Magma Pathways Research Articles

Geological controls on the natural CO2 accumulation in the Surennuoer Oilfield of the Hailar Basin, China

Naturally occurring CO2 often coexists with hydrocarbon accumulations on continental margins around the world, the presence of which may affect hydrocarbon production as a greenhouse gas. In this paper we focus on the natural CO2 in the Surennuoer Oilfield of the Hailar Basin, and investigate CO2 origin, distribution, related tectonic controls and accumulation mechanisms using geochemical, petrophysical and seismic data. Results show that the CO2 is of inorganic origin due to the degassing of magma from the deep crust or mantle, and is mainly stored in structural traps of N1 Member. There are close spatial and temporal relationships among CO2 distribution, major fault distribution and underneath igneous bodies, indicating a genetic link. The magmatic events and major faults are proposed to be the key controlling factors of CO2 distribution, which provided CO2 supply and migration pathways for both magma and degassed CO2. Additionally these faults also play a role in forming structural traps to store CO2. The sandstone reservoirs of low porosity and low permeability may play a limited role due to their poor reservoir quality. Two CO2 accumulation models are outlined in the Surennuoer strike-slip fault zone and the S3 fault zone. The former is characterized by shallower magmatic igneous body and a small amount of CO2 in the shallower N2 Member in addition to N1 Member. The Surennuoer strike-slip faults may have further channelled magma upwards which initially intruded along the basin-controlling fault, while the magma under the S3 fault zone did not migrate further upward along the S3 faults. Generally this study provides a detailed understanding of CO2 distribution, accumulation models and geological controls which may guide future hydrocarbon production in the study area.

Translithospheric magma plumbing system of intraplate volcanoes as revealed by electrical resistivity imaging

AbstractThe magma plumbing systems of volcanoes in subduction and divergent tectonic settings are relatively well known, whereas those of intraplate volcanoes remain elusive; robust geophysical information on the magma pathways and storage zones is lacking. We inverted magnetotelluric data to image the magma plumbing system of an intraplate monogenetic volcanic field located above the stagnant Pacific slab in northeast China. We identified a complex, vertically aligned, low-resistivity anomaly system extending from the asthenosphere to the surface consisting of reservoirs with finger- to lens-like geometries. We show that magma forms as CO2-rich melts in a 150-km-deep asthenospheric plume crossing the whole lithosphere as hydrated melt, inducing underplating at 50 km depth, evolving in crustal reservoirs, and erupting along dikes. Intraplate volcanoes are characterized by low degrees of melting and low magma supply rates. Their plumbing systems have a geometry not so different from that of volcanoes in subduction settings.

Extreme Curvature of Shallow Magma Pathways Controlled by Competing Stresses: Insights From the 2018 Sierra Negra Eruption

AbstractEruptions at shield volcanoes often occur from radially aligned linear fissures fed by blade‐like magma‐filled cracks (dykes). The fissures of the 2018 Sierra Negra eruption were scattered on the flank of the volcano. Space‐borne radar interferometric data (interferometric synthetic aperture radar) revealed that, unexpectedly, part of the eruption was fed by a 15 km long, tortuous and flat‐lying crack (sill). Here we develop a framework that captures the full three‐dimensional (3D) kinematics of non‐planar intrusions. This includes both an analytical and comprehensive numerical scheme. We constrain the models such that they match the observed ground deformation at Sierra Negra. We show that the peculiar sill trajectory is due to the competing stress gradient magnitudes being close to one another throughout its propagation. By accounting for the interaction of all these factors, these 3D models open the possibility to understand and simulate the geometry of magma transport at volcanic systems.

Reactivation of Magma Pathways: Insights From Field Observations, Geochronology, Geomechanical Tests, and Numerical Models

