The transport of volatile elements from the exosphere to the mantle via subduction, and their long-term storage in Earth’s interior - versus re-entry into the atmosphere viamagmatism and tectonic degassing - remain poorly constrained. The lithospheric mantle (LM), at the interface between these two major reservoirs, represents a potentially important sink and source of volatiles, and its role is only starting to be systematically addressed [1]. Here, we consider the effects of the orogenic cycle on the volatile element inventory of LM that evolved in a palaeo-convergent plate boundary affected by subduction, tectonic accretion, collapse and rifting. We take the European Variscan Orogen (EVO) and subsequent development of the European Cenozoic Rift System (ECRIS) through parts of the EVO as an example, keeping in mind the evolutionary diversity of this vast terrain.
LM is initially stabilised by decompression melting, where H2O behaves like a highly incompatible element, and redox-melting will extract CO2 initially stored in refractory graphite/diamond. Sulfur extraction efficiency depends on S solubility in the melt, S content and melt fraction, and refractory residues are predicted to be very S-poor. The effect of continental subduction (and preceding oceanic subduction) on the volatile element inventory of the mantle wedge is gauged by tectonically exhumed peridotite, which suggests a net addition of COHS via introduction of carbonate, hydrous and sulphide minerals accompanied by moderate oxidation [2]. The emplacement ages of syn-/late-/post-orogenic Mg-K-rich magmas (orogenic lamprophyres and lamproites) testify to the protracted (10s Ma) subsequent remobilization of earlier-formed hydrous mantle metasomes [3], the low solidi of which facilitate melting during heating and/or decompression [4]. The effects on C and S are unclear, but ƒO2 remained mostly below sulphate stability and sulphides may have persisted in the residue. Tectonic reconstructions [5] and basalt-borne xenoliths with garnet break-down microstructures [6] concordantly point to crustal thinning and exhumation of LM by up to 30 km. This brought huge volumes of carbon-enriched garnet-facies LM to depths where decarbonation and associated mantle-CO2degassing could occur if subsequently heating ± decompressed. We estimate this mantle volume at 7.2 Mio km3 in the French EVO alone, corresponding to a total of ~24 × 106 Mt C for a C concentration ~1,000 mg/g ([2]).
During ECRIS development and minor associated LM thinning, magma affinities had shifted to OIB-like, reflecting (partial) consumption of earlier-formed volatile element-rich metasomes, leaving the LM largely below its solidus. Interaction with carbonated silica-undersaturated basalts and associated wehrlitisation of shallow LM suggests a CO2flux of 1.7±1.1 Mt yr-1 in the ECRIS [7]. Notwithstanding evidence in some EVO xenoliths for introduction of hydrous and sulphide minerals during rift-related carbonated melt metasomatism, stable isotope data are required to understand the inherited subduction- vs. rift-related mantle metasomatic origin of volatile elements in the LM.
[1] Gibson SA & McKenzie D 2023 EPSL; [2] Förster et al. 2024 EPSL; [3] Krmíček et al. 2020 JPet; [4] Prelević et al. 2024 ESR; [5] Vanderhaeghe et al. 2020 BSGF; [6] Puziewicz et al. 2025 Lithos; [7] Aulbach et al. 2020 GPL
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How to cite: Aulbach, S., Puziewicz, J., Vanderhaeghe, O., Casetta, F., and Prelević, D.: Tracking volatile elements in the lithospheric mantle through the orogenic cycle, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8812, https://doi.org/10.5194/egusphere-egu25-8812, 2025.
Fossil subduction zones are critical for studying the deep geochemical cycles of carbon (C), oxygen (O), and sulfur (S). This study focuses on graphite-sulfides-magnetite-bearing garnet clinopyroxenites from the External Ligurian region (Northern Apennines, Italy) as indicators of deep recycling processes of subducted crust. These rocks crystallized from eclogite-derived melts (P ≥ 3 GPa and 1100 °C) after undergoing prolonged recycling in the mantle. Their unique composition provides valuable insights into the redox state and partitioning of Fe3+/Fe2+ associated with carbon and sulfur during subduction and subsequent mantle processes.
Using TEM-EELS and Synchrotron micro-Mössbauer analyses, we observed significant heterogeneities in Fe3+/Fe2+ distribution and its partitioning among mineral phases. Clinopyroxenites exhibit three generations of clinopyroxenes: unexsolved crystals in garnet cores with Fe3+/ΣFe = 0.16–0.38, clinoenstatite-exsolution-bearing grains with Fe3+/ΣFe = 0.03–0.10, and Al-poorer rims devoid of Fe3+. In contrast, garnets show Fe3+/ΣFe-poor cores (0–0.03) and slightly higher ratios in the rims (0.04–0.07). These variations indicate a progressive redistribution of Fe3+ between garnets and clinopyroxenes in response to temperature decreases from 1100 to 950 °C.
Calculated oxygen fugacities (fO2) reveal notable variations. At 3 GPa, the samples range from oxidized (ΔFMQ = -1.25 to 0) to reduced (ΔFMQ = -4.2 to -1.6). At 1.5 GPa, values span from -1.2 to -0.6 to below -5, suggesting that graphite likely formed through the reduction of previously oxidized carbon phases. This redox evolution is attributed to sub-solidus decompression in a closed system, with no significant fluid or melt-rock interaction.
The findings highlight the potential of these clinopyroxenites to record the intricate interplay of redox conditions, temperature, and pressure during subduction. The results also underscore the importance of mantle recycling processes in governing the fate of carbon and sulfur in the Earth's interior. By shedding light on these processes, this study opens new perspectives on the geochemical cycles of volatile elements within the convective mantle.
How to cite: Malaspina, N., Langenhorst, F., Pollok, K., Cerantola, V., Murri, M., Longa, C., Bersani, D., and Montanini, A.: The redox state of the heterogeneous mantle: insights from deeply recycled C-S-bearing crustal materials, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9307, https://doi.org/10.5194/egusphere-egu25-9307, 2025.
Dehydration of serpentinites in subduction zones is a major process that releases water at mantle depth. Tracking the dynamics and composition of the fluid produced is a critical step in constructing robust models of subduction processes. We analysed fluid-mobile elements and oxygen isotopes at the microscale as geochemical proxies for fluid production and circulation in high-pressure serpentinites in the Erro Tobbio and Zermatt-Saas units (Western Alps).
In the Erro Tobbio ultramafic rocks, oceanic lizardite is metastable at the peak metamorphic conditions of ~2 GPa, 550–600°C, suggesting that dehydration reactions may be delayed in low strain zones of the subducted slabs. In high strain zones, lizardite is completely replaced by antigorite. The oxygen isotopic composition of antigorite (δ18O of +6 to +8 ‰) is relatively uniform compared to the large variations observed in lizardite (0 to +12 ‰), indicating homogenisation at the sample scale during prograde metamorphism. In the Zermatt-Saas serpentinites, antigorite is the only serpentine phase and its fluid mobile elements (As, Sb and B) and O isotopic compositions still preserve evidence of different conditions during oceanic serpentinisation.
