Investigation of rock-forming processes in the Earth’s crust and mantle spans from the nano- to the orogen-scale and encompasses diverse techniques and approaches, including but not limited to field-based studies, petro-geochemical analysis and petrological and geodynamic modelling. All our observations in the rock record are the end-product of metamorphism, metasomatism, and deformation events that occurred during a terrane’s geological evolution. The study of metamorphic rocks is thus the key to decipher large scale and long-lasting tectonic processes, such as crustal thickening and exhumation, or the composition of geologic fluids and their role in geochemical cycling and deformation. Furthermore, reactions between fluids and rocks have a fundamental impact on many of the natural processes occurring in crustal settings, i.e. metamorphism and associated rheological weakening, localization of deformation, earthquake nucleation caused by high pressure fluid pulses and metasomatic fronts.
This session will focus on novel approaches to address key geological questions at a wide range of temporal and spatial scales using a multidisciplinary approach combining field, structural, petrological, geochemical techniques and thermodynamic simulations.
vPICO presentations: Tue, 27 Apr
Serpentine veins are ubiquitous in hydrated and deformed ultramafic rocks, and have previously been used to track fault kinematics and understand the evolution of environmental conditions during vein formation. However, difficulties in unambiguously identifying and mapping serpentine types at sub-micron to mm scales has limited our understanding of vein precipitation kinetics and growth histories. Using recently developed techniques of Raman spectroscopy mapping, combined with scanning- and transmission-electron microscopy, we describe a new type of mineralogically banded serpentine crack-seal vein in six samples from different settings around the world. In all of the studied samples, individual bands comprise a thin layer (~0.4–2 µm) dominated by chrysotile and a much thicker layer (~0.5–30 µm) dominated by polygonal serpentine/lizardite. Existing field and experimental data suggest that disequilibrium conditions immediately following crack opening may favour rapid precipitation of chrysotile along one of the crack margins. Subsequently, diffusional transport of elements favours slower precipitation of polygonal serpentine/lizardite which leads to crack sealing. The similarities in layer thicknesses and mineralogy exhibited by samples collected from extension and shear veins, dilational jogs, foliation surfaces, and the margins of phacoids, suggest that a common set of processes involving crack opening and sealing are active in a range of different structural sites within serpentinite-dominated shear zones, potentially associated with frequent and repetitive stress drops such as those recorded during episodic tremor and slow slip.
How to cite: Tarling, M. S., Smith, S. A. F., Rooney, J. S., Viti, C., and Gordon, K. C.: Serpentine crack-seal veins: a unique record of fluid conditions during faulting, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-425, https://doi.org/10.5194/egusphere-egu21-425, 2021.
Geophysical observations show that the Alpine Fault in New Zealand is characterised by mid-crustal off-fault recurring tremor events and off-fault regions of continuous deformation. While geodesy indicates that deformation is distributed across the South Island, evidence from the rock record shows deformation accommodated in a region within several km from the fault. This zone is characterized by a 100-300 m wide mylonitised central fault zone and an approximately 8--10km, wide deformation region marked by the presence of Alpine foliation. Magnetotelluric surveys of the Southern Alps indicate a mid-crustal, high signal area coinciding with the location of the recurring tremors.
While the mylonites and their associated mechanisms have been extensively studied in the field area, the wider off-fault deformation region has not had the same scrutiny. In the latter region, we observe frequent layer parallel, folded and crosscutting quartz veins. Quartz vein orientation and geometries are consistent with fracturing in the presence of fluid within an overall tectonic stress regime. The observed overprinting of older veins by younger vein generations, as well as their successive reorientations, indicate recurring fracturing within a continuously deforming region. Quantitative analysis of vein geometries including their width and displacement shows that vein formation may trigger the observed mid-crustal tremor signal. Microstructural signatures within the host rock are consistent with dissolution-precipitation creep as the main deformation mechanism in the host rock and pre-existing veins.
In summary, according to field evidence both geophysically observed transient and continuous deformation take place in the presence of fluid and occur contemporaneously. This implies that strain accommodation in the host rock facilitated by dissolution-precipitation creep is insufficient; consequently, stress is build-up over time triggering intermittent fracturing.
How to cite: McGrath, J., Piazolo, S., Morgan, R., and Elliott, J.: Field evidence for fluid facilitated fracturing and Dissolution-Precipitaion creep explains observed off-fault tremor and continuous deformation: Field examples from the Alpine Fault, New Zealand , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8214, https://doi.org/10.5194/egusphere-egu21-8214, 2021.
Strain accommodation in upper crustal rocks is often accompanied by fluid-mediated crystallization of phyllosilicates, which influence rock strength and shear zone formation. The composition of these phyllosilicates is commonly used for pressure-temperature-time constraints of deformation events, although it is often highly heterogeneous. This study investigates the reactions producing a phyllosilicate, chlorite, in and below greenschist-facies conditions and the variations in chlorite composition, along a strain gradient in the Bielsa granitoid (Axial Zone, Pyrenees). Compositional maps of chlorite (including iron speciation) are compared to nanostructures observed by transmission electron microscopy in increasingly-strained samples and related to mechanisms of fluid percolation and scales of compositional homogenisation. In the Bielsa granitoid, altered at the late Variscan, Alpine-age shear zones are found with high strain gradients. The undeformed granitoid exhibits local equilibria, pseudomorphic replacement and high compositional heterogeneities in chlorite. This is attributed to: (i) variable element supply and reaction mechanisms observed at nanoscale and (ii) little interconnected intra- and inter-grain nanoporosity causing isolation of fluid evolving in local reservoirs. In samples with discrete and mm-sized fractures, channelized fluid triggered the precipitation of homogeneous Alpine chlorite in fractures, preserving late-Variscan chlorite within the matrix. In low-grade mylonites, where brittle-ductile deformation is observed, micro-, nano-cracks and defects allows the fluid percolating into the matrix at the scale of hundreds of µm. This results in a more pervasive replacement of late-Variscan chlorite by Alpine chlorite. Local equilibria and high compositional heterogeneities in phyllosilicates as chlorite are therefore preserved according (i) matrix-fracture porosity contrasts at nanoscale and (ii) the location and interconnection of nanoporosity between crystallites of phyllosilicates that control reaction mechanisms and element mobility. In low grade mylonites, mineral and compositional replacement remains incomplete despite the high strain.
How to cite: Airaghi, L., Dubacq, B., Verlaguet, A., Bourdelle, F., Bellahsen, N., and Gloter, A.: From static alteration to mylonitization: a nano- to micrometric study of chloritization in granitoids with implications for equilibrium and fluid percolation length scales, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1053, https://doi.org/10.5194/egusphere-egu21-1053, 2021.
Determining the stress state during metamorphism is a key challenge in metamorphic petrology as the effect of differential stress on metamorphic reactions is currently debated. Conventional piezometry generally gives stresses that correspond to overprinting deformation rather than to mineral growth of high-grade metamorphism, so an alternative approach is required. Garnetite lenses from the ultrahigh-pressure, low-temperature metamorphic Lago di Cignana unit (Western Alps, Italy) record compaction by a high degree of mineral dissolution in the fluid-rich environment of a cold subduction zone. This work combines microstructural analysis of deformed garnet with elastic strains of quartz inclusions to study the stresses in these metasedimentary rocks.
Garnet exhibits abundant evidence for incongruent pressure solution (IPS), most notably as truncated zones that mismatch across grain boundaries, interlocking structures, and shape-preferred orientation (SPO). The gap in garnet compositions represented by overgrown truncated zonation corresponds to undeformed garnet with inclusions of quartz and coesite, indicating that IPS operated during prograde to peak metamorphism. The distribution of aspect ratios in the garnet grain population suggests that pressure solution preferentially affected smaller grains. SPO analysis of many subregions across a garnetite sample reveals a complex distribution, however the local SPO is consistent with the stress orientation expected for local microstructures such as layering, garnet stacks, or fine-grained internal fluid pathways. Locally, two different preferential orientations are observed, interpreted as the result of two subsequent deformation stages under different stress configurations.
Quartz inclusions in prograde euhedral garnet, grown on the outer margin of coevally deformed garnetite, were analysed with Raman spectroscopy. Elastic strains obtained for these inclusions are in agreement with predicted strains for entrapment along the prograde P-T path for the Lago di Cignana unit (~1.5–2.0 GPa; ~450–500 °C), whereas significant differential stress during entrapment is expected to result in deviating strain components.