AbstractField observations and unmanned aerial vehicle surveys from Caldera Taburiente (La Palma, Canary Islands, Spain) show that pre‐existing dykes can capture and re‐direct younger ones to form multiple dyke composites. Chill margins suggest that the older dykes were solidified and cooled when this occurred. In one multiple dyke example, an 40Ar/39Ar age difference of 200 kyr was determined between co‐located dykes. Petrography and geomechanical measurements (ultrasonic pulse and Brazilian disc tests) show that a microscopic preferred alignment of plagioclase laths and sheet‐like structures formed by non‐randomly distributed vesicles give the solidified dykes anisotropic elastic moduli and fracture toughness. We hypothesize that this anisotropy led to the development of margin‐parallel joints within the dykes, during subsequent volcanic loading. Finite element models also suggest that the elastic contrast between solidified dykes and their host rock elevated and re‐oriented the stresses that governed subsequent dyke propagation. Thus, the margin‐parallel joints, combined with local concentration and rotation of stresses, favored the deflection of subsequent magma‐filled fractures by up to 60° to form the multiple dykes. At the edifice scale, the capture and deflection of active intrusions by older ones could change the organization of volcanic magma plumbing systems and cause unexpected propagation paths relative to the regional stress. We suggest that reactivation of older dykes by this mechanism gives the volcanic edifice a structural memory of past stress states, potentially encouraging the re‐use of older vents and deflecting intrusions along volcanic rift zones or toward shallow magma reservoirs.

Prolonged basaltic magmatism and short-lived magmatic sulfide mineralization in orogenic belts

Economically important Ni-Cu sulfide deposits occur in orogenic belts worldwide. However, the duration of timing and geological processes critical for generation of high-Mg basaltic magma and Ni-Cu sulfide mineralization during orogeny have not been well addressed. Here, we report a detailed geochronological study on the Huangshan-Jingerquan Ni-Cu metallogenic belt, NW China. The complexes were regarded as the results of a prolonged basaltic magmatism from 380 Ma to 270 Ma according to previous zircon U-Pb ages. Most of the zircons for the previous age dating were separated from mafic facies and only a few from ultramafic rocks. However, the Ni-Cu sulfide ore bodies are mostly hosted within the ultramafic facies. Thus, there are debates on tectonic setting of the magmatism associated with the Ni-Cu sulfide mineralization. In this study, the zircons separated from the sulfide-mineralized mafic/ultramafic facies of eleven complexes display U-Pb ages ranging from 285 to 280 Ma, indicating a short-lived high-Mg basaltic magmatism associated with the Ni-Cu mineralization. It is likely that the olivine-free gabbros formed in 380–300 Ma and 280–270 Ma occurred in subduction and post-collisional periods, respectively. Collision combined with regional strike-slip shearing at the latest Carboniferous induced slab break-off and asthenosphere upwelling in early Permian. Decompression melting of the upwelling asthenosphere was extensively intensified by addition of water released from the broken slab and generated voluminous high-Mg basaltic magma beneficial to the short-lived Ni-Cu sulfide mineralization in 285–280 Ma. The regional strike-slip shearing created fracture networks as pathways for magma ascending. The sulfides carried by the ascending basaltic magma deposited in the fractures and magma chambers in different depths and produced the sulfide-mineralized dykes and complexes, respectively.

Robust Bayesian Joint Inversion of Gravimetric and Muographic Data for the Density Imaging of the Puy de Dôme Volcano (France)

Imaging the internal structure of volcanoes helps highlighting magma pathways and monitoring potential structural weaknesses. We jointly invert gravimetric and muographic data to determine the most precise image of the 3D density structure of the Puy de Dôme volcano (Chaîne des Puys, France) ever obtained. With rock thickness of up to 1,600 m along the muon lines of sight, it is, to our knowledge, the largest volcano ever imaged by combining muography and gravimetry. The inversion of gravimetric data is an ill-posed problem with a non-unique solution and a sensitivity rapidly decreasing with depth. Muography has the potential to constrain the absolute density of the studied structures but the use of the method is limited by the possible number of acquisition view points, by the long acquisition duration and by the noise contained in the data. To take advantage of both types of data in a joint inversion scheme, we develop a robust method adapted to the specificities of both the gravimetric and muographic data. Our method is based on a Bayesian formalism. It includes a smoothing relying on two regularization parameters (an a priori density standard deviation and an isotropic correlation length) which are automatically determined using a leave one out criterion. This smoothing overcomes artifacts linked to the data acquisition geometry of each dataset. A possible constant density offset between both datasets is also determined by least-squares. The potential of the method is shown using the Puy de Dôme volcano as case study as high quality gravimetric and muographic data are both available. Our results show that the dome is dry and permeable. Thanks to the muographic data, we better delineate a trachytic dense core surrounded by a less dense talus.