In both units, metamorphic olivine is formed by the brucite + antigorite dehydration reaction and generally shows isotopic equilibrium with antigorite, with ∆18OAtg-Ol of 1.5–2.5 ‰ at 550–600 °C. In the Erro Tobbio serpentinites, metamorphic olivine has homogeneous δ18O values of +4 to +5 ‰, and shows isotopic equilibrium with antigorite, regardless of the degree of deformation. However, in the Zermatt-Saas samples, metamorphic olivine and antigorite show different degrees of equilibration depending on the texture (Ulrich et al. 2024). Olivine in structures associated with fluid flow is either (i) in isotopic equilibrium with antigorite when the fluid responsible for olivine crystallisation is internally derived, or (ii) in isotopic disequilibrium with antigorite when olivine is formed by infiltration of an externally derived fluid released from serpentinite with a different isotopic composition. The occurrence of non-equilibrated olivine only in shear bands, shear zones and olivine veins suggests that these structures act as channels for large-scale fluid mobilisation. Channelling of the fluid flow is expected because the replacement of antigorite by olivine leads to a reduction in volume and thus changes the porosity and the fluid pressure at the microscale, while deformation increases permeability. The interaction of such fluids, which have a low δ18O of 4–5 ‰, with the overlying altered metabasalts and metasediments can result in a significant lowering of the oxygen isotopic composition and can be used to trace the pathways of serpentinite-derived fluids in the subducted slab (Bovay et al. 2021, Rubatto et al. 2023). Our study shows that the combined study of structures, textures, trace elements and oxygen isotopes at the microscale allows the reconstruction of fluid production and transport in subduction zone environments.
References
Bovay T, Rubatto D, Lanari P (2021) doi:10.1007/s00410-021-01806-4
Rubatto D, Williams M, Markmann T, Hermann J, Lanari P (2023) doi:10.1007/s00410-023-02060-6
Ulrich M, Rubatto D, Hermann J, Markmann T, Bouvier A-S, Deloule E (2024) doi:10.1016/j.chemgeo.2024.121978
How to cite: Rubatto, D., Ulrich, M., Vesin, C., Hermann, J., and Scambelluri, M.: Microscale oxygen isotope tracing of fluid production and circulation in subducted serpentinites , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4200, https://doi.org/10.5194/egusphere-egu25-4200, 2025.
Volatiles, such as H2O, CO2, Cl, F, and S, play a critical role in the petrological processes that drive Earth's dynamic systems, particularly in melting, mineral stability, and mass transfer of elements. While the fluid-melt immiscibility is well-documented in lower crustal settings (mostly granulites), its occurrence and implications at ultra-high pressure (UHP) conditions remain poorly understood. Here we present the formation of microdiamonds and primary melt inclusions in metapelitic gneiss from the Heia region of the Arctic Caledonides, Norway (see Janák et al., 2024 for more details).
The migmatitic Heia gneisses comprise garnet, kyanite, biotite, white mica, K-feldspar, plagioclase, and quartz, with accessory minerals including rutile, monazite, zircon, and apatite. Two types of inclusions coexisting in the same cluster were identified in garnet porphyroblasts: Type I (multiphase fluid inclusions) and Type II (primary melt inclusions). Type I inclusions contain microdiamond, rutile, apatite, Fe-Mg carbonates, and Al-phyllosilicates (muscovite-paragonite and pyrophyllite) as solid phases; the fluid phase is dominated by residual CO₂. Melt inclusions (Type II) contain muscovite, paragonite, phlogopite, K-feldspar, plagioclase, albite, quartz and kyanite. Excluding kyanite, the mineral assemblage suggests that the trapped melt was most likely granitic and derived from partial melting of the gneiss; kyanite, based on microstructural observations, is an accidentally trapped mineral. The coexistence of diamond-bearing fluid inclusions and melt inclusions in garnet provides evidence of partial melting and fluid-melt immiscibility under UHP conditions.
The occurrence of diamonds with carbonates and pyrophyllite as an OH-bearing phase suggests its crystallization from a C-O-H fluid, saturated by carbon potentially derived from an organic compound dissolved in the fluid. The fluid was likely derived internally through the devolatilization of hydrous silicates and decomposition of organic carbon during subduction and prograde metamorphism. Fluid-melt immiscibility at UHP conditions of 4.0–4.5 GPa and 840–900°C has been identified in both Åreskutan paragneisses from the Swedish Caledonides where microdiamonds were previously documented and melt inclusions were experimentally re-homogenized (Klonowska et al., 2017; Slupski, 2023), and in Heia. These findings are some of the first global discoveries, highlighting the role of subduction in transporting volatiles from the surface to the deep Earth, with significant tectonic implications.
Janák, M., Borghini, A., Klonowska, I., Yoshida, K., Dujnič, V., Kurylo, S., Froitzheim, N., Petrík, I., & Majka, J. (2024). Metamorphism and partial melting at UHP conditions revealed by microdiamonds and melt inclusions in metapelitic gneiss from Heia, Arctic Caledonides, Norway. Journal of Petrology, 65(11), egac114.
Klonowska, I., Janák, M., Majka, J., Petrik, I., Froitzheim, N., Gee, D. G. & Sasinková, V. (2017). Microdiamond on Åreskutan confirms regional UHP metamorphism in the Seve Nappe Complex of the Scandinavian Caledonides. Journal of Metamorphic Geology, 35, 541–565.
Slupski, P. M. (2023). Former melt inclusions in garnet from UHP gneisses of the Seve Nappe Complex, Scandinavian Caledonides. PhD Thesis, Università degli Studi di Padova, Department of Geosciences, p. 113.
How to cite: Klonowska, I., Janák, M., Borghini, A., Yoshida, K., Dujnič, V., and Majka, J.: Fluid-melt immiscibility at ultra-high pressure conditions: a case study from diamond-bearing metapelites in the Scandinavian Caledonides , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16389, https://doi.org/10.5194/egusphere-egu25-16389, 2025.
The formation and evolution of the Bangong-Nujiang Tethyan Ocean played a key role in the evolution of the Tibetan plateau before the India-Asia collision. However, the timing of the Bangong-Nujiang Ocean’s subduction initiation and the resulting magmatism remain subjects of ongoing debate. In this study, we focus on new identified andesites from the Amdo area, Southern Qiangtang Terrane. Using zircon U-Pb isotopes, bulk rock geochemical data, and whole-rock Sr-Nd isotopic data, we attempt to temporally and petrogenetically constrain the magmatism associated with the subduction initiation of the Bangong-Nujiang Tethyan Ocean. LA-ICP-MS zircon U-Pb ages demonstrate that the Zhaquxiang andesites were generated during the Late Triassic (ca. 211.5 Ma). They exhibit geochemical features resembling those of arc magmatic rocks, characterized by enrichment in large ion lithophile elements and depletion in high field strength elements. The calculated εNd(t) values and initial 87Sr/86Sr ratios for the andesites are from 2.6 to 4.4 and from 0.7048 to 0.7050, respectively. The andesites have low Mg# (31-40), Cr (30-38 ppm) and Ni (21-24 ppm) contents. These geochemical characteristics suggest that the Zhaquxiang andesites were most probably produced by partial melting of mafic lower crust, with sediment-derived melt incorporated into their source. Combining our new data and the field investigation we conclude that the formation of these andesites was triggered by the northward-subducting Bangong-Nujiang Tethyan oceanic lithospheric during the Late Triassic. Consequently, the subduction initiation of the Bangong-Nuijiang Ocean would be the Late Triassic (ca. 211.5Ma).
