Chlorine (Cl) is a highly hydrophile and incompatible element which may provide insights into the transfer of elements from the slab to the surface in subduction zone settings. Bulk rocks data have shown that Cl stable isotopes (δ³⁷Cl) are effective tracers of subducted fluids influence in volcanic rocks, with δ³⁷Cl variation up to 3‰ along an arc (Barnes et al., 2009). Nevertheless, a more profound comprehension is needed to identify the specific contributions from the different slab lithologies. Recent advancements in secondary ion mass spectrometry (SIMS) enable precise determination of δ³⁷Cl values at high spatial resolution. In situ measurements of olivine-hosted melt inclusions provide a first order constraint on the δ³⁷Cl of primary magmas since these melt droplets are unaffected by near surface processes.

Chlorine isotopes measurements in melt inclusions from arc samples have revealed large variation within a single rock sample (more than 2‰), and even larger considering melt inclusions from different rock samples along a single arc (up to 5‰). The combination of these data with O and B stable isotopes or with trace elements, measured within the same melt inclusions, suggest that the intra-sample and along-arc variations are related to variable influences of different Cl sources (Bouvier et al., 2019; Bouvier et al., 2022a,b). Indeed, the lowest δ³⁷Cl values (down to -3.4‰) usually reflect the imprint of subducted sediments, whereas the highest δ³⁷Cl values (up to -3.1‰) might reflect the presence of amphibole in the mantle source. Intriguingly, when we compare δ³⁷Cl values from bulk rocks with those obtained in situ in melt inclusions from the same volcano, discrepancies occasionally emerge. These deviations cannot be ascribed solely to instrumental biases. Instead, the difference between bulk rocks and melt inclusions suggests that the latter preserve undegassed signatures which might be lost in bulk rocks. In situ measurement of δ³⁷Cl in melt inclusions can thus be very useful to: (i) better constrain the behavior of Cl and δ³⁷Cl in subduction zone settings, in particular during fluid-rock interaction within the mantle wedge and during degassing, and (ii) track the influence of crystallization/dissolution of Cl-rich minerals in the context of arc magma genesis and differentiation.

References:

Barnes et al. (2009), G3, doi:10.1029/2009GC002587; Bouvier et al. (2019), EPSL 507, doi:10.1016/j.epsl.2018.11.036; Bouvier et al (2022a), EPSL 581, doi:10.1016/j.epsl.2022.117414; Bouvier et al. (2022b), Front. Earth Sci., doi:10.3389/feart.2021.793259

How to cite: Bouvier, A.-S., Rose-Koga, E., Portnyagin, M., Nichols, A., Flemetakis, S., and John, T.: Chlorine cycle in subduction zone settings: insights from chlorine isotopes in olivine-hosted melt inclusions and bulk rocks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17921, https://doi.org/10.5194/egusphere-egu24-17921, 2024.

Magmatic CO₂ emissions can affect the atmosphere composition, thereby driving long term global climate changes. Early Cenozoic climate trends are generally associated with changes in global silicate weathering related to the India-Asia convergence and collision, whereas changes in degassing from Neo-Tethyan magmatic arcs and their likely climatic effects are largely dismissed. Here, we characterize the petrography and measure the volatile content (e.g. CO₂, H₂O, F, Cl and S) of glassy, bubble-bearing and reheated melt inclusions within quartz, feldspar and pyroxene crystals from Early Cenozoic basalts, andesites and rhyolites from Ladakh (India), Tibet and Iran. Integrating our unprecedented measurements with modeling of the Neo-Tethyan geodynamics, we quantitatively assess the history of magmatic emissions from the Neo-Tethyan arcs and their contribution to Early Cenozoic climate changes. Assessing the Neo-Tethyan magmatic forcing of Early Cenozoic climate has major implications for our understanding of global volatile cycling on geological timescales.

How to cite: Ostorero, L., Sternai, P., Esposito, R., Bouilhol, P., Müller, V., Malaspina, N., Tumiati, S., Ballato, P., Zhang, Z., Ji, W.-Q., Dai, J., and Frezzotti, M. L.: Quantification of the pre-eruptive CO2 budget of Neo-Tethyan magmas and their forcing on Early Cenozoic global climate, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-777, https://doi.org/10.5194/egusphere-egu24-777, 2024.

One of the primary locations of mafic magma production on Earth is the global mid-ocean ridge system, with basalts erupted from these ridges serving as valuable probes for assessing the compositional variability of the upper mantle and exploring the deep carbon cycle. However, directly measuring carbon contents in mid-ocean ridge basalts (MORBs) has proven challenging due to degassing during magma ascent. Early investigations indicate that some incompatible-trace-element- depleted and -enriched MORBs avoid heavy degassing, and show a narrow range of CO₂/Ba ratios, which was generally applied to reconstruct the primitive CO₂ content of global MORBs. However, increasing studies reveal significant variability in the CO₂/Ba ratios of MORBs. Here, we compiled a dataset including the geochemical compositions of MORB glasses and melt inclusions for which studies supported no significant degassing. Based on it, we constructed a supervised machine learning (ML) model capable of accurately predicting CO₂ contents in individual samples using the selected elemental contents. Applying our model to a global MORB database reveals that CO₂ contents and CO₂/Ba ratios of global MORBs are highly variable, highlighting the significance of mantle heterogeneity, which can be attributed to the interactions with deep-sourced plume, or the recycled components associated with the big subduction zone. Our findings underscore the potential of ML as a powerful tool for uncovering hidden structural patterns in complex geological data, shedding light on the intricate interplay between carbon, mantle composition, and Earth's long-term geological processes.

How to cite: Lei, T.-T., Liu, J., and Xia, Q.-K.: Machine learning for reconstructing the primary carbon contents of mid-ocean ridge basalts, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7266, https://doi.org/10.5194/egusphere-egu24-7266, 2024.

