GMPV2.2

The long-term global sea level depends on the balance of H₂O exchanges between the Earth's mantle and the surface through both volcanism (mantle degassing) and subduction of hydrous minerals (mantle regassing). The estimates of H₂O fluxes by the current thermopetrological subduction models predict that regassing exceeds degassing by 60%, which may lead to a sea-level drop of at least a hundred meters in the last 540 Ma [Parai & Mukhopadhyay, 2012, Earth Planet. Sc. Lett., 317, 396-406. https://doi.org/10.1016/j.epsl.2011.11.024. These models further imply a moderate ( Tg/Myr) global input of H₂O at the subduction trenches. In contrast, geological constraints suggest a near-steady state of long-term sea level while geophysical observations advocate for a larger global H₂O input, especially given the large amounts of hydrated lithospheric mantle that are inferred at present-day subduction trenches. To address this paradox, we revise the subduction-H₂O flux calculations using recently published experimental data on natural hydrated peridotites at high-pressure conditions, which suggest that all hydrated phases destabilize below 800˚C for pressures higher than 8 GPa [Maurice et al., 2018, Contrib. Mineral. Petrol, 173(10), 86. https://doi.org/10.1007/s00410-018-1507-9 ]. Our reassessed thermopetrological models show that a prominent global H₂O input ( Tg/Myr), mainly conveyed by the layer of subducted serpentinized mantle, is compatible with a limited global H₂O retention in subducted slabs at mid-upper mantle depths ( Tg/Myr), including in models that consider some worldwide variability of the input serpentine. We also show that the global H₂O retention at mid-upper mantle depths is only driven by the hydrated mantle of coldest subducting plates. Overall, our models show that the present-day global water retention in subducting plates beyond mid-upper mantle depths barely exceeds the estimations of mantle degassing, and thus quantitatively support the stable-sea level scenario over geological times.

How to cite: Arcay, D., Cerpa, N., and Padrón-Navarta, J. A.: Reconciling extensive mantle hydration at subduction trenches and limited deep H2O fluxes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13287, https://doi.org/10.5194/egusphere-egu22-13287, 2022.

Subduction zones are principal pathways for the cycling of volatiles such as  hydrogen and carbonfrom the Earth’s surface to the mantle and back to the atmosphere. This cycling has significant long-term effects on Earth’s climate. However, the processes that lead to volatile release during subduction and total volatile fluxes are poorly understood. In our study, we will quantify and characterize the network architecture of dehydration pathways exhibited as mineralized olivine-bearing metamorphic veins in the exhumed meta-serpentinites from the Erro-Tobbio unit, Italy [1]. Applying network analytical methods and graph theory both macroscopically and microscopically can provide the mode of propagation and describe the controlling factors affecting the evolution of these dehydration networks. Furthermore, multiscale observations can confirm the scalability of the vein network and if quantitative results such as permeability or volatile flux can be extrapolated to larger scales.

Along with 2-D network analysis, these vein networks will be analyzed in 3-dimensions using X-ray tomography and sophisticated machine-learning methods, such as generative adversarial networks. The results of both will be compared, which can then assure whether current machine-learning methods can effectively create statistically equivalent copies of these networks. Lastly, the synthesis of 2-D and 3-D multiscale results should provide meaningful parameters for accurate calculations of volatile flux during the dehydration of subducting slabs. 

[1] Plümper et al. (2017) Nature Geoscience 10(2), 150-156.

How to cite: Arias, A., Beinlich, A., Eberhard, L., Scambelluri, M., and Plümper, O.: Multidimensional Analysis of Serpentinite Dehydration Networks and Implications for Volatile Flux in Subduction Zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2584, https://doi.org/10.5194/egusphere-egu22-2584, 2022.