How to cite: Hu, C., Wang, J., and Shen, L.: Resolving the subduction initiation of the Bangong–Nujiang Tethyan Ocean: New insights from the andesites in the southern margin of southern Qiangtang, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3992, https://doi.org/10.5194/egusphere-egu25-3992, 2025.
Since the late 1970s, significant efforts have been devoted to studying the phase transitions of hydrous minerals and their stability fields within the altered ultramafic portion of the subducting slab at high-T and high- to ultrahigh-P conditions. The deep subduction of hydrated ultramafic rocks in a cold thermal regime stabilizes the so-called dense hydrous magnesium silicates (DHMSs), which play a crucial role in influencing the deep-water cycle. In recent years, the role of the DHMSs as geochemical reservoirs has gained attention, particularly concerning boron (B), a key element for understanding geological processes involving serpentinized materials. For instance, the genesis of blue B-bearing diamonds in the lower mantle has been proposed as witness for the deep recycling of serpentinized materials via DHMSs (e.g., Regier et al., 2023). Investigating the geochemical behaviour of DHMSs remains a challenging task, hindering our understanding of the trace element budget transferred to depth through the cooler portions of the subducting slabs.
Here, we present the main results of a project aiming to address this knowledge gap using an experimental petrological approach that combines crystal-chemistry with in-situ geochemical investigations to unravel the potential of DHMSs to incorporate B (Cannaò et al., 2023). Using a Walker-type Multi Anvil apparatus, we synthetized high-P (olivine and humite) and ultrahigh-P (Phase-A, Mg-sursassite, Phase 11.5) phases in B-rich MSH and MASH systems. The synthetized phases were characterized for major and trace element concentrations with EMPA and LA-ICP-MS, respectively, while crystal-chemical investigations were conducted using single-crystal XRD and micro-Raman techniques. We document significant B enrichment, from hundreds to thousands of µg/g, in DHMSs, as well as in olivine and humite, suggesting that B can structurally be incorporated into these high-P and ultrahigh-P phases. These findings indicate that along cold prograde subduction paths, the destabilization of both antigorite and chlorite can transfer significant amounts of B to depth, either through olivine/humite or DHMSs. This work extends current knowledge of the B cycle and opens new perspectives to better disclose the deep recycling of elements, shedding light on the origin of the geochemical heterogeneity of the Earth’s mantle.
Cannaò, E., Milani, S., Merlini, M., Tiepolo, M., & Fumagalli, P. (2023). Phase-A as boron carrier in the Earth's interior. Lithos, 452, 107211.
Regier, M. E., Smit, K. V., Chalk, T. B., Stachel, T., Stern, R. A., Smith, E. M., Foster, G. L., Bussweiler, Y., DeBuhr, C., Burnham, A. D., Harris, J. W. & Pearson, D. G. (2023). Boron isotopes in blue diamond record seawater-derived fluids in the lower mantle. Earth and Planetary Science Letters, 602, 117923.
How to cite: Cannaò, E., Chrappan Soldavini, B., Merlini, M., Fumagalli, P., and Tiepolo, M.: Boron recycling beyond arcs: the role of DHMSs, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8810, https://doi.org/10.5194/egusphere-egu25-8810, 2025.
Heterogeneous assemblages within the lithospheric mantle represent some of the most enriched domains within the earth for alkaline, rare-earth and volatile elements. Volatile-rich mica-bearing pyroxenites are among these assemblages and were crucial in the recognition of metasomatism as a mantle process. However much of the experimental work until recently has focused on four-phase peridotites which are largely devoid of volatiles (i.e. C, H, N, S) and moderately volatile elements like fluorine unless they have been metasomatised. In domains where peridotite and pyroxenites coexist the chemical fingerprint of metasomatism is challenging to untangle, principally due to a lack of foundational phase relation studies in complex systems. Using a complex synthetic mica-pyroxenite system we will present new experimental results of melting and phase relations, and melt chemistry. Our experiments span 900 – 1400°C at pressures of 1, 2.5 and 5 GPa in a system containing carbon, water, sulfur, and fluorine, as well as 29 trace elements. Across all pressures we see the generation of a fluid phase at low temperature that acts as the precursor to a silicate or carbonated melt. The lower pressure silicate melts border the foidite field in TAS and straddle the leucitite and shoshonite divide in K2O vs. SiO2 space, while maintaining a K2O/Na2O value ranging from 0.9–3 in most experiments. The higher pressure carbonated melts from 5 GPa initially show low silica (~37wt%) that decreases to a low of ~27 wt% SiO2 as magnesite is consumed before increasing to roughly 41 wt% once major melting occurs at higher temperature. K2O/Na2O for these experiments ranges between 7 – 9 for the higher SiO2 melts, and up to 17 for the lowest SiO2 melt. The melts we've generated from our mica-pyroxenite assemblage can contribute to the explanation of a range of alkaline magmas while also having significant metasomatic potential at the point of melt generation.
How to cite: Lanati, A., Rohrbach, A., Tiraboschi, C., Berndt, J., Klemme, S., and Foley, S.: Experimentally determined melting and phase relations in a volatile bearing mica-pyroxenite system – implications for mantle metasomatism and alkaline volcanism. , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18420, https://doi.org/10.5194/egusphere-egu25-18420, 2025.
At subduction zones, mantle metasomatism induced by metamorphic fluids is known to have a noticeable control on the rheology, seismicity, volcanic activity and heat/mass transfer. Although geophysical, geochemical and petrological observations have greatly improved our knowledge about this process, significant uncertainties remain concerning the chemical composition and the extent of element redistribution within the mantle wedge. This is mainly due to the complexities of multicomponent fluid speciation and fluid migration dynamics, as well as to the uncertainties concerning thermal structure and rheological behavior of subduction zones. Herein, we incorporate forward thermodynamic modeling (Backcalc algorithm, Galvez et al. (2015), and MAGEMin software, Riel et al. (2022)) of fluid-rock chemical interactions with a 2D thermomechanical code (I2VIS, Gerya and Yuen (2003)) to present the first-order redistribution patterns of rock-forming elements.
The composition of multicomponent H2O-CO2 fluids emanated from slabs evolve with slab depth from diluted Si-Na solution to relatively complicated (alkali+Ca) aluminosilicate-rich solution. The associated alteration zones are characterized by a decrease in the phase proportions of antigorite, chlorite, and an increase of that of talc and carbonates with depth near the slab-mantle interface. Lithological boundaries with steep compositional gradients often undergo intensive fluid-mediated alterations, generating characteristic (talc-rich) metasomatites. This is because the slab-derived elements, such as C and Si, are mostly absorbed along the lithological boundaries. Ultimately, substantial carbon-poor fluids infiltration near sub-arc depths decomposes talc as slab surface temperatures approach the solidus. Talc together with antigorite and chlorite can potentially play a significant role in element circulation and mechanical properties (e.g., seismic activity) along the plate interface within subduction zones.
