In this session, we welcome contributions focusing on the application of microstructural analyses to solve problems in igneous and metamorphic petrology. We seek studies that showcase the development and integration of new microstructural and analytical techniques, such as studies combining traditional (e.g., universal stage) and modern (e.g., EBSD and/or numerical tools for quantitative petrology such as XMapTools) methods, studies focused on advances in 3D and 4D imaging, and numerical modeling involving microstructural and/or textural development. We also encourage contributions that combine microstructural analysis with results from other disciplines in order to better and more broadly solve petrological problems.
Orals: Mon, 28 Apr | Room 0.16
The plutonic lithologies at mid-ocean ridges provide a unique opportunity to investigate processes occurring in igneous reservoirs during oceanic accretion. Mineral modes, textures, microstructures, and in situ geochemistry are valuable tools used to reconstruct the complex differentiation processes within these mush-dominated environments [1]. A wealth of evidence at the crystal scale points to the involvement of melt-mush reactions at various stages of reservoir formation. Yet, melt-mush reactions take place by definition at a local scale, and their significance for melt differentiation at a larger scale is not straightforward. In this contribution, I will present the results of a series of studies that describe the impact of melt-mush reactions in cumulate gabbroic sections from different locations along slow-spreading ridges. The ubiquitous presence of melt-mush reactions at the scale of the entire magmatic units or reservoirs advocates for their significant impact on differentiation. This observation together with microstructural evidence led to the development of an alternative model of cumulate formation for open mush systems that undergo both repetitive melt recharges and melt-mush reactions, a process we call the melt flush [2]. Comparison between different crustal sections reveals the local variability in the reaction regimes (variable assimilation to crystallization ratios), despite the similarity in the reactions impacting the crustal sections. Variable regimes are likely caused by different melt fractions present in the mush during the reactions. Relying on these observations and previous studies, we conclude that the reaction regime is most likely controlled by the melt flux during the formation of the magmatic systems [3]. Such model paves the way for the characterization of past reservoir dynamics, provided a better quantification of “instantaneous” melt-mush reactions is available.
[1] Lissenberg, MacLeod & Bennett, Phil.Trans.R. Soc, 2019. http://dx.doi.org/10.1098/rsta.2018.0014
[2] Boulanger & France, JPet, 2023. https://doi.org/10.1093/petrology/egad005
[3] Boulanger et al., G3, 2024. https://doi.org/10.1029/2023GC011409
How to cite: Boulanger, M., Bakouche, M., Ferrando, C., France, L., Godard, M., Ildefonse, B., Laubier, M., and Médard, E.: Deciphering the plutonic record at mid-ocean ridges: from crystal-scale processes to crustal-scale accretion, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16754, https://doi.org/10.5194/egusphere-egu25-16754, 2025.
Alkali feldspar megacrysts (commonly named K-feldspar) are a distinctive feature of many plutonic rocks. Yet, there is still no consensus on their formation mechanism. Two main contrasting explanations have been proposed so far: (i) the megacrysts form late during the crystallization history of the plutonic rock in which they occur and attain their size in a melt-poor - even subsolidus- environment, or (ii) the megacrysts crystallize at an early stage and grow in the presence of large volumes of melt. In this research we address this two contrasting hypotheses looking at the microstructural features within the megacrysts from the granitic Castellaccio Pluton of the Asinara Island (NW Sardinia, Italy). Combining the petrographic observations, EMPA elemental maps and phase maps elaborations we observe a systematic occurrence of included and corroded relict of K-bearing mineral (muscovite and biotite) and plagioclase always mantled by a rim of anorthoclase and K-rich albite. The temperatures calculated from the Ti-in-Kfs thermometer indicate that the megacrysts have crystallized in the T range of 745-860 °C data. The strong positive europium anomaly and enrichment in barium (~ 4000 ppm) with respect to the whole-rock compositions (~350 ppm) exclude a former significant plagioclase fractionation, suggesting that K-feldspar megacrysts formed at an early stage of the crystallization of the melt. In this framework, the nucleation of the K-Feldspar is likely triggered by the assimilation of K and Al from biotite and muscovite. The presence of these two relict phases locally provides the necessary stoichiometry to start the crystallization of the anorthoclase rim and then the crystallization of the K-feldspar itself. The uptake of Ba from the melts by the K-Feldspar extends its thermal stability field beyond the thermal range predicted by the pseudosection models, allowing its euhedral growth till a megacrystic size (up to 20 cm).
How to cite: Idini, A., Ferrero, S., Fancello, D., and Casini, L.: Clues from the microstructural features of the K-Feldspar megacrysts growth mechanism in granites., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6780, https://doi.org/10.5194/egusphere-egu25-6780, 2025.
The duration of assembly of igneous bodies controls the thermal evolution of the system, which in turn controls the spatial distribution of melt throughout the incremental construction. A preliminary method to reconstruct temporal evolution of magma storage within a reservoir is to perform thermal simulations from which the magmatic activity duration, melt fraction distribution and magma cooling rates can be quantified. In addition, rock microstructures (i.e., mineral morphologies, crystallisation sequence and dihedral angle at three-grain junctions) also carry important information on kinetics of magma solidification. In particular, the geometry of these dihedral angles can be used to decode magma cooling rates variations through an igneous body and, in combination with thermal simulations, provide valuable information on both emplacement and magma solidification kinetics.