Dehydrated fluids expelled from serpentinized mantle in the subducted slab are gradually recognised as a vital role in generating arc magmatism and element cycling in the Earth. However, it remains not clear about their recycling at various depth in subduction zones and if slab serpentinite-derived fluids contribute to the genesis of lavas from the back-arc basins. Here, we study the magnesium (Mg) isotopic compositions of lavas from the Okinawa Trough (OT) and Lau basin (LB) as Mg isotopes have shown great potential to trace dehydration of slab serpentinites in recent years. Overall, lavas from the OT and LB have averagely heavier Mg isotopic compositions relative to the mid-ocean ridge basalt (MORB) mantle, which could be attributed to the involvement of slab serpentinite-derived fluids rather than crustal assimilation or input of subducted sediments as indicated by the isotopic modelling results. The δ²⁶Mg values of the southern OT (SOT) and southern LB (SLB) are generally higher than the middle OT (MOT) and northern LB (NLB), respectively, with an average of -0.11 ± 0.06‰ (2SD, n=5) for the SOT, -0.20 ‰ ± 0.04 (2SD, n=5) for the MOT, -0.13 ‰ ± -0.08 for the SLB (2SD, n=6) and -0.19 ‰ ± 0.06 (2SD, n=10) for the NLB. The binary modelling results have shown that various amounts of serpentinite-derived fluids could explain the variations in Mg isotopic compositions observed in the OT and LB. Combined published δ²⁶Mg values in subduction zones with our data, the thermal structure of inter-subduction zone may play a first control on the signal of Mg-rich serpentinite-derived fluids. By contrast, the contributions of these fluids to different segments in a specific subduction zone may depend on the slab depth beneath magmatic activity sites.

How to cite: Ding, Y., Jin, X., Li, X., Li, Z., Liu, J., Wang, H., Zhu, J., Zhu, Z., and Chu, F.: Magnesium isotopic composition of back-arc basin lavas and its implication for the recycling of serpentinite-derived fluids, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-877, https://doi.org/10.5194/egusphere-egu22-877, 2022.

Sulfur is a key element in the subduction zone-volcanic arc system; however, the mechanism(s) that recycle sulfur from the slab into the overlying volcanic arc are debated. Here we summarize recent advances in quantifying this component of the deep sulfur cycle. First, primary metamorphic or inherited sulfides in oceanic-type eclogites are only rarely observed as inclusions and are typically absent from the rock matrix. Additionally, sulfides are relatively common in rocks metasomatized at the slab-mantle interface by slab-derived fluids during exhumation. Combined, these two observations suggest that sulfur loss from subducted mafic crust is relatively efficient. Thermodynamic modeling in Perple_X using the Holland and Powell (2011) database combined with the Deep Earth Water model suggests that the efficiency and speciation of sulfur loss varies depending on the degree of seafloor alteration prior to subduction and the geothermal gradient of the slab. In relatively cold subduction zones, such as Honshu, slab-fluids derived from subducted mafic crust are predicted to exhibit elevated concentrations of HSO₄^-, SO₄^2-, HSO₃^-, and CaSO_4(aq), whereas hot subduction zones, such as Cascadia, are predicted to produce slab fluids enriched in HS^- and H₂S at lower pressures. The oxidation of sulfur expelled from subducted pyrite is balanced by the reduction of Fe³⁺ to Fe²⁺, consistent with the low Fe³⁺/SFe of exhumed eclogites relative to blueschists and altered oceanic crust. Where oxidized S-bearing fluids are produced, they are anticipated to interact with more reduced rocks at the slab-mantle interface and within the mantle wedge, resulting in sulfide precipitation and significant isotopic fractionation. The δ³⁴S values of slab fluids are estimated to fall between -11 and +8 ‰. Rayleigh fractionation during progressive fluid-rock interaction results in fractionations of tens of per mil as oxidized species are depleted and sulfides are precipitated, resulting in δ³⁴S values of sulfides that easily span the -21.7 to +13.9 ‰ range observed in metasomatic sulfides in exhumed high-pressure rocks. However, in subduction zones where reduced species prevail, the S isotopic signature of slab fluids is expected to reflect their source and will exhibit a narrower range in δ³⁴S values. As a result, the δ³⁴S values measured in arc magmas may not always be a reliable indicator of the contribution of different components of the slab, such as sediments vs. AOC. Additionally, the impact of S recycling on the oxygen fugacity of arc magmas is expected to vary both spatially and temporally throughout Earth history.

How to cite: Walters, J., Cruz-Uribe, A., and Marschall, H.: Sulfur in the slab: A sulfur-isotopes and thermodynamic-modeling perspective from exhumed terranes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11697, https://doi.org/10.5194/egusphere-egu22-11697, 2022.

