Magma Flow Research Articles

Trans-crustal structural control of CO2-rich extensional magmatic systems revealed at Mount Erebus Antarctica

Erebus volcano, Antarctica, with its persistent phonolite lava lake, is a classic example of an evolved, CO2-rich rift volcano. Seismic studies provide limited images of the magmatic system. Here we show using magnetotelluric data that a steep, melt-related conduit of low electrical resistivity originating in the upper mantle undergoes pronounced lateral re-orientation in the deep crust before reaching shallower magmatic storage and the summit lava lake. The lateral turn represents a structural fault-valve controlling episodic flow of magma and CO2 vapour, which replenish and heat the high level phonolite differentiation zone. This magmatic valve lies within an inferred, east-west structural trend forming part of an accommodation zone across the southern termination of the Terror Rift, providing a dilatant magma pathway. Unlike H2O-rich subduction arc volcanoes, CO2-dominated Erebus geophysically shows continuous magmatic structure to shallow crustal depths of < 1 km, as the melt does not experience decompression-related volatile supersaturation and viscous stalling.

Syntectonic magmatism and reactivation of collisional structures during late-Variscan shearing (SW Sardinia, Italy)

The Arbus igneous complex (SW Sardinia, Italy) represents a good example of a short time lived post-collisional composite pluton emplaced at shallow crustal level in the external zone of the Variscan chain. The pluton almost consists of granodiorite and leucogranite rock-suites emplaced at 304 ± 1 Ma within a main NW trending thrust separating the metamorphic wedge from the fold and thrust belt foreland. The pluton emplaced into a dilatational step over connecting two NW–SE dextral shear zones which belongs to a regional network of post-collisional strike-slip structures marking the transition from collision to post-collisional extension. The microstructure observed for quartz and K-feldspar confirms the lack of significant post-emplacement deformation, indicating only limited high-temperature sub-solidus recrystallization. Anisotropy of magnetic susceptivity data and field-structural analysis have been carried out to reconstruct the geometry of the pluton and the trajectories of magmatic flow in relation to regional deformation structures. Overall, the magmatic and the magnetic fabrics are broadly discordant with the metamorphic foliation of the country rocks, defining an EW trending elliptical asymmetric sill rooted in the SW quadrant. The reconstructed architecture combined to petrologic observation indicates that accretion of the pluton involved injection of multiple dykes through a sub-vertical feeder zone, combined to lateral flow of the roof controlled by inherited collisional structure. The duration of magmatic activity and the cooling history of the contact metamorphic aureole have been evaluated through a suite of 2D thermal models. All these observations, together with the available geochronological constraints are suggestive of very rapid construction of the pluton. The proposed emplacement model is fully consistent with the regional phase of strike-slip tectonics and widespread magmatism accommodating the large rotation of the Corsica-Sardinia block during the Carboniferous-Permian transition.

Mush ado about the Ratagain Complex, NW Scotland: insights into Caledonian granitic magmatism and emplacement from magnetic fabric analyses

The anisotropy of magnetic susceptibility (AMS) is used to reveal subtle mineral alignment fabrics in apparently isotropic crystalline lithologies, including granites. Such petrofabrics can be produced by emplacement-related magma flow or post-emplacement tectonic strain. However, discriminating between flow-related and tectonic fabrics using field observations alone may be challenging and is usually a broad and arbitrary interpretation. In this contribution, we employ a range of magnetic analyses to characterize the origin of the petrofabric in the c. 425 Ma Ratagain Complex, NW Scotland, a composite Late Caledonian granitic intrusion. Our detailed magnetic analyses reveal that whilst all intrusive units carry an ambient tectonic overprint, critically, this has not developed into an obvious tectonic fabric and contains a horizontal shortening component indicative of transpression. This appears at odds with the well-defined Silurian (Scandian phase) regional transtensional tectonic regime from c. 420–415 Ma onwards. Accordingly, we suggest that either the complex is younger than previously thought or that it existed as a crystal-mush close to the magmatic solidus for a protracted period after its initial emplacement. This study lays the foundations for much-needed further investigations into the detailed emplacement mechanisms, timescales and petrogenesis of individual granitic intrusions, to aid understanding of Late Caledonian tectonics. Supplementary material : Supplementary data to this article are available at https://doi.org/10.6084/m9.figshare.c.5941375 Thematic collection: This article is part of the Early Career Research collection available at: https://www.lyellcollection.org/cc/SJG-early-career-research