We studied the 900 m thick incrementally-emplaced Beauvoir rare-metal granite (Central Massif, France) in which the size and sequence of intrusion of the 18 individual sills have recently been recognised from compositional variations of Li-mica (i.e., lepidolite). The construction of the composite Beauvoir intrusion was numerically simulated, with each successive sill emplaced once the entire reservoir cooled below a critical temperature. Resulting values in emplacement rates are therefore linked to the chosen value of critical temperature. These simulations indicate that ~10 kyr likely elapsed between the emplacement of the first sill and the solidification of the last droplet of melt. The solidification time for each sill ranges from tens to thousands of years; this duration progressively increases during pluton construction. In such configurations, the magma cooling rate, and in particular that of the marginal regions of each sill, is high (e.g., >0.1 °C.yr-1), resulting in a disequilibrium geometry of three-grain junctions involving two grains of plagioclase and one of lepidolite (measured at the edges of the platy lepidolite grains). These dihedral angles (Θlpp) have median and standard deviation values slightly lower than would be expected from an impingement texture. This evidence of early and rapid crystal growth under diffusion-limited conditions following a sill injection is supported by the presence of skeletal cores in lepidolite as well as plagioclase hopper-like morphologies.
This study demonstrates the power of a combined approach, using both thermal simulations, and rock microstructures to reconstruct the Beauvoir pluton assembly and to extract information about solidification kinetics through time. As the application of dihedral angles has so far been limited to mafic magmas, this study unpicks the use of dihedral angles in felsic magmas, opening up perspectives for their use to better understand magma storage and solidification kinetics in felsic bodies.
How to cite: Esteves, N., France, L., Bouilhol, P., Annen, C., and Holness, M.: Assembly duration, cooling kinetics and associated microstructures of a small sized granite pluton, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9916, https://doi.org/10.5194/egusphere-egu25-9916, 2025.
The spatial distribution of mineral phases or pores in magmatic, metamorphic and deformed rocks bear genetic information on crystallization, reaction or transport processes amongst others. In general, the spatial distribution of phases can be categorized into random, clustered or anti-clustered types. For example anti-clustered distributions in a deformed, metamorphic rock can be related to heterogeneous nucleation, while clustered distributions can originate from transport limited mineral reactions. Similarly, crystallization processes have the potential to produce either random or clustered phase distributions hinting on crystallization sequence or reaction history. The deviation from randomness towards (anti-)clustering in bi- or multiphase system can be measured in a quantitative way giving the opportunity to address involved processes not only limited to a descriptive way.
Many metamorphic rocks exhibit an either deformation- and/or reaction-induced foliation and also primary foliations may be present in magmatic rocks. Addressing the phase distribution in an isotropic way may degrade the result of a microstructure quantification by camouflaging the spatial ordering of a phase with respect to one specific sample direction in an otherwise isotropic distribution. For example, K-feldspar may appear regularly spaced within quartz-rich layers, while in any other sample direction, this periodicity of K-feldspar is not present. In order to tackle anisotropy of spatial phase distributions, extensions of isotropic methods are presented. Newly derived descriptions of anisotropic phase distributions based on contact normals, a modified center-to-center method and a Fourier transform-based approach will be compared and based on natural examples, their advantages and shortcomings will be discussed.
How to cite: Kilian, R.: Anisotropy of spatial phase distributions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16854, https://doi.org/10.5194/egusphere-egu25-16854, 2025.
The relative magnitude of the energies of grain boundaries and heterophase interfaces in texturally equilibrated materials can be measured using the dihedral angle method, which is based on the control of the geometry of three-grain junctions involving two different phases by the relationship γAA = 2γAB cos (Θ/2), where Θ is the dihedral angle, γAA is the energy of the AA grain boundary and γAB is that of the heterophase interface. We have compiled a dataset of dihedral angles between 35 mineral pairs found in well-equilibrated granulite-facies rocks. Such a dataset permits the ranking of grain boundary energies, and we find that this ranking correlates with that of the crystalloblastic series. More significantly, we also find that for almost all mineral pairs, both dihedral angles (i.e. those at both AAB and BBA junctions) are <120˚, meaning that the energy of the heterophase interface is lower than that of either grain boundary. An analogous situation occurs in nearly all binary systems of metals. One notable exception is the Zn-Sn pair for which one angle is <120˚ and the other is >120˚, meaning that the energy of the heterophase interface is intermediate between that of the two grain boundaries. This intermediate situation is also observed in experimental mineral charges created by hot-pressing powders, and matches the predictions of a simple model of interfaces involving randomly oriented crystal lattices. That neither metamorphic rocks nor most metals fit this theoretical framework must be a consequence of the creation and preservation of low-energy heterophase interfaces during solidification, reaction, deformation and grain growth. Preliminary work shows that epitaxy governs the location and orientation of nucleation in most metamorphic reactions, resulting in low-energy heterophase interfaces. One corollary of our results is that textural maturation of metamorphic rocks generally results in phase mixing rather than separation and layer formation. Furthermore, that dislocations can cross epitaxial heterophase interfaces, and that the rate of migration of interfaces is dependent on their energy, means that an understanding of rock rheology and microstructural development derived from experimental studies using initially hot-pressed powders may not be directly applicable to natural systems.
How to cite: Holness, M. and Dyck, B.: The energy of grain boundaries and interfaces in metamorphic rocks: comparison of natural materials with experimental charges, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-305, https://doi.org/10.5194/egusphere-egu25-305, 2025.
Open system metasomatic processes generally involve change of volume. Volumetric strain resulting from fluid-mediated mass transfer in rocks is commonly inferred from bulk-rock chemical mass-balance analysis. But the manner in which such volume change is accommodated in deeper crust remains enigmatic. The occurrence of geometrically well-defined reaction zones at the boundary between country rock (pelitic garnet-mica schist) and metamafic dikes (epidote-amphibolites) in the Archean Singbhum craton of eastern India provides an opportunity to address this problem.
The boundary between the two lithounits is marked by two metasomatic reaction zones (MRZs) in the sequence metamafic dike – Zone 1 (amphibole-epidote-sodic-plagioclase-quartz and chlorite) – Zone 2 (Zone 1 assemblage minus amphibole) – pelitic schist, with well-defined boundaries between each unit. We studied these using bulk-rock geochemistry, mineral chemistry, thermodynamic modelling and EBSD analysis.