Comprehensive High‐Precision Relocation of Seismicity on the Island of Hawai‘i 1986–2018

AbstractAbundant seismicity beneath the Island of Hawai‘i from mantle depths to the surface plays a central role in understanding how volcanoes work, grow, and evolve at this intraplate oceanic hotspot. We perform systematic waveform cross‐correlation, cluster analysis, and relative relocation of 347,445 events representing 32 years of seismicity on and around the island from 1986 to 2018. We successfully relocate 275,009 (79%) events using ∼1.7 billion differential times (P and S) from ∼128 million similar‐event pairs. The results reveal a dramatic sharpening of seismicity along faults, streaks, rings, rift zones, magma pathways, and mantle fault zones; seismicity delineating crustal detachments on the flanks of Kīlauea and Mauna Loa is particularly well‐resolved. The resulting high‐precision spatio‐temporal image of seismicity captures almost the entire 1983–2018 Pu‘u ‘Ō‘ō‐Kūpaianaha eruption of Kīlauea with its numerous distinct episodes and wide‐ranging activity, culminating in the 2018 lower East Rift Zone eruption and summit collapse.

A Systematic Approach to Mapping Regimes of Earthquake‐Induced Static Stress Changes Acting on Magmatic Pathways

AbstractLarge earthquakes alter the crustal stress field across great distances (hundreds to thousands of kilometers) over geologically short timescales (seconds to years). These stress changes can affect magmatic systems, triggering (or suppressing) volcanic unrest and eruption, along with other deeper processes. We use simple kinematic source models in an isotropic elastic half‐space to assess earthquake‐induced static stress changes (>1 kPa) over the entire thickness of the lithosphere and consider the implications for magma ascent and storage. Modeling subduction zone earthquakes, we calculate static normal stress changes with depth on three mutually perpendicular end‐member magma pathways: vertical arc‐parallel, vertical arc‐perpendicular, and horizontal. From this, we define seven stress change regimes within the adjacent volcanic arc. Three of these regimes may strongly encourage magma ascent in dykes by inducing unclamping (decreased compressive normal stress) of vertical pathways which increases in magnitude toward the surface and clamping of horizontal pathways. Two of the regimes may encourage stalling and storage of magma in sills near the base of the crust by inducing unclamping of horizontal pathways at depth. The spatial distribution of the regimes is largely dependent on earthquake magnitude, but also varies with slip distribution and interface dip. We show how the responses of magmatic systems to earthquakes also depends on the stress change magnitude and the state of the magmatic system, with a greater impact expected for larger stress changes acting on weaker, more thermally mature systems.

The rebirth and evolution of Bezymianny volcano, Kamchatka after the 1956 sector collapse

Continued post-collapse volcanic activity can cause the rise of a new edifice. However, details of such edifice rebirth have not been documented yet. Here, we present 7-decade-long photogrammetric data for Bezymianny volcano, Kamchatka, showing its evolution after the 1956 sector collapse. Edifice rebirth started with two lava domes originating at distinct vents ~400 m apart. After 2 decades, activity became more effusive with vents migrating within ~200 m distance. After 5 decades, the activity focused on a single vent to develop a stratocone with a summit crater. We determine a long-term average growth rate of 26,400 m3/day, allowing us to estimate the regain of the pre-collapse size within the next 15 years. Numerical modeling explains the gradual vents focusing to be associated with loading changes, affecting magma pathways at depth. This work thus sheds light on the complex regrowth process following a sector collapse, with implications for regrowing volcanoes elsewhere.

Mechanical Imaging of a Volcano Plumbing System From GNSS Unsupervised Modeling

AbstractIdentification of internal structures in an active volcano is mandatory to quantify the physical processes preceding eruptions. We propose a fully unsupervised Bayesian inversion method that uses the point compound dislocation model as a complex source of deformation, to dynamically identify the substructures activated during magma migration. We applied this method at Piton de la Fournaise. Using 7‐day moving trends of Global Navigation Satellite System (GNSS) data preceding the June 2014 eruption, we compute a total of 15 inversion models of 2.5 million forward problems each, without a priori information. Obtained source shapes (dikes, prolate ellipsoids, or pipes) show magma migration from 7–8 km depth to the surface, drawing a mechanical “tomography” of the magma pathway. Our results also suggest source geometries compatible with observed eruptive fissures and seismicity distribution. In case of finite magma volume involved in final dike injection, source volume estimates from this method allow forecasting volumes of erupted lava.

