strombolian phases had a much larger crystal content (&gt;55 vol%) with respect to the magma discharged during the fire-fountain phases (~35 vol%). Rheological modelling shows that the crystal-rich magma falls in a region beyond a critical crystal content where small addition of solid particles causes an exponential increase of the effective magma viscosity. When implemented into the modeling of steady magma ascent dynamics (as assumed for the fire-fountain activity), a large crystal content as the one found for products of Strombolian eruption phases results in a one order of magnitude decrease of mass flow-rate, and in the onset of conditions where small heterogeneities in the solid fraction carried by the magma translate into highly unsteady eruption dynamics. We argue that crystallization on top of the magmatic column during the intermediate phases when magma was not discharged favoured conditions corresponding to Strombolian activity, with fire-fountain activity resuming after removal of the highly crystalline top. The numerical simulations also provide a consistent interpretation of the association between fire-fountain activity and emergence of lava flows from the crater flanks.

A new petrological and geophysical investigation of the present‐day plumbing system of Mount Vesuvius

A model of the electrical resistivity of Mt. Vesuvius has been elaborated to investigate the present structure of the volcanic edifice. The model is based on electrical conductivity measurements in the laboratory, on geophysical information, in particular, magnetotelluric (MT) data, and on petrological and geochemical constraints. Both 1‐D and 3‐D simulations explored the effect of depth, volume and resistivity of either one or two reservoirs in the structure. For each configuration tested, modeled MT transfer functions were compared to field transfer functions from field magnetotelluric studies. The field electrical data are reproduced with a shallow and very conductive layer (∼0.5 km depth, 1.2 km thick, 5 ohm.m resistive) that most likely corresponds to a saline brine present beneath the volcano. Our results are also compatible with the presence of cooling magma batches at shallow depths (&lt;3–4 km depth). The presence of a deeper body at ∼8 km depth, as suggested by seismic studies, is consistent with the observed field transfer functions if such a body has an electrical resistivity &gt; ∼100 ohm.m. According to a petro‐physical conductivity model, such a resistivity value is in agreement either with a low‐temperature, crystal‐rich magma chamber or with a small quantity of hotter magma interconnected in the resistive surrounding carbonates. However, the low quality of MT field data at long periods prevent from placing strong constraints on a potential deep magma reservoir. A comparison with seismic velocity values tends to support the second hypothesis. Our findings would be consistent with a deep structure (8–10 km depth) made of a tephriphonolitic magma at 1000°C, containing 3.5 wt%H2O, 30 vol.% crystals, and interconnected in carbonates in proportions ∼45% melt −55% carbonates.

Two Competing Effects of Volatiles on Heat Transfer in Crystal-rich Magmas: Thermal Insulation vs Defrosting

The build-up and eruption of crystal-rich ignimbrites, commonly referred to as ‘Monotonous Intermediates’, has been attributed to the incremental addition of new magma batches and the episodic partial melting and reactivation of rheologically locked crystal-rich magma bodies (mushes). In this study, we explore the role of volatiles exsolved from a hot intrusion underplating a crystal mush on the thermal evolution of the coupled mush–intrusion system. We solve the enthalpy conservation equation for the mush and the intrusion and investigate the exsolution of volatiles from the intrusion and their transport through the mush in one dimension. Our calculations span a range of pressures (from 1 to 4 kbar), mush composition (andesite to rhyolite) and initial water contents (from 1·5 to 6 wt %). The mobility of volatiles in the mush is controlled by the volume fraction of the pore space they occupy, and, as a consequence, by the amount of melting and the injection rate of volatiles from the intrusion to the mush. We find that volatiles affect the melting of the mush (or defrosting) in two opposite ways depending on pressure and the initial water content of the intrusion. When the intrusion volatile content is high and pressure relatively low (>4 wt % H2O at 2 kbar), the mass transfer of volatiles from the intrusion to the mush carries enthalpy beyond the melting front and can thus enhance defrosting and possibly remobilization of the mush. For lower initial volatile contents (< 4 wt % H2O) in the intrusion and/or higher pressure (3–4 kbar), volatiles stall at the interface between the two magma bodies and prevent defrosting as they thermally insulate the mush from the intrusion. We propose that the dual role played by volatiles during the thermal evolution of crystal-rich magmas reheated by an underplating intrusion can explain the presence of crystal-rich ignimbrites in arcs and their absence in drier hotspots or extensional regions.