The objective of this work is to study the fluid rock-interactions at low metamorphic grade in subduction zones. We focused in particular on the evolution of metapelites from the base of the seismogenic zone (⁓250℃) to the down-dip transition to the aseismic domain (⁓330℃). In the three examples examined here (Kodiak Complex in Alaska, Shimanto Belt in Japan, the French Alps), we followed the variations in mineralogy, trace element budget, as well as fluid inclusion elemental and isotopic (δ⁷Li) composition.

In the Kodiak and Shimanto belt, the mineralogy remains constant with temperature increase, with the dominance of phyllosilicates (white mica and chlorite), quartz and plagioclase. In more deformed zones of higher-T samples (330 ± 16℃ and 3 ± 0.4 kbar for Kodiak and 320 ± 14℃ and 3.9 ± 0.4 kbar for Shimanto belt) quartz and plagioclase are completely dissolved, while large white mica and chlorite grains crystallized. Also, the chlorite/white mica ratio is higher with temperature increase.

White mica is a main host for B, LILE and to smaller extent for Li. Plagioclase hosts the same elements but in lower concentrations. Chlorite is a main host for Li ± B and quartz hosts Li to smaller extent than chlorite and mica. Bulk rock analysis revealed partial loss in B, Rb, Sr and Cs with temperature increase, in contrast to the retention of Li and Ba. Mass balance based on trace element concentrations of individual phases and their proportion point to a reorganization of elements released during quartz and plagioclase dissolvement and phyllosilicate recrystallization: Rb, Cs and Ba released from plagioclase are incorporated in higher grade mica, Li released from mica and quartz is incorporated into chlorite. In the lack of newly formed phase as a host, B and Sr are probably released into a fluid.

The salinity at 250°C is around 2wt.% NaCl eq., i.e. lower than original pore-filling seawater. The freshening can be accounted for by smectite dehydration and transformation into illite. From 250 to 330°C, a salinity increase is observed, up to 3.5wt.%, possibly related to the chlorite crystallization requiring higher amount of water. The fluid is highly enriched in fluid-mobile elements in comparison with seawater. δ⁷Li values of fluid inclusion leachates are distinct for each locality: +8.1 to +17.07‰, in the Kodiak, +2.53 to +10.39‰ in the Shimanto belt and -1.54 to +9.54 ‰ in the western Alps. δ⁷Li of fluids is independent of other parameters, such as temperature or Li concentration.

Mineral reactions and fluid-mobile elements concentration in phases point to overall a local redistribution of fluid-mobile elements between phases, except for minor release of B and Sr. Lithium isotopes, which show that δ⁷Li of fluid is possibly buffered by host rock, confirm the fact that the rocks behaved to a large extent as a closed system during burial and subsequent exhumation.

How to cite: Rajic, K., Raimbourg, H., Richard, A., Lerouge, C., Millot, R., and Herviou, C.: Elemental and lithium isotopic signature of fluids in metapelites from ancient subduction zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11784, https://doi.org/10.5194/egusphere-egu22-11784, 2022.

Mechanisms such as crystallization differentiation, subduction erosion, delamination, or relamination that are responsible for the formation and modification of modern crust with an on average andesitic composition are actively debated (Hacker et al. 2015). Isotope fractionation associated with igneous processes is documented for many non-traditional stable isotope systems, making them promising tools to advance our understanding of modern arc crust formation. Titanium isotopes are especially promising, as volcanic and plutonic arc rocks show a trend from light to heavy isotope values with increasing SiO₂ concentration due to the fractionation of minerals with light Ti isotopes.

We present new Ti isotope data on medium K calc-alkaline to shoshonitic magmatic differentiation suites from the Adamello Batholith (N-Italy), Kos (Agean arc), Torres del Paine (Patagonia) and the Dolomites (N-Italy) in addition to crust-derived mafic cumulates. The Ti isotopic composition of dacites and granites range between δ⁴⁹Ti_OL-Ti ≈ 0.3 to 1.1‰, with heavier values for more alkaline granitic melts in agreement with published data (Hoare et al. 2020). Mafic cumulates from related and additional localities are overall isotopically lighter than (their) granitic counterparts ranging between δ⁴⁹Ti_OL-Ti ≈ -0.15 and +0.08‰. Cumulates of studied crustal sections enriched in Fe-Ti oxides (>5 modal %) show δ⁴⁹Ti values lighter than the depleted MORB mantle (DMM, δ⁴⁹Ti_OL-Ti ≈ +0.002 ± 0.007‰) and counterbalance the isotopically heavy composition of felsic rocks. The occurrence of cumulates heavier than DMM may have several reasons: (i) “heavy” cumulates may represent late-stage relicts of progressive magma differentiation containing trapped intercumulus melt or (ii) they experienced overprinting, e.g., by mafic rejuvenation.