The transition from Zone-1 to Zone-2 is gradational and marked by a gradual decrease in amphibole modal content. The discontinuity (~ 200 to 8) in variation of bulk-rock Ti/Cr ratio across the zones suggests that the original contact between pelite-dike was within MRZs. Bulk-rock mass balance suggests MRZs formed by Na-metasomatism (gain of ~325 g/100g of protolith), which facilitated the exchange between pelite and mafic-dike and removal of elements (Ca-K-Fe-Ti-Mg) from the MRZs to an external system. Pseudosection modeling shows that the region was cooled isobarically (at 5-6 kbar and 600 -> 300 °C) during the mass transfer process. Major oxide compositions of amphiboles and epidotes show systematic variations within the MRZs.
The euhedral and equant quartz and plagioclase grains exhibit polygonal mosaic texture with anhedral epidote grains at the grain boundaries and triple junctions in the MRZs. Quartz intragrain misorientation analysis from the two MRZs suggests deformation temperatures below 500°C. The absence of relict grains and low CPO strengths indicates that the recrystallization of quartz and plagioclase occurred under fluid-present conditions. EBSD-based crystallographic vorticity axis analysis shows that the quartz and plagioclase grains of the MRZs, amphiboles in the dike and Zone 1, and the retrograde minerals (chlorite, ilmenite, and magnetite) in the pelite record pure shear-dominated deformation signatures. In contrast, the quartz grains in the pelite and the dike were deformed under a simple shear-dominated regime.
Micro-structural analysis and mineral-chemistry variation indicate that quartz-plagioclase recrystallization and amphibole-epidote formation via mass transfer are interconnected processes. The presence of idioblastic-poikilitic amphiboles and epidotes (at the grain boundaries and triple-junctions) indicates that new grains of epidote and amphibole formed by dissolution re-precipitation processes during metasomatic transport by fluids in the MRZs. Evidence for such fluid-mediated exchanges are observed at the microscale as interconnected intergranular fluid pathways within the MRZs and at the outcrop scale as fracture-filled amphibole-bearing-quartz veins within the mafic dike. We propose that the fluid-mediated mass transfer (via dissolution-precipitation) helped to accommodate the volume changes during retrograde cooling reactions through the creation of localized stress fields.
How to cite: Behera, S., Chakraborty, S., Dutta, D., and Misra, S.: Deformation signatures of mass transfer processes: Insights from metasomatic reaction zones in the Archean Singbhum craton in eastern India using geochemical and microstructural analysis, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2759, https://doi.org/10.5194/egusphere-egu25-2759, 2025.
Dumortierite, an aluminous borosilicate mineral, is relatively rare in Earth’s crust, but it is the second most abundant aluminous borosilicate after tourmaline. Boron is an element with many uses in modern societies worldwide, from health products to wind turbine blades for clean energy, but of limited availability and at future risk of supply. Only a limited number of studies have been published on dumortierite mineralization processes and the factors that control its abundance and distribution remain poorly understood. Here we present the first comprehensive electron backscatter diffraction (EBSD) study of dumortierite mineralization mechanisms in kyanite-muscovite-dumortierite veins occurring in the metapelites of the Amgaon Gneiss Supracrustals, in the Girola hill area, Sakoli region, Central India. Advanced microstructural and chemical analyses show that mixed-mode brittle-viscous deformation in kyanite, by fracturing and easy glide on (100)[001], facilitates fluid-rock interactions with reactive hydrothermal fluids rich in B, F, and K and containing Na, Ti, Mg, Fe and Pt. These interactions cause the dissolution of kyanite along fracture and cleavage surfaces and the precipitation of muscovite (F = 0.32 wt%) and topaz (F = 16.48 wt%). The motion of ripplocation defects in muscovite facilitates fluid migration along cleavage surfaces and crystallisation of dumortierite needles within these surfaces. Fluid flux removes silica in solution from the system, so reactions may continue. The observed mineral assemblage, the microstructural signature of kyanite and muscovite, the moderate fluorine content of topaz, and low fluorine content of muscovite, together suggest that dumortierite mineralization results from hydrothermal activity, possibly in a transitional magmatic-hydrothermal environment, possibly at P > 2.5 kbar and 400°<T < 550°, that could be linked with granite intrusions in the area. Using EBSD we demonstrate that dumortierite mineralization is focussed along muscovite basal cleavage surfaces and dumortierite needle elongation is likely controlled by the fluid flux direction. These new results advance our understanding of dumortierite mineralization mechanisms and conditions and have important implications for our understanding of the distribution of this aluminous borosilicate mineral in muscovite-rich rocks.
How to cite: Mariani, E., Dandekar, S., Dandekar, T. R., Khatirkar, R. K., Pande, K., Gardner, J., Bagshaw, H., Randive, K., and Peshwe, D.: Kyanite-muscovite-dumortierite vein mineralisation mechanisms from advanced microstructural analysis using EBSD, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18596, https://doi.org/10.5194/egusphere-egu25-18596, 2025.
Pegmatite, which forms in various tectonic environments and crystallizes through developed magma, is a significant source of green energy transition metals like Li-Cs-Ta and REEs (Müller et al., 2022). Lithium is one of the most important metals for making high-energy batteries and battery storage systems (Müller et al., 2022). It is also the lightest metal on the periodic table and has unique properties that make it ideal for use in batteries.