Magmatic tubes, pipes, troughs, diapirs, and plumes: Late-stage convective instabilities resulting in compositional diversity and permeable networks in crystal-rich magmas of the Tuolumne batholith, Sierra Nevada, California

A complex array of widespread, but domainally developed, structures is preserved in the Tuolumne batholith, including stationary and migrating tubes, pipes, troughs, diapers, and plume heads. These structures, all formed by local magma flow through crystal-mush host magmas, are often associated with the formation of schlieren rich in accessory and mafic minerals, and are associated with filter pressing and accumulations of crystals with diverse magma histories and ages. Together they represent a network in which channelized flow occurred in an existing chamber of crystal-rich magmas, resulting in local compositional and structural diversity. These structures also are useful structural tools for evaluating the internal evolution of magma chambers. For example, the consistently steep tube and pipe axes indicate that neither the pluton nor features in the pluton were tilted during growth, thus excluding models in which subhorizontal layers tilted to form the existing steep contacts. Although the overall direction of younging established by geochronologic studies is toward the batholith center, local younging directions determined from troughs cutoffs indicate that outward growth occurred in many zones. The highly variable movement directions of local diapirs and plumes require interactions between buoyancy forces and other gradients. The existence and characteristics of these structures have several other implications. Interpretations derived about crystal residence times in chambers and about crystal mixing during eruptions need to be treated with caution, since mixed crystal populations existed well prior to eruptions, and in the case of the Tuolumne batholith involved crystals with ages ranging over ~10 m.y. A likely solution is that crystals in subvolcanic chambers become armored (rimmed) by other crystals or exist in crystal clusters that, in spite of changing environmental conditions, prevent rapid chemical communication with the surrounding melts. These structures also challenge many aspects of the incremental chamber growth model resulting in sheeted bodies championed by Glazner, Bartley, Coleman, and colleagues for the Tuolumne batholith. The thousands of preserved internal structures provide clear evidence against late annealing and removal of internal contacts, and are difficult to reconcile with either vertical sheeted or subhorizontal laccolith models; however, they are permissive of early pulsing leading to one or more large magma chambers.

Multiple inflation and deflation events at Kenyan volcanoes, East African Rift

The presence or absence of magma exerts a fundamental control on the distribution of strain in continental rift zones, yet the time scales of magma intrusion remain unconstrained. Using more than a decade of measurements from interferometric synthetic aperture radar (InSAR), we detected geodetic activity at four of the eleven central rift volcanoes in the Kenyan sector of the East African Rift. Subsidence of 2–5 cm occurred at Suswa and Menengai over the period 1997–2000, ~9 cm of uplift was recorded at Longonot in 2004–2006, and ~21 cm of uplift occurred at Paka during 2006–2007. The deformation is episodic, and no deformation was observed at these volcanoes during other time periods. Preceding the infl ation at Paka, we observed transient uplift and subsidence of a second, nearby source, likely associated with fl ow through a complex plumbing system. The best-fi tting source models for each episode include infl ation or defl ation of a horizontal penny-shaped crack at a depth of 2–5 km. The episodic nature of the activity, its lack of correlation with seasons, and the preferred source geometry are all consistent with activity in the volatile-rich cap to a crystal-rich magma chamber beneath each of the four volcanoes. The presence of active magmatic systems beneath more than 40% of the volcanoes, and the 2007–2008 explosive eruptions at nearby Oldoinyo Lengai volcano, have implications for models of continental rifting, caldera volcanoes, geothermal resources, and volcanic and seismic hazard in rift zones.