In summary, metasomatism induced by mobile volatile elements often results in notable petrological records wherever hydrous minerals are stable in the mantle wedge. The redistribution of non-volatile elements involving the addition of common peridotite phases(e.g. clinopyroxene) is stealthier except for the lithological boundaries. This improved petrological-thermomechanical modeling strategy provides a promising tool for studying the complex interplay among geodynamics of subduction zones, geochemical recycling of shallow planetary interior, and magmatic processes.
How to cite: Ren, J., Faccenda, M., Zhong, X., Galvez, M. E., Yang, J., and Nicolas, R.: Fluid-mediated forearc mantle metasomatism: Insights from petrological-thermomechanical modeling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14329, https://doi.org/10.5194/egusphere-egu25-14329, 2025.
The decarbonation process of subducting slab plays a key role in the carbon cycle of subduction zones. This process is influenced by several factors, including thermal structure, bulk-rock composition and thickness of lithologic layers. However, the mechanisms, flux and efficiency of subduction-related decarbonation remain widely debated. To better quantify the carbon release within subducting slab, two end-member subduction zones are selected in this study: Tonga and Colombia, which represent ultra-cold and ultra-hot oceanic subduction zone, respectively. Based on the numerical simulations using a newly developed coupled thermomechanical-metamorphism-dissolution decarbonation model in open system, combined with observational data on gas emissions and geochemical signatures from volcanic arcs, we systematically investigate the slab decarbonation processes in Tonga and Colombia subduction zones. This study reveals contrasting decarbonation patterns between Tonga and Colombia, highlighting the influence of thermal structure and bulk-rock composition on slab decarbonation.
Model results indicate that, thermal structure is first-order control on decarbonation. The cumulative decarbonation flux of hot Colombia is three times higher than that of cold Tonga. The overall decarbonation efficiency shows a greater contrast, with values of 1.3 % for Tonga and 9.7 % for Colombia. The dominant decarbonation mechanism also differs: carbonate dissolution dominates in Tonga, while metamorphic decarbonation prevails in Colombia. Additionally, the bulk-rock composition of lithologic layers also plays roles. In Tonga, CO2-poor sediments and adequate surface slab temperature facilitate efficient carbon release, whereas in Colombia, sediments are highly enriched in carbon and calcium, hindering metamorphic decarbonation, and only minor carbon is released via carbonate dissolution.
We further collected annual gas emission data to constrain the CO2 outflux from arc volcanic degassing. Volcanic CO2 outflux from Tonga arc is significantly lower than that of Colombia arc, consistent with the trend predicted by model. However, in Colombia, the predicted total carbon outflux released from slab is lower than the observed volcanic CO2 outflux. Besides, compiled geochemical data from overlying arc rocks reveals obvious signals of sediment melt contributions to mantle sources in Colombia. Based on results of experimental petrological studies, we propose that a portion of sediments in Colombia forms diapirs that ascend into the mantle wedge, generating melting-induced decarbonation. This could account for the discrepancy between the model predictions and observed volcanic CO2 outflux in Colombia.
How to cite: Zhang, H.-R., Li, Z.-H., and Wang, Y.: Contrasting Decarbonation Patterns in Ultra-Cold Tonga and Ultra-Hot Colombia Subduction Zones: Insights from Thermal-Petrological Modeling, Gas Emissions, and Geochemical Observations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17844, https://doi.org/10.5194/egusphere-egu25-17844, 2025.
In the deep Earth cycle of carbon, CO2 is thought to play a key role. The Helgeson-Kirkham-Flowers (HKF) standard state thermodynamic properties of CO2 dissolved in water are well established by experiment and equations of state at ambient conditions and at upper crustal pressures and geothermal temperatures (Shock et al., 1989). They have been widely used to compute aqueous equilibria and mineral-fluid equilibria with other carbon species in codes such as SUPCRT92. Extrapolation of the HKF standard state Gibbs free energy equation of state of CO2 to higher pressures and temperatures, widely used in the Deep Earth Water (DEW) model, has not been adequately tested. Experimentally determined mineral solubilities provide such a test. However, under deep Earth conditions, the solubilities of mineral assemblages, such as classic decarbonation equilibria involve large amounts of dissolved CO2. As a consequence, model solubilities depend on the aqueous activity coefficient of CO2 as well as its standard state free energy. Fortunately, the activity coefficients for aqueous CO2 have been measured (Aranovich and Newton, 1999). In the same study, the decarbonation equilibria give us measured solubilities of CO2 when the mole fractions of CO2 are converted to molalities. Knowledge of the experimental activity coefficients and solubilities enable a direct test of predicted standard state free energies.
For example, at 1.0 GPa and 800°C, using the hypothetical 1.0 m standard state for aqueous CO2, experimentally measured activity coefficients and solubilities in molality can be combined to give experimental activities of aqueous CO2. For two different equilibria at 1.0 GPa and 800°C, wollastonite-calcite-quartz (high CO2) and enstatite-magnesite quartz (low CO2), the experimental CO2 activities are close to two orders of magnitude lower than the values computed using the HKF equation of state The same discrepancy at 1.0 GPa and 800°C is obtained using the experimental solubility of graphite (Tumiati et al., 2017). The consistency of these three tests, all at 1.0 GPa and 800°C, requires a substantial revision to the HKF prediction of the aqueous standard state free energy of CO2. The latter becomes more positive than previously at elevated pressures and temperatures. In turn, a revised equation of state characterization of aqueous CO2 will imply less of the molecule CO2 relative to other aqueous carbon-bearing species under deep Earth conditions.
Shock, E. L., H. C. Helgeson and D. A. Sverjensky (1989). "Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Standard partial molal properties of inorganic neutral species." Geochimica et Cosmochimica Acta 53: 2157-2184.
Aranovich, L. and R. Newton (1999). "Experimental determination of CO2-H2O activity-composition relations at 600-1000 C and 6-14 kbar by reversed decarbonation and dehydration reactions." American Mineralogist 84(9): 1319-1332.
Tumiati, S., C. Tiraboschi, D. A. Sverjensky, T. Pettke, S. Recchia, P. Ulmer, F. Miozzi and S. Poli (2017). "Silicate dissolution boosts the CO2 concentrations in subduction fluids." Nature Communications 8(1): 616.
How to cite: Sverjensky, D.: Revision needed for the predicted standard free energy of aqueous CO2 at elevated temperatures and pressures, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18314, https://doi.org/10.5194/egusphere-egu25-18314, 2025.
Posters on site: Tue, 29 Apr, 14:00–15:45 | Hall X2
The metamorphic rocks from the Pohorje Mountains represent parts of the Austroalpine metamorphic units of the Eastern Alps, which are thought to experience the peak P-T in the ultrahigh-pressure zone during the Cretaceous orogeny, mainly near to or in the diamond stability field (Vrabec et al., 2012; Janák et al., 2015 and references therein). Gneisses are the most common occurring lithologies. They host garnet porphyroblasts, which contain numerous fluid and solid inclusions, among which diamonds have also been identified. To better understand their precipitation from COH fluids, thermodynamic modelling was performed and to elucidate the structure of diamond-bearing inclusions, they were investigated at the atomic scale using a plethora of TEM techniques.