Magma-limestone interaction is thought to be an important source of carbon in volcanic arc emissions (1). To better understand the production of volatiles and their behaviour in silicate melts during magma-limestone interaction, we performed Raman and FTIR spectroscopic analysis of bubbles and glasses in the products of a time-series of high pressure-temperature experiments (2). The experiments were designed to simulate entrainment and assimilation of limestone (CaCO₃) xenoliths in mafic magma using starting materials from an iconic example of a limestone-hosted arc volcano (Mt. Merapi, Sunda arc, Indonesia) (3). The experimental conditions were T = 1200 °C and P = 0.5 GPa, with run-times ranging from t = 0 s to t = 300 s. Our shortest run-time experiment (t = 0 s) reveals formation of CO₂-rich bubbles (± C, CO, N₂, H₂, H₂O, CH₄) in and around the magma-limestone reaction site and fast diffusion of CO₃^2- and CO₂ molecules throughout the host melt (qualitatively faster than Ca diffusion). Longer run-time experiments (up to t = 300 s) show that bubbles evolved to become larger and richer in CO₂ close to the reaction site and that they grew by extracting CO₂ from the surrounding melt. Magma-limestone interaction thus rapidly mobilizes CO₃^2- andCO₂ and promotes formation of compositionally evolving CO₂-rich fluids, which could migrate along fractures, faults, or other fluid escape pathways to contribute to atmospheric fluxes of CO₂ at volcanic arcs.

References

(1) Mason E., Edmonds M., Turchyn AV (2017) Remobilization of crustal carbon may dominate volcanic arc emissions. Science 357, 290-294, doi: 10.1126/science.aan5049

(2) Deegan FM, Troll VR, Freda C, Misiti V, Chadwick JP, McLeod CL, Davidson JP (2010) Magma-carbonate interaction processes and associated CO₂ release at Merapi volcano, Indonesia: Insights from experimental petrology. Journal of Petrology 51, 1027-1051, doi:10.1093/petrology/egq010

(3) Deegan FM, Troll VR, Gertisser R, Freda C (2023) Magma-carbonate interaction at Merapi volcano. In: Gertisser R., Troll VR, Walter T, Agung Nandaka IGM, Ratdomopurbo A (Eds.) Merapi volcano: Geology, eruptive activity, and monitoring of a high-risk volcano (Volcanoes of the World Book Series). Springer Verlag, Berlin, Heidelberg, New York. Chapter 10, 291-321, doi:10.1007/978-3-031-15040-1_10

How to cite: Deegan, F., Capriolo, M., Weis, F., Callegaro, S., Troll, V., Freda, C., Misiti, V., Aradi, L., Skogby, H., Darmawan, H., and Geiger, H.: Tracking CO2 mobility during magma-limestone interaction: insights from spectroscopic analysis of experiments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7943, https://doi.org/10.5194/egusphere-egu24-7943, 2024.

The nature and the fraction of carbon that are recycled to the deep Earth via the subduction factory remain an active and controversial research frontier. While many studies have attempt to establish the budgets and distribution of inorganic carbon in the subducting slab at high pressure (HP), relatively little is known about the organic counterpart. Here, we explore the nature and diversity of solid organic compounds trapped in HP ultramafic rocks from the Monviso meta-ophiolite. We show that the eclogitic ultramafic rocks record strong variations of redox conditions (i.e., oxygen fugacity, fO₂) during subduction. Such variations influenced carbon distribution and redox state. In particular, reducing conditions associated with brucite breakdown in meta-ophicarbonate promoted the formation of an unexpected diversity of abiotic carbon-based materials, ranging from disordered carbonaceous matter and organic minerals, never described in HP environments, to nanodiamonds. These newly-formed organic compounds could subsequently be recycled in the deep mantle, with the potential to play a major role on the deep carbon cycle, therefore calling for a thorough examination of the diversity, abundance and stability of solid organic phases under deep Earth.

How to cite: Debret, B., Ménez, B., and Caurant, C.: High pressure diversity of solid organic compounds in subduction zones, a window to the deep organic carbon cycle., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8477, https://doi.org/10.5194/egusphere-egu24-8477, 2024.

Efficient transfer of S and chalcophile metals through the Earth’s crust in arc systems is paramount for the formation of large magmatic-hydrothermal ore deposits and can strongly affect the Earth’s climate. The formation of sulfide-volatile compound drops has been recognized as a potential key mechanism for such transfer but their fate during dynamic arc magmatism remains cryptic. We report evidence of compound drops preserved in the active Christiana-Santorini-Kolumbo volcanic field. The observed compound drops are micrometric sulfide blebs associated with vesicles trapped within silicate phenocrysts. The compound drops accumulate and coalesce at mafic-felsic melt interfaces where larger sulfide ovoids form. These ovoids are subsequently oxidized to magnetite during sulfide-volatile interaction. Comparison of metal concentrations between the sulfide phases and magnetite allows for determination of element mobility during oxidation. The formation and evolution of compound drops is an efficient mechanism for transferring S and chalcophile metals into shallow magmatic-hydrothermal arc systems.

How to cite: Patten, C. G. C., Hector, S., Kilias, S. P., Ulrich, M., Peillod, A., Beranoaguirre, A., Nomikou, P., Eiche, E., and Kolb, J.: Transfer of sulfur and chalcophile metals via sulfide-volatile compound drops in the Christiana-Santorini-Kolumbo volcanic field, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7404, https://doi.org/10.5194/egusphere-egu24-7404, 2024.

Magmatic-hydrothermal ore deposits form in response to metal precipitation from fluids released from crustal magma reservoirs. Following release from the magma, these fluids often go through phase separation involving the condensation of a saline liquid phase (=brine). The brine tends to stay in the deeper part of the hydrothermal system, whereas the vapor ascends to form epithermal ore deposits. Therefore, chemical contrast between these two phases may be deterministic for the spatial distribution of mineralization in magmatic-hydrothermal systems, and also affects volcanic sulfur outputs during quiescent periods. As sulfur is a key constituent for ore metal transport and precipitation, understanding its partitioning between the vapor and brine phases is critically important.