Seismic tremor reveals slow fracture propagation prior to the 2018 eruption at Sierra Negra volcano, Galápagos

Seismic tremor observed near active volcanoes is an important tool for volcano monitoring as it often appears shortly before eruptions. Although tremor can be generated by a variety of physical processes it is usually interpreted as direct evidence for flowing magma in the sub-surface. These interpretations typically feed into risk assessments for potential eruptions. Using the temporal evolution of tremor amplitude and spectral data from a distributed seismic network that captured the 2018 eruption at Sierra Negra in Galápagos, we determine that tremor is not directly related to sub-surface fluid movement. Instead at Sierra Negra tremor likely indicates a slowly propagating fracture, which is later exploited as a pathway for silent magma flow. Distinct differences in the source migration and the spectral character of pre-eruptive and co-eruptive tremor allow both a location estimate of the future eruption site and a precise timing of the eruption onset.

Reservoir depressurization driven by passive gas emissions at Ambrym volcano

Despite being a widespread and common process, the impact of passive volcanic degassing on the pressurization state of a magma reservoir is not well understood. If mass loss due to gas emissions results in reservoir depressurization and surface subsidence, the pressure difference between a shallow reservoir and deep magma source may result in magma recharge and eventually trigger an eruption. It is therefore important to determine how a simplified reservoir-conduit system responds to such degassing processes. Here we use an extreme example of persistent volcanic degassing—Ambrym—as a case study to relate sulphur dioxide mass flux with reservoir depressurization and edifice-scale subsidence, both measured from satellite-based remote sensing observations. A geodetic inversion of surface displacements measured with Interferometric Synthetic Aperture Radar modeled using the Boundary Element Method provides bounds on the reservoir pressure change during an episode of subsidence at Ambrym from 2015 to 2017. These results are input into a lumped parameter theoretical model developed by Girona et al. (2014), and the free parameters (e.g., reservoir size and conduit radius) are systematically explored. We find that the 2015–2017 subsidence episode is consistent with pressure decreasing at a rate of −5.2 to −2.0 MPa year−1 in a reservoir at ∼2 km b.s.l., as a result of passive degassing. The subsidence episode is observed to end abruptly in October 2017, and no significant deformation is detected in the 14 months leading up to a rift zone intrusion and submarine eruption in December 2018, despite substantial degassing. We explain this lack of pre-eruptive deformation by an influx of ∼0.16 km3 of magma into a shallow (<2 km b.s.l.) reservoir that counterbalances the depressurization caused by degassing. This recharge volume is comparable with the volume of magma subsequently extracted from Ambrym's reservoir in December 2018. We conclude that at some open-vent passively degassing volcanoes, deflation caused by degassing may reduce or even cancel any inflation signal caused by magma influx. Nonetheless, detection of pre-eruptive recharge can be achieved by monitoring changes in the long-term deformation rate.

Focused flow during the formation and propagation of sills: Insights from analogue experiments

Geophysical observations of sills suggest magma can be transported hundreds of kilometres within these thin (<100 m thick) horizontal sheet-like bodies. However, existing magma flow models struggle to explain how sills propagate so extensively. We conducted scaled analogue experiments to explore the dynamics of magma flow in sills. Two fluids (magma analogues) with different rheology (water, a Newtonian fluid, or xanthan gum, a shear-thinning fluid) were injected through the base of a clear-Perspex tank that contained two superposed gelatine layers with contrasting stiffness and a weakly bonded interface (crustal analogue). To visualise and map fluid flow over time, we applied particle image velocimetry to passive tracer particles that fluoresced under laser light and were suspended in the fluid. Flow within the feeder dyke varied with the injected fluid; fluid recirculation occurred around the margins in the Newtonian dyke, whereas fluid at the shear-thinning dyke margins was stagnant, and both formed a centralised vertical jet. A region of focused flow was observed within the shear-thinning sill and there was no evidence of focused flow within the Newtonian sill. These experiments suggest that focused flow may be common in shear-thinning magma intrusions and highlights the coupling of flow dynamics between the feeder dyke and sill. If such focused flow occurs in nature, enhanced by a non-Newtonian rheology, this could explain how larger sills develop with magma transport focused through a lower viscosity region localised within a larger volume of potentially eruptible magma mush.