By combining microstructural analysis of garnet with elastic-strain analysis of quartz inclusions, stress orientations obtained from deformed garnet are combined with the stress magnitude for coeval garnet growth. The results indicate that the garnetite lenses were deformed and metamorphosed under low differential stress of variable orientation during subduction. These results are in agreement with a system where garnet is wet by a fluid phase that allows IPS.
Acknowledgements: This project has received funding from the European Research Council under the H2020 research and innovation program (N. 714936 TRUE DEPTHS to M. Alvaro)
How to cite: van Schrojenstein Lantman, H., Wallis, D., Gilio, M., Scambelluri, M., and Alvaro, M.: Elastic strains of quartz inclusions and microstructures from pressure solution in garnet reveal orientation and low magnitude of differential stress during subduction metamorphism, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13793, https://doi.org/10.5194/egusphere-egu21-13793, 2021.
Fluid activity in the crust is a key process controlling the generations of earthquakes, magmas, ore deposition formation and deep geothermal activities. Although high pore fluid pressure has been recognized by geophysical observations and geological observations of mineral filled fractures, the actual fluid pressure, their durations and associated permeability are controversial and remain largely unknown. Here we propose a new methodology estimating the duration, fluid pressure gradients and permeability recorded in fluid-rock reaction zones, by utilizing thermodynamic analyses in conjunction with halogen (Cl, F) profiles along the reaction zones.
We have analyzed exceptionally well-exposed crustal fluid–rock reaction zones at Sør Rodane mountains, East Antarctica. The thermodynamic analyses of granitic dike–granulite-facies crust reaction zone at 0.5 GPa, 700°C (Uno et al., 2017) and amphibolite-facies hydration reaction zones around mineral-filled fractures at ~0.3 GPa, 450°C (Mindaleva et al., 2020) reveals extremely high fluid pressure gradients of ~100 MPa/10cm or ~1 MPa/mm across the reaction zones. The reactive transport analysis suggest that fluid activity lasted for 100–250 days and ~10 hours, respectively. These extremely high fluid pressure gradients represent the low permeability of the intact amphibolite and granulite host rocks without fractures. The estimated permeabilities of the host rocks are 10−20–10−22 m2, and are several orders smaller than the widely accepted crustal permeability model (~10−18 m2; e.g., Ingebritsen and Manning, 2010). On the other hand, permeability along the fractures are estimated as high as 10−11–10−16 m2, which is analogous to the permeability estimated for the hypocenter migration for the crustal earthquake swarms (~10−14–15 m2; e.g., Nakajima and Uchida, 2018). Our observation supports that low permeability of intact crust promotes fluid accumulation and subsequent fracturing in the crust and/or underlying plate boundaries.
Nakajima, J., Uchida, N., 2018. Nature Geoscience 11, 351–356.
Ingebritsen, S.E., Manning, C.E., 2010. Geofluids 10, 193–205.
Uno, M., Okamoto, A., Tsuchiya, N., 2017. Lithos 284–285, 625–641.
Mindaleva, D., Uno, M., Higashino, F. et al., 2020. Lithos 372–373, 105521.
How to cite: Uno, M., Mindaleva, D., Okamoto, A., and Tsuchiya, N.: Crustal fluid pressure gradients and permeability evolutions estimated from metamorphic fluid-rock reaction zones (Sør Rondane Mountains, East Antarctica), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9762, https://doi.org/10.5194/egusphere-egu21-9762, 2021.
The Proterozoic gneisses of the Bamble lithotectonic domain (south Norway) underwent intense scapolitisation caused by K- and Mg-rich fluids and extensive albitisation with formation of numerous ore deposits.
By detailed studies of mineral reaction fabrics we document release of the chemical active Mg, K and Fe-components forming the metasomatic fluid: Breakdown of biotite to muscovite releases K, Mg, Fe, Si and H2O. As reaction products tiny Fe-oxide needles are present in the transforming rock. H2O is reacting with K-feldspar to produce additional amounts of white mica and quartz. During a subsequent reaction muscovite is replaced to sillimanite again releasing quartz and a K-rich fluid. The reactions form the peculiar sillimanite-nodular quartzite, but also well-foliated sillimanite-mica gneiss.
Optical and EBSD microfabric studies reveal a shape preferred orientation for quartz, but despite of a pronounced foliation, quartz does not show a crystallographic preferred orientation. A crystallographic preferred orientation is present for mica and sillimanite. Coarse micas show sutured boundaries to quartz, implying low nucleation rates, no crystallographic or surface-energy control during growth and no obvious crystallographic relationship to quartz.
Our study illustrates the transformation of a quartzofeldspatic lithology into sillimanite-bearing quartzite. The mineral replacement and deformation show ongoing metamorphic reactions during deformation. The microfabric data indicates reaction at non-isostatic stress condition. The deduced mineral replacement reactions document a source of K-, Mg- and Fe-rich metasomatic fluids necessary to cause the pervasive scapolitisation and Fe-deposition in the area. The mineral reactions and deformation produce rocks with a new mineralogy and structure; an increased understanding of these processes is important for the modelling of crustal building and geological history.
How to cite: Engvik, A. K., Trepmann, C. A., and Austrheim, H.: Mg-K-Fe fluid producing mineral reactions, metasomatism and microfabric development during formation of nodular sillimanite-gneiss in the middle crust, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2909, https://doi.org/10.5194/egusphere-egu21-2909, 2021.
The overall rates of multi-component reaction networks are known to be controlled by feedback mechanisms. Feedback mechanisms represent loop systems where the output of the system is conveyed back as input and the system is either accelerated or regulated (positive and negative feedback respectively). In other words, feedback mechanisms control the rate of a reaction network without external influences. Feedback mechanisms are well-studied in a variety of reaction networks (e.g. bio-chemical, atmospheric); however, in fluid-rock interaction systems they are not researched as such. Still, indirect evidence, theoretical considerations and direct observations attest to their existence [e.g. 1, 2, 3]. It remains unknown how mass and energy transport between distinct reaction sites affect the overall reaction rate and outcome through feedback mechanisms. We propose that feedback mechanisms are a missing critical ingredient to understand reaction progress and timescales of fluid-rock interactions. We apply the serpentinization of ultramafic silicates as a relatively simple reaction network to investigate feedback mechanisms during fluid-rock interactions. Recent studies show that theoretical timescale-predictions appear inconsistent with natural observations [e.g. 4, 5]. The ultramafic silicate system is ideal for investigating feedback mechanisms as it is relevant to natural processes, is reactive on timescales that can be explored in the laboratory, and natural peridotite typically consists of less than four phases. Our preliminary observations indicate a feedback between pyroxene dissolution and olivine serpentinization. Olivine serpentinization appears to proceed faster in the presence of pyroxene. Furthermore, the bulk system reaction rate increases with increasing fluid salinity, which is opposite to the salinity effect on the monomineralic olivine system. Dunite (>90% olivine) is rare, which is why it is crucial to explore the more common pyroxene-bearing systems. The salinity effect is important to investigate due to the inevitable increase in fluid salinity from the boiling-induced phase separation and OH-uptake in the formation of serpentine. Here we present preliminary textural and chemical observations, which will subsequently be used for kinetic modelling of feedback.
 Ortoleva P., Merino, E., Moore, C. & Chadam, J. (1987). American Journal of Science 287, 997-1007.
 Centrella, S., Austrheim, H., & Putnis, A. (2015). Lithos 236–237, 245–255.
 Nakatani, T. & Nakamura, M. (2016). Geochemistry, Geophysics, Geosystems 17, 3393-3419.
 Ingebritsen, S. E. & Manning, C. E. (2010). Geofluids 10, 193-205.
 Beinlich, A., John, T., Vrijmoed, J.C., Tominaga, M., Magna, T. & Podladchikov, Y.Y. (2020). Nature Geoscience 13, 307–311.
How to cite: Aarrestad, I., Plümper, O., Roerdink, D., and Beinlich, A.: Feedback mechanisms in mineral replacement networks: an experimental investigation of the ultramafic model system, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8633, https://doi.org/10.5194/egusphere-egu21-8633, 2021.