Transient processes in Stromboli’s shallow basaltic system inferred from dolerite and magmatic breccia blocks erupted during the 5 April 2003 paroxysm

We describe the mineralogy, geochemistry, and mesomicrostructure of fresh subvolcanic blocks erupted during the 5 April 2003 paroxysm of Stromboli (Aeolian Islands, Italy). These blocks represent ∼50 vol.% of the total erupted ejecta and consist of fine- to medium-grained basaltic lithotypes ranging from relatively homogeneous dolerites to strongly or poorly welded magmatic breccias. The breccia components are represented by angular fragments of dolerites entrapped in a matrix of vesiculated (lava-like to scoriae) crystal-rich (CR) basalt. All of the studied blocks are cognates with the CR basalt of the normal Strombolian activity or lavas and they are often coated by a few-centimeter thick layer of crystal-poor (CP) basaltic pumice erupted during the paroxysm. We suggest that they result from the rapid increase of pressure and related subvolcanic rock failure that occurred shortly before the 5 April 2003 explosion, when the uppermost portion of the edifice inflated and suffered brecciation as the result of the sudden rise of the gas-rich CP basalt that triggered the eruption. Dolerites and magmatic matrix of the breccias show major and trace element compositions that match those of the CR basalts erupted during normal Strombolian activity and effusive events at Stromboli volcano. Dolerites consist of (a) phenocrysts normally found in the CR basalts and (b) late-stage magmatic minerals such as sanidine, An60-28 plagioclase, Fe–Mn-rich olivines (Fo68-48), phlogopite, apatite, and opaque mineral pairs (magnetite and ilmenite), most of which are never found both in lava flows and scoriae erupted during the persistent explosive activity that characterizes typical Strombolian behavior. Subvolcanic crystallization of the Stromboli CR magma, leading to slowly cooled equivalents of basalts, could result from transient drainage of the magma from the summit craters to lower levels. Fingering and engulfing of the material that collapsed from the summit crater floor into the shallow basaltic system during the late evening of 28 December 2002 coupled with the short break in the summit persistent explosions between December 2002 and March 2003 permitted the CR magma pockets to solidify as dolerites, which were confined to the uppermost portion of the system and thus not involved in the ongoing flank effusive activity. Crystal size distribution of the basaltic blocks and crystallization of the finer-grained (<0.1 mm) mafic minerals of the dolerites over a time interval of ∼100 days closely agrees with the above interpretation. Vesicle filling (miarolitic cavities) locally found in some dolerites, with minerals deposited as vapor-phase crystallization is a result of continuous gas percolation through the rocks of the uppermost portion of the volcanic system. Poorly welded magmatic breccias formed during syn-eruptive processes of 5 April 2003, when the paroxysm strongly shattered the shallow subvolcanic system and many dolerite fragments were entrapped in the CR magma. In contrast, the high degree of welding between the dolerite clasts and the CR basaltic matrix in the strongly welded magmatic breccias provides a snapshot of subvolcanic intrusions of the CR basalt into the dolerite when, after a 2-month break in activity, CR magmas started to rise again to the summit craters. Blocks similar to these subvolcanic ejecta of 5 April 2003 were also erupted during previous paroxysms (e.g., 1930) suggesting that changes in the usual Strombolian activity (e.g., short breaks in the persistent mild explosions and/or flank effusive activity) lead to transient crystallization of dolerites in the shallow plumbing system.

Magma Ascent and Crustal Accretion at Ultraslow-Spreading Ridges: Constraints from Plagioclase Ultraphyric Basalts from the Arctic Mid-Ocean Ridge