In Africa, Li is mined from hard rock deposits such as pegmatites. These deposits are often small, covering only a few hundred square meters, and are usually found in Li-Cs-Ta pegmatites (London, 2018). The main Li-bearing minerals in these pegmatites are spodumene, petalite, and lepidolite (Müller et al., 2022). Li-Cs-Ta pegmatites are typically hosted in metamorphosed rocks formed under upper greenschist to lower amphibolite facies conditions (Bradley et al., 2017). However, the origin of the melt or fluid responsible for forming Li-Cs-Ta pegmatites is poorly understood. It is still unclear whether this melt formed through extreme fractionation of a cooling parental granite or came directly from the dehydration of metasedimentary rocks during metamorphism (Müller et al., 2017). To better understand the origin of the Li-rich melt and why some pegmatites contain Li-Cs-Ta minerals while others do not, this study will combine trace element analysis with SIMS insitu oxygen isotope data from quartz. Preliminary trace element results suggest some crustal origin; however, this will be tested more by the oxygen isotope work. The study focuses on pegmatites in the Richtersveld Subprovince, part of the Namaqua Metamorphic Belt in South Africa.
This area is of particular interest because it contains both Li-mineralized and non-mineralized pegmatites. The pegmatites are hosted by both metasedimentary and igneous rocks in the amphibolite facies and are part of a belt bordered by LCT and NYF pegmatites. This research will help explain the processes that control Li mineralization in pegmatites in this region.
References
Bradley, D.C., et al., 2017. Mineral-deposit model for lithiumcesium-tantalum pegmatites. In: Mineral Deposit Models for Resource Assessment. U.S. Geological Survey, Reston, Virginia, pp. 1–48. https://doi.org/10.3133/sir20105070O.
London, D. (2018) ‘Ore-forming processes within granitic pegmatites’, Ore Geology Reviews, 101, pp. 349–383. Available at: https://doi.org/10.1016/j.oregeorev.2018.04.020.
Müller A., et al., 2017. The Sveconorwegian Pegmatite Province – thousands of pegmatites without parental granites. Canadian Mineralogist , 55, 283–315, https://doi.org/10.3749/canmin.1600075.
Müller, A. et al., 2022. GREENPEG–exploration for pegmatite minerals to feed the energy transition: first steps towards the Green Stone Age. Geol. Soc. London Spec. Publ. 526, 27. https://doi.org/10.1144/SP526-2021-189
How to cite: Madlakana, N.: Mineralogical and petrogenetic characterization of the Witkop pegmatite, Northern Cape, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21085, https://doi.org/10.5194/egusphere-egu25-21085, 2025.
Rocks found at the Earth’s surface typically undergo long and complex transformations within the Earth’s crust. In response to varying pressures and temperatures deep in the crust, rocks undergo recrystallization, involving the formation or dissolution of minerals and changes in the equilibrium composition of existing phases. The underlying processes, such as inter-crystalline chemical diffusion and recrystallization, are irreversible, as they continually modify the properties of evolving rocks. While such processes can preserve evidence of past geological conditions, they can also erase earlier equilibration stages. Understanding these processes is challenging because the inverse problem of reconstructing past conditions is inherently non-unique (ill-posed).
This presentation showcases different modeled examples of re-equilibration, revealing that interpretations of past equilibria can sometimes be misleading. In addition, inverse diffusion modeling can estimate the duration of geological processes, although it remains prone to ambiguity. Forward modeling of physical processes helps to identify sources of non-uniqueness and provides regularization, which reduces the model-parameter space. Therefore, integrating geophysical, geodynamic, and petrological inversion methods offers distinct advantages by providing a more robust framework for testing hypotheses and reducing uncertainty.
How to cite: Moulas, E.: Petrology as an ill-posed inverse problem, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3116, https://doi.org/10.5194/egusphere-egu25-3116, 2025.
Geologists have always used a wide variety of microscopy and microanalysis tools for a broad range of rock and mineral characterisation. As datasets become larger and projects become grander in scale, we are increasingly seeing the use of machine learning techniques to streamline all kinds of data acquisition and processing. These techniques have often highlighted the inconsistent nature of data handling by the geoscience community. This has commonly resulted in having lots of data but not “big data”, precluding the use of modern data analysis.
In many ways this is not the fault of the geologist. The complex nature of rock samples, covering more orders of magnitude in scale and requiring a detailed understanding of texture and chemistry, goes far beyond that of other materials in the physical sciences. Combined with analytical systems that were often designed for other sciences, this often results in a personal approach in how to interrogate our samples.
Light microscopy in particular provides a fantastic example of the personal nature of geological sample interpretation. Petrography training, and the subsequent ability to identify and interpret the mineralogy of a thin section, is the epitome of a standard geoscience task that has proved exceedingly hard to automate. Much of this is due to the vast number of minerals with overlapping optical properties, of which we use a dynamic (rotating polarisation) understanding to interpret a thin section. This is a combination of a huge amount of information with which we train our human brains, and standard data processing, even standard machine learning techniques are simply not “smart enough” to replicate.
Right now we are seeing the rapid emergence of deep learning neural networks (DLNN) across a range of 2D and 3D applications. In light microscopy these DLNN models can segment petrography data in ways that have never been possible before, with clear separation of minerals with similar appearance, distinguishing grain boundaries from fractures/cleavage, and determining boundaries of low relief minerals. They also have advantages over traditional segmentation in terms of lithology classification, where different combinations/textures of even the same minerals can have geological meaning but were previously hard to computationally separate. By taking these tasks that are relatively simple for human brains, but have been hard to upscale to large datasets, we can start to make consistent approaches across the geoscience community.
In addition, we now have the capability to generate large amounts of petrography data with automated slide scanners, meaning both data acquisition and processing can happen at the speed and scale necessary to make automated “big data” petrography a real tool for the geologist. Both data storage and the training of DLNN models can now be moved online, which means not only greater access of these tools to our community, but the ability to contribute to, modify, and assess the quality of future workflows.
How to cite: Taylor, R.: How deep learning is changing Geoscience microscopy, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17379, https://doi.org/10.5194/egusphere-egu25-17379, 2025.