The C-O-H fluid system undergoes evolution during metamorphism as a result of variations in pressure and temperature. The saturation of the C-O-H fluid with carbon depends on the P-T-fO2 conditions. Carbon saturation of the C-O-H fluid can be represented with carbon saturation isopleths. Assuming that the fluids, trapped in the metapelitic rock share common P-T path, the relationship between the carbon saturation isopleths and the P-T path is essential.
P-T diagrams were calculated at various logfO2 values to determine the threshold at which the system's behaviour changes. The modelling revealed that it is highly sensitive to the change of fO2. Due to the shape of the P-T path of metapelite, graphite precipitates in all cases up to approximately 700 °C and 2.5 GPa, where the P-T path changes its slope. Up to the fO2 value of QFM –1.5, the P-T path is parallel to the isopleths; therefore, the carbon doesn’t precipitate. When the fluid is more reduced, reaching QFM –1.6, the P-T path intersects the isopleths throughout its range. If the C-O-H fluid is even more reduced, the P-T path crosses more isopleths, resulting in even more abrupt precipitation of carbon. This means that ~ QFM –1.6 was the maximum fO2 value at which the diamonds from the Pohorje metapelites could have precipitated.
The behaviour of the COH fluids predicts dissolution of the diamonds during the retrograde metamorphism, which didn’t occur. TEM analysis of the diamond-bearing inclusion revealed the presence of an amorphous phase, which enclosed the diamonds and prevented dissolution processes. Furthermore, the amorphous phase influenced the internal structure of the diamonds, which precipitated after the amorphous solid. Electron diffraction and high-resolution TEM showed that some diamonds are polycrystalline, composed of numerous nanocrystallites. The crystallisation of metamorphic diamonds suggests complex dynamics within the microsized system, which was partially revealed by our study. However, further studies are needed to draw more precise conclusions.
Janák, M., Froitzheim, N., Yoshida, K., Sasinková, V., Nosko, M., Kobayashi, T., Hirajima, T. & Vrabec, M. Diamond in metasedimentary crustal rocks from Pohorje, Eastern Alps: A window to deep continental subduction. J. Metamorph. Geol. 33, 495–512 (2015).
Vrabec, M., Janák, M., Froitzheim, N. & De Hoog, J. C. M. Phase relations during peak metamorphism and decompression of the UHP kyanite eclogites, Pohorje Mountains (Eastern Alps, Slovenia). Lithos 144, 40–55 (2012).
How to cite: Sotelšek, T., Janák, M., Semsari Parapari, S., Šturm, S., and Vrabec, M.: Diamond precipitation from COH fluids: a case study from Pohorje, Eastern Alps, Slovenia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5255, https://doi.org/10.5194/egusphere-egu25-5255, 2025.
Subduction is an efficient process that allows the transfer of elements and interaction between the crust and the mantle. During subduction elements concentrated in the crust, such as volatiles (e.g., H2O, CO2, Cl, F, S and N2) and other incompatible elements, can be released in melts and fluids that then interact with the overlying mantle. The most direct way to investigate crust-mantle interaction is to target melts and fluids responsible for the elements transfer. In this contribution, we report the occurrence of micrometric volatiles-bearing fluid inclusions in clinopyroxene and garnet of a mantle body in Góry Sowie (SW Poland).
In Góry Sowie, migmatitic gneisses with subordinate granulites host bodies of garnet peridotites and metabasites (Kryza and Pin, 2002). The garnet clinopyroxenite hosting the inclusions is associated with other ultramafic rocks and it mainly contains garnet, clinopyroxene, amphibole, and locally orthopyroxene porphyroblasts in a fine-grained matrix.
The inclusions in clinopyroxene are randomly distributed in the inner part of the crystal, thus they are primary, and they were trapped while the host was growing in the presence of a fluid phase. The main phase assemblage in the inclusions was determined with micro-Raman spectroscopy and it includes two carbonates (dolomite and calcite), N2, cristobalite, CH4, and pyrophyllite. In garnet, the inclusions are primary/pseudosecondary (i.e., distributed along fractures occurring during garnet growth) and they contain CO2, dolomite, pyrophyllite, N2, and CH4. The presence of carbonates, OH-bearing phases, and N2 suggests that the trapped fluid is COHN; hence the garnet clinopyroxenite formed in the presence of such fluid.
Further studies will allow us to better constrain the nature of the fluid and quantify the concentration of carbon in the different C-bearing phases and N2. COH(N) fluids in subduction zones can interact with the mantle metasomatizing it and our data will help to better constrain their importance for volatiles mobilization and transfer to the mantle during subduction.
This research is part of the project No. 2021/43/P/ST10/03202 co-funded by the National Science Centre of Poland and the European Union Framework Programme for Research and Innovation Horizon 2020 under the Marie Skłodowska-Curie grant agreement No. 945339.
Kryza and Pin (2002). Mafic rocks in a deep-crustal segment of the Variscides (the Góry Sowie, SW Poland): evidence for crustal contamination in an extensional setting. Int J Earth Sci (Geol Rundsch), 91, 1017-1029.
How to cite: Borghini, A., Majka, J., Walczak, K., and Włodek, A.: COHN fluid inclusions in garnet clinopyroxenite of Góry Sowie (SW Poland), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18537, https://doi.org/10.5194/egusphere-egu25-18537, 2025.
Carbon represents an essential element for the origin and evolution of life and profoundly contributes to the well-being and sustainability of our planet. Over recent years, understanding carbon cycling on a global scale has become a central objective within the Earth science community, and the study of subduction zone fluids has become a crucial topic, as this geological setting represents the primary carbon input into the mantle. At subsolidus conditions, carbon transfer is mediated by mineral dissolution, triggered by aqueous fluids released from the subducting slab. While carbonate solubility has been extensively investigated, the contribution of reduced carbon forms, such as graphite and amorphous carbon, has been only recently taken into consideration, and their role in the deep carbon cycle is still unconstrained. Several issues remain open, especially whether carbon-rich fluids, generated from reduced carbon dissolution, can be transferred across oxidized conditions in the subduction mélange and eventually reach the mantle wedge.
Here we present in-situ results on the solubility of glass-like carbon, considered a proxy for disordered subducted organic material, in aqueous fluids and in equilibrium with quartz at pressures up to 2 GPa and 800 °C. Experiments were conducted in Hydrothermal Diamond Anvil Cells1, employing Rhenium gaskets to ensure oxidized conditions2 and mimic fluids released by dehydrating slabs in fore to back-arc settings. The solubility in aqueous fluids of glass-like carbon and quartz was determined by in-situ observations of the complete dissolution of samples, while the speciation of the fluids was monitored by Raman spectroscopy. Our results constrain the mutual solubility of carbon and silica in natural slab fluids and provide new constraints for the transfer of carbon operated by aqueous fluids.
