The structure of the magmatic system beneath subaerial volcanos, including the architecture and distribution of the bodies where magma is stored and the network of conduits that transport melt between these accumulations and the surface, plays a fundamental role in all aspects of volcano construction and evolution, from igneous differentiation to hazard assessment. However, due to inaccessibility, little is known about the geometry of the magma bodies residing beneath subaerial volcanos. 

Mid-ocean ridges host the most extensive magmatic system on Earth, with 98% of its length below the ocean surface, which makes them an ideal target to be scanned by controlled-source marine seismic techniques. Beneath some portions of this vast system, the shallowest magma bodies are present and represented by long-linear Axial Magma Lenses (AML). It is at these shallow-most AMLs where dikes nucleate and connect the magma accumulations to the surface to result in an eruption. To explore the magma plumbing systems at mid-ocean ridges, we use 3-D multichannel seismic data across a mid-ocean ridge environment and apply advanced marine seismic techniques to develop the highest resolution reflection images of the AMLs so far. The data were collected across a magmatically dynamic portion of the East Pacific Rise at 9°50’N with documented dike intrusion and eruptions in 1991/1992 and 2005/06.

The observations indicate that the magma reservoirs in the shallow crust are not represented by smooth bodies, but show strongly lineated topography that is spatially linked to the distribution of eruptive fissures and erupted lavas above. In the detailed topography, we find evidence for: 1) a dike root zone beneath where a caldera-like axial eruptive fissure zone is present, 2) deep excavation of this root zone within the primary eruption site for the last documented eruption, and 3) dikes rupturing from edges as well as the center of magma lenses. We also demonstrate that the distribution of additional, off-axis crustal magma accumulations further impact the stresses and melt budget at shallow-level magma accumulations leading to more frequent eruptions. Our results show that the mechanism behind eruptions along mid-ocean ridges is predominantly bottom-up and not fundamentally different from the eruptions’ mechanism at subaerial volcanoes. Considering the fine-scale morphology of shallow magma bodies will be critical for future generations of more realistic numerical models to aid in effective global volcanic hazard assessment and mitigation.

How to cite: Marjanovic, M., Carbotte, S., Stopin, A., Singh, S., Plessix, R.-É., Marjanović, M., Nedimović, M., Canales, J. P., Carton, H., Mutter, J., and Escartín, J.: Links Between Volcanic Eruptions and Magma Body Geometry Revealed by Seismic Reflection Imaging at the East Pacific Rise, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6783, https://doi.org/10.5194/egusphere-egu23-6783, 2023.

IC-FEMRES (Imperial College Finite Element Magma REservoir Simulator), is a finite-element based numerical code for simulating the 3D dynamic behaviour of a two-phase, multi-component magma reservoir with chemical reaction.  The code is built upon the open-source IC-FERST package (http://multifluids.github.io/) which includes advanced numerical features such as dynamic mesh optimization, to allow fine-scale solution features to be captured while simulating in a large domain.

The model solves for velocity using a finite-element approach, and for transport using a control-volume scheme to ensure the conservation of energy, mass, and components.  Solid, melt and volatile phases are modelled as Stokes fluids with very different Newtonian viscosities.  Individual crystals in the solid matrix are incompressible, but the solid phase is compressible to account for changes in melt fraction.  The formulation captures viscous compaction and convection of the solid matrix, and flow of melt and volatiles via a Darcy-type formulation at low melt fraction, and a hindered-settling type approach at high melt fraction.  It also captures heat transport by conduction and advection, and component transport by advection.  A chemical model is used to calculate phase fraction and composition.  The numerical package sequentially solves for: 1. Melt and solid velocity (mass and momentum conservation); 2. Enthalpy and component transport (energy and component conservation); 3. Phase fraction and composition (chemical model).  Material properties such as density and viscosity can be coupled to solution fields such as melt fraction and composition to yield a highly non-linear system of coupled equations that are solved iteratively.

We demonstrate here the validation of the formulation against well-constrained test cases, and example results for a magma reservoir in the continental crust obtained using a simple two-component chemical model created by fitting a binary phase diagram to experimental melting data.  Solutions show significant deviations from the predictions of 1- and 2D thermal models, or 1D models that include magma dynamics, and may explain some hitherto poorly understood aspects of magma reservoir formation, dynamics and chemical differentiation. 