Dyke Architecture, Mineral Layering, and Magmatic Convection; New Perspectives From the Younger Giant Dyke Complex, S Greenland

AbstractIgneous sheet intrusions are a fundamental component of volcano plumbing systems. Identifying how sheet intrusion emplacement and geometry controls later magmatic processes is critical to understanding the distribution of volcanic eruptions and magma‐related ore deposits. Using the Younger Giant Dyke Complex (YGDC), a Mesoproterozoic suite of large (&lt;800 m wide) mafic dykes in southern Greenland, we assess the influence sheet of emplacement and geometry on subsequent magma flow and mush evolution. Through structural mapping, petrographic observations, and anisotropy of magnetic susceptibility fabric analyses, we show that the YGDC was emplaced as a series of individual dyke segments, which following coalescence into a sheet intrusion remained largely isolated during their magmatic evolution. Through petrographic evidence for liquid‐rich growth of cumulus phases, concentric magnetic fabrics, and the detailed study layered zones within the YGDC, we infer magma convection occurred within the cores of each dyke element. We particularly relate layering to hydrodynamic sorting processes at a magma‐mush boundary toward the base of each convection cell. Overall, our work demonstrates that the initial geometry of sheet intrusions can constrain magma flow patterns and affect the distribution of crystallization regimes.

Stretching, Shaking, Inflating: Volcanic-Tectonic Interactions at a Rifting Silicic Caldera

Silicic caldera volcanoes are frequently situated in regions of tectonic extension, such as continental rifts, and are subject to periods of unrest and/or eruption that can be triggered by the interplay between magmatic and tectonic processes. Modern (instrumental) observations of deformation patterns associated with magmatic and tectonic unrest in the lead up to eruptive events at silicic calderas are sparse. Therefore, our understanding of the magmatic-tectonic processes associated with volcanic unrest at silicic calderas is largely dependent on historical and geological observations. Here we utilize existing instrumental, historical and geological data to provide an overview of the magmatic-tectonic deformation patterns operating over annual to 104 year timescales at Taupō volcano, now largely submerged beneath Lake Taupō, in the rifted-arc of the Taupō Volcanic Zone. Short-term deformation patterns observed from seismicity, lake level recordings and historical records are characterized by decadal-scale uplift and subsidence with accompanying seismic swarms, ground shaking and surface ruptures, many of which may reflect magma injections into and around the magma reservoir. The decadal-scale frequency at which intense seismic events occur shows that ground shaking, rather than volcanic eruptions, is the primary short-term local hazard in the Taupō District. Deformation trends near and in the caldera on 101–104 yr timescales are atypical of the longer-term behavior of a continental rift, with magma influx within the crust suppressing axial subsidence of the rift basin within ∼10 km of the caldera margin. Examination of exposed faults and fissures reveals that silicic volcanic eruptions from Taupō volcano are characterized by intense syn-eruptive deformation that can occasionally extend up to 50 km outside the caldera structure, including ground shaking, fissuring and triggered fault movements. We conclude that eruption and unrest scenarios at Taupō volcano depend on the three-way coupling between the mafic-silicic-tectonic systems, with eruption and/or unrest events leading to six possible outcomes initially triggered by mafic injection either into or outside the magma mush system, or by changes to the tectonic stress state.