Posters on site: Tue, 29 Apr, 10:45–12:30 | Hall X2
Igneous rocks are significant sources of rare Earth elements. Their composition is affected by processes in magma chambers, liquid reservoirs residing beneath the surface of Earth. Crystallization of these reservoirs is typically assumed to be fractional, but it is unclear what is the primary mechanism behind separating the solid phase from the liquid: i) in-situ crystallization along the walls of the intrusion, or ii) sedimentation of crystals from suspension within the liquid magma. The latter scenario should be imprinted in the crystal size distribution of the deposit, as differently sized crystals have different residence times in the liquid. Solidification of magmatic systems is a complex problem and its individual aspects are often investigated separately. Physics-based models that couple the dynamics of solidification, settling laws derived from particle-laden flow experiments, and kinetic laws of crystal growth and nucleation remain scarce.
Here, we build upon [1] and present a parameterized model of convection inside a magma chamber that explicitly treats crystal nucleation, growth, and gravitational settling in a vigorously and turbulently convecting fluid. The call for a new self-consistent model of a cooling magma chamber is driven, among others, by the recently formulated unified settling law that captures also the transition from particles that are well mixed by the convective currents to those sinking nearly vertically with their Stokes velocity, [2]. By invoking the energy balance and compositional evolution of the system (in this initial phase treated as a binary alloy), we show how the shape of the crystal size distribution and crystal grading in the sedimented lower part of the solidified body evolve as the fluid cools and the sediment layer increases in size. The model predicts dimensional results and aspires to model the microstructure of intrusions formed from authentic magmatic systems. To this end, we incorporate realistic laws of crystal growth and nucleation derived explicitly from thermodynamics principles. The model provides an initial framework for studying the link between flow dynamics inside the chamber, thermal/compositional evolution, and sediment signature, and can be easily built upon.
[1] Jarvis, R.A., Woods, A.W., 1994. The nucleation, growth and settling of crystals from a turbulently convecting fluid. J. Fluid Mech. 273, 83–107
[2] Patočka, V., Tosi, N. & Calzavarini, E. (2022). Residence time of inertial particles in 3D thermal convection: implications for magma reservoirs. Earth and Planetary Science Letters 591, 117622.
How to cite: Beran, V., Patočka, V., and Špillar, V.: Parameterized model of a cooling magma chamber, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5728, https://doi.org/10.5194/egusphere-egu25-5728, 2025.
Near Galpyeong Reservoir in northern Cheongsong County, mid-eastern Korean Peninsula, a rhyolitic stock with an elliptical shape is exposed, measuring about 2 km by 1 km. Surrounding this stock, rhyolitic dykes radially intrude the Cretaceous sedimentary rocks within a 4 to 5 km range. These dykes are mainly grayish-blue or brown, with occasional white dykes. They contain well-developed spherulitic textures, with spherulites up to 1 meter or more in diameter. Due to their diverse floral-like shapes, they have been traditionally called "flower stones" and are recognized as a key geosite of the Cheongsong Global Geopark.
The rhyolitic dykes, with widths of 0.5 to 3 m, display non-linear, curved paths. Brown dykes crosscut grayish-blue ones, and larger spherulites occur in dyke cores, while smaller ones are located at the margins. Flow banding, induced by shearing, is more prominent at the margins and in the brown dykes. Elongated cavities are frequently observed along flow bands in brown dykes, but these bands do not penetrate the spherulites. The spherulites in brown dykes are generally larger and often encased in mafic outer crusts, facilitating easy separation from the host rock.
The chemical composition of the rhyolitic dykes corresponds to the subalkaline series of rhyolites. Spherulites show relatively lower SiO₂ content than the matrix, but in the spider diagram, there are minimal compositional differences between the spherulites and the matrix.
Spherulites primarily formed through a combination of radial and spherical quartz growth. Their nucleation centers include quartz, orthoclase, or flow structures, though in some cases, no nucleus is identifiable. Some spherulites exist as individual units, while others are aggregates of smaller spherulites forming larger ones. In the brown dykes, most large spherical spherulites are clusters of numerous smaller ones.
Chemical composition analysis from the center of the spherulites to the matrix indicates SiO₂ and Na₂O contents are highest in the spherulite interiors, decreasing sharply in the nearby matrix and stabilizing at low levels at greater distances. In contrast, Al₂O₃, K, and FeO contents are lowest in the spherulite interiors, peaking at the surfaces, and decreasing gradually to relatively high levels further away.
The zircon saturation geothermometer suggests that in grayish-blue dykes, spherulites and matrix formed at nearly the same temperature, while in brown dykes, spherulites formed at higher temperatures than the matrix.
These findings indicate that the margins of a rhyolitic magma chamber cooled rapidly, enabling spherulites to form under relatively high internal temperatures. The initial dykes likely formed without incorporating chamber spherulites. In a subsequent phase, dykes transported pre-existing spherulites, during which flow structures developed around the already solid spherulites.
How to cite: Woo, H. and Jang, Y.: Microstructural and Geochemical Evolution of Spherulitic Rhyolitic Dykes in Cheongsong, South Korea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7560, https://doi.org/10.5194/egusphere-egu25-7560, 2025.
Laboratory Diffraction Contrast Tomography (LabDCT) is an emerging 3D non-destructive characterisation technique that has been widely utilised in materials science but remains relatively novel in geoscience. This research illustrates its potential for complex natural materialsthrough applications to chromite sands, chromitite from the Bushveld Complex, and other geomaterials such as olivine, rock salt, and diopside. By comparing identical 2D slices from LabDCT with EBSD results, our findings demonstrate good agreement, with average disorientation angles between DCT and EBSD consistently within 0.3° (Chen et al., 2023). This comparison validates the accuracy and reliability of LabDCT. Furthermore, LabDCT uniquely provides comprehensive 3D data, including volumetric and textural information, offering deeper insights into igneous and metamorphic processes that are beyond the reach of traditional 2D methods.