[1] Bassett W.A., Shen A.H., Bucknum M. & Chou I.M. Rev. Sci. Instrum. 64, 2340–2345 (1993)
[2] Foustoukos D.I. & Mysen B.O. Am. Mineral. 100, 35–46 (2015)
How to cite: Tiraboschi, C. and Sanchez-Valle, C.: Carbon transfer by slab-fluids: in-situ experimental constraints, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10872, https://doi.org/10.5194/egusphere-egu25-10872, 2025.
In the models of the geological carbon cycle, the contribution from crustal magmas is generally overlooked. However, organic matter that has been transformed to graphite through metamorphism could represent an important source of carbon in the lower crust, which can be re-mobilized during partial melting (Cesare et al., 2005). A COH fluid produced solely by dehydration in the presence of graphite will retain the H/O ratio of H2O (H/O = 2) and thus maximize the H2O activity and form of a ternary H2O-CO2-CH4 mixture (Connolly and Cesare, 1993). Previous solubility studies have mainly considered oxidizing conditions (H/O < 2) and cannot be used to interpret graphitic systems, in which conditions are more reducing (Carvalho et al., 2023). Therefore, it is essential to obtain new solubility data for graphite-saturated silicate melt coexisting with a ternary H2O-CO2-CH4 fluid.
In this study, solubility experiments were carried out in a single-stage piston cylinder apparatus at 5 to 10 kbar and in a temperature range of 800-1000°C. To simulate an anatectic melt formed in the mid to lower metasedimentary crust, a haplogranitic glass was synthesized, and the experimental charge was loaded with graphite and H2O as the source for the COH fluid. To buffer the fluid composition at the condition H/O = 2 during the run, the double-capsule technique was utilized, and graphite and H2O was added in the outer capsule. The speciation of the experimental fluid was analyzed ex-situ by a capsule-piercing quadrupole mass spectrometer (Tiraboschi et al., 2016). In all experiments H2O was the major fluid component, in accordance with thermodynamic predictions, followed by variable amounts of CO2 and CH4. The relative amount of H2O to carbonic species in the fluid changes with pressure and temperature, and the experiments cover a range of XH2Ofluid = 0.67-0.99.
The experimental glasses contain bubbles which have been analyzed by micro-Raman spectroscopy, revealing that they mainly consist of pure CH4 or binary CH4-CO2 mixtures and graphite. The H2O content of the glasses have been determined by micro attenuated total reflectance Fourier transform infrared spectroscopy. Dissolved H2O generally increases with pressure, and there is no visible temperature dependence. To quantify the total carbon (CO2) content of the glasses, secondary ion mass spectrometry will be used. The new solubility data for carbon will complement the existing experimental dataset to allow better interpretation of complex systems where graphite, melts and fluids are present.
References:
Carvalho et al. (2023) Chem Geol 631
Cesare et al. (2005) Contrib Mineral Petrol 149, 129-240
Connolly, J.A.D. & Cesare, B. (1993) J Metamorph Geol 11, 379-388
Tiraboschi et al. (2016) Geofluids 16, 841-855
How to cite: Krona, M., Tumiati, S., Toffolo, L., Bartoli, O., Carvalho, B. B., Sorger, D., Dingwell, D. B., and Cesare, B.: Solubility of CO2 and H2O in graphite-saturated haplogranitic melt, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12413, https://doi.org/10.5194/egusphere-egu25-12413, 2025.
Phlogopite (nominally AKMMg3T(AlSi3)O10X(OH)2) is the magnesian trioctahedral endmember of the biotite solid-solution series. This phyllosilicate can accommodate substantial amounts of fluorine, usually up to 5 weight percent (wt%) and up to 8.7 wt% in extreme cases (Gianfagna et al., 2007), which substitutes for hydroxyl (OH) groups at the anionic site. Given that phlogopite is a commonly found hydrous silicate, acting as an important F reservoir in ultramafic and ultrapotassic upper mantle lithologies and an accessory mineral in various igneous and metamorphic rocks, it can contribute to the Earth’s volatile cycles. Therefore, the deeper understanding of the temperature-induced phlogopite breakdown into pyrope and forsterite (Trønnes, 2002) and the role of F in the crystal structure of fluorophlogopite during its structural collapse can provide valuable information in several Earth’s dynamic systems. For instance, in metasomatic processes in the peridotitic mantle wedge of subduction zones, mineral stability at upper mantle conditions, and complex volcanic systems. Additionally, the high-temperature atomic dynamics of complex hydrous silicates containing Fe2+, characterized by considerable structural anisotropy, as studied by Raman spectroscopy, can offer valuable information about the thermal activation of charge carriers (delocalized H+ and e-), thus about lithosphere conductivity anomalies (Bernardini et al., 2023).
This study focuses on the temperature-induced changes in the local structure and atomic vibrations of Fe2+-containing fluorophlogopite. For this purpose, a fluorophlogopite sample from Cardiff, Ontario in Canada was subjected to in situ Raman spectroscopy in the temperature range of 300-1450 K. The exact chemical formula of this mineral specimen was determined by wavelength-dispersive electron microprobe analysis (EMPA) and it is A(K0.93Na0.06Ba0.01)M(Mg2.81Fe2+0.15Ti0.01Mn0.01)T(Si2.99Al0.99Ti0.02)O10X(F1.60OH0.40). We show that fluorophlogopite undergoes stepwise structural and chemical changes, which can be monitored by the evolution of the Raman-active phonon modes at ~93 cm-1 (interlayer vibrations), 195, 279, and 325 cm-1 (dominated by octahedral vibrations) as well as at 684 and 739 cm-1 (TO4-ring mode vibrations). Near 500-600 K an interlayer structural instability occurs, which most probably results in a rearrangement of the layer stacking sequence. This activates the mobility of K+ cations in the interlayer space in the temperature range between 600 and 1000 K. Two independent heating-cooling runs to 1100 and 1450 K indicate a partial loss of K+ above 1000 K, which was confirmed by subsequent WD-EMPA. Oxidation of MFe2+ takes place between 900 and 1300 K. A partial thermal decomposition of fluorophlogopite occurs above 1300 K, leading to the formation of a minor amount of nanosized forsterite within the phlogopite matrix, but the overall biotite structure type persists up to 1450 K.
References
- Bernardini, G. Della Ventura, J. Schlüter, B. Mihailova, Geochem. 2023, 83, 125942.
- Gianfagna, F. Scoradri, S. Mazziotti-Tagliani, G. Ventruti, L. Ottolini, Am Mineral. 2007, 92, 1601.
- R.G. Trønnes, Mineral Petrol. 2002, 74, 129.
How to cite: Aspiotis, S., Reinberg, C., Peters, S., and Mihailova, B.: Temperature-induced structural transformations in fluorophlogopite studied by in situ high-temperature Raman spectroscopy, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6307, https://doi.org/10.5194/egusphere-egu25-6307, 2025.