Sulfide Ore Formation of the Kalatongke Ni-Cu Deposit as Illustrated by Sulfide Textures

Abstract The Kalatongke magmatic Ni-Cu deposit features high Ni-Cu grades compared with other Ni-Cu deposits in the Central Asian orogenic belt. The sulfides, mainly hosted by olivine norite and gabbronorite, are characterized by high Cu/Ni ratios. There is wide variety of textural relationships in the mineralized rocks, including globular, sulfide matrix, emulsion, disseminated, net-textured, and semimassive to massive textures. Quantitative textural measurements reveal that more than 65 vol % of the total sulfide volume in disseminated ore (defined as containing 4–10 vol % sulfide) and more than 90 vol % of the total sulfide volume in net-textured ore (typically 16 vol % sulfide) are hosted in few interconnected networks with equivalent sphere diameters (ESDs) larger than 6 mm. This illustrates that sulfide coalescence is a critical process. The remaining sulfide blebs define two groups of log-linear particle size distribution (PSD), i.e., a finer group (ESD of 0.080–&amp;lt;0.529 mm) and a coarser group (ESD of 0.529–4.084 mm). The PSD of the finer group differs slightly among different types of mineralization. The origin of this group of sulfides is attributed to sulfide nucleation simultaneously with crystallization of the olivine-orthopyroxene-plagioclase phases from the ore-forming magma in the current magma chamber. The PSD of the coarser group from net-textured ores is parallel to that from the disseminated ore but has lower intercept values. Modeling results show that aggregation of ~40–70% sulfide blebs of different grain size from the disseminated ore into the networks can generate the coarser group sulfide PSD of net-textured ore. In addition, monosulfide solution (MSS, pyrrhotite component)-enriched sulfide globules are commonly located close to sulfide matrix ore breccias and emulsion-textured ores, rimming the net-textured and massive orebodies. These globules are different from the sulfide component that was in equilibrium with the mafic magma, suggesting they were formed by mechanical remobilization of a cumulus MSS-enriched component from a previously segregated and partially crystallized sulfide pool. These observations, combined with the ubiquitous chilled margin xenoliths in the high-grade ores, suggest that the Kalatongke deposit was the result of voluminous magma flow through the current location accompanied by sulfide reworking and percolation. All these textural characteristics could be explained by emplacement within a laterally propagating bladed dike. Moreover, the deformation recorded in the emulsion-textured and massive ores suggests the fault system remains active during magma solidification, driving downward migration of sulfide from the sulfide pool at magmatic temperature to form vein-type massive ore. We suggest that the Kalatongke deposit formed by magma pulses injecting into the current location and that the tectonic movement remains active during the solidification of the intrusion. The syntectonic emplacement model, which may be a common feature in the Central Asian orogenic belt Ni-Cu deposits, indicates that the fault systems beneath the intrusion are of great exploration interest for high-grade ores.

Volcanic temperature changes modulated volatile release and climate fluctuations at the end-Triassic mass extinction

Emplacement of the Central Atlantic Magmatic Province (CAMP) is thought to have triggered global environmental changes and the end-Triassic mass extinction (ETE). However, the mechanisms linking volcanism and environmental change are unclear. Here we provide new insight into these linkages by measuring the abundance of both sedimentary five- to six-ring polycyclic aromatic hydrocarbons (PAHs) and mercury from strata deposited in shallow marine environments across the ETE at the GSSP Kuhjoch section in Austria and St. Audrie's Bay section in the UK. To contextualize these data, we report results from laboratory experiments measuring the production of SO2 and CO2 during heating of limestone and mudstone. ETE sediments record parallel enrichments of mercury and five- to six-ring PAHs, which could have been produced by intrusive magma (mainly sills) and lava flows during the early stage of the CAMP emplacement; these data indicate a direct link between massive gas emission from sill contact metamorphism and the ETE. The fraction of coronene – a highly condensed six-ring PAH that requires greater energy to form compared to smaller PAHs – accumulated in the sediments during these initial volcanic events is low, and it coincides with the terrestrial plant turnover and initial marine extinction. Coronene increases to medium values coinciding with the final marine extinction level. Our heating experiments of typical carbonate and mudstone materials show that relatively low temperature heating (>350°C) by sills releases massive amounts of SO2 on a 100 yr time scale, whereas higher temperature heating (500–600°C) forms more CO2 on the same time scale. The combination of our end-Triassic geochemical data and laboratory results implies that low heating by sills caused SO2-dominated gas emission to the stratosphere and low CO2 emission, inducing global cooling that could have precipitated the mass extinction. The subsequent increase in coronene content indicates higher volcanic temperature that would have volatilized CO2 rich gas; the consequence was a switch to greater CO2 release and long-term (>105 yr) global warming.