We therefore find that the Ti isotopic composition of cumulate rocks and likely also the magmatic lower continental crust is influenced by their mineralogical composition. How this impacts the Ti isotopic composition of the bulk continental crust in the light of delamination and relamination processes needs further work.

REFERENCES

Hacker, B. R., Kelemen, P. B., & Behn, M. D. (2015). Continental lower crust. Annual Review of Earth and Planetary Sciences, 43, 167-205.

Hoare, L., Klaver, M., Saji, N. S., Gillies, J., Parkinson, I. J., Lissenberg, C. J., & Millet, M. A. (2020). Melt chemistry and redox conditions control titanium isotope fractionation during magmatic differentiation. Geochimica et Cosmochimica Acta, 282, 38-54.

How to cite: Storck, J.-C., Greber, N. D., Müller, A., Pettke, T., and Müntener, O.: Titanium isotopic fractionation of arc derived melts and cumulates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9783, https://doi.org/10.5194/egusphere-egu22-9783, 2022.

Aqueous and saline fluids have a fundamental role in subduction zones and represent a major vector of mass transfer from the slab to the mantle wedge. In this setting, assessing the mobility of redox sensitive elements, such as iron, in aqueous fluids and melts is essential to provide insights on the oxygen fugacity conditions of slab-derived fluids and the oxidation state of arc magmas.

We experimentally investigate the solubility of magnetite and hematite in water-saturated haplogranitic melts, which represent the felsic melt produced by subducted eclogites. Experiments were conducted at 1–2 GPa and temperature ranging from 700 to 950 °C employing an endloaded piston cylinder apparatus. Single gold capsules were loaded with natural hematite, magnetite and synthetic haplogranite glass. Two sets of experiments were conducted: a first set with pure H₂O and a second set with a 1.5 m H₂O-NaCl solution. After quench, the presence of H₂O in the haplogranite glass was verified by Raman spectroscopy, while iron and major element contents were determined by electron microprobe analysis.

Results show that a significant amount of FeO is released from magnetite and hematite equilibrated with hydrous melts, up to 1.96 ± 0.04 wt% at 1 GPa and 950 °C. In the presence of NaCl, we observed an increase in the amount of iron in the haplogranite glass, e.g. from 1.04 ± 0.12 wt% to 1.50 ± 0.31 wt% of FeO_tot at 800 °C. These concentrations are substantially higher than the iron solubility in aqueous and saline fluids predicted by thermodynamic modelling (DEW model, Sverjensky et al., 2014), likely due to formation of Fe- and Si-bearing complex in the haplogranite-bearing fluid at run conditions. Our results suggest that hydrous melts can effectively mobilize iron from Fe-oxides even at relatively low-pressure conditions. Slab-derived hydrous melts can thus represent a valid agent for mobilizing iron from the subducting slab to the mantle wedge, and can strongly influence the geochemical cycles of Fe and the redox conditions of subduction zone fluids.

Sverjensky, D. A., Harrison, B. and Azzolini, D. (2014) Water in the deep Earth: The dielectric constant and the solubilities of quartz and corundum to 60kb and 1200°C, Geochim. Cosmochim. Acta, 129, 125–145

How to cite: Tiraboschi, C., Arno, R., Stephan, K., Jasper, B., and Carmen, S.-V.: Iron mobility in slab-derived hydrous silicate melts at sub-arc conditions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11854, https://doi.org/10.5194/egusphere-egu22-11854, 2022.