Reaction-induced fracturing may occur when dry rocks are exposed to water and undergo mineral transformations that involve a volume change. The volume increase associated with hydration reactions in rocks, like the hydration of periclase to brucite, requires a stable water film to be present at reactive grain boundaries. Recent experiments on the hydration of periclase observed that the replacement reaction slows down dramatically when the effective mean stress exceeds 30 MPa. We hypothesise that a stable fluid film is required for the brucite-forming reaction to progress, and when the applied pressure overcome the hydration force, the fluid film will collapse and be squeezed out of the grain contacts which will prevent formation of brucite. To quantify this effect, we run molecular dynamics simulations where our setup consists of two interfaces of either periclase or brucite surrounded by water, and we study the behaviour of the water film confined between two surfaces subject to compressive stress. The simulations are carried out using the ClayFF force field and the single point charge (SPC) water model in the molecular dynamics simulation program LAMMPS. Our simulations show that when the pressure reaches a few tens of MPa, the water film collapses. The process reduces the water film thickness to one or two water layers and reduces the self-diffusion coefficient of the water molecules by a factor of eight. When the water film thickness is less than two water layers, the water film thickness is smaller than the hydration shell around Mg2+-ions, which will limit the ion-transportation. The observed collapse of the water film to a single layer at a normal pressure of 25-30 MPa might explain the observed slow-down of reaction-induced fracturing in the periclase-brucite system.
How to cite: Guren, M. G., Sveinsson, H. A., Hafreager, A., Jamtveit, B., Malthe-Sørenssen, A., and Renard, F.: Molecular dynamics study of confined water in the periclase-brucite system under conditions of reaction-induced fracturing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10766, https://doi.org/10.5194/egusphere-egu21-10766, 2021.
The advancement in analytical imaging techniques, including atomic force microscopy (AFM) and scanning and transmission electron microscopies (SEM and TEM), has allowed us to observe processes occurring at mineral surfaces in situ at a nanoscale in real space and time and hence giving the possibility to elucidate reaction mechanisms. Classical crystal growth theories have been established for well over 100 years and while they can still be applied to explain crystal growth in many growth scenarios, we now know that these models are not always an accurate description of the mechanism of all crystal/mineral growth processes, especially where nanoparticle formation is observed. Consequently there is a current challenge at the forefront of understanding crystal/mineral growth mechanisms. This work describes experimental observations of non-classical crystallization processes at the nanoscale. Using AFM as well as SEM and TEM imaging, we demonstrate that many minerals commonly grow by the attachment of nanoparticles on an existing mineral surface, often resulting from the coupling of dissolution of a parent phase and the precipitation of a new product mineral. Through varied examples of crystal/mineral growth, including calcite and other carbonates, barite, brucite, and apatite, we define the importance of the mineral-fluid interface and the aqueous fluid interfacial (boundary) layer in the control of crystal growth. Whether a crystal will grow by classical monomer attachment resulting in step advancement or by the formation, aggregation and merging of nanoparticles, will be controlled by the aqueous fluid composition at the mineral-fluid interface. The processes described also allow for the development of porosity within the new mineral and hence have important consequences for fluid movement and element mobility within the Earth. Additionally an understanding of natural mineral growth has implications for geomimetic applications for the manufacture of functional engineered materials.
How to cite: Putnis, C. V., Wang, L., Ruiz-Agudo, E., Ruiz-Agudo, C., and Renard, F.: Crystallization via non-classical pathways: Nanoscale imaging of mineral surfaces , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9044, https://doi.org/10.5194/egusphere-egu21-9044, 2021.
Rodingites are metasomatic rocks, frequently found in ophiolitic complexes. They offer important information about the interaction between ultramafic-mafic rocks and metasomatizing fluids, as well as about the post-magmatic evolution of ophiolitic suites (Tsikouras et al., 2009; Hu & Santosh, 2017; Surour, 2019; Laborda-Lopez et al., 2020). Metasomatism, such as rodingitization, is a very intricate process, which depends on the mineralogy of the initial rock, the nature of the metasomatic agent, the fluid/rock ratio, the duration of metasomatism and the chemical disequilibrium at the time of metasomatism between the host rock and the metasomatic medium (Poitrasson et al., 2013). Rodingites from the Veria-Naousa and Edessa ophiolites, in Northern Greece, were geochemically analyzed and characterized by substantial overprint of primary textures. Their field observation, their neoblastic mineral assemblages and metasomatic textures reveal that they derived from ultramafic and mafic protoliths. The mineral phases in the ultramafic derived rodingites (UDR) include mainly diopside, garnet, chlorite, epidote, tremolite and Fe-Ti oxides whereas mafic derived rodingites (MDR) consist of diopside, garnet, vesuvianite, chlorite, quartz, prehnite and actinolite. The studied rodingites present δ65Cu values varying from -0.17‰ to 0.62‰ and for ultramafic and mafic parent-rocks from -0.49‰ to +0.50‰. The UDR and MDR from both ophiolites display δ66Zn range from -0.06‰ to 0.74‰ and their photoliths present a narrower range from +0.04‰ to +0.41‰. Rodingitization affects in different way UDR and MDR samples. On one hand, Cu isotope ratios are systematically heavier in rodingites compared to their respective protoliths, except for one rodingite sample that requires confirmation due to large error bar. On the other hand, Zn isotopes show enrichment in light isotopes (group 1: comprising all UDR and some MDR samples), or in heavy isotopes (group 2, only MDR samples). Intriguingly, the same protolith can lead to both group 1 and 2 rodingites, as defined here. No mineralogical or geochemical trend can be found to understand the dual behavior of Zn stable isotopes during rodingitization so far. Fe isotopes do not show any significant fractionation of δ56Fe, ranging from +0.07‰ to +0.19‰ for the rodingites and from +0.12‰ to +0.23‰ for their protoliths, indicating that Fe isotopes are highly resistant to rodingitization. Our study shows that rodingitization enriches metasomatized samples in heavy Cu isotopes and has no impact on Fe isotopes. It remains unclear why Zn isotopes can be affected both ways.
How to cite: Zaronikola, N., Debaille, V., Rogkala, A., Petrounias, P., Mathur, R., Pomonis, P., Hatzipanagiotou, K., and Tsikouras, B.: Rodingitization of mafic and ultramafic rocks in ophiolites from Northern Greece, seen by non-traditional stable isotopes, such as Cu, Fe and Zn., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2025, https://doi.org/10.5194/egusphere-egu21-2025, 2021.
Multi-stage tourmalines are widely developed in granitic gneisses and hydrothermal veins from the Laojunshan metamorphic dome, Southwest China. These tourmalines exhibit variable petrographic characteristics and microstructures by ductile deformation to brittle deformation, which offers a great opportunity to understand the fluid and structural evolution during exhumation of the Laojunshan metamorphic dome. Three types of tourmalines have been recognized, including disseminated tourmaline distributed in granitic gneisses (Tur-G), elongated and broken tourmalines in quartz veins (Tur-QV), needle-columnar and fine-grained tourmaline with micro-shear zone in tourmaline veins (Tur-TV). All the tourmalines belong to the alkali group representing dravite-schorl solid solution series. The former two types belong to schorl and the latte type contains more Mg-rich components. Models of occurrence and chemical varieties including Al-occupation at the Y-site suggest that the Tur-G type and Tur-QV type tourmalines crystallized from magmatic fluids and the Tur-TV type tourmalines are hydrothermal origin. Hydrothermal tourmalines are characterized by higher Mg/(Mg + Fe) ratios, more pronounced positive Eu anomalies, higher Li, Sr, HREE contents and lower Na/(Na + Ca) ratios, lower Nb, Zr, Hf, LREE contents compared with magmatic tourmalines. The increase of Mg/(Mg+Fe) ratios from the Tur-QV to Tur-TV type tourmalines is associated with the crystallization of Fe-rich mineral during hydrothermal stage. In the Tur-QV types, the decrease of Mg/(Mg+Fe) ratios and increase of Al and LREE contents from core to rim suggest the contamination from surrounding strata. The δ11B values of Tur-G, Tur-QV, Tur-TV type tourmalines are ranging from -13~-7.9‰, -15.5~-7.5‰, -18.6~-11.6‰ respectively, which suggests that the boron was mainly derived from granitic melt and exsolved hydrothermal fluid. Boron isotopic variations of tourmaline are mainly controlled by temperature and exsolved fluid. All the results of observations from outcrop to thin section scales and chemical analysis indicate the formation of disseminated tourmaline distributed in granitic gneisses (Tur-G) should have been associated with late stage of magma evolution before regional exhumation, while tourmalines in hydrothermal veins (Tur-QV and Tur-TV) have been formed by the magmatic-hydrothermal events during exhumation of Laojunshan metamorphic dome. The primary tourmalines experienced shearing and fracturing, and then were replaced by chlorite, potassium feldspar and epidote. The ductile-brittle deformation of tourmalines was produced by progressive strain localization accompanied by the alkaline, B-undersaturated fluids, indicating episodes of brittle fracturing, possibly as a consequence of faulting at depths and subsequent fluid flow during exhumation of the dome.