The building blocks of igneous sheet intrusions: Insights from 3-D seismic reflection data

Abstract The propagating margins of igneous sills (and other sheet intrusions) may divide into laterally and/or vertically separated sections, which later inflate and coalesce. These components elongate parallel to and thus record the magma flow direction, and they can form either due to fracture segmentation (i.e., “segments”) or brittle and/or non-brittle deformation of the host rock (i.e., “magma fingers”). Seismic reflection data can image entire sills or sill-complexes in 3-D, and their resolution is often sufficient to allow us to identify these distinct elongate components and thereby map magma flow patterns over entire intrusion networks. However, seismic resolution is limited, so we typically cannot discern the centimeter- to meter-scale host rock deformation structures that would allow the origin of these components to be interpreted. Here, we introduce a new term that defines the components (i.e., “elements”) of sheet-like igneous intrusions without linking their description to emplacement mechanisms. Using 3-D seismic reflection data from offshore NW Australia, we quantify the 3-D geometry of these elements and their connectors within two sills and discuss how their shape may relate to emplacement processes. Based on seismic attribute analyses and our measurements of their 3-D geometry, we conclude that the mapped elements likely formed through non-elastic-brittle and/or non-brittle deformation ahead of the advancing sill tip, which implies they are magma fingers. We show that thickness varies across sills, and across distinct elements, which we infer to represent flow localization and subsequent thickening of restricted areas. The quantification of element geometries is useful for comparisons between different subsurface and field-based data sets that span a range of host rock types and tectonic settings. This, in turn, facilitates the testing of magma emplacement mechanisms and predictions from numerical and physical analogue experiments.

Cтруктурно-петрологическая эволюция Cнежнинского габбро-сиенитового массива (Южное Прибайкалье)

The article deals with the structural and petrological evolution of one of the many gabbroid massifs of the Southern Baikal region - Snezhninsky, located near the river of the same name, characterized by the most typical structural and metamorphic paragenesis: comfort with layering of containing rocks, located in a steep twisted wing of asymmetrical folds of latitudinal extension with horizontal hinges and axial planes inclined to the north. the array is complicated by a longitudinal oblique upsurge with a powerful zone of tectonites. The northwestern end of the massif is complicated by the upsurge of the northeastern extension. To a depth, the massif sinks to 4-5 km and there is cut off by another fault of the northwestern extension. The internal structure of the array is characterized by a rather monotonous development of small structural forms: the structures of the magma flow, emphasized by the elongation of flat xenoliths, the asymmetrical folds of the right pattern in the plan, the gradual intertransitions between all varieties of rocks composing the array, indicating a significant warming of all rocks during their formation and assimilation.

Volcanic tremor associated with the Surtsey eruption of 1963–1967

The formation of the island of Surtsey over 3.5 years, remains one of the best-documented volcanic, island-forming eruptions to date. The basaltic submarine volcanic activity was detected on November 14, 1963, where ocean depth was 130 m prior to the eruption at the southern end of the Vestmannaeyjar archipelago. The eruptions occurred in several phases involving explosive and effusive activity, including the initial submarine phase on November 12–13, 1963. Separate phases of subaerial volcanic activity occurred during November 14, 1963–January 1964, January–April 1964, April 1964–May 1965, May–October 1965, December 1965–August 1966, and August 1966–June 1967. Seismic data quality from this period is inferior compared to that of modern monitoring systems. Four permanent seismic stations were operated in Iceland at the time, whereof only two, located at 115 and 140 km distance, had the sensitivity to record tremor from Surtsey. Nevertheless, the scanned analog seismograms (http://seismis.hi.is/) show that the eruptive activity was accompanied by considerable seismic activity, both earthquakes, and volcanic tremor. Earthquakes were primarily associated with changes in vent location. Both spasmodic and harmonic tremor was identified, both with low (&lt;3 Hz) and higher (3–5 Hz) characteristic frequencies. The results indicate a complicated relationship between tremor and magma flow rate or style of activity. During the explosive eruption, the highest magma flow rates occurred in the first 10–20 days, a period with little observed tremor. The highest tremor is observed in December 1963–March 1964, after the discharge rates had dropped substantially, and on a timescale of hours-to-days, no clear relationship between tremor and eruption style is observed. The same applies to the effusive activity, where no seismic tremor was observed during most of the effusive eruption of Surtungur, despite the fact that magma flow rates were ~3 times higher than during later phases where some tremor was observed. Keywords: Submarine volcanism, eruption precursors, volcanic tremor, precursory tremor, continuous uprush eruptions