How to cite: Hu, H., Salinas, P., and Jackson, M.: A 3D finite element magma reservoir simulator, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5441, https://doi.org/10.5194/egusphere-egu23-5441, 2023.

The prevalence, durability and physical significance of crystal mushes in crustal magmatic systems is a topic of current interest in igneous petrology. Fragments of mushes brought to the surface by basaltic eruptions provide a snapshot of the temporal evolution of crustal magmatic systems.  Petrographic and geochemical analysis of such fragments give valuable insights into basaltic magma reservoirs, including information about magma storage conditions and possible eruption triggers. A detailed petrological and geochemical study was carried out on gabbroic mush nodules from the Brandur, Fontur and Saxi tuff cones to understand the processes that occur before large fissure eruptions in the Bárðarbunga system, Iceland.

Petrographic studies of the mush nodules, from QEMSCAN images, reveal a bimodal phenocryst population in a glassy vesicular groundmass. Probe analyses confirm the bimodal population consists of a primitive and evolved assemblage. The former is composed of large equant crystals of high-anorthite plagioclase (An~88), high-forsterite olivine (Fo~86) and high Mg# clinopyroxene (Mg#~86) forming an interconnected solid framework. The evolved assemblage consists of low-anorthite plagioclase (An~75), low-forsterite olivine (Fo~77) and low Mg# clinopyroxene (Mg#~79) crystallising in the pore space of the mush framework and on the rims of the primitive macrocrysts. The textures and compositions seen suggest the nodules experienced two stages of crystallisation: primitive macrocrysts crystallised first and were stored in crystal mushes. Then a later event caused a change in PTX conditions and triggered relatively rapid crystallisation in the pore-spaces of the mushes.

The quenched glass in the pore spaces of the nodules has the composition of a basaltic liquid that in chemical equilibrium with the evolved assemblage of crystals. Thermobarometry based on equilibrium between this liquid and the phases indicates that the final stage of crystallisation occurred at pressures of ~2 kbar. A putative interstitial liquid composition was reconstructed under the assumption of closed system growth of the evolved assemblage by using the QEMSCAN pixel maps to add the evolved crystals to the interstitial glass composition. This reconstructed liquid is far from chemical equilibrium with the primitive crystals in the mush framework, indicating that the assumption of simple closed system crystallisation from an initial mush liquid in equilibrium with the primitive solids is not correct. Therefore, the phase mapping and compositional relationship provide constraints on open-system processes in mushes.

The failure of the closed system models to match the observations is significant in two ways. First, the lack of equilibrium between mush liquid and cumulus plagioclase is consistent with the expected sluggish diffusion of NaSi-CaAl in plagioclase. This disequilibrium poses challenges for numerical models of magmatic systems that use the assumption of crystal-melt equilibrium to link temperature, melt fraction and phase compositions.  Second, bubble expansion during pre-eruptive ascent forces mush liquid out of solid framework in the nodules and may provide observational constraints on the physics of multiphase flow in deep magmatic systems.

How to cite: Maclennan, J., Boyes, X., and Mutch, E.: Disequilibrium during mush evolution in the Bárðarbunga volcanic system, Iceland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13522, https://doi.org/10.5194/egusphere-egu23-13522, 2023.

Dispersive grain pressure (Bagnold, 1954) is commonly used to explain the observed axial concentrations of phenocrysts in dykes and sills via flow differentiation (Komar, 1972). The idea was formulated for particle fractions exceeding 0.13 by volume. A dispersive pressure is proposed that is greatest near the intrusion walls, forcing crystals to move inward, towards the centre of the magmatic flow where shear strains are low. However, Barriere (1979) argued that this phenomenological ‘Bagnold effect’ should be confined only to narrow (<<100 m) wide intrusions. His reasoning was that in larger channels, the wall effect driving the dispersive pressure diminishes swiftly, nullifying the dispersive pressure. This is true where the relevant length scale of the problem scales with the ratio W/d, where W is the full channel width and d is particle diameter.

Here we show that for congested magma (0.5 > Φ > 0.8), with the rheology decomposed into scalar and vector components, particle fluctuations (in velocity) are dependent critically on the distance gap (h) between nearest neighbour that imparts a particle pressure. Thus, the critical ratio becomes d/h. It is fluctuations in the interparticle gap distance arising during shear in the flowing suspension that causes migration, irrespective of the channel width. We show that for a fixed particle size, d/h scales with crystal fraction (Φ) and the migration effect is enhanced as W/d increases.   We focus here on particle (crystal) migration as opposed to segregation or particle size sorting, although the latter are both amenable to analysis through modifications to our mathematical model.    