Fluids are the primary agents for mass transfer in subduction zones. These fluids can be captured as primary inclusions within minerals crystallizing during subduction processes. Some of these inclusions, referred to as multiphase solid inclusions (MSI), are characterized by the high proportion and variety of minerals, hence by a high concentration of solute in the trapped fluid. Kotková et al. (2021) have described primary MSI in garnets of subduction-related ultra-high pressure (UHP) peridotites (P-T of 1030-1150 ºC/3.6-4.8 GPa) of the Bohemian Massif. MSI range in size between ≈5-40 µm and are mostly composed of hornblende, the barian mica kinoshitalite, dolomite and magnesite. MSI have been interpreted as trapped residual liquids produced after partial UHP crystallization of carbonate-silicate melts that now form garnet pyroxenite veins in the peridotites. Experimental re-melting of MSI is the best procedure to investigate the precise nature of trapped fluids. We have conducted re-melting experiments of MSI present in garnets of a lherzolite, taking the inclusions to P-T around their entrapment conditions at or close to host rock peak P-T, 4-4.5 GPa and 1000-1225 ºC. The inclusions (re-)crystallized into a garnet fringe at the boundary between inclusions and host garnet, barian mica and carbonatite melt towards the center of the inclusion, and a large irregular and empty space in between the garnet fringe and the central silicate-carbonate component. Microstructures and mass balance indicate that the empty space was occupied by a Na-K-Cl-F-rich saline aqueous fluid (brine). Hence experiments did not produce a single melt at any experimental conditions, but systematically show the stability and coexistence of barian mica + carbonatite melt + brine at the entrapment conditions, and a garnet fringe indicating reaction between trapped fluids and host garnet. This suggest that growing garnet trapped a carbonatite melt and a saline aqueous fluid coexisting in the matrix, together with solid crystals of barian mica likely produced by metasomatism of the percolating fluids through the host peridotite. It is intriguing, however, that neither single mica crystals nor separate former carbonate melt and brine have been found included in garnets. Mass balance shows that carbonate melt is the main host for incompatible elements such as Ba. This presentation will discuss the bearings of the experimental results on the nature and origin of these MSI, potential links to diamond formation and their implication on mass transfer processes in subduction zones.

Kotkova et al. (2021) Lithos 398-399, 106309

How to cite: Acosta Vigil, A., Kotková, J., Copjaková, R., Wirth, R., and Hermann, J.: Experimental constraints on the nature of multiphase solid inclusions and their bearing on mantle wedge metasomatism, Bohemian Massif, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13297, https://doi.org/10.5194/egusphere-egu22-13297, 2022.

Eclogite-hosted metamorphic veins mark former fluid migration pathways during a subduction-exhumation cycle, and allow to trace fluid-mediated element transfer across lithologies, to ultimately metasomatize the mantle wedge. Fluids can be generated by dehydration reactions at different P-T conditions in various lithologies, all influencing how different chemical constituents are dissolved and re-precipitated. Here we present a study of eclogite-hosted quartz-rich metamorphic veins in the Variscan Münchberg Massif. The eclogites probably represent subducted continental crust that was variably hydrated and subjected to amphibolite facies conditions before reaching eclogite facies peak conditions of ca. 3 GPa and 700°C.

Isolated, mm-sized quartz pockets with euhedral high-pressure minerals are common in the Münchberg eclogites, but continuous veins that may have allowed focused fluid flow beyond specimen scale are rare. Nevertheless, where such veins occur, they can contain high-pressure minerals such as garnet and omphacite, but also rutile, zircon, and allanite, indicating high field strength-element (HFSE) mobility at least on the specimen scale. Garnet- and omphacite-bearing veins are typically 1-10 mm thick with average crystal sizes of 1 mm and less. A different vein type is mostly similar in thickness, but consists of quartz + phenocrysts (sometimes >1 cm long) of kyanite, phengite, and/or rutile. Symplectite-rich selvages surrounded by mostly fresh host eclogite are common.

Oxygen isotope thermometry of quartz-garnet, quartz-phengite, and quartz-kyanite pairs yield temperatures around 700°C, interpreted to represent vein crystallization. δ¹⁸O values of vein quartz (+6.1 to +10.5‰) from all vein types are identical to δ¹⁸O values of host rock quartz (the latter were predicted from mass balance modelling at 700°C based on host rock δ¹⁸O values from +4.0 to +7.9‰). While it is evident that the garnet- and omphacite-bearing veins were formed under eclogite facies conditions, pressures are uncertain for the quartz-rutile, quartz-phengite, and quartz-kyanite veins. Still, vein formation at relatively high pressures seems probable, as solubilities of chemical components tend to increase with pressure, facilitating HFSE mobilization from the source rock. We propose that internal fluids were generated by dehydration of phengite and/or zoisite and/or amphibole from the eclogites. Isolated quartz-rich pockets formed in eclogites that may have released only small amounts of fluid, whereas continuous metamorphic veins were formed in eclogites that produced more fluid, probably reflecting a more intense hydration before eclogite facies metamorphism. The internal origin of the fluids supported by oxygen isotope evidence argues against fluid transport over large distances. The fluids may have largely remained in place before being consumed for symplectite formation upon retrogression to amphibolite facies conditions.