Crustal Melting Evidence in Migmatites of Higher Himalayan Crystallines (HHC) along Bhagirathi, Dhauliganga Valleys, and Sikkim Himalaya, India

Abstract An attempt has been made to evaluate crustal melting evidence through textural studies within a narrow zone of migmatite present in NW Himalaya along Bhagirathi and Dhauliganga valleys and two zones close to MCT and throughout within in the Sikkim Himalayas, NE Himalaya. It appears that partial melting was initiated by muscovite dehydration melting with a positive volume change driving melt segregation and discontinuous crystallization of peritectic biotite in the leucosome. Further, during retrogression due to cooling, a certain amount of melt was consumed. The occurrence of isolated pseudomorphosed melt pockets and lack of euhedral magmatic flow textured feldspar further indicates that the melt fraction was low for the alignment of crystals. The processes of melt segregation and migration could have been limited. The migmatite leucosomes and a small volume of in situ tourmaline-bearing leucogranite along extensional crenulation cleavages and melt pods indicate water-saturated melting of pelitic metasedimentary rocks. The formation of migmatites happened at around 46 Ma, corresponding to a peak metamorphic event due to collisional tectonics of the Himalayan orogeny. The presence of feeder dikes for main tourmaline-bearing leucogranite indicates that the source for the main body could be migmatite which is also supported by the similarity in REE patterns of the main body and in situ tourmaline-bearing leucogranite.

Magmatic, hydrothermal and ore element transfer processes of the southeastern Archean Superior Province implied from electrical resistivity structure

Although magmatic- and metamorphic- derived fluids are widely recognized mineralizing agents, the role of crustal architecture in defining source and sink zones within the middle to lower crust and upper mantle of ancient orogens remains enigmatic. The globally largest and best-preserved Archean greenstone belts lie in the Superior Province, Canada. They provide an ideal location to investigate the influence of igneous construction and subsequent syn-deformational plutonism and metamorphism on the localization of metal-rich melts and fluids throughout the crustal column. Integration of three-dimensional magnetotelluric modelling and seismic reflection sections across the Abitibi subprovince reveals details of a 'whole-of-crust' magmatic and hydrothermal system. East-west low resistivity structures broadly underlie the surface traces of the major deformation zones that are host to significant gold endowment (>200 Moz), while mid-crustal cross trends suggest mineralized fluids flowed along 'pipes' within fault planes. Most low resistivity structures are inferred to represent domains containing interconnected zones of graphite and/or sulfide. These delineate relict mantle source/transit domains and crustal pathways enriched by the flow of magmas or metamorphic fluids genetically related to a late-stage pulse of ore-bearing magmatism, possibly as a result of slab break-off or delamination. Using the combined MT and seismic data, we develop a 3-D crustal-scale model which highlights how evolving orogenic architecture-controlled mass transfer and metallogenic processes developed.

Deciphering syn- and post-emplacement processes in shallow mafic dykes using magnetic anisotropy