The release and transport of volatiles, including sulfur-bearing species, by subduction related dehydration fluids are some of the key mechanisms of the deep sulfur cycle and link the surface with the crust and mantle sulfur budgets. However, parameters like the speciation of sulfur and the prevailing redox conditions during dehydration-related and fluid-mediated transport are still highly debated. To gain a deeper understanding of sulfur transport, distribution and speciation driven by different fluid migration processes at the blueschist-eclogite facies transition, we investigate samples from the New Caledonian Pouébo eclogite mélange (Taetz et al., 2016). These samples comprise two generations of garnet-quartz-bearing veins with adjacent omphacite-rich reaction halos in a blueschist metabasalt matrix. The veins are interpreted as both internal dehydration veins and external transport veins formed during prograde to peak metamorphism of the oceanic slab (Taetz et al., 2016). Therefore, these samples offer an ideal opportunity to link sulfur transport and speciation with different fluid migration processes at HP/LT subduction conditions.
Using the sulfur mineral distribution within and at varying distance to the veins, combined with in situ sulfur isotope and sulfide trace element compositions, three generations of pyrite formation and two main stages of sulfur mobilization at peak metamorphic conditions are inferred. The first stage of sulfur mobilization is linked to the formation of dehydration veins at low fO2 conditions, by the breakdown of water-bearing phases. At this stage, the primary wall rock pyrite (δ34S = -3.4‰ to -35.7‰) is partially leached from the wall rock and reprecipitates as fine-grained pyrite aggregates with δ34S values averaging at -12.4‰ in and along the newly forming, small-scale quartz-garnet-bearing dehydration veins. The second stage of sulfur mobilization takes place during the infiltration of an external fluid and formation of the transport vein. Due to a significant decrease of sulfide minerals towards the transport vein and the occurrence of Fe-oxide decomposition rims around the selvage pyrites, we infer an oxidizing character of the external fluid. This causes the development of a redox gradient between more oxidizing vein and more reducing matrix during selvage formation and enables sulfur mobilization and oxidation in the selvage area balanced by the reduction of omphacite Fe3+. The absence of sulfur-bearing minerals in the transport vein itself indicates, furthermore, that dissolved sulfate was removed from the investigated vein system by the passing external fluid.
Based on the studied vein systems, we imply that in subduction zones at the blueschist-eclogite facies transition fluid-mediated sulfur mobilization and speciation is mainly controlled by the fluid-rock ratio. While internally-buffered rock dehydration fluids carry sulfur in its reduced form, fluid-dominated transport veins may carry sulfur mostly in its oxidized form.
REFERENCES
Taetz, S., John, T., Brocker, M., & Spandler, C. (2016). Fluid-rock interaction and evolution of a high-pressure/low-temperature vein system in eclogite from New Caledonia: insights into intraslab fluid flow processes. Contributions to Mineralogy and Petrology, 171(11). doi:ARTN 9010.1007/s00410-016-1295-z
How to cite: Scherzer, S., Schwarzenbach, E. M., Pettke, T., and Scicchitano, M. R.: Intraslab sulfur mobilization in different co-occurring redox regimes at HP/LT conditions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9962, https://doi.org/10.5194/egusphere-egu25-9962, 2025.
Volcanic eruptions are driven by bubble growth in magma, caused by the exsolution of volatiles within the melt. The most important magmatic volatiles are H2O and CO2, as they are the most abundant and exert the largest control on bubble growth. As a result, most experimental work involving magma degassing involves a simplified H2O-only, or H2O-CO2 system. However, the most important magmatic volatile used in volcano monitoring is SO2, because it is much easier to identify as unambiguously volcanogenic compared to CO2 and H2O. Because of the relative scarcity of research into sulfur degassing, most interpretations made from SO2 emission data assume equilibrium conditions; however, given the relatively slow diffusion of S in silicate melts, it is likely that disequilibrium S degassing is common in natural systems.
In this contribution, we explore the nature of sulfur degassing in magmatic systems. We extend a bubble growth model to include sulfur (creating a general framework that could be used to incorporate other volatile phases). Preliminary results indicate that sulfur has a negligible influence on bubble growth physics under typical eruption conditions, but, importantly, it shows that sulfur degassing is typically in disequilibrium in basaltic melts. This runs contrary to the typical assumption of equilibrium degassing when interpreting SO2 emissions, which has implications for the use of SO2 data as a proxy for magma degassing.
How to cite: Willar-Sheehan, S., Llewellin, E., and Sullivan, P.: Disequilibrium Sulfur degassing – a mixed volatile bubble growth model , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18516, https://doi.org/10.5194/egusphere-egu25-18516, 2025.
Mobility of S in subduction zones is complex with far reaching impacts on the formation of large hydrothermal ore deposits in arc environments, volcanic S emissions into the atmosphere potentially impacting the Earth’s climate, S transfer into the mantle via the subducting slab and accompanying potential remodification of the S mantle budget. Sulfur flux and sulfides in subducting slabs have been extensively investigated, but most studies focus on HP metamorphism from the zone de mélanges at the mantle-crust transition, thus lacking a comprehensive overview of the slab perspective. In particular, a knowledge gap exists regarding the early stages of the S cycle within the subduction zone from sub-greenschist to epidote blueschist/eclogite facies. This study presents new data on the control of pyrite growth during subduction-related prograde metamorphism and the implications on the flux of S as well as related elements (Ag, As, Bi, Co, Cu, Mo, Pb, Sb, Se, Te, Tl, Zn; hereafter referred to as related elements) in subduction zones.
The Cyclades, Greece, are part of the Hellenides subduction system. The Cyclades host the Cycladic Blueschist Unit (CBU), which contains a passive margin and an ophiolitic sequence hosting metasedimentary and metavolcanic rocks metamorphosed from pumpellyite to eclogite facies and are thus particularly well suited to investigate mass transfer in a subducting slab during prograde metamorphism. We selected samples from islands within the CBU to obtain insights into the S mobility along the subducting slab.
Petrological observations show that pyrite growth and abundance increase during prograde metamorphism from the pumpellyite/greenschist (~300°C, ~7 kbar) to the blueschist/ecologite facies (500–600°C, ~22 kbar). Partial resorption and hematisation/magnetisation of pyrite seems to have occurred at the transition of peak HP metamorphism to early exhumation at >500–600°C. In-situ analyses on pyrite (LA-ICP-MS) and trace element maps distribution (EPMA) within prograde pyrite shows complex concentric patterns. Based on geothermobarometric investigations of inclusions in pyrite we observed two main temperature ranges of 350–400°C and 550–600°C where metal mobilisation happenned (Au, Sb, Pb, Te, Bi, Ni, and Co). Following this, pyrite can serve as a new powerful tool to indirectly constrain metal mobility during prograde metamorphism.
How to cite: Peillod, A., Patten, C. G. C., Drüppel, K., Beranoaguirre, A., Hector, S., Kleine-Marshall, B. I., Borchardt, P. M., Majka, J., and Kolb, J.: Pyrite growth during slab subduction: implications for S flux, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12541, https://doi.org/10.5194/egusphere-egu25-12541, 2025.