We conducted experiments to determine the effect of oxygen fugacity on the partitioning of sulfur between vapor and brine at P-T conditions relevant for the magmatic-hydrothermal transition by trapping coexisting vapor and brine phases as synthetic fluid inclusions in quartz and subsequently analyzing them for sulfur concentrations by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). The results show that at relatively low fO₂ at which sulfur is predominantly present in 2- oxidation state, sulfur shows strong preference for the vapor phase with liquid/vapor partition coefficients around 0.2 at a liquid/vapor salinity contrast above a factor of 6. However, with increasing fO₂, sulfur partitioning into the liquid phase increases corresponding to the transition of the redox state of sulfur from 2- to 4+ and 6+. In relatively alkaline fluids at the fO₂ of the MnO-Mn₃O₄ buffer, the preferential partitioning of sulfur to the liquid phase is nearly as strong as that of NaCl. Liquid condensation from SO₂-rich single-phase aqueous fluid at T = 875^oC and P = 200 MPa were observed by in situ Raman spectroscopic experiments and sulfate was identified as the dominant sulfur species in the condensed liquid. It is thus apparent that liquid condensation in oxidized magmatic hydrothermal systems is a redox process that yields sulfate-bearing brines.

A consequence of the above observations is that brines in typical porphyry Cu (-Au) ore forming systems will have the capacity to precipitate ore metal sulfides without externally derived sulfur, provided that some sulfate can be reduced to sulfide and the HCl produced during metal sulfide precipitation is consumed via fluid-rock interaction or is carried away by the vapor phase to higher levels of the system. In addition, a large fraction of the sulfur budget of the primary magmatic volatile phase in oxidized active arc volcanic systems may be stripped by brine condensation during quiescent degassing periods and may never reach the Earth’s atmosphere.

How to cite: Zajacz, Z. and Farsang, S.: Redox dependent sulfur partitioning between liquid and vapor: implications for magmatic-hydrothermal ore genesis and volcanic degassing, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13176, https://doi.org/10.5194/egusphere-egu24-13176, 2024.

The opening of sedimentary basins is often accompanied by magmatic activity, and the emplacement of magmatic sills in the sediments. Contact metamorphism at the edges of the sills releases  large amounts of volatiles (H₂O, CO₂, CH₄, and H₂S), disrupting the carbon and sulfur cycle within the basin. The volatiles then migrate toward the seafloor and may be released into the oceans and atmosphere. While the reactions of dehydration, decarbonation, and desulfurization during contact metamorphism are well understood, the interactions between the produced fluids and the rocks remain unclear, especially when the fluids remain trapped within the metamorphic aureoles. This study, based on samples collected during the IODP Expedition 385 in the Guaymas Basin, aims to quantify the fluids produced and trapped in the metasediments during the emplacement of magmatic sills. Geochemical and mineralogical characterization of sediments in contact with the sill indicates metamorphic reactions, primarily affecting silica polymorphs (opal-A and opal-CT), diagenetic sulfides (pyrite), and detrital minerals (mainly quartz, clays, and feldspars), and cracking of the organic matter. The transitions from opal-CT to metamorphic quartz and from pyrite to pyrrhotite are good markers of the metamorphic aureoles. The size and the mineralogical assemblages differ in the upper and the lower aureoles suggesting major fluid rock interactions below the sill.  Indeed in this metamorphic aureole, the precipitation of carbonate-pyroxene-pyrrhotite patches within a quartz-plagioclase matrix is a good indicator of fluid-rock interactions at about 200-400°C. These newly formed carbonate patches indicate the trapping of metamorphic fluids below the sill. Isotopic data (δ¹⁸O and δ¹³C) were used to estimate the peak temperature of metamorphism and precipitation of carbonates. Additionally, thermodynamic modeling was conducted to understand the conditions of these reactional textures in the metasediments below the sill and quantify the amount of carbon and sulfur trapped under the sill. We show that a third of the carbon released to the fluid by organic matter cracking is directly trapped under the sill as carbonates. These results indicate that a significant fraction of the carbon initially released upon emplacement of the sill is sequestered deeply and doesn't rise up to the ocean floor.

How to cite: Cheviet, A., Buatier, M., Choulet, F., Goncalves, P., Galerne, C., Riboulleau, A., Bach, W., and Vennemann, T.: Carbon and sulfur trapping during contact metamorphism: Impact on volatile transfers in sedimentary basins with magmatic activity, example of the Guaymas Basin., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9119, https://doi.org/10.5194/egusphere-egu24-9119, 2024.

A large body of work has challenged the paradigm of carbonate stability at the forearc and subarc (> 80 km) conditions in subducted slabs and revealed a variety of complex processes that play an important role in the so-called slow C cycle. Serpentinite-hosted carbonate rocks (i.e., ophicarbonates) are an important rock type for the deep C cycle because they can occur either in the slab or in the mantle wedge. The question revolves around phase stability and metamorphic reactions upon subduction that can lead to a change in carbonate phase assemblage and fluid composition. Moreover, the phase relation between carbonates, silicates, oxide and sulfide minerals in ophicarbonates can be informative about the redox conditions during prograde metamorphism.

We present a case study of ophicarbonate rocks from the Zermatt-Saas unit, Western Alps, that were subducted up to eclogite facies conditions at 2.5 GPa, 560° C. In the study area, ophicarbonates overlie a large body of partially dehydrated serpentinites. This allows us to understand whether fluids released from the serpentinites infiltrated the ophicarbonates or not, and to what extent decarbonation reactions occurred in an open or closed system. We investigated three carbonate-bearing rock types: ophicarbonates, olivine-carbonate veins, and a talc-magnesite reaction rind at the contact between ultramafic and mafic/felsic lithologies. Our petrological and geochemical investigation, as well as thermodynamic modelling, reveal that the metamorphic evolution of the ophicarbonate was in a closed system, where calcite/aragonite was replaced by metamorphic dolomite and diopside, and that this reaction is nearly CO₂ conservative, with the released fluid composition close to pure water. In situ LA-ICP-MS trace element analyses also show that carbonate in olivine-carbonate veins was most likely sourced from the ophicarbonates. Our thermodynamic modelling indicates that the talc-magnesite reaction zone was most likely formed during early exhumation between 9-13 kbar and 530-460° C, at X_CO2 between 0.007 and 0.009. Lastly, we will discuss how the silicate-oxide-sulfide redox buffering assemblage indicates that all three rock types were equilibrated at redox conditions < FMQ.

In conclusion, our study demonstrates that in the absence of external fluid infiltration, carbonates in ultramafic lithologies are stable at subduction conditions. This suggests that ophicarbonate have a potential important role in the deep, long term, carbon cycle.