Dykes form key pathways for the transport and emplacement of magma within the crust. We have identified syn- and post-emplacement processes recorded across a ~2 m thick basaltic dyke on the Isle of Skye, Scotland. We measured the rock magnetic properties, anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic magnetisation (AARM) across two dyke-thickness profiles spaced 13 m apart along the dyke strike (sites G5 and G6). At 20–25 cm intervals, our samples are very closely spaced compared to standard sampling protocols. Our results show that the dyke's magnetic fabrics originate from two mineral groups: titanomagnetite, which is abundant in the central dyke region at site G5, and iron sulphides (pyrite and pyrrhotite) that dominates the margin regions at both sites. The titanomagnetite occurs in unaltered dyke rock; its magnetic fabrics are primary, having formed during magma solidification, and record lateral magma flow. The pyrrhotite occurs in jointed and hydrothermally altered dyke rock; its petrological and magnetic fabrics are secondary, having originated from a sulphide-rich fluid which infiltrated cooling joints oriented perpendicular to the dyke margins and locally modified the primary magnetic fabrics. At site G6, pyrrhotite also occurs in the dyke centre, suggesting that locally the post-emplacement sulphide-rich fluid permeated into this region; this site is located close to a branch in the dyke and has increased joint frequency, which could explain the enhanced alteration. The presence of syn- (primary) and post-emplacement (secondary) fabrics was only identified due to our high frequency sampling regime and use of both AMS and AARM techniques. We highlight that future magnetic anisotropy studies of dykes may benefit from high sample frequency combined with sampling along-strike and across-thickness. Using both AMS and AARM techniques to detect more variations in magnetic fabrics can reveal more complete syn- and post-emplacement dyke histories.

The Four‐Stage Evolution of Martian Mantle Inferred From Numerical Simulation of the Magmatism‐Mantle Upwelling Feedback

AbstractTo understand how mantle dynamics exerts control over the history of magmatic activities and surface environment on Mars, I used a numerical model of magmatism in the convecting mantle that is nominally anhydrous and internally heated. Magmatism occurs as an upward permeable flow of basaltic magma generated by decompression melting through matrix. The mantle evolves in four stages in preferred models. In Stage I, high initial temperature in the uppermost mantle causes an extensive magmatism intensified by two types of positive feedback that operate between magmatism and mantle upwelling flow, the MMUb and MMUc feedbacks: the buoyancy and volume change of matrix, respectively, caused by migrating magma that a mantle upwelling flow generates intensify the flow itself to generate more magma. The stratification suppresses mantle convection and magmatism for the next tens to hundreds of millions of years, allowing heat to build up in the mantle by internal heating (Stage II). Eventually, magma is generated at depth, and the MMUb feedback operates to cause an episodic plume magmatism that releases water from the interior of Mars (Stage III). The plume magmatism also stirs the mantle to make it more homogeneous and extracts heat producing elements from the deep mantle to let the magmatism itself wane and cease. In the final stage IV, mantle convection becomes more akin to a thermal convection. The episodic magmatism and water outgassing in Stage III account for the volcanism and clement surface environment observed for early Mars.

Magma Chamber Formation by Dike Accretion and Crustal Melting: 2D Thermo‐Compositional Model With Emphasis on Eruptions and Implication for Zircon Records

AbstractWe present a 2D thermo‐elastic model of a magma body formation in a granitic crust by injection of rhyolitic or basaltic dikes and sills. Elastic analytical solutions enable the computation of rock displacement in response to magma intrusion and evacuation during volcanic eruptions. Phase diagrams for magma and rocks govern melting/crystallization behavior and temperature evolution in 16 million computational cells. Calculated temperature histories are used to predict zircon crystallization/dissolution, their ages, and isotopic ratios within individual batches of magma and rocks. Incremental dike injection naturally generates magma batches of melt that form kilometer‐wide clusters appearing in different parts of the domain with diverse shapes, horizontal and vertical interconnectivity. The volume of eruptive melt strongly depends on magma influx rates Q, the width of the injection region W, and eruptions. For example, rhyolitic dike injection with Q = 0.125 m3/s with W = 5 km during 100 ka generates ∼50 km3 of eruptive melt while no significant melt forms if W = 10 km. Injection of basaltic dikes into the granitic crust generates comparable amounts of rhyolitic melt from dike‐residual and country rocks. High injection intensities produce magma reservoirs capable of large eruptions, while repetitive eruptions lead to shrinkage of magma bodies. High heat input causes host rock zircons to lose a significant portion of their old cores. Magmatic zircons in the periphery form quickly due to rapid cooling of intruded dikes while in the central part crystals can grow during several hundreds of ka. The model predicts highly heterogeneous O and Hf isotopic ratios recorded in zircons.