How to cite: Li, W., Cao, S., Nakamura, E., Ota, T., Kunihiro, T., and Liu, Z.: Chemical and boron isotopic variations of deformed tourmaline in the Laojunshan metamorphic dome, Southwest China: Implication for magmatic-hydrothermal evolution during exhumation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11830, https://doi.org/10.5194/egusphere-egu21-11830, 2021.
Water-rock interactions at elevated pressures and temperatures may mobilize chromium from chromite to produce a variety of Cr-silicate minerals. Common Cr-silicates include fuchsite (KCr2(AlSi3O10)(OH)2), kämmererite ((Mg5Cr)(AlSi3)O10(OH)8), tawmawite (Ca2CrAl2Si3O12(OH)), and uvarovite (Ca3Cr2Si3O12). Here we assess the geochemistry and calculate the thermodynamic properties of a variety of Cr-silicates to elucidate their formation as well as how they may contribute chromium to the environment. Chromium-silicates follow an idealized 1:1 relationship with regards to Cr(III) and octahedral Al, except for kämmererite. Kämmererite can have Al in excess of 1:1 to Cr(III), substituting into the Mg site. FTIR and Raman analyses demonstrate that Cr(III) enrichment is distinguishable between respective end member minerals. Thermodynamic properties were calculated using established estimation algorithms and unit-cell measurements. Overall, we provide an extensive assessment of Cr-silicates that addresses the formation of Cr-silicates and fate of chromium in the environment.
How to cite: Dall, J., Oze, C., Celestian, A., and Rossman, G.: Geochemistry of Chromium-Silicate Minerals, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1595, https://doi.org/10.5194/egusphere-egu21-1595, 2021.
Molecular hydrogen (H2) released during serpentinization of oceanic mantle is one of the main fuels for chemosynthetic-based deep life. Hydrogen is produced during the oxidation of ferrous to ferric iron, and the amount of H2 generated strongly depends on rock type, fluid composition, alteration temperature, and water-to-rock ratio.
Progress has been made in understanding serpentinization and related H2 production at slow-spreading mid-ocean ridges (MORs). Less attention has been paid to the hydration of mantle rocks at passive continental margins where different rock types are involved (lherzolite instead of harzburgite/dunite at MORs) and the alteration temperatures tend to be lower (<200°C vs. >200°C). To close this knowledge gap we investigated serpentinization and H2 production using drill core samples from the West Iberia margin (Ocean Drilling Program Leg 103, Hole 637A).
Lherzolitic compositions indicate that the exhumed peridotites represent sub-continental lithospheric mantle. The rocks are strongly serpentinized and mainly consist of serpentine with little magnetite and are generally brucite-free. Serpentine can be uncommonly Fe-rich, with XMg = Mg/(Mg+Fe) < 0.8, and shows distinct compositional trends towards a cronstedtite endmember. Bulk rock and silicate fraction Fe(III)/∑Fe ratios range from 0.6–0.92 and 0.58–0.8, respectively. Our data show that more than 2/3 of the ferric Fe is accounted for by Fe(III)-serpentine. Mass balance and thermodynamic calculations suggest that the initial serpentinization of the samples at temperatures of <200°C likely produced about 100–250 mmol H2 per kg rock, which is 2–3 times more than previously estimated.
These results lead us to suggest that the generation potential of H2 evolves from continental break-up to ultraslow and eventually slow MOR spreading. The observed metamorphic phase assemblages systematically vary between these different settings, which has consequences for H2 yields during serpentinization. At passive margins and ultraslow-spreading MORs, the main phase hosting Fe(III) appears to be serpentine, and H2 yields of 100–250 mmol and 50–150 mmol H2 per kg rock, respectively, may be expected at temperatures of <200°C. At slow-spreading MORs, in contrast, serpentinization of harzburgite may produce 200–350 mmol H2 per kg most of which is related to the formation of magnetite at >200°C. Within the same (low) temperature range, larger volumes of serpentinite should form at passive margins than at slow-spreading MORs, owing to lower geothermal gradients. Relative to both slow- and ultraslow-spreading MORs, serpentinization at passive margins likely produces more H2 and under conditions closer to/within the habitable zone. These sites may hence be suitable environments for hydrogenotrophic microbial life.
How to cite: Albers, E., Bach, W., Pérez-Gussinyé, M., McCammon, C., and Frederichs, T.: How much energy for life (H2) is generated by serpentinization at passive continental margins?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1469, https://doi.org/10.5194/egusphere-egu21-1469, 2021.
The Permian was a time of strong crustal extension in the area of the later-formed Alpine orogen. This involved extensional detachment faulting and the formation of metamorphic core complexes. We describe (1) an area in the Southern Alps (Valsassina, Orobic chain) where a metamorphic core complex and detachment fault have been preserved and only moderately overprinted by Alpine collisional shortening, and (2) an area in the Austroalpine (Schneeberg) where Alpine deformation and metamorphism are intense but a Permian low-angle normal fault is reconstructed from the present-day tectonometamorphic setting. In the Southern Alps case, the Grassi Detachment Fault represents a low-angle detachment capping a metamorphic core complex in the footwall which was affected by upward‐increasing, top‐to‐the‐southeast mylonitization. Two granitoid intrusions occur in the core complex, c. 289 Ma and c. 287 Ma, the older of which was syn-tectonic with respect to the extensional mylonites (Pohl, Froitzheim, et al., 2018, Tectonics). Consequently, detachment‐related mylonitic shearing took place during the Early Permian and ended at ~288 Ma, but kinematically coherent brittle faulting continued. Considering 30° anticlockwise rotation of the Southern Alps since Early Permian, the extension direction of the Grassi Detachment Fault was originally ~N‐S and the sense of transport top-South. In this area, there is no evidence of Permian strike-slip faulting but only of extension. In the Schneeberg area of the Austroalpine, a unit of Early Paleozoic metasediments with only Eoalpine (Cretaceous) garnet, the Schneeberg Complex, overlies units with two-phased (Variscan plus Eoalpine) garnet both to the North (Ötztal Complex) and to the South (Texel Complex). The basal contact of the Schneeberg Complex was active as a north-directed thrust during the Eoalpine orogeny. It reactivated a pre-existing, post-Variscan but pre-Mesozoic, i.e. Permian low-angle normal fault. This normal fault had emplaced the Schneeberg Complex with only low Variscan metamorphism (no Variscan garnet) on an amphibolite-facies metamorphic Variscan basement. The original normal fault dipped south or southeast, like the Grassi detachment in the Southern Alps. As the most deeply subducted units of the Eoalpine orogen (e.g. Koralpe, Saualpe, Pohorje) are also the ones showing the strongest Permian rift-related magmatism, we hypothesize that the Eoalpine subduction was localized in a deep Permian rift system within continental crust.
How to cite: Froitzheim, N. and Klug, L.: Permian rifting and detachment faults and their role in Alpine collisional tectonics, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16307, https://doi.org/10.5194/egusphere-egu21-16307, 2021.
Long-lived high- to ultra-high temperature (HT-UHT) terranes formed mostly during the Paleo-Proterozoic and are often associated to supercontinent cycles. Yet the detailed processes and conditions involved in their formation remain largely unresolved. Here we highlight the importance of the specific geothermal conditions necessary to form migmatitic to granulitic crusts. An analytical resolution of the heat equation highlights the interdependency of the thermal parameters controlling the crustal geotherm, i.e. the Moho temperature, when deformation occurs at thermal equilibrium. We further perform thermo-mechanical experiments mimicking an orogenic cycle, from shortening to gravitational collapse, to study the effect of deformation velocity that affects the crustal thermal equilibrium. We show that the formation of HT-UHT terranes is promoted by an elevated radiogenic heat production in the crust. Finally, the interplay between the thermal parameters and the orogenic cycle duration explain the difference in orogenic style through time and why some terranes are preferentially granulitic or migmatitic.