Slab Detachment, Mantle Flow, and Crustal Collision in Eastern Sicily (Southern Italy): Implications on Mount Etna Volcanism

AbstractNew data and interpretations of the geodynamics of eastern Sicily point to deep crustal shortening taking place in the area. Reconstructions of the lithospheric system, seismicity distribution, and stress state in the crust indicate that deformation is expressed by a large thrust‐ramp cutting through the entire lower plate. The tectonic structure is propagating directly beneath the Mount Etna volcano, one of the few active volcanoes in Europe. Geostructural interpretation of tomographic sections allows for interpretations of the compressional structure as originating in response to trench‐parallel breakoff of the Ionian slab. Following the simple assumption that if a slab retreats, it must either be compensated or alternatively pushed by the fore‐arc mantle, we argue that the opening of a gateway in the slab has encouraged the fore‐arc mantle to flow toward the Mount Etna region. Mantle mobilization has had a twofold influence on both magmatic source mixing and the inception of underplating processes beneath the Mount Etna. A shortening prevailing over extension in the crust below the volcano seems to have a significant impact on the dynamics of the Mount Etna volcanic system, which manifested through anomalous signals over the last thousands of years. Since a tectonic inversion of previous dilatational magma pathways is expected in such a converging setting, the documented variations are believed to be consistent with a volcano experiencing a declining phase. Comparison with other extinct volcanic systems in the southern Tyrrhenian margin, lying atop a detached slab and involved in contraction, provides insights into the evolution of Mount Etna.

Active Volcanism Revealed from a Seismicity Conduit in the Long-resting Tatun Volcano Group of Northern Taiwan

Abundant earthquakes clustered within a particular zone often reflect an active geological feature, such as clustering seismicity along a fault zone and a huge number of volcanic-earthquakes around the erupting conduit. Herein we perform a double-difference tomographic inversion and relocate the seismicity at the long-resting Tatun volcano group (TVG) in northern Taiwan. A dramatic improvement of the earthquake location model surprisingly show that, from 2014 to 2017, two clustered seismic zones are identified in the TVG. One major group of events (>1000) persistently clustered within a ~500 m diameter vertical conduit with a ~2 km height. The clustering seismicity conduit is just located nearby Dayoukeng, one of the strongest fumaroles in the TVG, and is connected to a fracture zone characterized by low Vp/Vs in the shallow crust. The other group of events is clustered within a sphere-like zone beneath Mt. Chihsin around the depths between 0.5 km and 2 km. Both seismic zones are probably triggered by the significantly volcanic gases and fluids ascending from the deep magma reservoir. Combined with a variety of results from literature, the seismicity conduit near the strong fumarole is the evidence for an active volcano and also identifies a likely pathway for ascending magma if the TVG erupts again in the future. But possibility of developing different magma pathways at other clustered seismic zones such as beneath Mt. Chihsin may not be totally excluded.

Seismic tomography of compressional wave velocity and attenuation structure for Makushin Volcano, Alaska

Located in the central portion of the Alaska-Aleutian subduction zone, Makushin Volcano is among the most active volcanoes in the United States and has been classified as a very high threat by the National Volcano Early Warning System based on its eruptive history and proximity to the City of Unalaska and international air routes. In 2015, five standalone seismic stations and three mini seismic arrays of 15 stations each were deployed on Unalaska Island to supplement the Alaska Volcano Observatory permanent seismic network. This temporary array was operational for one year. Taking advantage of the increased azimuthal coverage and the array's increased earthquake detection capability, we developed body-wave Vp and Vp/Vs seismic images of the velocity structure beneath the volcano. Our results show a complex structure with the upper 4 km of the crust. The shallow high-Vp features possibly delineate remnant magma pathways or conduits. Low-Vp regions are found east of the caldera at approximately 4–8 km depth. This is in agreement with previous seismic tomographic work and InSAR-based geodetic models, which had identified this region as a possible long-term source of magma. A three-dimensional compressional wave attenuation model complemented the velocity models. In general, we observe regions of corresponding low P-wave velocity and high attenuation east of the summit caldera between 5 and 7 km depth, supporting the presence of a melt-rich zone. The absolute Vp values (5.5–6.0 km/s) in this region are consistent with the presence of a magma body with a mafic signature. We suggest that a triggering mechanism that involves mafic injections into a more evolved shallow magma chamber is representative of the current magmatic system below the volcano, similarly to what petrological studies have inferred for past eruptions.