Variable magma flow in sills: Can a magma source be constrained?

It has been increasingly recognized that emplacement of mafic sills plays an important role in magma transport throughout the crust. However, further understanding of sill propagation and merging of flow within sill complexes in large igneous provinces is needed. This study focuses on outcrop along the KwaZulu-Natal North Coast (South Africa), where dolerite sills of the Karoo Large Igneous Province intruded shales, siltstones, and sandstones of the Permian-aged Vryheid Formation. These sills intruded at high crustal levels and are associated with significant vesiculation, host rock fluidization and brecciation. An integrated approach is used to examine sill morphology, intrusive relationships, and inherent structures in the sills in relation to the magma flow processes. Mesoscale magma flow indicators preserved in the sills include bridge structures, intrusive steps, deformed vesicles, and magma lobes. The predominantly prolate magnetic fabrics determined by anisotropy of magnetic susceptibility techniques indicate magma flow in the sills was oriented NW, NE–SW and SSE, and largely locally coaxial with field evidence. The mesoscale magma flow indicators developed from linkage between numerous sill segments and variation in the magma propagation direction within the plane of intrusion and show directional variation within (and between) sills. The local flow dynamics within each sill can be revealed in this way; however, the possible variation in flow in the intrusion must be considered when interpreting the gross magma flow direction of the entire sill network. The sills occur in close vertical proximity and are inferred to have intruded through lateral and upward magma flow within merging sill segments creating an interconnected sill network, fed from an eastward local magma source. Interpretation of magma flow in sills should be done with care, to avoid oversimplification regarding regional flow, and highly variable flow in sills is best reconciled at the field scale.

Petrological evidence of rapid evolution of the magma plumbing system of Bezymianny volcano in Kamchatka before the December 20th, 2017 eruption

We used petrological data on samples of the December 20th, 2017 eruption of the Bezymianny volcano (Kamchatka, Russia) to investigate the evolution of the magmatic system during the repose period and the following reactivation of the volcano. The eruptive products are diverse in bulk rock and matrix glass compositions, but are similar in phenocryst proportion, composition, and zoning. Three rock types are distinguished by the presence of either silica-phase (cristobalite or tridymite) or their absence. Rocks without silica phase (Silica-phase Free Host Rocks) are characterized by ~55–56 wt% bulk SiO2 content and have predominantly glassy groundmass with rare microlites in dacitic matrix glass (65.5–70.5 wt% SiO2 and 2.5–3.5 wt% K2O). Cristobalite-bearing rocks are characterized by ~56.6–57.5 wt% bulk silica content and highly crystalline groundmass with many small microlites and rhyolitic matrix glass (76.5–78 wt% SiO2 and 3.8–4.2 wt% K2O) between them. Tridymite-bearing rocks are characterized by ~58.8 wt% bulk silica content and glassy groundmass with rare microlites in rhyolitic matrix glass (77–77.5 wt% SiO2 and ~3.6 wt% K2O).Rhyolite-MELTS simulation and mineral thermobarometry indicate that silica-phase-bearing rock last equilibrated under low-pressure conditions (~15 Mpa) corresponding to the conduit depths, whereas rocks without silica phase crystallized at higher pressure corresponding to shallow magma storage depths (>40–60 Mpa). Bulk rock mass-balance calculations indicate that silica-phase-bearing magmas were produced by 15% of crystal fractionation from the magma without silica phase. Based on this petrological data, we reconstruct the evolution of the magmatic system as follows. During the volcano repose period, undisturbed magma fractionated and stratified due to buoyant ascent of fluid and melt to the chamber roof. Rejuvenation of magmatic system caused pushing the differentiated magma from the top magmatic reservoir to the conduit, where silica phases crystallized. We argue that cristobalite-bearing rocks are likely the remnants of conduit plug, whereas tridymite-bearing rocks represent magma trapped in the conduit under the plug. The renewal of explosive activity in December 2017 was triggered by magma influx from deeper levels of the plumbing system shortly before the explosion, which is supported by the presence of amphibole-bearing mafic enclaves of mid-crustal origin (520–850 Mpa).