Plagioclase ultraphyric basalts (PUBs) with up to 54% plagioclase phenocrysts were dredged in the rift valley and adjacent flanks of the ultraslow-spreading Mohns and Knipovich ridges. The PUBs show large variations in crystal morphologies and zoning. The large variations suggest that single basalt samples contain a mixture of plagioclase crystals that aggregated at different levels in the magma conduits. Resorbed crystals and repeated reverse zones suggest that the magma reservoirs were replenished and heated several times. Thin concentric zones with melt inclusions, and sharp reductions in the anorthite content of 3–7%, are common between the reverse zones. These zones, and skeletal crystals with distinctly lower anorthite contents than massive crystals, are interpreted to be the result of rapid crystalliztion during strong undercooling. The changes between short periods of cooling and longer periods with reheating are explained by multiple advances of crystal-rich magma into cool regions followed by longer periods of gradual magma inflow and temperature increase. The porphyritic basalts are characterizd by more depleted and more fractionated compositions than the aphyric basalts, with lower (La/Sm)N, K2O and Mg-numbers. This relationship, and the observation that PUBs are sampled only close to segment centres along these ridges, suggests that the PUBs formed by higher degrees of melting and evolved in more long-lived magma reservoirs. We propose that the zoning patterns of plagioclase crystals and crystal morphologies of these PUBs reflect the development and flow of magma through a stacked sill complex-like conduit system, whereas the aphyric equivalents represent later flow of magma through the conduit. The formation of voluminous higher-degree melts may trigger the development of the magma conduits and explain the generally depleted compositions of PUB magmas.

Modelling shear bands in a volcanic conduit: Implications for over-pressures and extrusion-rates

Shear bands in a volcanic conduit are modelled for crystal-rich magma flow using simplified conditions to capture the fundamental behaviour of a natural system. Our simulations begin with magma crystallinity in equilibrium with an applied pressure field and isothermal conditions. The viscosity of the magma is derived using existing empirical equations and is dependent upon temperature, water content and crystallinity. From these initial conduit conditions we utilize the Finite Element Method, using axi-symmetric coordinates, to simulate shear bands via shear localisation. We use the von Mises visco-plasticity model with constant magma shear strength for a first look into the effects of plasticity. The extent of shear bands in the conduit is explored with a numerical model parameterized with values appropriate for Soufrière Hills Volcano, Montserrat, although the model is generic in nature. Our model simulates shallow (up to approximately 700 m) shear bands that occur within the upper conduit and probably govern the lava extrusion style due to shear boundaries. We also model the change in the over-pressure field within the conduit for flow with and without shear bands. The pressure change can be as large as several MPa at shallow depths in the conduit, which generates a maximum change in the pressure gradient of 10's of kPa/m. The formation of shear bands could therefore provide an alternative or additional mechanism for the inflation/deflation of the volcano flanks as measured by tilt-metres. Shear bands are found to have a significant effect upon the magma ascent rate due to shear-induced flow reducing conduit friction and altering the over-pressure in the upper conduit. Since we do not model frictional controlled slip, only plastic flow, our model calculates the minimum change in extrusion rate due to shear bands. However, extrusion rates can almost double due to the formation of shear bands, which may help suppress volatile loss. Due to the paucity of data and large parameter space available for the magma shear strength our model results can only allow for a qualitative comparison to a natural system at this stage.

Petrologic constraints on the decompression history of magma prior to Vulcanian explosions at the Soufrière Hills volcano, Montserrat

A series of 88 Vulcanian explosions occurred at the Soufrière Hills volcano, Montserrat, between August and October, 1997. Conduit conditions conducive to creating these and other Vulcanian explosions were explored via analysis of eruptive products and one-dimensional numerical modeling of magma ascent through a cylindrical conduit. The number densities and textures of plagioclase microlites were documented for twenty-three samples from the events. The natural samples all show very high number densities of microlites, and > 50% by number of microlites have areas < 20 μm 2. Pre-explosion conduit conditions and decompression history have been inferred from these data by comparison with experimental decompressions of similar groundmass compositions. Our comparisons suggest quench pressures < 30 MPa (origin depths < 2 km) and multiple rapid decompressions of > 13.75 MPa each during ascent from chamber to surface. Values are consistent with field studies of the same events and statistical analysis of explosion time-series data. The microlite volume number density trend with depth reveals an apparent transition from growth-dominated crystallization to nucleation-dominated crystallization at pressures of ∼ 7 MPa and lower. A concurrent sharp increase in bulk density marks the onset of significant open-system degassing, apparently due to a large increase in system permeability above ∼ 70% vesicularity. This open-system degassing results in a dense plug which eventually seals the conduit and forms conditions favorable to Vulcanian explosions. The corresponding inferred depth of overpressure at 250–700 m, near the base of the dense plug, is consistent with depth to center of pressure estimated from deformation measurements. Here we also illustrate that one-dimensional models representing ascent of a degassing, crystal-rich magma are broadly consistent with conduit profiles constructed via our petrologic analysis. The comparison between models and petrologic data suggests that the dense conduit plug forms as a result of high overpressure and open-system degassing through conduit walls.