To address artefacts such as inaccurate grain boundaries and partially indexed grains, we developed a novel post-processing workflow validated through comparisons with EBSD data (Chen et al., 2024). This workflow refines grain boundary definitions, improves reconstructions of partially indexed grains, and rectifies morphological inaccuracies. Results from both resin-mounted chromite and natural chromitite samples demonstrated a significant enhancement in the accuracy of LabDCT outputs, not only reducing unindexed volumes but also restoring precise crystallographic data.
This research demonstrated the application of LabDCT to a series of natural rocks, underscoring LabDCT’s revolutionary potential in mineralogy. Its capabilities are particularly valuable to understand the genesis of ore deposits, advancing the recovery of critical metals, and assessing rock textures in 3D.
References
Chen, X., Godel, B., & Verrall, M. (2023). Comparison of Laboratory Diffraction Contrast Tomography and Electron Backscatter Diffraction Results: Application to Naturally Occurring Chromites. Microscopy and Microanalysis, 29 (6), 1901-1920.
Chen, X., Godel, B., & Verrall, M. (2024). Postprocessing Workflow for Laboratory Diffraction Contrast Tomography: A Case Study on Chromite Geomaterials. Microscopy and Microanalysis, 30 (3), 440-455.
How to cite: Chen, X., Godel, B., and Verrall, M.: Diffraction contrast tomography: unlocking its potential for mineral resources application, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8067, https://doi.org/10.5194/egusphere-egu25-8067, 2025.
There is a large body of field, textural and chemical evidence from mafic layered intrusions indicating that crystals predominantly nucleate and grow in situ, i.e., at solidification fronts along the margins of evolving magma chambers. Campbell was the first to provide an explanation for this fundamental mode of crystallization in which new nuclei appear on existing crystals along chamber margins, a process he referred to as ‘heterogeneous self-nucleation’. Holness et al. (2023) have recently argued, however, that this nucleation mechanism is implausible because continued growth of existing crystals is more kinetically favourable than nucleation of new grains of the same phases. As an alternative explanation, Holness et al. (2023) have returned to the classical settling model in which crystals nucleate and grow throughout the entire magma chamber. Massive chromitites in the Bushveld Complex then arise due to gravity-induced settling of chromite clusters from the convecting resident melt on the chamber floor, with formation of the clusters being due to ‘synneusis’ – random collision of suspended grains leading to adherence. This mechanism is, however, inconsistent with existing field, textural and chemical observations from massive chromitites and other igneous rocks of the Bushveld Complex. Most of these observations are indicative of the rock forming via some form of repeated nucleation and growth of new crystals on existing ones at the chamber floor. A challenge is thus to identify the mechanism of in situ nucleation and growth of crystals along the margins of layered intrusions that would be reconcilable with the ground-truth observations. In this study we suggest that the most likely candidate is secondary nucleation caused by seed crystal surfaces - a process that has never been invoked in igneous petrology - but plays a key role in the formation of crystals in industrial crystallizers. During this process, Van Der Waal’s attractive forces in the vicinity of a crystal face can stabilize new nuclei of the same phase in proximity of the original crystal. We present experimental evidence of growth of chromite clusters from chromite-saturated mafic magma in support of this argument. Since secondary nucleation is induced by pre-existing parent crystals acting as catalysts for further nucleation, one may logically expect it to actively operate along the crystal-rich solidification fronts at the floors, walls, and roofs of evolving magma chambers.
How to cite: Latypov, R., Chistyakova, S., Barnes, S., Godel, B., Iacono-Marziano, G., and Kruger, W.: Secondary nucleation at crystal surfaces as a potential mechanism for in situ nucleation and growth of crystals at magma chamber margins , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8245, https://doi.org/10.5194/egusphere-egu25-8245, 2025.
Komatiites are extrusive volcanic rocks with 18 wt % Mg, and are almost exclusively of Archean age (Arndt 1994). These high-Mg lavas have very low viscosity (0.1-10 Pa) and are expected to have flowed in a turbulent manner (Huppert and Sparks 1985). However, there is little field evidence for such flow behaviour (Cas et al. 2024). Additionally, as such hot lavas no longer erupt on Earth, we know little of the initial crystal cargo they may have transported. To address these questions, we investigated komatiite lava flows from the Reliance Formation, Belingwe Greenstone belt, Zimbabwe, using quantitative microstructural analyses together with electron backscatter diffraction (EBSD).
We investigated misorientation axes between neighbouring olivine grains in crystal clusters, to gain information about their crystallisation history and mode of aggregation. We found a systematic misorientation axis, [100], and three dominant misorientation angles (4˚, 40˚ and 60°). The low-angle misorientation axes are associated with olivine crystals with dendritic morphology, and are associated with regions of the crystals where branching occurs. In contrast, the 40˚ misorientation angles are found in crystal clusters formed by synneusis, whereas the 60° misorientation angles are associated with grains showing a twinning relationship (Wieser et al. 2019). We also investigated the crystallographic relationship between enclosed grains of Cr-rich spinels and their host olivine crystals. These show an epitaxial relationship; [100]Ol [111]Sp and [001]Ol [110]Sp.
Vance (1969) suggests that the likelihood of random collisions between olivine crystals is most likely in a turbulent flow, but Schwindinger & Anderson (1989) point out that such random collisions will not produce aggregates with systematic crystallographic alignment. Instead, they infer that synneusis with systematic alignments requires a “flowing fluid” (i.e. laminar flow). Therefore, we suggest that the relationships we observe in the Belingwe olivine clusters result from synneusis of crystals carried from their magmatic source in a laminar flow. Additionally, the observed epitaxial relationship between olivine and spinel suggests that spinel either exsolved from olivine, or nucleated heterogeneously on the olivine substrates with an epitaxial relationship. This will require further investigation.
Reference:
Arndt, N. T. In Developments in Precambrian Geology, vol. 11, pp. 11-44. Elsevier, 1994.
Huppert, H. E., and Sparks. R.S.J. " Journal of Petrology 26, no. 3 (1985): 694-725.Cass et al 2024
Cas, R., Wright, J. V., & Giordano, G. (2024). Cham: Springer International Publishing.