How to cite: Cenki-Tok, B., Rey, P. F., and Arcay, D.: How high to ultra-high temperature terranes form, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1750, https://doi.org/10.5194/egusphere-egu21-1750, 2021.
The generation of melt during exhumation of UHP–HP metamorphic rocks is an important variable in our full understanding of the fluid–melt-fluid evolution during the subduction cycle and the exhumation mechanism of deeply subducted continental crust (Wang et al., 2020; Sizova et al., 2012). It is generally believed that the partial melting of deeply subducted eclogite is controlled by the mineral assemblage, particularly the presence of any hydrous minerals, and the metamorphic P–T path. Here we report results from the Sulu belt, which was formed by the deep subduction of the passive margin of the Yangtze Craton under the North China Craton, with exhumation occurring during the Mesozoic (240–210 Ma). Recent studies in this belt have shown that phengite-bearing UHP eclogites can develop a solute-rich supercritical fluid or melt along grain boundaries by dehydroxylation of nominally anhydrous minerals during the early stage of decompression and/or trigger partial melting by breakdown of phengite and/or omphacite during the later stage of exhumation (Wang et al., 2014; Wang et al., 2020; Feng et al., 2021). However, the capacity of bimineralic eclogite to melt remains enigmatic due to the anhydrous mineral assemblage, indicating a low primary bulk water content, and the absence of studies reporting evidence of melting.
To determine whether bimineralic eclogite can produce melt during exhumation we undertook a comprehensive study, including petrology, microstructure and geochronology, on a bimineralic eclogite boudin within gneiss from a locality in the northeast of the Sulu belt. The margin of the eclogite boudin is extensively retrogressed, whereas the core is well preserved with distinctive garnet-rich and pyroxene-rich layers. The Ca-rich clinopyroxene in the boudin core exhibits abundant exsolved quartz formed during exhumation. In the pyroxene-rich layers micrometer-scale intergranular pockets composed of euhedral Ca-rich hornblende and Ca-rich plagioclase, and accessory barite and apatite, are interpreted as leucosome. Comparing the calculated bulk composition of the leucosome pockets, which is diorite, with the clinopyroxene, garnet and accessory mineral compositions from the host, suggests that the melt formation is dominated by the breakdown of clinopyroxene rather than garnet or the accessory minerals, based on the trace element characteristics. Symplectitic intergrowths of hornblende and plagioclase occur along boundaries between the garnet-rich and pyroxene-rich layers and extend into both.
LA-ICP–MS analysis of metamorphic zircon from the eclogite with leucosome pockets yields an age range of 230–210 Ma. Ti-in-zircon thermometry yields a wide range of temperatures from 800 to 500°C. By contrast, temperatures calculated from the rock-forming minerals yield 890–830°C (Grt–Cpx thermometry at 3 GPa), 880–820°C (Amp–Pl (from leucosome pocket) thermometry at 1 GPa), and 700–650°C (Amp–Pl (from symplectite) thermometry at 1.0–0.5 GPa). Overall, we interpret the partial melting of the bimineralic eclogite in the northeastern Sulu belt to record breakdown of clinopyroxene during decompression from UHP–HP metamorphic conditions. This represents the first detailed micro-scale study of in situ melting of UHP bimineralic eclogite.
How to cite: Wang, Z., Wang, L., Brown, M., and Johnson, T.: Partial Melting of Bimineralic Eclogite by Clinopyroxene Breakdown , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14165, https://doi.org/10.5194/egusphere-egu21-14165, 2021.
Intracrystalline diffusion is an efficient mechanism in high-grade rocks. Therefore, growth zoning in garnet is erased and the evidence for prograde path is lost. However, information recorded by the textures may store significant clues for deciphering part of the P-T path. An example is provided here from the migmatitic paragneisses from the Mont Mary nappe (Western Alps).
The latter is made of a pre-Alpine basement consisting of an upper and a lower unit. The upper unit is made of paragneisses, marbles and amphibolites similar to those of the Valpelline Unit and of the Ivrea Zone. The lower unit displays granitic orthogneisses, paraschists (with muscovite, biotite, garnet with local occurrences of staurolite, kyanite and andalusite) (Dal Piaz et al. 2015). In this unit, we discovered a hectometre-sized volume with no Alpine overprint, preserving migmatitic paragneisses, the topic of this study.
The paragneisses display quartzo-feldspathic leucocratic layers interpreted as crystallized melts. The leucosomes are separated by biotite- and sillimanite-rich layers, with conspicuous garnet porphyroblasts. In addition, fresh cordierite crystals are found in these layers. Sillimanite included in garnet rims has the same orientation than the one in the matrix. There, the foliation is defined by the shape fabric of biotite and sillimanite, wrapping both garnet and cordierite crystals.
Such textures may be used to propose a P-T path. A sequence of prograde reactions, including dehydration-melting of muscovite, then biotite, result in the production of a large amount of sillimanite. Garnet growth was continuing during incongruent melting. However, intracrystalline diffusion has erased the prograde chemical zoning, as well as the distribution and shape of mineral inclusions. The late replacement of garnet and cordierite by biotite and sillimanite indicates near-isobaric cooling, also recorded by chemical zoning along garnet rims.
Chemical data on coexisting minerals will be used to provide quantitative constraints on the P-T path. In addition, preliminary geochronological data suggest that detrital zircons grains were significantly reset during the HT metamorphism, which could have taken place c. 270 Ma ago. To conclude, the studied paragneisses offer another example of Permian near-isobaric cooling in the middle crust of the Adriatic plate.
Dal Piaz G.V., Bistacchi A., Gianotti F., Monopoli B., Passeri L., Schiavo A. & collaboratori (2015) – Note illustrative della carta Geologica d’Italia alla scala 1:50.000. Foglio 070, Monte Cervino. ISPRA, Servizio Geologico d’Italia, 070, 1-431.
How to cite: Ballèvre, M., Poujol, M., Rousseau, S., and Manzotti, P.: Documenting isobaric cooling in the lower crust using cordierite breakdown textures (Mont Mary nappe, Western Alps), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7433, https://doi.org/10.5194/egusphere-egu21-7433, 2021.
The basement units of the Alps offer an excellent example to study how the Palaeozoic continental crust was recycled during the Alpine orogeny. The reconstruction of the pre-Alpine evolution of the continental basement is challenging and mainly relies on information provided by low-strain volumes, where mineralogical relics and isotopic data on accessory minerals can be safely investigated.
The knowledge of the pre-Alpine history of the Palaeozoic basement places severe constraints on its behaviour during the Alpine continental subduction. Firstly, its location in the (lower, middle, or upper) crust has implications for material balance during the Alpine orogeny. Secondly, its mineral content will determine how much water is needed for its transformation into an equilibrium eclogite-facies assemblage, with major implications for its metastability, hence its density and rheology during the Alpine history.
Here we investigate the pre-Alpine continental basement of the Dora Maira Massif (Western Alps), worldwide renowned for its Alpine quartz- to coesite-eclogite facies metamorphism. However, little is known about its pre-Alpine history. Spectacular polycyclic garnet-staurolite micaschists associated with garnet-biotite orthogneisses represent exceptional witnesses for reconstructing the Palaeozoic evolution of this region. Both lithologies contain mineralogical relics, such as a first generations of garnet, staurolite, muscovite and biotite indicative of a regional pre-Alpine amphibolite-facies metamorphism. Thermodynamic modelling on the micaschists constrains this pre-Alpine metamorphism at 640-660 °C, 6-7 kbar. Detrital zircon geochronology indicates that the youngest age population in the micaschists ranges from 450 Ma to 600 Ma and represents the maximal depositional age for the Palaeozoic sediment. U-Pb zircon geochronology in the garnet-biotite orthogneisses points to crystallization of the magma in the earliest Silurian (442 ± 2 Ma).
Detrital zircons in the micaschists display metamorphic overgrowths, characterized by high U content and very low Th/U ratios, as reported previously in amphibolite to granulite facies rocks. These metamorphic overgrowths yield U-Pb ages of 303 ± 2 Ma. These data constrain the timing of the Barrovian metamorphism in the Dora Maira Massif and confirm the hypothesis of a genetic link between this metamorphic episode and the Variscan orogeny.
The eclogite-facies polycyclic rocks from the Dora-Maira Massif therefore derive from upper crustal late Carboniferous lithologies, similar to those found in the Gran Paradiso and Monte Rosa, but different from the granulite-facies, lower crustal, rocks found in the Sesia Zone.