How to cite: Piccoli, F., Hutter, A., and Hermann, J.: Ophicarbonates transport carbon to the deep mantle: a case study from the Zermatt-Saas ophiolite, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5032, https://doi.org/10.5194/egusphere-egu24-5032, 2024.

The continental lithosphere is an enormous reservoir for carbon, but its sequestration and release over geological time are poorly understood. Recent advances indicate that estimates of the amount of carbon released by gradual degassing from the mantle need to be revised upwards, whereas the carbon supplied by plumes may have been overestimated in the past. Variations in rock types and oxidation state may be very local, exerting strong influences on carbon storage and release mechanisms. Deep subduction of thick sedimentary packages may be prevented by diapirism, whereas thinner sequences may be subducted. Carbonates stored in the transition zone melt when they heat up, a process which is recognised by coupled stable isotope systems (e.g. Mg, Zn, Ca). There is no uniform “mantle oxygen fugacity”, and heterogeneous oxidation conditions are likely to exist, particularly at the thermal boundary layer and in the lowermost lithosphere where very local mixtures of rock types coexist. The infiltration of carbonate-rich melts from either subduction or melting of the uppermost asthenosphere leads to trapping of carbon by redox freezing or as carbonate-rich dykes in this zone. Deeply-derived, reduced melts may form additional diamond reservoirs, recognised as polycrystalline diamonds associated with websteritic silicate minerals.

Carbon is released by either edge-driven convection, which tears down sections of the thermal boundary layer and lower lithosphere so that they melt by a mixture of heating and oxidation, or by lateral advection of solids beneath rifts. Both mechanisms are concentrated at changes in lithosphere thickness and result in carbonate-rich melts, explaining the spatial association of craton edges and carbonate-rich magmatism. High-pressure experiments on individual rock types, and increasingly on reactions between rocks and melts, are fine-tuning our understanding of processes and providing unexpected results that are not seen in experiments on single rocks. Future research should concentrate on elucidating local variations and integrating these with the interpretation of geophysical signals. Global concepts such as average sediment compositions and a uniform mantle oxidation state are not appropriate models for small scale processes; an increased focus on local variations will help to refine carbon budget models.

How to cite: Jacob, D., Chen, C., and Foley, S.: Local variations in lithology, thermal conditions and redox state control mechanisms of carbon storage and release in the continental mantle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6836, https://doi.org/10.5194/egusphere-egu24-6836, 2024.

Metasomatism, a process whereby infiltrating fluids and melts rich in trace elements and volatiles interact and alter mantle rocks is a common mantle process, as reflected by the mineralogy and chemical composition of mantle-derived samples from all continents. However, the metasomatic agent itself is only rarely available as fluid/melt inclusions for direct analyses; and thus, in many cases, the nature of metasomatism remains elusive.

High-density fluid (HDF) microinclusions in diamonds provide a unique record of the compositions and nature of deep mantle carbon- and water-bearing (COH) fluids. Their high volatile content: 8-20 wt.% H₂O and 4-32 wt.% CO₂, and their enrichment in incompatible elements make them a key player in mantle metasomatism. The most common HDFs vary in compositions between four major types: silicic, rich in Si, Al, K and water; low- and high-Mg carbonatitic, rich in Ca, Mg, Fe, K and carbonate; and saline, rich in Cl, K, Na and water. Few lines of evidence indicate the relation of saline HDFs and subducted surface material: their K/Cl ratio overlaps the range of altered oceanic crust; pronounced positive Eu and Sr anomalies reflect the involvement of plagioclase during low-pressure crustal processes of protolith formation; and, low ³He/⁴He isotope ratios (2.7–4.4 Ra) further strengthen a connection with recycled surface material. In addition, recent radiogenic isotope data points to the involvement of distinct mantle sources and subducted components in the formation of varying HDF types. For example, the isotopic composition of silicic to low-Mg carbonatitic HDFs in a suite of diamonds from Canada indicate the contribution of two distinct sources within the continental lithosphere: one with relatively primitive isotopic compositions characterized by εNd of −0.2, ⁸⁷Sr/⁸⁶Sr of 0.7044 and ²⁰⁶Pb/²⁰⁴Pb of 17.52, and another with more unradiogenic εNd < −16 and radiogenic ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb > 0.713 and 18.3, respectively. The latter reflects an old metasomatic event in the continental lithosphere involving fluid addition from a subducting slab, most probably in the Paleoproterozoic. In comparison, isotopic compositions of HDFs in South African diamonds suggest that saline HDFs record the involvement of metasomatized Archaean lithosphere and subducting surface material that includes recent sediments.

HDFs are the deepest mantle fluids we have at hand. Due to their high mobility, they can migrate and react with different mantle lithologies over a range of depths. Such HDFs-rock interaction leads to the formation of new metasomatic phases and enriches depleted mantle rocks in volatile and incompatible elements, thereby impacting their density, rheology and melting behavior. Indeed, the mineralogy and chemical composition of xenoliths/xenocrysts and some alkaline magmas suggest a prevalent role of HDFs in mantle metasomatism and deep Earth processes.

How to cite: Weiss, Y.: Micro-windows to deep COH-rich planetary fluids and associated mantle processes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5041, https://doi.org/10.5194/egusphere-egu24-5041, 2024.

Water is a key component at Earth’s surface controlling geomorphological processes and establishing an environment capable of supporting life, but it is also present in the mantle as point defects in the lattice of nominally anhydrous minerals (NAMs). Even in trace amounts (i.e., ppm wt), H₂O exerts a profound influence on rock properties such as decreasing the melting temperature, which allows melting to occur at much greater depths with respect to volatile-free conditions, impacting the geochemical evolution of the deep mantle. Therefore, precisely modelling melting processes as a function of volatiles content is of great interest in petrology and requires a precise knowledge of the behavior of volatiles at mantle pressure and temperature.

A fundamental parameter used to investigate hydrous melting is the partition coefficient of H₂O (D_H2O^min/melt = C_H2O^min/C_H2O^melt), which can be studied at high-pressure high-temperature by performing laboratory experiments. A large number of studies have previously determined the D_H2O^min/melt for major mantle phases and mantle conditions in a hydrous system. However, a more comprehensive investigation of the distribution of H₂O in the mantle requires also the consideration of CO₂, which is the 2^nd most abundant volatile in the mantle and is expected to alter the activity of H₂O of the system and, consequently, the D_H2O^min/melt. Notably, the effect of CO₂ on D_H2O^min/melt at mantle conditions remains largely unconstrained today.