Wieser, P. E., Vukmanovic, Z., Kilian, R., Ringe, E., Holness, M. B., Maclennan, J., & Edmonds, M. (2019). Geology, 47(10), 948-952.
Vance, J. A. (1969). On synneusis. Contributions to Mineralogy and Petrology, 24(1), 7-29
Schwindinger, K. R., & Anderson Jr, A. T. (1989). Contributions to Mineralogy and Petrology, 103(2), 187-198.
How to cite: Vukmanovic, Z., Holness, M., and Nicoli, G.: Transport of crystal cargo in high-Mg lava flows: a microstructural study of Reliance Formation komatiites, Belinqwe Belt, Zimbabwe, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12063, https://doi.org/10.5194/egusphere-egu25-12063, 2025.
The Vihanti-Pyhäsalmi region hosts major volcanic massive sulfide (VMS) deposits within the Precambrian Fennoscandian shield in Finland. The Pyhäsalmi Cu-Zn-Ag-Au VMS deposit has undergone various degrees of metamorphism and deformation that have altered the composition and texture of the ore minerals resulting in ore minerals remobilization. This study aims to constrain the P-T-t path utilizing the metamorphic index minerals and in situ Lu-Hf garnet ages (acquired at the University of Adelaide) from the Paleoproterozoic ore potential Pyhäsalmi region. Moreover, our study also focuses on the remobilization history of ore minerals during polyphase deformational and metamorphic events of the Pyhäsalmi deposit’s halo region through petrographic, mineralogical, compositional and 3D micro-CT analyses. Additionally, we study the microstructures in 2D and 3D to tie regional polyphase deformations with porphyroblast’s growth and ore mineral’s remobilization. Understanding the metamorphism of the Pyhäsalmi region will provide new information for the remobilized ore deposits and other ore potential regions.
The preliminary studies from the supracrustal rocks from the drill core samples of the Pyhäsalmi deposit’s halo region suggest medium to high amphibolite facies metamorphic condition with two metamorphic peak conditions such as (1) peak condition one (M1) at 550-600°C and 2-3 kbar represented by staurolite, garnet and cordierite assemblages and (2) the second peak condition (M2) at 650-700°C and 4-6 kbar characterized by sillimanite, cordierite, garnet, biotite assemblages. Moreover, petrographical and mineralogical studies highlight late stage retrograde hydrothermal fluid activity represented by intense chloritization of sillimanite, garnet, biotite, anthophyllite and cordierite and epidotization of the plagioclase. In situ Lu-Hf inverse isochron ages of 1895 ± 53 Ma from cluster type Fe-Mn rich homogenous garnets likely represent the earliest metamorphic event, whereas Lu-Hf ages of 1824 ± 16 Ma from corona type garnets and Lu-Hf ages of 1802 ± 7 Ma from elongated garnets with Mn rich core denotes the second metamorphic event. Additionally, based on textural and mineralogical study accompanying with Flinn strain diagram from micro-CT analysis, we propose that the Pyhäsalmi deposit experienced two distinct periods of ore mineral remobilization, corresponding to two compressional deformation stages. The first compressional stage (D1–D2, at ~1.91 Ga) is associated with the collision of the Svecofennian volcanic arc with the Archean crust. During this stage, mechanical remobilization processes predominated, including cataclastic flow, as evidenced by brecciated pyrites and pyrrhotites, and translational gliding, which produced elongated pyrites and pyrrhotites. The second compressional stage (D4, at 1.82–1.79 Ga) is characterized by post-collisional intense shearing within the Oulujärvi Shear Zone (OjSZ). This stage resulted in mixed-state remobilization processes that involved both chemical and mechanical mechanisms. Evidence for these processes includes fracture-filling sulfides (pyrite, pyrrhotite, chalcopyrite), recrystallization, and the spatial redistribution of disseminated pyrrhotites around the grain boundaries of silicate porphyroblasts (indicative of plastic flowage). Additionally, flattened, foliated, and folded sulfides (pyrite, pyrrhotite, sphalerite) are observed along the hinge zones of crenulated cleavage, further supporting the occurrence of mixed-state remobilization during this period.
How to cite: Islam, S. R., Heilimo, E., Cutts, K., and Kuva, J.: Modelling of metamorphic P-T conditions and in situ Lu-Hf garnets dating from the Paleoproterozoic Pyhäsalmi region, Central Finland: insights into polymetamorphic-multiphase deformational events and ore mineral’s remobilization, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15898, https://doi.org/10.5194/egusphere-egu25-15898, 2025.
Quantifying phase proportions (Φ), grain size, crystal area number density (NA), and surface area to volume ratio (SvP) is essential to understanding igneous crystallisation. Electron backscatter diffraction (EBSD) offers advantages over backscattered electron (BSE)-based image analysis: it segments grains based on crystal structure and orientation, and involves a smaller interaction volume. However, EBSD can be time-consuming, and grain reconstructions depend on post-processing parameters and workflows, especially in glass-rich samples. We evaluated strengths and weaknesses of microstructural analysis of glass-rich samples using EBSD.
Crystallisation experiments synthesised and analysed via BSE imaging by Pontesilli et al. (2019) were re-analysed using EBSD. The samples are two synthetic trachybasaltic glasses, one nominally anhydrous, the other with 2 wt% added H2O, heated to a superliquidus temperature of 1300°C at 400 MPa and fO2 close to NNO+2 in a piston cylinder apparatus. After 30 minutes the samples were cooled at 80°C min-1 to 1100°C (considerably below their liquidus temperatures) and held there for 30 minutes before quenching. The samples contain dendritic to skeletal clinopyroxene (Cpx) crystals in a glass matrix, clustered with smaller skeletal to anhedral titanomagnetite (Tmt) grains.