Flow differentiation via particle migration is likely to be just as effective in wider channels (W >> 100m) than in narrow ones, eliminating the need to invoke other fluid dynamical or thermal explanations (convection, multiple intrusion, gravitational settling) to explain the central concentration of phenocrysts in dykes and sills exceeding several metres in width.  As the (multiphase) migration effect exerts a strong control on both magma rheology and composition, flowage differentiation as a mechanism for compositional variation during magma emplacement in large intrusions is open for re-evaluation. 

References

Bagnold, RA, (1954). Experiments on gravity-free dispersion of large solid spheres in a Newtonian fluid under shear. Proc. Roy. Soc. London 225, 49-63.

Barriere, M, (1976). Flowage differentiation: limitation of the Bagnold effect to the narrow intrusions. Contrib. Min. Pet. 55, 139-145. 

Komar, P, (1972). Mechanical interactions of phenocrysts and flow differentiation of igneous dykes and sills. Geol. Soc. Amer. Bull. 83, 973-988.

How to cite: Koenders, C. and Petford, N.: Flow differentiation in dykes and sills NOT limited by intrusion width, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2116, https://doi.org/10.5194/egusphere-egu23-2116, 2023.

Deep low-frequency seismic events are detected in the East Eifel Volcanic Field (EEVF) since 2013. To well detect and locate such events the Deep Eifel Earthquakes Project - Tiefe Eifel Erdbeben (DEEP-TEE) started in July 2014 which now is composed of ca. 10 permanent and 15 mobile recording stations. Up to now, the DEEP-TEE seismic dataset contains eight years of continuous seismic records and the network has been reconfigured and continuously developed to achieve an optimum configuration regarding detection and location of seismic events.

In order to detect the weak deep low-frequency (DLF) events we developed a seismic event detector and found ca. 330 localizable DLF events in 2014-2021. The DLF hypocenter distribution outlines a near-vertical structure close to the Laacher See Volcano (LSV) which erupted about 13,079 years ago. The hypocenters are as deep as ca. 45 km, close to the assumed lithosphere-asthenosphere boundary, and reach to about 5-8 km depth. Most events occur close to the Moho and in the lower crust what is interpreted as magmatic underplating and deep crustal intrusion. In the same depth range but further to the west, we find seismic reflections with a negative polarity. These are also interpreted as magmatic pockets in the lower crust and the Moho region.

We try to estimate the mass flux (magma and volatiles) which is related with the seismicity. For this we apply Aki et al.'s model (JVGR, 1977) for describing the magma movement (a so-called chain of cracks connected by narrow channels) and estimate the related magma intrusion volume rate in the EEVF lithosphere. We assume an initial set of model parameters and evaluate the sensitivity and stability of the modelling results by allowing a reasonable range of each individual input parameter. Our results give an estimate of about 2,000-16,000 cubic meters of melt per year which is transported in the lithosphere.

How to cite: Ritter, J., Koushesh, M., and Eickhoff, D.: Melt Detection and Estimation of the Current Magma Intrusion Rate beneath the East Eifel Volcanic Field, Germany, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15245, https://doi.org/10.5194/egusphere-egu23-15245, 2023.

In NE Türkiye, an almost 30,000 km² area is covered by young volcanic rocks, ranging in age from Miocene to Quaternary and spanning the whole compositional spectrum from basanites/tephrites to high silica rhyolites. The region exhibits a plateau morphology, known as the Erzurum-Kars Plateau,  at ~2 km above sea level. That volcanic plateau continues far beyond the state border into Georgia (ie., the Samtskhe-Javakheti plateaus). Although there are a few studies, the petrological evolution of the these volcanic plateaus is still not well known. To better understand the origin, magmatic history, and geodynamic setting of the volcanism on these plateaus, we, Turkish and Georgian researchers, have been conducting a joint cross-border research project (i.e., TÜBİTAK- SRGNSF project #118Y272) across the region. The volcanic units making up those plateaus are composed of numerous volcanic cones of different shapes and sizes, lava domes, pyroclastic layers, and widespread plateau-forming lavas.