How to cite: Nosenzo, F., Manzotti, P., Poujol, M., Ballèvre, M., and Langlade, J.: Characterization of P-T conditions and age of the mid-crustal material involved in the Alpine continental subduction (Dora Maira Massif), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1827, https://doi.org/10.5194/egusphere-egu21-1827, 2021.
Heat transfer during and after the emplacement of tectonic nappes within an orogeny is controlled by three fundamental processes: advection, diffusion and production of heat. Production is mainly caused by radioactive decay and shear heating. The relative importance and timing of these processes is often contentious. For example, in the Lepontine Dome of the Central European Alps the timing of the thermal evolution and the relative importance of advection, diffusion and shear heating is disputed. To better constrain and understand heat transfer in the Lepontine Dome, we apply a combined approach of petrological and structural analysis, zircon dating of migmatites and theoretical modelling.
We use data from an almost vertical transect (in the Ticino’s valleys) cutting from bottom-to-top the Simano, Cima Lunga and Maggia gneissic nappes. These nappes show an extremely pervasive mineral and stretching lineation (NW-SE directed) indicating non-coaxial deformation during shearing at amphibolite facies metamorphic conditions. The transition from the Simano to the Cima-Lunga nappe is marked by a progressive change in the texture of gneisses, in which the porphyroblasts become more stretched from the bottom to the top. Locally, at the tectonic contacts, syn-tectonic migmatites have been found. Their leucosomes contain metamorphic zircons with ages spreading from 40 to 31 Ma (U-Pb dating).
The widespread paragneisses frequently contain garnets of different sizes and internal microstructure. Published and own petrological data of these garnet-bearing rocks attest an inverted metamorphic gradient from ca. 700°C to 650-600 °C at intermediate pressures below the Cima Lunga unit during the peak-T amphibolite facies condition.
Overall, the field data depict a major km-scale shear zone that generated an inverted metamorphic gradient during the peak-T amphibolite facies condition between 40 and 31 Ma. These results hint that fast advection of heat or shear heating (or both component contempraneously) contributed to imprint the regional amphibolite facies metamorphism during nappe emplacement.
To take another step towards unravelling the controlling heat transfer processes in the Lepontine Dome and to test the relative importance of production, diffusion and advection, we employ three theoretical approaches with increasing complexity. First, we perform a dimensional analysis estimating dimensionless numbers, such as Peclet and Brinkman, for a range of reasonable parameters for the Lepontine Dome. Second, we apply numerical 2D thermo-kinematic simulations of trishear thrust-ramp evolution to test, for example, the impact of temperature-dependent viscosity and the geometrical relationship between temperature isogrades and nappe boundaries. Third, we apply state-of-the-art numerical 2D thermo-mechanical simulations of subduction and collision to investigate heat transfer and the resulting metamorphic facies distribution during the formation of an orogenic wedge.
Finally, we combine our modelling results with the available structural, age and metamorphic results to discuss potential scenarios for the heat transfer through the Lepontine dome.
How to cite: Tagliaferri, A., Schmalholz, S. M., and Schenker, F. L.: Investigating heat transfer through the Lepontine Dome (Central European Alps) with a combined petrological, structural, dating and modelling approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1974, https://doi.org/10.5194/egusphere-egu21-1974, 2021.
Metapelitic rocks of the Irumide Domain in central Malawi contain detrital and metamorphic zircons. U-Pb zircon geochronology yielded two age populations, which have been dated at c. 1995 Ma and 1050 Ma. The ages demonstrate that the precursor sediments to these rocks were derived from erosion of the Palaeoproterozoic Ubendian Domain, which is adjacent to the north, and at a later stage were affected by the Irumide orogeny. The metapelitic rocks are characterised by garnet + sillimanite + biotite ± muscovite ± K-feldspar mineral assemblages. Phase equilibria modelling shows that they equilibrated under pressure-temperature conditions of about 7 kbar and 700–740˚C. In combination with the metamorphic ages this is interpreted to record late Mesoproterozoic (c. 1050 Ma) accretion of a juvenile island arc, the South Irumide Domain, to the southern margin of the Tanzania-Congo Craton.
How to cite: Böhme, S. C. and Boger, S. D.: New insights into the age and metamorphic evolution of the Irumide orogeny in Malawi, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12571, https://doi.org/10.5194/egusphere-egu21-12571, 2021.
Our refined ability to estimate metamorphic conditions incurred by rocks has increased our understanding of the dynamic earth. Calculating pressure (P), temperature (T) and time (t) histories of these rocks is vital for reconstructing tectonic movements within subduction zones. However, large disparities in peak P within a structurally coherent tectonic unit poses difficulties when attempting to resolve a tectono-metamorphic history, if a depth dependant lithostatic P is assumed. However, what is clear is that pressure, or mean stress, in a rock cannot exactly be lithostatic during an orogeny due to differential stress, required to drive rock deformation or to balance lateral variations in gravitational potential energy. Deviations from lithostatic P is commonly termed tectonic pressure, and both its magnitude and impact on metamorphic reactions in disputed.
For the ‘Queen of the Alps’ (the Monte Rosa massif), estimates for the maximum P recorded during Alpine orogenesis remain enigmatic. Large disparities in published estimates for peak P exist, ranging between 1.2 and 2.7 GPa. Moreover, the highest P estimates (2.2 - 2.7 GPa) are for rocks that comprise only a small percentage (< 1%) of the total volume of the nappe (whiteschist bodies and eclogitic mafic boudins). We present newly discovered whiteschist lithologies that persistently exhibit higher P conditions (c. 2.2 GPa) compared to metagranitic and metapelitic lithologies (c. 1.4 - 1.6 GPa). Detailed mapping and structural analysis in these regions lack evidence for tectonic mixing. Therefore, we suggest that a ΔP 0.6 ± 0.2 GPa during peak Alpine metamorphism could potentially represent tectonic pressure. Furthermore, we outline possible mechanisms that facilitate ΔP, namely mechanically- and/or reaction-induced. We present data from numerical models that exhibit significant ΔP (c. 0.4 GPa) during a transient period of high differential stress prior to buckling and subsequent exhumation of viscous fold nappes, similar to exhumation mechanisms suggested for the Monte Rosa nappe. As well as this, we present new routines for calculating metamorphic facies distribution within numerical models of subduction zones that agree with natural distributions within orogens.
The maximum burial depth of the Monte Rosa unit was likely significantly less than 80 km (based on the lithostatic pressure assumption and minor volumes of whiteschist at c. 2.2 GPa). Rather, the maximum burial depth of the Monte Rosa unit was presumably equal to or less than c. 60 km, estimated from pressures of 1.4 - 1.6 GPa recorded frequently in metagranite and metapelitic lithologies. In order to understanding, more completely, a rocks metamorphic history, consideration of the interplay between tectonic and metamorphic processes should not be overlooked.
How to cite: Vaughan Hammon, J. D., Luisier, C., Candioti, L. G., Schmalholz, S. M., and Baumgartner, L. P.: Deciphering the coupling between tectonic and metamorphic processes in the Monte Rosa nappe (Western Alps), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11159, https://doi.org/10.5194/egusphere-egu21-11159, 2021.
The eclogites from Vårdalneset, Western Gneiss Region, Norway, show an exceptional large variety of reaction and deformation microfabrics that document the processes and conditions during burial and exhumation. Coarse grained eclogites comprise about 35% omphacite, 25% garnet and 20% amphibole with various amounts of white mica, zoisite, kyanite, rutile, zircon and pyrite. Their fabric is characterized by few mm long and several hundred µm wide amphibole and omphacite grains aligned in the foliation plane with zoned garnet porphyroblasts up to several mm in diameter. In contrast, finer-grained mylonitic eclogites with grain diameters of few hundred µm comprise systematically higher amounts of garnet (45%) and omphacite (35%) and generally less amphibole (< 5%) but similar amounts of zoisite, white mica, rutile and quartz. In the coarse-grained eclogite, amphibole shows evidence of dislocation creep as indicated by undulatory extinction, subgrains and recrystallized grains in necks of boudinaged coarse amphibole layers as well as in contact to garnet. The large garnet porphyroblasts generally show a complex zonation with an inclusion-rich Fe-poor and Mg-rich inner core surrounded by a zone with Fe- and Ca-rich patches and a broad Mg-rich, Ca- and Fe-poor rim. Only at contact to coarse amphibole an additional, a few tens of µm thin serrated rim further enriched in Mg can occur. At the direct contact to such serrated Mg-rich rims, amphibole is partly replaced by a fine-grained quartz-kyanite ± rutile aggregate, indicating dehydration reactions of amphibole. Quartz - kyanite ± rutile aggregates are surrounding garnet also in contact to omphacite, zoisite and to other garnet crystals. The microstructures suggest that deformation and dehydration of amphibole are coupled and played an important role during deformation of the eclogites finally leading to the mylonitic eclogites with higher amounts of garnet and omphacite. Deformation is suggested to have triggered the dehydration reaction by a slight and local increase in temperature. Furthermore, deformation provided additional pathways for the escaping fluids along the increased grain and phase boundary area, as indicated by commonly present quartz within interstitials between recrystallized amphibole grains. In all samples, few µm wide amphibole rims replacing garnets document restricted rehydration-reactions at a later stage. The large variety of the deformation and reaction microfabrics exemplarily show that both deformation and metamorphic reactions did not proceed at long-term continuous conditions, but that both are coupled and occurred episodically.