Fast ascent rate during the 2017–2018 Plinian eruption of Ambae (Aoba) volcano: a petrological investigation

In September 2017, after more than a hundred years of quiescence, Ambae (Aoba), Vanuatu’s largest volcano, entered a new phase of eruptive activity, triggering the evacuation of the island’s 11,000 inhabitants resulting in the largest volcanic disaster in the country’s history. Three subsequent eruptive phases in November 2017, March 2018, and July 2018 expelled some of the largest tropospheric and stratospheric SO2 clouds observed in the last decade. Here, we investigate the mechanisms and dynamics of this eruption. We use major elements, trace elements, and volatiles in olivine and clinopyroxene hosted melt inclusions, embayments, crystals, and matrix glasses together with clinopyroxene geobarometry and olivine, plagioclase, and clinopyroxene geothermometry to reconstruct the physical and chemical evolution of the magma, as it ascends to the surface. Volatile elements in melt inclusions and geobarometry data suggest that the magma originated from depth of ~ 14 km before residing at shallow (~ 0.5 to 3 km) levels. Magma ascent to the surface was likely facilitated by shallow phreatic eruptions that opened a pathway for magma to ascend. Succeeding eruptive phases are characterised by increasingly primitive compositions with evidence of small amounts of mixing having taken place. Mg–Fe exchange diffusion modelling yields olivine residence times in the magma chamber ranging from a few days to a year prior to eruption. Diffusion modelling of volatiles along embayments (melt channels) from the first two phases of activity and microlite number density suggests rapid magma ascent in the range of 15–270 km/h, 4–75 m/s (decompression rates of 0.1 to ~ 2 MPa/s) corresponding to a short travel time between the top of the shallow reservoir and the surface of less than 2 min.

Temporal evolution of magma and crystal mush storage conditions in the Bárðarbunga-Veiðivötn volcanic system, Iceland

The depth(s) of magma storage reservoirs beneath active volcanic regions may change with time. Determining the rates and causes of millennial-scale changes in magmatic system architecture is critical for the development of realistic time-integrated models of crustal evolution. Here we examine a suite of samples from the exceptionally well-exposed Bárðarbunga-Veiðivötn volcanic system in central Iceland in order to resolve the temporal evolution of magma storage conditions within one of Iceland’s most productive volcanic systems. We have measured the major and minor elemental composition of glass, mineral and melt inclusion from five erupted units that span a full glacial cycle, from a <100 ka subglacial eruption to a historical eruption in 1477 AD. All samples contain macrocrysts (>500 μm), polymineralic clots and high-crystallinity nodules, consistent with derivation from crystal mush bodies. Macrocryst rims are in chemical equilibrium with their respective carrier melts, while macrocrysts cores are too primitive to have crystallized from these melts. Each sample records a distinct range of macrocryst compositions, indicating that the composition and/or eruptibility of stored crystal mush has changed with time. Macrocrysts from the oldest units are the most primitive, and the macrocryst compositional range becomes wider and, on average, more evolved, with time. Clinopyroxene-melt and melt-based (OPAM) geobarometers reveal temporally invariant crystallization conditions of 1.9–2.2 ± 0.7 (1σ) kbar pressure, corresponding to depths around 6.8–7.8 ± 2.5 km. All the samples also contain melt inclusions trapped at mid-crustal pressures of ∼2.6 kbar (9.6 km). In addition, melt inclusions hosted in most primitive olivines and plagioclases from subglacial and early Holocene eruptions preserve evidence of crystallization in a lower-crustal storage level(s) located at 17.5 km (4.9 kbar). This petrological record of deep crystallization may be linked to a surge in eruption rates, tapping of lower-crustal magma reservoirs, consistent with a crustal response associated with postglacial isostatic rebound. In contrast, the absence of a deep crystallization signature in the younger eruptive units may reflect lower magma production rates under steady-state conditions of the crust, and new magma pathways favouring melt storage in the mid-crust.