Standardising the confidence index of EBSD pixels to the highest value for each grain strongly influences quantification results. Standardisation leads to overestimation of crystallinity and grain size, but delivers better estimates of NA and SvP values. For non-standardised scans, varying the minimum confidence index threshold used for cleaning affects all microstructural parameters studied, whereas varying minimum grain size threshold and step size strongly affects only NA and SvP. Varying boundary smoothing only affects SvP.
For Cpx, EBSD and BSE are in excellent agreement for ΦCpx in both samples and SvPCpx in the hydrous sample, while EBSD-derived SvPCpx is 50% higher than the BSE-derived value for the anhydrous sample. For both samples, EBSD-derived maximum Cpx length is ~ 100% higher than the BSE result. For Tmt, EBSD systematically finds slightly elevated ΦTmt and Tmt grain size, and for the anhydrous sample only, significantly higher Tmt NA.
Despite larger maximum lengths, calculated Cpx growth rates from EBSD are within 10% of BSE-derived values, because the calculation employs the square root of length times width. The large differences in NA and SvP found for the anhydrous sample derive from its finer, dendritic microstructure, and the smaller (200 nm) step size of the EBSD scans compared to BSE imaging. For the more euhedral and coarser-grained hydrous sample, BSE and EBSD return similar results, and an EBSD step size of 1 µm is sufficient. The systematically larger Tmt sizes obtained from EBSD are overestimates due to signal from Tmt below the sample surface.
In conclusion, care must be taken applying EBSD to glass-rich samples. Thresholds must be carefully chosen by comparing reconstructed grains and image quality maps, different processing workflows are required to obtain different microstructural parameters, and phase-specific over-/underestimates of parameters may occur. EBSD delivers most improvement for microstructures with sub-micrometer length scales.
Pontesilli et al. (2019), Chem Geol 510:113-129. 10.1016/j.chemgeo.2019.02.015
Funded by the Austrian Science Fund (FWF): P 33227-N
How to cite: Griffiths, T., Pontesilli, A., Peres, S., and Masotta, M.: The quantification of microstructural parameters of glass-bearing samples: electron backscatter diffraction mapping versus backscatter electron imaging , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16781, https://doi.org/10.5194/egusphere-egu25-16781, 2025.
The Bushveld Complex is widely known as the world’s largest igneous intrusion, spanning an area of 550km and a depth of 8km (Cawthorn & McCarthy, 2023). The Upper Zone of the Busvheld Complex is characterised with the massive (> 1 m thick) magnetitite layers. The magnetitite layers contain between 70 and 30 vol. % of magnetite (Fe3O4) and ilmenite(FeTiO3). The Upper Zone contains 24 distinct layers of magnetitite with surrounding contacts of anorthosite on the hanging and foot walls of each. The development of these layers is heavily disputed with arguments for fractional crystallization (Reynolds, 1985), in-situ crystallization (Kruger & Latypov, 2020), and crystal mush magmatic emplacement (Vukmanovic, et al., 2019).
In this study we analysed samples from three distinctive cores from Khuseleka Mine, investigating layers 1, 13 and 14 above the Main Magnetitite layer (Reynolds, 1985). Our study relies on quantitative microstructural and geochemical data. Electron backscatter diffraction (EBSD) has been used to investigate rock microstructure such as crystal orientation and intragrain microstructure; and electron probe micro analysis (EPMA) to investigate chemical variations between crystal rims and cores. Orientation analysis revealed an unexpected, but mild crystallographic preferred orientation in both magnetite and ilmenite crystals, exhibiting point maxima at (100) and at [10-10] respectively. Crystallographic preferred orientations in magnetite are rare in oxide phases due to cubic symmetry, however, the CPO exhibited suggests that the CPO is generated from deposition through a flow in the melt, generation through post-depositional deformation and recrystallization (Pilchin, 2011) or conversely, through topotactic reactions between magnetite grains (Barbosa & Lagoeiro, 2010). The hypothesis for post-depositional deformation is further supported by the evidence of recrystallisation and low-angle boundaries in magnetite grains.The investigation of crystallographic relationship between magnetite and ilmenite has epitaxial relationship between the two phases, this is evidenced by grains of ilmenite displaying parallel poles with adjacent grains of magnetite . Ilmenite shows less intragrain microstructure than magnetite, hence the mild CPO in ilmenite (a trigonal phase) could be explained by crystallographic control between magnetite and ilmenite during oxide crystallisation, or ilmenite deposition from a magma flow (Till & Rybacki, 2020).
EPMA data reveals variations in geochemistry between figures for crystal rims and cores and exhibits consistent zoning of TiO2 in ilmenite samples across separate cores (51.5core wt% and 52.1rim wt%). The vertical depletion of Cr, discovered by Kruger & Laytpov, 2020, and Cawthorn & McCarthy, 2023 is challenged by this study as no correlation between vertical displacement and Cr concentrations has been acknowledged, suggesting that the theory of in-situ crystallization at the examined magnetite layers does not apply.
This data acquired from this study suggests that the magnetite layers in RLS have been subjected to post-depositional deformation which has disrupted the primary texture of the oxides. The variations of minor elements in magnetite does not support in-situ crystallisation. However further analyses are needed to decipher the early magmatic history of these enigmatic bodies.
How to cite: Grimshaw, T. and Vukmanovic, Z.: Microstructural and geochemical characterisation of magnetitite layers from the Khuseleka Mine, Bushveld Complex, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20334, https://doi.org/10.5194/egusphere-egu25-20334, 2025.
Posters virtual: Tue, 29 Apr, 14:00–15:45 | vPoster spot 1
EGU25-17567 | ECS | Posters virtual | VPS22
Evidence of syntectonic muscovite and garnet porphyroblast development in metapelites and its implications for the ~ 1 Ga tectonic event of the Chotanagpur Granite Gneissic Complex (CGGC), IndiaTue, 29 Apr, 14:00–15:45 (CEST) | vP1.1