How to cite: Pohlner, J., El Korh, A., Klemd, R., Pettke, T., and Grobéty, B.: Eclogite-hosted metamorphic veins in the Münchberg Massif (Germany), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11108, https://doi.org/10.5194/egusphere-egu22-11108, 2022.

It is generally accepted that subduction zones are important sites for element recycling into the Earth’s mantle. This does in particular also include carbon, which is transported in the form of organic carbon and carbonates. While organic carbon is expected to effectively fix carbon in the slab, carbonates are often entitled as an important CO₂ source for arc magmatism. The exact composition of the total subducted carbon load, in terms of oxidised and reduced carbon material, changes between different slabs and consequently the total released carbon varies significantly among suduction zones. An important mechanism for carbon release is the dissolution of carbonates in aqueous fluids. Ophicarbonates, containing both serpentine and carbonate minerals, are thus of special interest: The fluid released through serpentine dehydration reactions interacts with carbonates and causes the release of carbon. However, to better constrain the carbon release it is essential to understand the release of fluid in carbonated systems.

In this study we present a detailed experimental analysis on the effect of carbonates on the fluid release from serpentinites. We performed multi-anvil experiments on model ophicarbonates. Our starting material was a mixture between natural antigorite and Ca-carbonate and/or graphite. We also conducted thermodynamic calculations on various serpentinite-carbonate systems. Our experimental results show that serpentine dehydrates at temperatures <600 °C at 2.5 GPa, which is lower with respect to uncarbonated serpentinites. For a serpentinite with 20 wt% CaCO₃ the dehydration of serpentine thus takes place at 50 - 60 km depth. In the absence of CaCO₃ the fluid is released at 60 - 70 km depth. In cold subduction zones this shift in dehydration depth is even more extreme: In a carbonated system the serpentine was found to dehydrate at 80 - 110 km depth, in comparison to 110 - 130 km depth in the uncarbonated system. We found that this shift is mainly due to Ca-Mg exchange reactions between the carbonate and silicate fraction. The experimental run products show distinct dehydration mineralogy, forming Ca-silicates and Mg-bearing carbonates. In combination with mass balance calculations we show that the total carbonate-fraction does not decrease over the whole experimental temperature range. In conclusion, serpentinites with a high Ca-carbonate content are expected to dehydrate earlier in the subduction zones, whereas the carbon remains in the slab. The presence of Ca-carbonate thus has the potential to prevent subduction of water into deeper levels of the Earth’s mantle.

How to cite: Eberhard, L., Plümper, O., and Frost, D. J.: Experimental constraints on low temperature dehydration induced by mineral reactions in calcite-bearing ophicarbonates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1215, https://doi.org/10.5194/egusphere-egu22-1215, 2022.

The formation of carbonated serpentinites (serpentine, Mg-Ca carbonates) and listvenites (quartz, Mg-carbonate) by reactions between exhumed mantle peridotites and percolating CO₂-bearing fluids is a major sink for carbon from spreading ridges to ophiolites and orogenic suture zones. During ICDP Oman Drilling Project, the transition from the base of the Semail Ophiolite to its metamorphic sole was drilled at Hole BT1B (Wadi Mansah), allowing to recover ~200 m of variously carbonated serpentinites and listvenites, and underlying metabasalts. Mineralogical and geochemical investigations indicate that carbonation at the expense of the Wadi Mansah peridotites was triggered by the migration of multiple fluid batches along the basal thrust at shallow depths and low temperatures (50-250 °C). To better constrain the impacts of fluid source(s) and protolith compositions on reaction pathways and oxidation state during carbonation, we carried out iron and zinc isotopes study of 19 variously carbonated peridotites (13 listvenites, 5 carbonated serpentinites, one serpentinized harzburgite) and of 6 underlying metamorphic samples from Wadi Mansah area (including 3 BT1B samples).