Seismic activity during the 2013–2015 intereruptive phase at Lascar volcano, Chile

SUMMARYIn addition to enabling the physical processes of volcanic systems to be better understood, seismology has been also used to infer the complexity of magma pathways and plumbing systems in steep-sided andesitic and stratovolcanoes. However, in these volcanic environments, the application of seismic location methods is particularly challenging and systematic comparisons of common methods are lacking. Furthermore, little is known about the characteristic seismicity and deep structure of Lascar volcano, one of the most historically active volcanoes in northern Chile known to produce VEI-4 eruptions. To better understand the inner processes and deep structure of Lascar, the local broad-band seismic monitoring network was densified during a temporal installation in 2014–2015. Herein, we focus on the local seismicity during the 2014–2015 unrest episode, during which we recorded numerous seismic events mainly classified as long-period (LP) type, but also denote volcano-tectonic (VT) activity. Specifically, a long-lasting phase of LP activity is observed over a period of ∼14 months that starts in tandem with a pulse of VT activity. The LP rate and amplitude are modulated over time; they are lower in the initial phase, rise during the intermediate period from October 2014 to July 2015, and finally slowly decay while approaching the eruption time. The location of LPs is challenging due to the typical lack of clear seismic onsets. We thus encompass this problem by comparing a broad range of different standard and novel location techniques to map the source region of LPs by fitting the amplitude decay, polarization patterns, coherence of characteristic functions and cross-correlation differential times. As a result, we principally constrain LP locations within the first 5 km depth below the summit extending downward along a narrow, conduit-like path. We identify different regions of complexity: VTs dominate at depth, both VTs and LPs cluster in an intermediate depth region (down to 1.5 km), suggesting a change in the plumbing system geometry, and LPs dominate the shallowest region. Based on these results, we infer the presence of a subvertical conduit extending down to a depth of ∼5 km, and a region of path divergence, possibly accommodating a magma plumbing system, at a depth of ∼3 km beneath the volcano summit. Identifying the locations of complexities in the magma pathways at Lascar may help identify future unrest. The results are compared with independent observations, demonstrating the strength of the location method used herein that will be tested at volcanoes elsewhere.

Stress inversions to forecast magma pathways and eruptive vent location.

When a batch of magma reaches Earth's surface, it forms a vent from which volcanic products are erupted. At many volcanoes, successive batches may open vents far away from previous ones, resulting in scattered, sometimes seemingly random spatial distributions. This exposes vast areas to volcanic hazards and makes forecasting difficult. Here, we show that magma pathways and thus future vent locations may be forecast by combining the physics of magma transport with a Monte Carlo inversion scheme for the volcano stress history. We validate our approach on a densely populated active volcanic field, Campi Flegrei (Italy), where we forecast future vents on an onshore semiannular belt located between 2.3 and 4.2 km from the caldera center. Our approach offers a mechanical explanation for the vent migration over time at Campi Flegrei and at many calderas worldwide and may be applicable to volcanoes of any type.

The spatial distribution of Mercury's pyroclastic activity and the relation to lithospheric weaknesses

Mercury's surface preserves a rich history of volcanism, impact cratering, and tectonic deformation. Geological observations show that the earliest evidence of thrust faulting that was induced by the secular cooling and resulting global contraction of the planet coincided with the waning stages of effusive volcanism, but that explosive volcanism continued beyond this point. Stresses from global contraction, however, would have precluded efficient vertical magma ascent. Sites of pyroclastic activity—manifest as irregular depressions surrounded by diffuse, spectrally distinct halos—spatially coincide with lithospheric discontinuities, such as faults or those associated with impact craters. The vast majority of explosive vents are situated on the floors, rims, central peaks, or peak rings of impact structures. A substantial portion of such vents is also proximal to thrust faults: they are most spatially concentrated at or within 20 km of faults, with ever fewer vents progressively farther from tectonic structures. We statistically evaluated the spatial distribution of sites of pyroclastic activity with respect to faults and impact craters by generating sets of random point locations of equal count to those volcanic sites, computing their spatial relationship to the mapped faults and craters, and comparing them to our observations. We find that although the observed proximity of vents to faults is indistinguishable from a random distribution, their spatial association with impact craters is non-random. To examine the interrelatedness of several geospatial relationships of lithospheric weaknesses and pyroclastic activity, we performed a principal component analysis that tested correlations between vent size, the presence of vents within a crater, the diameters and degradation states of those craters, and vent distance from mapped faults, which help tie together interpretations of magma volumes and eruption energies, repeated utilization of magma pathways, and durations of eruptive events in the geological context of global contraction. Results reveal a predominance of small-sized vents indicative of short-lived, low-volume pyroclastic activity that are consistent with suppressed volcanism after the onset of global contraction. Greater size ranges of vents are found in large impact craters and when faults are nearby, which points to denser fracture networks facilitating magma ascent.