We present the results of a series of 2 GPa-1200°C piston cylinder experiments investigating an eclogitic system, where clinopyroxene (+garnet) crystals were equilibrated with melt. The experiments were conducted at hydrous and hydrous-carbonated conditions with different amounts of CO₂. The experimental charges were recovered, and minerals and melts were investigated by electron microprobe analyses, to determine chemical composition and Fourier Transform Infra-Red and Raman spectroscopy to determine the H₂O contents. The experiments highlight a strong effect of CO₂ content on decreasing D_H2O^min/melt. The new results are used to discuss melting processes interesting eclogitic lithologies in the deep mantle and at representative chemical compositions.

How to cite: Curtolo, A., Condamine, P., Bolfan-Casanova, N., Schiavi, F., and Novella, D.: Melting of hydrous and hydrous-carbonated eclogite in the mantle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15241, https://doi.org/10.5194/egusphere-egu24-15241, 2024.

Subduction zone fluids play a critical role in the global element cycle. During subduction and progressive heating of the subducting slab, hydrous minerals become unstable, which releases water (Schmidt and Poli 1998). The released water is the trigger for element transport within the subducting slab, along the plate interface, and in the overlying mantle wedge (Bebout 2007). Such released fluid composition has been changed due to fluid-rock interaction during the metamorphism, which has been provided by fluid inclusion behavior in high-pressure (HP) minerals in eclogite facies rock (Bayarbold et al., 2023). Moreover, element mobility along the subduction might be highly dependent on volatile compounds in the metamorphic fluid (Tanis et al. 2016). Related to the size of the trapped fluid and fluid-mineral interaction after being trapped in HP minerals, the reconstructed fluid composition from eclogite facies has not yet been properly quantified. In this study, we report the results of our investigation of fluid inclusions (FIs) in the Khungui eclogite, found in western Mongolia.

Based on the main mineral assemblages of the Khungui eclogite, it can be classified into two types: (i) dry eclogite (<15 vol.% for hydrous minerals) and (ii) wet eclogite (>45 vol.% for hydrous minerals). Numerous primary FIs are present in the omphacite (Omp) from dry eclogite. Omp in dry eclogite is clearly distinguished into two types (Omp1 and Omp2) by their textures. Omp1 occurs within large (up to 100 µm) elongated quartz crystals in the matrix. The fractured Omp1 contains mineral inclusions (amphibole and quartz) and abundant primary FIs. In contrast to Omp1, Omp2 shows an equilibrium boundary with barroisite and garnet in the matrix. Furthermore, no mineral inclusion or FIs occur in Omp2, which is partially replaced by hornblende and plagioclase, whereas Omp1 is not.  FIs in Omp1 consist of variable phases such as liquid, vapor, and several solid phases with various phase ratios. The solid phases in FI within Omp consist of halite (Hl), amphibole, and calcite. These observations reveal that (1) the peak pressure-temperature (P-T) of dry eclogite is higher than the eclogite facies P-T condition of wet eclogite (2.1–2.2 GPa, 580–610°C); (2) high-saline and CO₂ saturated fluid interacted with rock at eclogite facies; and (3) Hl in primary FIs in Omp suggests that the salinity of fluid might be up to 26.3 wt.% NaCl equivalent.

How to cite: Bayarbold, M., Okamoto, A., Uno, M., Kotov, A., Agroli, G., and Tsuchiya, N.: Multiphase inclusions in HP rocks (Western Mongolia): Remnants of high saline–CO2 fluids released during deep subduction, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13634, https://doi.org/10.5194/egusphere-egu24-13634, 2024.

Fluid-induced metasomatic changes and origin of patchy charnockite via CO₂ influxed dehydration of biotite and amphibole is frequently encountered in different high-grade terrains. The current study reports fluid-mediated transition of garnet-bearing quartzofeldspathic gneiss (GG) to orthopyroxene-bearing quartzofeldspathic gneisses (charnockitic gneisses; CG) from parts of Eastern Ghats mobile belt. Field study reveal that irregular patches of charnockitic gneisses have developed within a foliated garnet-bearing quartzofeldspathic gneiss. The foliation in the garnet-bearing quartzofeldspathic gneiss is continuous and pervasive in the charnockitic gneiss while the contact between the two lithotypes is transitional which advocates for a fluid-induced transformation. Petrography suggest that the peak metamorphic mineralogy in the garnet-bearing quartzofeldspathic gneiss comprises garnet+ plagioclase+ alkali-feldspar (perthite) + quartz+ ilmenite. Garnet (Alm₇₆Py₁₈Gr₅Sps₁ to Alm₆₈Py₂₆Gr₅Sps₁) grains in the garnet-bearing quartzofeldspathic gneisses are dominantly anhedral to subhedral, often occur as aggregates and define the melanosomes. Small, polygonal garnet grains often show recrystallized boundaries, indicating presence of deformation. In the charnockitic gneisses, large orthopyroxene (X_Mg ~0.54 to 0.58; Al: 0.26 to 0.28 a.p.f.u.) grains have inclusions of polygonal garnet grains (Alm₆₈Py₂₈Gr₄Sps₁)along with ilmenite and quartz. Plagioclase in both the rocktypes are dominantly albitic and plagioclase in garnet-bearing quartzofeldspathic gneiss is more sodic than that of charnockitic gneiss (An₃₁Ab₆₉ in GG and An_38-40Ab_62-60 in CG). In both the rocktypes biotite develops as secondary minerals replacing both garnet and orthopyroxene.

Reaction modelling suggest the following reactions that possibly led to the development of orthopyroxene:

1.9 Garnet + 8.3 Plagioclase _GG + 5.5 Quartz + 3.1 Mg²⁺+ 1 Ca²⁺ = 4.7 Orthopyroxene + 8.3 Plagioclase _CG

The reaction predicts mobilization of Mg and Ca being essential in forming the charnockitic patches.

