GMPV6.1

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.

[References]

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 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.

[1] Ortoleva P., Merino, E., Moore, C. & Chadam, J. (1987). American Journal of Science 287, 997-1007.

[2] Centrella, S., Austrheim, H., & Putnis, A. (2015). Lithos 236–237, 245–255.

[3] Nakatani, T. & Nakamura, M. (2016). Geochemistry, Geophysics, Geosystems 17, 3393-3419.

[4] Ingebritsen, S. E. & Manning, C. E. (2010). Geofluids 10, 193-205.

[5] 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.

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.

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.

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.

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.

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.

REFERENCES
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).

References:

• 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.