Preliminary findings of our research have revealed that the composition and structure of the lithospheric domains below the plateau might have significant effects on the geochemical character and the lithological features of the volcanics. The volcanic succession covering the Pontide Block in the north is dominated by Late Miocene-Pliocene calc-alkaline andesitic and dacitic lavas, which mostly form medium-sized volcanic edifices. These edifices are partially overlain by Upper Pliocene to Quaternary aged low-viscosity, plateau-forming basic lavas which are also calc-alkaline. Notably, pyroclastics are scarce in the north.

The portion of the plateau that overly the Northeastern Iranian Block and the ophiolitic mélange in the south consists of a much wider variety of lava and pyroclastic lithologies. It starts with a ~5.5 Mys old acid pyroclastic layer at the base, consisting of rhyolitic pyroclastics, domes, and obsidian. It is overlain by the plateau-forming basic to intermediate lavas, Pliocene in age. In turn, the plateau sequence is overlain by a previously unknown caldera-like volcanic complex, which we named “the Digor volcanic complex”, located between Kars and Digor. It has a diameter of ~60 km and consists of lavas and pyroclastics of Late Pliocene to Quaternary in age, displaying both calcalkaline and alkaline character.

All those volcanics contain a clear inherited subduction signature from previous subduction events (i.e., Pontide Arc in the north). Our petrological melting modellings revealed that the magmas were possibly derived from two contrasting metasomatized lithospheric mantle sources: (1) a spinel peridotite with or without minor amphibole and, (2) a pyroxenitic mafic source with a minor amount of phlogopite. Our data indicate that the melts derived from these two sources were mixed into each other en route to the surface. Most of the plateau lavas might have been derived from the first type (i.e., spinel-peridotite) while the younger alkaline Digor volcanics were dominantly from the second type (i.e., pyroxenite). The thinning of the lithospheric mantle by delamination and the gradual increase of heat coming from the upwelling asthenospheric mantle might be responsible for these variations. Our FC and AFC models show that plateau lavas experienced intense amphibole±garnet fractionation and moderately assimilated continental crust.

How to cite: Keskin, M., Aysal, N., Yılmaz, İ., Hanilçi, N., Okrostsvaridze, A., Koral, H., Kasapçı, C., Şişman Tükel, F., and Bochenko, G.: Deep magmatic processes beneath an active collision zone: Petrological and geochemical evidence from the volcanic plateaus in northeastern Türkiye and western Georgia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14990, https://doi.org/10.5194/egusphere-egu23-14990, 2023.

Central Asian Orogenic Belt (CAOB) with multiple blocks and suture zones is a key locality for understanding the process of plate tectonics. Extensive studies are mainly on the western CAOB, but less on the eastern side. Many questions remain unclear due to the lack of obvious structural records and ophiolite assemblages. In this study, we report the andesites sampled from Laolongtou Formation in the eastern CAOB with detailed geochronology and geochemistry analyses. The andesites are characterized by high Mg# values at their intermediate SiO₂ contents, which are defined as typical high Mg# andesites. Zircon U-Pb ages show they erupted at the Late Triassic (~236 Ma) and the Ti-in-zircon thermometer indicates a potential high primary magma temperature. Geochemically, they show relatively high contents of Al₂O₃, Na₂O, Cr, and Ni, with enrichment in light rare earth elements and depletion in high field strength elements. Besides, they are markedly depleted in Nb and Ta, enriched in Sr, Ba contents, and significantly differentiated in Th and U contents. They have homogeneous depleted Sr-Nd isotopic compositions that fall into the range of MORB and mantle-derived ranges. Together with the depleted zircon Hf isotopic compositions, showing the possible addition of a hot and depleted component. We propose that they were formed by interactions of components derived from a subducting slab and the overlying mantle wedge. The slab-derived components are most likely a low degree of partial melting of subducted oceanic crust that was able to stabilize garnet and rutile, without plagioclase in the melt residue. They subsequently interacted with the overlying mantle wedge, which resulted from an post-collisional setting related to the final closure of Paleo-Asian Ocean. The upwelling of the upper mantle triggered by the oceanic slab break-off may explain the genesis of the high Mg# andesites and the formation of the continental crust in northeast China.

How to cite: Zhang, L., Huang, F., Xu, J., and Liu, X.: Post-collision Extension in the Eastern Central Asian Orogenic Belt: Insight from the Late Triassic High-Mg Andesites, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12573, https://doi.org/10.5194/egusphere-egu23-12573, 2023.