Conventional geothermobarometry constrain the temperature-pressure conditions of orthopyroxene formation between 800 to 850°C and 7-8 kbar. Pseudosection constructed in NCKFMASHT (Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂) system and the intersection of compositional isopleths of garnet from each rocktypes further yield a similar physical conditions corresponding to the observed mineral assemblages. High metamorphic temperatures also corroborates well with the high Al₂O₃ (4.30-5.53 wt.%) contents of the orthopyroxene. Mass balance calculations using the whole rock compositions predict extensive mobilization of Mg which corroborates the inference from the modelled reaction. T-X_Mg and P- X_Mg diagram indicate that the stability of the orthopyroxene is strongly influenced by X_Mg rather the physical conditions. T-X_H2O diagram further suggest that low H₂O (possibly high CO₂ or brine-rich) fluids favour the stabilization of orthopyroxene in the observed assemblage and at ~800°C and ~7 kbar, increasing H₂O content even up to 2.5 wt.% does not hinder the stability of the orthopyroxene suggesting that temperature and the Mg-influx were the probable parameter that triggered the transition. Low H₂O and high CO₂ fluid presumably facilitated the process.

How to cite: Mukherjee, S. and Singh, P.: Formation of patchy charnockite from garnet bearing quartzofeldspathic gneiss: evidences of fluid induced metasomatism under granulite facies conditions , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18325, https://doi.org/10.5194/egusphere-egu24-18325, 2024.

Aqueous fluids released by metamorphic dehydration reactions are key components for magmatism, seismicity, creep, and geochemical cycling in subduction zones. How these fluids drain and migrate towards the mantle wedge is not fully understood, partly because the recognition and interpretation of deep fluid pathways in the exhumed rock record is challenging. Serpentinites are among the most important H₂O carriers in subducted slabs, with dehydration occurring for example during the lizardite to antigorite transition (ca. 300 – 350 °C), brucite breakdown (ca. 500 °C) and antigorite dehydration (620 – 670 °C). Fluid production and the related formation of interconnected porosity allowing fluid migration are influenced by pre-existing chemical and mineralogical heterogeneities, as well as microstructure and porosity. In oceanic and low-grade metamorphic serpentinites such heterogeneities are very common. To investigate their effect on metamorphic dehydration and fluid migration during subduction to forearc conditions, we experimentally dehydrated natural serpentinite that contains abundant brucite formed during the prograde lizardite–antigorite transformation [1]. In the starting material, brucite occurs as veins and as intergrowths with serpentine in the matrix. We performed piston-cylinder experiments at conditions of brucite dehydration in subduction zones (520 – 570 °C; 1.5 GPa), coupled with micro-tomography (µ-CT), Raman, electron microscopy and microstructural analysis. The experimental results show the formation of olivine as (i) veinlets along the rims of brucite veins, (ii) surrounding and replacing Fe-oxides, and as (iii) tabular grains growing in the serpentinite matrix at the hot spot of the sample cylinder. All olivine types are related to newly formed porosity visible at the resolution of the µ-CT (1.2 µm voxel size). Broad-ion beam polished FE-SEM analysis of the starting material indicates that veins of brucite (± serpentine, Fe-oxides) have significantly more nano-porosity than the serpentine matrix. This observation and the formation of olivine veinlets along the previous brucite vein walls in the experiment suggest that the presence of brucite veins –formed early during shallow forearc metamorphism of serpentinite– will influence fluid production and migration pathways during brucite and antigorite dehydration at deep forearc conditions. Incidentally, our results further demonstrate that preferential dehydration occurs when an external reducing agent (in the case of the experiment H₂ most likely derived from the graphite heater) triggers the replacement of serpentine + magnetite by olivine, in line with previous experimental and natural observations [2,3]. Incipient fluid release from serpentinite is thus heterogeneous at the microscale, and will cause local fluid pressure variations that may lead to flow and ultimately drainage. This process, possibly in combination with deformation, deviatoric stress and/or external fluid flux, may favour the development of commonly mono-mineralic olivine veins, which are inferred to form by an interplay of brucite dehydration and reactive fluid flow [4,5].

[1] Menzel et al., 2018, Lithos

[2] Eberhard et al., 2023, Journal of Petrology

[3] Padrón-Navarta et al., 2023, Nature Geoscience

[4] Plümper et al., 2017, Nature Geoscience

[5] Huber et al., 2022, G-cubed

Funding: M.D.M: Junta de Andalucía (Postdoc_21_00791) and project “RUSTED”, MCIU Spain (PID2022-136471N-B-C21 & 22). LE: NWO (VI.Vidi.193.030), EXCITE (TNA-C3-2023-13).

How to cite: Menzel, M. D., Eberhard, L., Padrón-Navarta, J. A., van Melick, H., and Plümper, O.: Heterogeneous fluid release in subduction zones – evidence from experimental dehydration of brucite vein networks in serpentinite, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17686, https://doi.org/10.5194/egusphere-egu24-17686, 2024.

Boron has two stable isotopes, ¹⁰B and ¹¹B, which are strongly fractionated during geological processes. They have been widely used to trace fluids in subduction zones. The temperature-dependent equilibrium boron isotope fractionation depends on boron coordination in the B-hosting minerals and fluids. In blueschist- and eclogite-facies high-pressure metamorphic rocks, omphacite (Cpx), amphibole and white mica are the dominant hosts of B. Yet, different crystallographic mechanisms of B substitution in Cpx have been proposed with first-order implications for B isotope fractionation during slab dehydration and eclogite formation. Hence, clarification of B coordination in clinopyroxene is desired, but a direct determination of boron coordination in silicates at the trace-element level is not technically possible. In this study, we have thus determined the B coordination in omphacite, glaucophane and mica by indirect means through the investigation of the B isotope fractionation in natural rocks.

We investigated a set of six different tourmaline-bearing reaction zone rocks from the high-pressure (HP) mélange on the island of Syros formed at approximately 0.7 GPa, 430 °C. The rocks show the paragenesis tourmaline + phengite + omphacite + glaucophane in textural equilibrium, which offers the opportunity to determine equilibrium B isotope fractionation among these minerals. The proportions of trigonally and tetrahedrally coordinated B in omphacite, glaucophane and phengite was then estimated from the respective boron isotope fractionation against tourmaline. The B isotope fractionation between phengite and tourmaline is -14.7 ±0.6 ‰, and -12.4 ±0.8 ‰ between omphacite and tourmaline. B isotope composition in omphacite is 2.5 ±1.6 ‰ heavier than in phengite. No significant difference was found between glaucophane and phengite. From these results, we conclude that boron in omphacite is dominantly in tetrahedral coordination (84 ±6 % of the total B) with a minor amount of B in trigonal coordination (16 ±6 %).