How to cite: Trepmann, C. A., Engvik, A. K., and Prince Gutierrez, E. G.: Deformation and reaction fabrics in eclogites from the Western Gneiss Region (Norway) - evidence of dehydration reactions attributed to episodic deformation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2303, https://doi.org/10.5194/egusphere-egu21-2303, 2021.
The interaction between ascending carbonic fluids and rocks at shallow depths in orogenic systems plays an important role in carbon flux regulation. In subduction zones, most works have focused on processes related to carbon release from the subducting slab or sequestration via high-pressure (HP) carbonation of mafic or ultramafic lithologies. A significant fraction of the carbonic fluids released by deep metamorphic reactions can also reach orogenic complexes and react with crustal and exhumed metamorphic rocks. However, the amount of fluid-mediated carbonation that may take place at crustal depths in orogenic complexes is still poorly constrained.
We report the occurrence of retrograde mafic eclogites and metasomatic marbles in UHP units in the Chinese Tianshan orogenic belt. The mafic eclogites recorded two successive, superimposed metamorphic–metasomatic stages: graphite precipitation along fractures and veins at eclogite facies (Stage#1) and pervasive rock carbonation (i.e., Stage#2: silicate dissolution and carbonate precipitation) at retrograde amphibolite to greenschist facies. This work focuses on Stage#2 carbonation, which consists of the transformation of Stage#1 graphite-bearing eclogites into carbonate + paragonite (± zoisite) + quartz. We present field, microstructural, petrological, and geochemical results of carbonic fluid–rock interactions affecting exhumed mafic eclogites. These results are supported by thermodynamic modeling for low-pressure carbonation of mafic eclogite obtained by means of EQ3/6 and the Deep Earth Water model. Carbon and oxygen isotopic data and thermodynamic modeling suggest an external metasedimentary source for the Stage#2 carbonation. This deep carbon sequestration event can be referred to retrograde, greenschist-facies conditions at about 10 kbar and 450 °C, and redox conditions similar or more oxidized than the quartz–fayalite–magnetite (QFM) buffer. Our findings provide new insights into the reactivity of metastable, exhumed metamafic rocks with ascending carbonic fluids in orogenic systems. We conclude that retrograde, fluid-mediated rock carbonation can significantly impact on carbon fluxes from active collisional belts.
How to cite: Hu, H., Vitale Brovarone, A., Zhang, L., Piccoli, F., Peng, W., and Shen, T.: Retrograde carbon sequestration in orogenic complexes: a case study from the Chinese Southwestern Tianshan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1154, https://doi.org/10.5194/egusphere-egu21-1154, 2021.
Phase equilibrium modelling offers a welcome window onto rock-forming processes. It underpins the principles of geothermobarometry, which today is commonly carried out via pseudosection calculations in software such as THERMOCALC and Perple_X. Increasingly, phase equilibrium modelling is combined with complementary approaches such as diffusion or geodynamical calculations, in order to simulate Earth processes.
However, as anyone with experience of pseudosection calculations will know, it is not always easy to make sense of a rock through phase equilibrium modelling. Problems may relate to: (1) in what way the assumption of thermodynamic equilibrium may, or may not, be applied; (2) uncertainties in compositional analysis; and (3) uncertainties in the composition-dependent equations of state (x-eos). The x-eos are the building blocks of the modelling – one x-eos is needed to represent each of the mineral and fluid phases in the calculation.
Of the problems listed above, (3) is the most opaque for the user. In this talk I will discuss the uncertainties associated with the x-eos, and the implications of those uncertainties for thermobarometry and the simulation of Earth processes. I will describe two tools, currently in development, for investigating x-eos-derived uncertainty in thermobarometry.
How to cite: Green, E. and Powell, R.: Model-derived uncertainties in the calculation of geological phase equilibria, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1734, https://doi.org/10.5194/egusphere-egu21-1734, 2021.
Volume changes during metamorphic reactions are key contributors to the physical changes of crystalline rocks. Assessing dehydration or hydration reactions in terms of conjugate V–T pseudosections provides indicators of transient departures in hydrostatic pressure and their impact on observed mineral equilibria. The expansion in volume of major dehydration events such as the breakdown of lawsonite or chlorite delineate zones of fluid overpressure that generate connectivity via fracturing. Net compressional reactions represent sinks for fluid consumption and the focussing of strain. The capacity of metamorphic rocks to generate or consume fluid along portions of the P–T–V path exerts a fundamental control on the distribution of stresses in the crust and the observed mineral assemblages. Coupling a phase equilibria approach to mechanical modelling provides a quantitative framework to assess these changes in fluid pressure that can be compared to prominent case studies in rocks from New Caledonia and New Zealand.
How to cite: Chapman, T., Clarke, G., Milan, L., and Vry, J.: The volume conjugate in progressive metamorphism, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-639, https://doi.org/10.5194/egusphere-egu21-639, 2021.
The development of activity-composition models for melt in phase equilibria modelling has enabled the study of crustal differentiation processes through partial melting. A number of minor and trace elements are not accommodated in the melt models relevant to aluminous sediments, but are of considerable petrological importance (e.g. Zr, P, Ti). In this study, a new methodology is presented for handling minor and trace components that are currently unable to be thermodynamically constrained in supersolidus conditions. A new feature is built into the thermodynamic modelling software Rcrust (Mayne, Moyen, Stevens, et al., 2016), called Component Packet, that can manipulate chemical components that are not accommodated within the activity composition models. Using this functionality, any such element of interest can be modelled for each PT point in Rcrust and partitioned between specified phases. In this study, the usefulness of this new functionality has been demonstrated using the behaviour of Ti. Ti is critical in stabilizing biotite at high temperature. Thus, the lack of Ti in some solution models for melt in aluminous systems, result in a reduced stability of biotite in magma modelled as having fractionated from the residuum. In order to overcome this, a component packet is employed to investigate the proportion of Titanium that would be partitioned to melt during the anatexis of an average amphibolite-facies metapelite. In this scenario, Titanium contents in melt are estimated by linear regression to titanium versus maficity in a global compilation of S-type granites generated for this purpose. The results are compared to that of an internally consistent thermodynamic model, which does include TiO2. The linear regression method produces trends that agree with the internally consistent model under certain conditions and produces TiO2 contents for “melt” that are within the lower range of S-type granites and matches the correlation of TiO2 vs FeO+MgO of S-type granites, indicating that it is built on a strong relation. Melts are extracted once 7 vol% is accumulated, with 1 vol% retained in the residuum. The phase assemblages in extracted melts were investigated through cooling at 3 kbar and 650°C. The presence of Titanium in melt via Component packet results in a biotite mode of up to 2.5 wt% greater than melts formed without TiO2 at high temperatures, but on average less than 1 wt%. In addition, extracted melts with Ti from Component Packet allow ilmenite to be a liquidus phase. This shows that the component packet can be used to more accurately model titanium in melt, which greatly affects the stability of Ti phases at emplacement. Furthermore, the petrological applicability of the Component Packet is such that the methodology used here could be applied to the approximation of other minor components that thermodynamic models are currently unable to handle for crustal melting, such as P2O5.
Mayne, M.J., Moyen, J.F., Stevens, G. & Kaislaniemi, L. 2016. Rcrust: a tool for calculating path-dependent open system processes and application to melt loss. Journal of Metamorphic Geology. 34(7):663–682.