Role of Crustal Contamination in Formation of the Jinchuan Intrusion and Its World-Class Ni-Cu-(PGE) Sulfide Deposit, Northwest China

The Jinchuan ultramafic body on the southwestern margin of the North China craton hosts a giant Ni-Cu-(PGE) deposit. This dike-like intrusion consists mainly of lherzolite, dunite, and minor pyroxenite. Fortly-seven vol% of the intrusion is composed of disseminated sulfide ore and minor massive and net-textured ores. Ore bodies occur commonly in the lower part of the intrusion; the largest contains about 50% of the total metallic resources in Jinchuan, and has a "flame-like" shape in vertical cross-section. The ultramafic rocks have high MgO (19-45 wt%), low CaO (<6.5 wt%) and Al2O3 (<7 wt%), and right-inclined chondrite-normalized REE patterns. These rocks contain variable Ir (0.4-17 ppb), Ru (0.6-21 ppb), Rh (0.05-8.4 ppb), Pt (0.6-196 ppb), and Pd (1.2-135 ppb), in general lower than those in the sulfide ores (2.9-110 ppb Ir, 3.3-260 ppb Ru, 1.5-237 ppb Rh, 6.9-3972 ppb Pt, and 15-532 ppb Pd). They have high Th/Nb (0.15-0.5) ratios and show primitive mantle-normalized trace element patterns with Nb-Ta negative anomalies, consistent with derivation from mantle magmas variably contaminated by crustal materials. Compositional heterogeneity of the disseminated sulfides cannot be accounted for by in-situ fractional crystallization, but is consistent with differentiation and sulfide segregation in a staging magma chamber at depth. We propose a compressive tectonic model in which injection of sulfide-poor, crystal-rich magmas from the upper part of the staging magma chamber was followed by injection of a sulfide-rich crystal-mush from the lower part of the staging magma chamber to form the Jinchuan body.

The December 2002–July 2003 effusive event at Stromboli volcano, Italy: Insights into the shallow plumbing system by petrochemical studies

On 28 December 2002, the persistent Strombolian activity at Stromboli was interrupted by the sudden onset of lava emission onto the Sciara del Fuoco slope, a horseshoe-shaped depression on the NW flank of the volcano. The effusive episode went on until 22 July 2003 and produced a cumulative volume of lavas of 11 × 10 6 m 3 ranking the event as the largest occurred in the past 30 years. The eruptive vents were mainly located in the NE sector of the Sciara del Fuoco depression, at an elevation of 550–600 m a.s.l. On 30 December, the eastern portion of the Sciara del Fuoco collapsed producing a tsunamigenic landslide. On 5 April 2003, a paroxysmal eruption occurred at the summit craters during which crystal-poor pumiceous products were emitted. The paroxysm did not interrupt the lava emission. The Strombolian activity at the summit craters gradually resumed starting from March 2003 and fully recovered by the end of July. Periodic sampling of the lava at the active vents was carried out during the entire effusive event. All the analysed samples are shoshonitic basalts (SiO 2 48.5–50.4 wt.%; K 2O 2.1–2.4 wt.%) in the range of composition observed in the products erupted during the past 20 years. They bear about 50 vol.% zoned crystals of plagioclase An 90–60, diopside–augite and olivine Fo 70–73 in a compositionally homogeneous shoshonitic groundmass. Bulk rock major and trace element contents measured with different methodologies at different laboratories show only minor variations. Sr and Nd isotope ratios are close to those of the crystal-rich scoriae erupted in the previous years. Despite of small compositional variations related to the emptying of the zoned topmost portion of the conduit, the data show that the switch from Strombolian explosive to effusive activity is not associated with changes of the textures and composition of the erupted products. Slight but somehow systematic variation of trace elements and isotope ratios between products erupted before and after the 5 April eruption are likely accounted by limited mixing between the fresh, volatile-rich, crystal-poor, magma erupted as pumice during the paroxysm, and the volatile-poor, crystal-rich magma feeding the lava flow. The uniform composition of the erupted lava indicates the presence of a large volume of well-mixed, crystal-rich, homogeneous magma residing in the shallow plumbing system of the volcano. Two possible trigger mechanism of the effusive event are here proposed: (i) a discrete input of fresh magma into the lower part of the shallow magmatic system occurred some months before the eruption and was followed by crystallisation, degassing and mixing with the crystal-rich shallow magma which re-homogenised the system. These processes eventually led to the rise of the magma at a higher level and failure of the conduit walls. (ii) The onset of the effusion may represent the consequence of a gradual rise of the magma level in the conduits occurred in the past two decades. The process of progressive refilling, initiated after the 1985 effusive eruption, ultimately culminated on 28 December 2002 with the failure of the conduit wall and the opening of vents 100–150 m below the summit craters.