How to cite: Xu, J., Marschall, H. R., and Gerdes, A.: Boron isotope fractionation during oceanic-crust dehydration in subduction zones: boron coordination in omphacite, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19863, https://doi.org/10.5194/egusphere-egu24-19863, 2024.

Aqueous fluids are essential agents of mass transport and geochemical cycling in various magmatic, metamorphic orogenic and subduction settings. Thermodynamic properties of aqueous solutes at high pressure have been modelled using several approaches: (1) electrostatic models (Sverjensky et al. 2014), (2) density extrapolations (Wohlers et al. 2011; Manning 2013), or (3) density models (Dolejš and Manning 2010; Dolejš and Salomone 2023). Critical comparison of experimental and predicted mineral solubilities in aqeuous fluids over wide range of temperatures and pressures provides the most accurate constraints on the functional performance of these approaches. This evaluation reveals that (1) electrostatic models offer greater fitting flexibility at costs of extrapolation accuracy and stability across the pressure-temperature space; (2) density models are more robust, require fewer parameters at costs of accuracy at low-temperature conditions. With increasing pressure, the solute concentrations rise and approach the critical transition to hydrous melts. Solute-solvent interaction indicate that mineral-fluid equilibria during critical approach are initially far from congruent and this pressure-temperature region strongly promotes metasomatic effects. The infinite dilution formalism is not physically not suited for reproducing these solubility trends. Instead, thermodynamic description of the critical fluid-melt transition requires consideration of: (1) competing effects of solute polymerization vs. association (hydration) in the dilute region, (2) transition from flexible liquid medium to structured aluminosilicate framework, and (3) speciation mechanism of H₂O in aluminosilicate melt. Using the model system H₂O-SiO₂ we demonstrate coupling between configurational, ideal and excess mixing effects using several molecular and structural formalisms. Use of universal thermodynamic conditions for critical point or curve effectively constrains the location of these phase diagram features and it reduces the number of independent parameters in the mixture equation of state.  In the simplest case, the solubility, melting and critical phase equilibria in the system H₂O-SiO₂ can be reproduced with a four-parameter equation of state for solute and one interaction parameter. These results reveal diverse heuristic aspects arising from classical thermodynamic constraints and indicate predictive capability and practical accuracy of these models for natural solute-rich fluids in high-pressure settings.

References:  Dolejš D., Manning C.E., 2010. Geofluids 10, 20-40.  Dolejš D., Salomone F., 2023. Proc. 17^th Bienn. SGA Meeting 1, 143-146.  Manning C.E., 2013. Rev. Mineral. Geoch. 76, 135-164.  Miron D. et al., 2017. Am. J. Sci. 317, 755-806.  Sverjensky D.A. et al., 2014. Geoch. Cosmoch. Acta 129, 125-145.  Sverjensky D.A., 2019. J. Geol. Soc. 176, 348-374.  Wohlers A. et al., 2011. Geoch. Cosmoch. Acta 75, 2924-2939.

How to cite: Dolejš, D.: High-pressure aqueous systems: experimental constraints and thermodynamic models of fluid-mineral-melt equilibria, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12887, https://doi.org/10.5194/egusphere-egu24-12887, 2024.

Large granitic batholiths developed in many mountain belts during the late stages of orogenesis. While current melting process research focuses on fluid-absent breakdown of hydrous minerals at amphibolite to granulite facies conditions, petrography, phase equilibrium modelling, and trace element evidence suggests that melt formation due to influx of fluids could be more widespread than previously thought. We investigate Crd bearing diatexites and Kfs bearing migmatitic paragneisses from two locations in the Moldanubian domain of the Bohemian Massif (Czech Republic) for signs of melt formation during water-fluxed or hydrous mineral breakdown melting. We use petrography of thin sections, mineral chemistry by EPMA and trace element compositions from LA-ICP-MS analysis. Samples from Nemojov consist of Crd + Bt + Plg + Qtz + Sill + Ilm ± Kfs and show signs of biotite breakdown to Kfs. It is assumed that pre-existing muscovite was consumed during water-fluxed melting and the subsequent peritectic reaction involves biotite, together with Sill, Plg, and Qtz to form Crd. Diffuse mesosome and leucosome boundaries and the absence of peritectic Kfs suggest water-fluxed melting. The Pohled sample has peritectic Kfs together with Plg + Bt + Ms + Qtz + Ilm ± Grt. This sample has a mm-sized foliation with a clear distinction between melanosome consisting of Plg+ Bt + Ms + Qtz + Ilm and the leucosome comprising Kfs + Qtz ± Grt, reflecting muscovite dehydration melting. Trace element data from both sites show significant differences in Rb, Ba, and Sr, which are dominantly incorporated in micas and feldspars. Variable Rb/Sr ratios can be an indicator for water fluxed melting: since Sr is mainly hosted in Plg and its involvement is stronger during water-fluxed melting, low Rb/Sr values (below 3.5) can be expected in Bt, Plg, and Kfs. This indicates muscovite involvement in Pohled and plagioclase-involvement in the Nemojov samples. Biotite hosts several elements such as LILE and metals like Sc, V, Cr, Co, Ni Nb, Ta, Sn, W. During melting, biotite was consumed via water fluxed melting via the reaction:

Bt + Plg + Sill + Qtz + H₂O --> Crd + Melt

Elements like Li and Be are redistributed mainly to Crd, but Rb, Cs, Ba, Sc, V, Cr, Co, Ni, Nb, Ta, Sn, and W were incorporated into the melt (assuming equilibrium between Ilm + Bt). Alkali feldspar and plagioclase have high Ba, Sr, and Pb concentrations, and Zn is enriched in plagioclase. All of this indicates fluid influx, where muscovite released Rb, Cs, and Sn that was taken up by the feldspars. On the other hand, fluid-fluxed plagioclase breakdown resulted in elevated Sr and Ba in biotite and alkali feldspar during melting reactions in Nemojov. Feldspar from Pohled similarly shows high Rb, Sn, and Cs values indicating the influence of muscovite breakdown.