How to cite: Hoffman, S., Mayne, M., and Stevens, G.: A new methodology for considering minor elements of geologic importance in phase equilibria modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10651, https://doi.org/10.5194/egusphere-egu21-10651, 2021.
Petrological models based on equilibrium thermodynamics have proven critical in assessing how mineral assemblages evolve with pressure (P) and temperature (T) conditions. Still, they remain limited for the investigation and simulation of fluid-rock interaction processes in open systems. The interaction between a reacting aqueous fluid and a (already water-saturated) rock at eclogite facies conditions, for example, can have no or very limited effects on the mineral assemblage—beyond eventually triggering re-equilibration. Therefore, pervasive fluid flows that are not associated to intense metasomatism cannot be modeled using phase diagrams and often remain hardly noticeable even to experienced petrologists. Unlike major and minor elements used for thermodynamic modeling, stable isotopes (e.g. oxygen) are known to be more sensitive for recording interaction with a fluid in isotopic disequilibrium.
In order to extend the existing modeling capabilities, an integrated modeling framework was developed applicable to multi-rock open systems combining thermodynamic and oxygen isotope fractionation modeling based on internally consistent databases (Vho et al. 2019, 2020). The petrological model quantifies the effect of dehydration reactions on the bulk δ18O of a rock during prograde metamorphism and can simulate different degrees of fluid-rock interaction with the surrounding rocks. This approach, in combination with the measurement of isotopic composition in key minerals, can be used for characterizing the behavior of open vs closed systems in natural settings and quantify the degree of fluid-rock interaction. Estimation of integrated fluid fluxes across geologic units of the Western Alps then allows permeability changes to be quantified along with the metamorphic conditions under which these changes occurred. Such results open the door to the dynamic simulation of reactive fluid flows in high-pressure environmentthat are controlled by the compaction pressure of the rock matrix.
This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 850530).
- Vho, A., Lanari, P., Rubatto, D. (2019). An internally-consistent database for oxygen isotope fractionation between minerals. Journal of Petrology, 60, 2101–2129
- Vho, A., Lanari, P., Rubatto, D., Herman, J. (2020). Tracing fluid transfers in subduction zones: an integrated thermodynamic and δ18O fractionation modelling. Solid Earth, 11, 307-328
How to cite: Lanari, P., Bovay, T., Rubatto, D., Dominguez, H., Markmann, T., Riel, N., and Vho, A.: Petrogeochemical tools for simulating fluid-rock interaction processes in high-pressure metamorphic terrains, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7938, https://doi.org/10.5194/egusphere-egu21-7938, 2021.
Corona texture between olivine-plagioclase is a common phenomenon in metabasic rocks and has been reported from different geological terrane of the world. However, the documented coronal phases from these terrane show significant variation in terms of number and composition. In this study, we have tried to explore the effect of different parameters like pressure, temperature, reactant bulk composition, availability of fluid, chemical potential gradient etc. on the genesis of such distinct coronal minerals. To address this question, we have compared three coronal assemblages developed between olivine and plagioclase from published literature (Gallien et al. 2012; Banerjee et al. 2019; Adak & Dutta, 2020). These three samples represent different terrane and have distinctly separate geological evolutionary history that led in formation of the texture. The samples are – i) #CGGC, a mafic intrusive from Chotanagpur Granite Gneissic Complex, India (Adak & Dutta, 2020); ii) #GTSI, an olivine bearing mafic dyke from Granulite Terrane of South India (Banerjee et al. 2019); and iii) #VFH, a troctolitic gabbro from Valle Fértil and La Huerta range, Argentina (Gallien et al. 2012). The layers in coronae of #CGGC and #GTSI are defined by three phases of separate composition; orthopyroxene and amphibole are common, but #CGGC contains spinel and #GTSI contains magnetite. Whereas, #VFH contains four phases, clinopyroxene in addition to orthopyroxene, spinel and amphibole. Besides evaluation of reactant composition and their effect, our methodology also incorporates Schrienemaker’s analysis through P-T and chemical potential diagrams. Considering the chemistry of both the reactant and product phases we have used a simplified CMASH system and calculated μCaO–μH2O, μMgO–μH2O, μCaO–μMgO diagram along with petrogenetic grid for each sample. The results show that along with change in P-T, factors like initial composition of the reactant minerals, behaviour of the system during reaction (open/closed) and P-T-t path of evolution also play significant role in determining the products in coronae formed from the reactant olivine and plagioclase.
How to cite: Banerjee, M., Adak, V., and Dutta, U.: Identifying parameters on genesis of coronal phases at olivine-plagioclase contact: A comparison from different geological terrane, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16052, https://doi.org/10.5194/egusphere-egu21-16052, 2021.
The deep continental crust's chemical makeup is central to the debate of crustal formation, evolution, strength, and bulk composition. The impenetrable depths and pressures of the deep (roughly > 10 km) crust force geoscientists to rely on indirect sampling methods, studying medium- to high-grade metamorphic terrains and xenoliths to ascertain the composition of the middle and lower continental crust. Analyzing the deep crust in situ requires geophysical data, such as seismic velocities: Vp, Vs, and the Vp/Vs ratio. Each method provides a different perspective on deep crustal composition, but alone, neither is definitive.
To address the nonuniqueness in crust composition modeling, we use thermodynamic modeling software (i.e. Perple_X) to relate observed seismic velocities to bulk compositions and mineralogies. We present a multidisciplinary model for the composition of Earth's deep crust, using geochemical and geophysical data. Through a Monte Carlo modeling approach, we determine the best-fit geochemical model for bulk middle and lower crustal compositions. For 12 different tectonic regimes, we quantify uncertainties in crustal composition, temperature, and seismic velocity while recognizing our own scientific biases. We present a global model of deep crustal composition conclude that regional scale geological variations benefit from a higher resolution model. Overall, our model forecasts 77% of the deepest continental crust has 45 to 55 wt.% SiO2; 15% 55 to 65 wt.% SiO2; 8% may have > 65 wt.% SiO2. Of perhaps equal or greater importance, however, we present a scalable, modular program that can be altered to incorporate additional petrological and geophysical constraints, allowing geoscientists to more easily compare different scenarios for the deep crust.
How to cite: Sammon, L., McDonough, W., and Mooney, W.: Estimating compositions of the deep continental crust, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6328, https://doi.org/10.5194/egusphere-egu21-6328, 2021.
Modern quantitative phase equilibria modelling techniques utilizing internally consistent datasets and activity-composition models have been successfully applied to a number of problems in metamorphic geology from hand sample to outcrop scale. Attesting to this the term “phase equilibria” appears in 1 548 articles in the Journal of Metamorphic Geology and one third of those are within the last 10 years. These techniques traditionally proceed either through the manual solution of non-linear equations or by a more automated Gibbs free energy minimization approach. However in order for these techniques to be scaled up to deal with crustal or planetary scale problems a number of hurdles still need to be overcome.
Spatial dimensions in a crustal or planetary model are estimated by grids with modelling conducted on individual cells. This allows processes within cells to effect chemical change to partner cells and thereby approximate open or conditionally open systems. Compositional constraints to the chemical system such as oxygen fugacity are pressure and temperature dependent therefore in order to model a planet wide set of conditions oxygen fugacity buffers are enabled that are dependent on the pressures and temperature of the individual grid cells. Stratigraphic layering is introduced by automating the procedure for setting the initial composition of cells and dependence relations determine the hierarchy of compositional change induced within crustal columns. Phase manipulations such as fluid, melt or crystal addition or extraction are defined by mechanistic parameters that simulate boundary conditions for example melt accumulation thresholds, fluid porosity threshold, rheological lockup conditions etc. Since certain key chemical parameters used in identifying crustal processes such as trace element ratios cannot be traditionally modelled due to their absence from the internally consistent thermodynamic datasets new methods of component approximation are introduced following the methods of trace element partitioning and accessory phase saturation for supersolidus systems.
Finally the increased complexity and number of calculations required to scale up phase equilibria modelling systems to the crustal or planetary scale provides an increased computational challenge therefore new potential strategies are explored for the optimizing of calculation load via parallel processing.
How to cite: Mayne, M.: Adapting phase equilibria modelling to crustal and planetary scale problems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8190, https://doi.org/10.5194/egusphere-egu21-8190, 2021.
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