Bulk chemical trends at arc volcanoes are not liquid lines of descent

Although the magmatic output of arc volcanoes defines clear chemical trends, the compositional sequence of eruption is often chaotic. At best, support for the concept of progressive evolution by fractionation/assimilation in a central chamber is ambiguous. Rather than defining a liquid line of descent, we suggest that chemical trends in arc systems are populated by discrete magma batches. These have been derived from a limited set of sources and undergone a common family of processes, but in different proportions. Crystal–liquid fractionation may occur at the mafic end of the spectrum in classical fashion by gravitational crystal extraction from melt-rich magma. However, at the silicic end where melt viscosity is high, fractionation may occur by forceful expulsion of interstitial melt from near-solidus arc plutons and adjacent crust. The andesite/dacite middle ground is commonly occupied by relatively crystal-rich magma batches with complex mixing and mingling histories. Such intermediate magmas appear to be stable residents of shallow arc crust—mechanically because of their neutral buoyancy, thermally because they are frequently recharged with basalt, and chemically because they resist large shifts in composition. Stored magma that “regresses” in an evolutionary sense by mixing or mingling during recharge is preferentially erupted and forward evolution during quiescence is inhibited by high viscosity of melt. The surface expression is the building of a stratocone or dome complex. In contrast, silicic magmas, once they have coalesced from interstitial melt in their plutonic sources through coupled porous flow and diking, tend to be erupted promptly, flushing out resident andesite/dacite magma batches in their path. The surface expression is caldera formation, which commonly terminates one episode of cone building and serves as a prelude to another.

Melt inclusion analysis of the Unzen 1991–1995 dacite: implications for crystallization processes of dacite magma

Dacitic magma, a mixture of high-temperature (T) aphyric magma and low-T crystal-rich magma, was erupted during the 1991–1995 Mount Unzen eruptive cycle. Here, the crystallization processes of the low-T magma were examined on the basis of melt inclusion analysis and phase relationships. Variation in water content of the melt inclusions (5.1–7.2 wt% H2O) reflected the degassing history of the low-T magma ascending from deeper levels (∼250 MPa) to a shallow magma chamber (∼140 MPa). The ascent rate of the low-T magma decreased markedly towards the emplacement level as crystal content increased. Cooling of magma as well as degassing-induced undercooling drove crystallization. With the decreasing ascent rate, degassing-induced undercooling decreased in importance, and cooling became more instrumental in crystallization, causing local and rapid crystallization along the margin of the magma body. Some crystals contain scores of melt inclusions, whereas there are some crystals without any inclusions. This heterogeneous distribution suggests the variation in the crystallization rate within the magma body; it also suggests that cooling was dominant cause for melt entrapment. Numerical calculations of the cooling magma body suggest that cooling caused rapid crystal growth and enhanced melt entrapment once the magma became a crystal-rich mush with evolved interstitial melt. The rhyolitic composition of melt inclusions is consistent with this model.