The dehydration of oceanic crust in subduction zones is a key control on subducting plate interface rheology and global element and fluid budgets. The “lower” grade metamorphic history of dehydration in subducted metabasalts in warm subduction zones is particularly important as these rocks may carry significant volumes of water into subduction zones, some of which may be released in the forearc at depths where deep slow slip and tremor occur in modern subduction zones, helping to generate local high pore fluid pressures. To explore the geologic record of such dehydration reactions, we synthesize petrologic observations, bulk rock and epidote major- and trace-element geochemistry, Sr-isotope data, and thermodynamic modeling of epidote-amphibolite facies metabasalts in the Catalina Schist of CA, USA. These metabasalts represent exhumed slices of oceanic crust that experienced peak P-T conditions of ~550°C and ~1 GPa and were underplated during Cretaceous subduction beneath North America. We focus on using epidote-group minerals in these rocks as a recorder of hydration and dehydration processes because epidote commonly forms during seafloor hydrothermal alteration and is a predicted reaction product in thermodynamic modeling of metabasalts during prograde subduction. Indeed, the Catalina Schist metabasalts include textural and geochemical evidence of seafloor hydration in the form of interpillow epidosite, epidote trace element patterns, and bulk-rock Sr-isotope values. However, these rocks also include metamorphic epidote porphyroblast and vein-like networks that developed during prograde metamorphism. Based on in-situ epidote trace element analyses, we suggest that metamorphic epidote in these rocks grew from pumpellyite breakdown, in some cases resulting in the development of epidote-rich zones that represent loci of dehydration and possible fluid pathways. We suggest these vein-like zones developed as a result of density changes during this reaction. Using thermodynamic models of the bulk rock composition of these metabasalts, we estimate that this pumpellyite to epidote reaction occurred at ~300°C and 0.5–0.7 GPa. The results of our thermodynamic modeling further suggest that the pumpellyite to epidote reaction resulted in a change in the water content of a hydrated Catalina Schist basalt from 5.5 wt. % H2O to 2.5 wt. % H2O, or a release of ~90 kg H2O per cubic meter of basalt. This corresponds to a flux of water from a 600 m-thick pile of altered basalts on the order of 104–105 kg m-2 Myr-1 in the region experiencing dehydration. In modern subduction zones, a dehydration pulse at these conditions would provide significant volumes of fluid at the updip end of the deep slow slip and tremor source region, and may provide a local source for inferred elevated pore fluid pressures. Our petrologic and geochemical observations paired with thermodynamic modeling provide insight into the metamorphic reactions that may deliver water to the plate interface at the conditions of slow slip, and show that epidote-group minerals are a powerful tools for exploring relatively “low” grade metamorphic processes in subduction zones (e.g., lithologies lacking garnet).
How to cite: Lindquist, P., Condit, C., Hoover, W., and Guevara, V.: The geologic record of hydration and dehydration in the subducting slab: Epidote minerals record alteration and metamorphism before and during subduction, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12613, https://doi.org/10.5194/egusphere-egu25-12613, 2025.
It is well known that the density of metamorphic reactions occurring in subduction zones, due to intense fluid-melt activity, is very high. One of the minerals resulting from all these reactions, present both in the downgoing slab and in the serpentinized mantle wedge, is magnetite. Its study is therefore crucial to understanding geodynamic processes, as it gives information about oxidized fluids, temperature, trace elements mobility etc... Its applicability encompasses not only metamorphic petrology (sensu lato) but also tectonic processes of accretion, exhumation and obduction (e.g., emplacement of the Samail Ophiolite), as well as ore geology, as it is an ubiquitous mineral in numerous ore deposit types. Being able to date magnetite, providing a temporal framework to all these reactions, is to put in one more piece of this big puzzle.
Advances in analytical techniques and instrumentation, above all regarding the laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), have made that currently, the U-Pb dating reaches far beyond the traditionally dated minerals (zircon, monazite, rutile, etc.). In this context, at the FIERCE laboratory of the Goethe University-Frankfurt, we have investigated the possibility of dating magnetite.
Magnetite from several localities (Greek Islands, Cyprus, Alps) have been studied, resulting in a variety of U and Pb contents (up to a few µg/g in the case of U) as well as a significant spread on the U/Pb ratios. This has allowed us to date the studied samples, with internal precisions as good as 1.5% in the best of the studied cases.
Recognising the possibility of dating such a mineral is only the first step in the implementation of the technique. Ideally, the availability of reference materials for magnetite analyses would be very advantageous. Currently (as of this EGU-abstract deadline), some of the samples dated by LA-ICPMS are being analysed using the U-Th/He method, which will allow us to compare the ages obtained by both methods and eventually, to use those magnetite as reference for future analyses.
How to cite: Beranoaguirre, A., Peillod, A., Patten, C., Dunkl, I., Hector, S., Ring, U., Kolb, J., and Gerdes, A.: First steps of LA-ICPMS U-Pb magnetite geochronology, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16826, https://doi.org/10.5194/egusphere-egu25-16826, 2025.
Subducted oceanic crust plays an important role in controlling the chemical budget of the crust and mantle and in the composition of arc lavas. Boron isotopes, 10B and 11B, are strongly fractionated during oceanic-crust dehydration in subduction zones. The temperature-dependent equilibrium boron isotope fractionation depends on boron coordination in the B-hosting minerals and fluids. Two approaches can be employed to quantify boron isotopic fractionation in subducted oceanic crust: modeling based on boron coordination in minerals and fluids, and direct measurement of the boron isotope budget of the devolatilized slab. To address this, simultaneous measurements of major and trace elements, as well as boron isotope ratios were conducted using a split-stream LA-SF-ICPMS setup at the Frankfurt Isotope & Element Research Center (FIERCE) at Goethe Universität Frankfurt.
We investigated the in-situ boron isotope compositions of minerals from reaction zone rocks from the high-pressure (HP) mélange on the island of Syros, which formed at approximately 0.7 Gpa, 415 ±15 °C. The paragenesis tourmaline + phengite + omphacite + glaucophane are 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 and glaucophane can be then estimated from the respective boron isotope fractionation against tourmaline and phengite. It is concluded that in clinopyroxene (omphacite), 88 ± 9% of boron is incorporated in tetrahedral coordination, for example via the B(F,OH)Si-1O-1 substitution, with the remaining 12 ± 9% entering by replacement of SiO4 tetrahedra with BO3 triangles. In contrast, B in glaucophane is exclusively incorporated in the tetrahedrally coordinated sites.
Bulk rock elemental abundances and boron isotopic compositions of oceanic metamorphic rocks, from Raspas Complex, Zambezi Belt, Cabo Ortegal Complex, Syros Island, and Tian Shan were measured as well. The boron isotopic composition of almost all samples (approximately -10 to +5 ‰) ranges from δ11B values close to that of fresh MORB to that of typical altered oceanic crust. Our results, thus, demonstrate that B isotopic fractionation in subducted oceanic crust is much smaller than predicted in previous studies.
How to cite: Xu, J. D., Marschall, H. R., Gerdes, A., Schmidt, A., and John, T.: Boron isotopic fractionation in subducted oceanic crust, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20179, https://doi.org/10.5194/egusphere-egu25-20179, 2025.