Long-period seismicity reveals magma pathways above a laterally propagating dyke during the 2014–15 Bárðarbunga rifting event, Iceland

The 2014–15 Bárðarbunga–Holuhraun rifting event comprised the best-monitored dyke intrusion to date and the largest eruption in Iceland in 230 years. A huge variety of seismicity was produced, including over 30,000 volcano-tectonic earthquakes (VTs) associated with the dyke propagation at ∼6 km depth below sea level, and large-magnitude earthquakes accompanying the collapse of Bárðarbunga caldera. We here study the long-period seismicity associated with the rifting event. We systematically detect and locate both long-period events (LPs) and tremor during the dyke propagation phase and the first week of the eruption. We identify clusters of highly similar, repetitive LPs, which have a peak frequency of ∼1 Hz and clear P and S phases followed by a long-duration coda. The source mechanisms are remarkably consistent between clusters and also fundamentally different to those of the VTs. We accurately locate LP clusters near each of three ice cauldrons (depressions formed by basal melting) that were observed on the surface of Dyngjujökull glacier above the path of the dyke. Most events are in the vicinity of the northernmost cauldron, at shallower depth than the VTs associated with lateral dyke propagation. At the two northerly cauldrons, periods of shallow seismic tremor following the clusters of LPs are also observed. Given that the LPs occur at ∼4 km depth and in swarms during times of dyke-stalling, we infer that they result from excitation of magmatic fluid-filled cavities and indicate magma ascent. We suggest that the tremor is the climax of the vertical melt movement, arising from either rapid, repeated excitation of the same LP cavities, or sub-glacial eruption processes. This long-period seismicity therefore represents magma pathways between the depth of the dyke-VT earthquakes and the surface. Notably, we do not detect tremor associated with each cauldron, despite melt reaching the base of the overlying ice cap, a concern for hazard monitoring.

Intrusive hyaloclastite and peperitic breccias associated to sill and cryptodome emplacement on an Early Paleocene polymagmatic compound cone-dome volcanic complex from El Guanaco mine, Northern Chile

This work explores the textural characteristics, morphology and facies architecture of well-preserved Paleocene hyaloclastic and peperitic breccias associated with subvolcanic intrusions at El Guanaco gold mine (Northern Chile). The El Guanaco mine volcanic sequence is part of a polymagmatic compound cone-dome volcanic complex grouping several dacitic domes and maar-diatremes, and subordinated subvolcanic intrusions of basaltic, andesitic and dacitic compositions. The Soledad-Peñafiel Fault System is a first order regional structure controlling the location and style of the volcanism in the region. Three different intrusive bodies (Basaltic sills, Dacitic cryptodomes and Andesitic cryptodomes) were found to intrude into a wet and poorly consolidated pyroclastic sequence representing the upper portions of a maar-diatreme. Consequently, extensive quench fragmentation and fluidization along their contacts occurred, leading to the formation of widespread breccia bodies enclosing a coherent nucleus. Differences in matrix composition allows to define two main breccias types: 1) poorly-sorted monomictic breccias (intrusive hyaloclastites) and 2) poorly-sorted tuff-matrix breccias (peperites). The observed facies architecture is interpreted as the result of the interplay of several factors, including: 1) magma viscosity, 2) the geometry of the intrusives, and 3) variations on the consolidation degree of the host rocks. Additionally, the overall geometry of each intrusive is interpreted to be controlled by the effective viscosity of the magmas along with the available magma volume at the time of the intrusions. The presence of three compositionally different subvolcanic bodies with intrusive hyaloclastite and peperite envelopes indicate, not only that all these intrusions occurred in a short period of time (probably less than 2–3Ma), but also that the volcaniciclastic pile suffer little or none compaction nor consolidation during that time. The presence of three compositionally variated synvolcanic intrusives can be explained either by the presence of a zoned magmatic chamber feeding the volcanic complex, or more likely by the influence of the Soledad-Peñafiel Fault Zone acting as a preferential pathway for different magma compositions/sources to rise to the surface.