How to cite: Alt, I., Oliveira da Costa, E., Kunz, B., Warren, C., Vroon, P. Z., and Brouwer, F. M.: Crustal fluid cycling triggers large-scale melting in Moldanubian migmatites, Bohemian Massif, Czech Republic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2162, https://doi.org/10.5194/egusphere-egu24-2162, 2024.

K₂O-poor or trondhjemite leucosome is a common feature of migmatites in the Himalayas and other orogens. The origin of K₂O-depleted leucosomes has been attributed to several melting scenarios, such as residuum left after melt extraction and magma accumulation. However, the process leading to the formation of leucosome of trondhjemite composition needs to be better understood. In the kyanite zone of the Higher Himalaya Crystallines (HHC) along the Bhagirathi valley, partial melting of metapelite resulted in the formation of two types of leucosomes: K₂O enriched granitic L-1 leucosomes formed stromatic veins of variable thickness and K₂O poor trondhjemitic L-2 leucosomes occur as patches and lenses. The L-1 leucosomes are characterized by positive and negative Eu anomaly, variable depleted REE content, and represent in-source leucosomes. The L-2 leucosomes are REE depleted, have small negative Eu anomaly (0.8-0.9), and show a clear trend for anatectic melt-residuum-protolith in major-oxide plots of compatible versus incompatible elements. The overall depletion of REE, Sc, V, Cr, Ni, Co, Ti, Th, Nb, Hf, P₂O₅, and Zr/Zr* <1 in both leucosomes is indicative for disequilibrium melting. Plagioclase composition in the L-1 leucosomes and the associated melanocratic layers show a narrow range for X_Ab (0.81-0.86). Pl composition measured for two leucosome layers in L-2 leucosomes show near-constant values between X_Ab =0.81 and X_Ab =0.85, but the melanosome shows reverse-zoned plagioclase grains with variable X_Ab (0.69-0.80). Phase equilibria modelling in the system MnNKCFMASTH (Perplex version 7.0.6, thermodynamic database: Holland and Powell (2011)) using the protolith composition correlated to L-2 leucosome shows a clockwise P-T path in the kyanite field. The evolution of melt composition modeled as a function of pressure and X(H₂O) at constant temperatures of 690, 710, and 750° C shows that X(H₂O) has little to no effect on melt composition. Melting at low pressure produces a granitic composition, and with increasing pressure at constant temperature and X(H₂O), melt composition evolves from granite to trondhjemite. The higher the temperature, the smaller the chemical effect.

We conclude that the K₂O-poor trondhjemite L-2 leucosomes were produced at incipient partial melting by disequilibrium melting of plagioclase by the reaction Mus + Pl₁ (Ab -rich) → melt + Pl₂ (Ab-poor). The melt volume is too small to form an interconnected network on which melt could escape, and thus, the melt formed patches and lenses. With increasing temperature, the melting reaction evolved to muscovite dehydration melting, producing a large melt fraction of granitic composition (L-1 leucosome). While previous studies consider K₂O poor-leucosomes as residuum melt after melt extraction or represent melt accumulation, we suggest that K₂O poor melts could be anatectic melts produced at the beginning of partial melting at high pressure in low melt fractions.

References:

Connolly, J.A., 2005. Earth and Planetary Science Letters, 236(1-2), pp.524-541.

Holland, T.J.B. and Powell, R.T.J.B., 2011.  Journal of metamorphic Geology, 16(3), pp.309-343.

How to cite: Kushwaha, A., Singh, S., Putlitz, B., and Baumgartner, L.: Origin of trondhjemite leucosome in a kyanite-grade migmatite from the Higher Himalaya Crystalline (Bhagirathi valley): Insight into leucosome forming processes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16973, https://doi.org/10.5194/egusphere-egu24-16973, 2024.

Micro-Raman spectroscopy was used to determine the inclusions in early-crystallised magmatic zircon from the Late Cretaceous (~82 Ma on zircon) acidic igneous rocks in the Slavonian Mts. (Mt. Papuk and Mt. Požeška Gora), located in the southwestern part of the Pannonian Basin (Croatia). The host rocks (granite and rhyolite) are predominantly peraluminous and alkali to calcic-alkali and were formed from high-temperature (up to 950 °C), ferroan and oxidised, dry A₂-type of magma.

The mineral inclusions detected in the zircon from granite and rhyolites are anatase, apatite, hematite, ilmenite and possibly magnetite. Although anatase is generally considered to form in low-temperature hydrothermal environments and is regarded as metastable, experimental studies in the field of materials science have shown that anatase, compared to rutile, is being stabilised in the systems under a high cooling rate.

Numerous multiphase solid inclusions contain various polymorphs of feldspars (albite, K-feldspar, kokchetavite and kumdykolite) and SiO₂ (quartz or cristobalite), hematite and phyllosilicates (kaolinite and muscovite). Such mineral association confirms the existence of an early granitic melt; therefore, this type of inclusion can be regarded as melt inclusions. Melt inclusions with such composition represent nanogranitoids, commonly found in peritectic garnets from high grade metamorphic rocks. Compared to the nanogranitoids of anatectic origin previously reported in partially melted rocks, here they represent primary inclusions of parental magma, i.e. magmatic nanogranitoids, that were protected from later equilibration with the melt or alteration by fluids. Furthermore, here we present the first finding of kokchetavite and kumdykolite in a magmatic zircon. This finding is a strong evidence that kumdykolite and kokchetavite do not require ultra-high pressure (UHP) to form and therefore should not be considered as exclusively UHP phases. Instead, together with anatase, these polymorphs are likely evidence of rapid uplift and consequent rapid cooling of hot oxidised magma generated in the deep (lower) crustal level. The rapid uplift was most likely aided by the weak zones in the continental crust, such as deep faults in a local back-arc extensional (half-)graben in the course of the Late Cretaceous regional geological event associated with the closure of the Neotethys Ocean in the area of the present-day Slavonian Mts.

How to cite: Schneider, P. and Balen, D.: Magmatic nanogranitoids in the Late Cretacous zircon from Slavonian Mts. (Croatia), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3452, https://doi.org/10.5194/egusphere-egu24-3452, 2024.