The chemical differentiation of mantle-derived magmas in subduction zones during the generation, transport, and emplacement has always been a concern, which is closely related to the petrogenesis of calc-alkaline granitoids. A systematic study of petrography, mineralogy, and geochemistry is conducted on typical arc granitoids and associated mafic microgranular enclaves (MME) from the Chinese Altai, Central Asian Orogenic Belt. Magma hybridization modeling using major and trace element compositions suggests that the parental magma of granitoids is a mixture of a mafic and a felsic endmember. The sharp decrease of plagioclase An values from cores to rims (e.g., from ca. 80 to 40) implies polybaric crystallization of water-saturated magmas accompanied by degassing. Petrographic evidence and plagioclase in situ Sr isotopic compositions ((⁸⁷Sr/⁸⁶Sr)i = 0.7053–0.7071) show the involvement of isotopically different magmas during the mineral crystallization. The positive zircon εHf(t) values of MME (+2.3 to +5.4) and granitoids (+0.6 to +4.6) further show that the mafic melts are mantle-derived, while felsic melts should originate from juvenile lower crust with the slightly more evolved isotopic composition. An evolution scenario of the mantle-derived mafic magma and formation of enclave-bearing calc-alkaline plutons in arc settings is demonstrated: Hydrous mantle melts rose to the deep crustal-mantle boundary, where they effectively mixed with juvenile lower crustal melts to form the hybrid parental magma of the granitoids. In the high crustal-level chambers, decompression-dominated crystallization, mingling, and limited mixing of mafic magma blobs and enclosing granitic melts ultimately determined the rock texture, mineral composition, and enclave morphology. This work was financially supported by Hong Kong RGC GRF (17302317), National Key R&D Program of China (2017YFC0601205), NSFC Projects (41730213, 42072264, 41902229, and 41972237).

How to cite: Cui, X., Sun, M., Zhao, G., Zhang, Y., Yao, J., and Wong, J.: Petrogenesis of the enclave-bearing granitoids from the Chinese Altai: implications for the differentiation of mantle-derived magmas and formation of calc-alkaline plutons in subduction zones, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-177, https://doi.org/10.5194/egusphere-egu23-177, 2023.

Silicate melt inclusions (SMI) in rhyolitic volcanic rocks in the ~2699 – 2697 Ma Bousquet Formation, Subprovince, Québec were studied through integration of a variety of microanalytical methods (petrography, laser Raman microspectroscopy, LA-ICP-MS) to explore links between magmatic metal/volatile endowment and the high gold content of mineral deposits in the world-class Doyon-Bousquet-LaRonde mining district. The study is the first to present melt inclusion data from felsic volcanic rocks of Archean age.

Rhyolitic SMI of primary origin were characterized from magmatic quartz phenocrysts from tholeiitic rhyolite sills and calc-alkaline flows near gold-rich volcanogenic massive sulfide deposits. Silicate melt inclusion trace element chemistry records a continuous transition from ocean ridge to volcanic arc tectonic affinity. SMI Sr-Y-La-Yb systematics are  inconsistent with Archean tonalite-trondhjemite-granodiorite (TTG; “adakitic”) compositional domains; rather, they are consistent with post-Archean TTG (“calc-alkaline”) suggesting significant compositional modification of TTG magmas through contamination and/or plagioclase fractionation during magma storage and ascent.  Thermobarometry suggests prolonged phenocryst residence at depth prior to eruption with SMI entrapment at ~10-12 km depth. Concentrations of Au in the SMI are variable and up to two orders of magnitude higher than in the host bulk volcanic rocks. This demonstrates that whole rock data are not representative of the composition of the original magmatic liquids and, thus, cautioning the traditional use of whole rock data as a proxy for volcanic assemblage fertility in such Archean environments. Moreover, SMI show melt co-entrapment with an immiscible, high density, carbonic fluid (CO₂-dominant), indicating that rhyolitic melts were saturated in CO₂. Saturation of this fluid phase may explain, in part, the variability observed in SMI metal contents, and demands consideration of the relative importance of early separation of magmatic volatile phases versus seafloor hydrothermal leaching of volcanic products in controlling the magmatic metal endowment of Archean exhalative ore-forming systems.  

How to cite: Hanley, J., Meagher, D., Neyedley, K., Mercier-Langevin, P., and Zajacz, Z.: First insight into gold enrichment associated with Archean magmatic processes in the deep crust through melt inclusion studies: an example from the Abitibi Subprovince, Québec, Canada, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8045, https://doi.org/10.5194/egusphere-egu23-8045, 2023.