The partially serpentinized harzburgite and carbonated serpentinites have δ56Fe and δ66Zn compositions ranging between -0.05 – +0.06 ‰ and -0.11 – +0.15, respectively, overlapping that of previously analysed abyssal (δ56Fe: -0.15 – +0.11 ‰; δ66Zn: +0.12 – +0.62 ‰), ophiolitic (δ56Fe: -0.27 – +0.14 ‰; δ66Zn: -0.56 – +0.38 ‰), orogenic (δ56Fe: -0.06 – +0.12 ‰; δ66Zn: +0.03 – +0.55 ‰), and fore-arc (δ56Fe: -0.26 – +0.09 ‰) peridotites. In contrast, listvenites display highly variable δ56Fe and δ66Zn values, between -0.33 – +0.2 ‰ and -0.46 – +0.64 ‰ respectively. Iron isotopes compositions show a positive correlation with bulk iron contents. Zinc isotope compositions are positively correlated to δ13C_TC values, suggesting a high mobility of Zn in carbonate-bearing fluids. The lightest δ66Zn values were measured in listvenites with minor amounts of fuchsite (Cr-mica), that often display evidences for breakdown of Cr-spinel. Metamorphic sole samples display isotopic compositions typical of mafic rocks (δ56Fe: +0.01 – +0.24 ‰; δ66Zn: +0.24 – +0.47 ‰), in agreement with an oceanic crust-derived protolith (MORB, δ56Fe: +0.06 – +0.18; δ66Zn: +0.27 – +0.30 ‰).

Our results suggest an important control of the protolith chemistry and complexation with dissolved carbon in reactive fluids on the Fe and Zn isotopes compositions.

How to cite: Decrausaz, T., Godard, M., Debret, B., and Martinez, I.: Carbonation of peridotites along the basal thrust of the Semail Ophiolite (OmanDP Hole BT1B): insights from Fe and Zn isotopes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3929, https://doi.org/10.5194/egusphere-egu22-3929, 2022.

In subduction zones, serpentinite-hosted ophicarbonates and their main dehydration and decarbonation reactions linked to prograde metamorphism are relatively well understood. On the contrary, the geological conditions and processes leading to the carbonation of subduction-zone metaserpentinites by fluid–rock interactions remain poorly constrained. At different arc depths, the reaction of decarbonation fluids derived from marble and carbonate‐bearing sediment with slab and mantle wedge serpentinites, as well as tectonic mixing and deformation along subduction zone interface, may produce magnesite-bearing rocks with a bulk composition similar to that of ophicarbonates. Subsequently, these hybrid metasomatic lithologies will undergo decarbonation reactions at prograde or retrograde conditions which may influence the cycling of C and other volatiles from the slab to the mantle wedge and the global estimates of C fluxes at convergent margins. This can be evaluated through the study of exposed paleo-subduction metamorphic suites.

Here we investigate the tectonic, textural and mineralogical evolution of marble layers and magnesite-rich lenses hosted in chlorite harzburgite (Chl-harzburgite) in the Cerro Blanco ultramafic massif (Nevado-Filábride Complex, Betic Cordillera, S. Spain), which records high-pressure alpine subduction metamorphism as evidenced by the transition from antigorite serpentinite (top of the body) to Chl-harzburgite (bottom) due to high-pressure deserpentinization. Chl-harzburgite is separated from a gneiss and mica schist crustal sequence by a footwall of strongly heterogenous mylonite (around 20 m thick) encompassing: transposed, foliated and brecciated marble layers, foliated Chl-harzburgite lenses (in some cases completely transformed to retrograde serpentinite), centimetre to several decimetre thick boudins of idiomorphic coarse to very coarse magnesite aggregates, associated to chlorite and magnetite and enveloped by the mylonitic foliation, and, finally, abundant almost monomineralic amphibole aggregates. We interpret this mylonite zone as a detachment leading to the exhumation of the Cerro Blanco massif after reaching peak subduction metamorphic conditions that formed the Chl-harzburgite assemblage.

Combined field, EDS-SEM, and EPMA data obtained from a detailed cross-section sampling of this mylonite zone reveal that, locally, metamorphic equilibrium was reached between Chl-harzburgite and the transformation products of the magnesite boudins during the mylonite foliation development. Thermodynamic modelling of these assemblages allows inferring the relationship between deformation and metamorphic conditions during exhumation, including possible decarbonation reactions.

We thank the Universidad de Jaen 1263042 FEDER-UJA grant, funded by the European Social Fund and the European Regional Development Fund.

How to cite: López Sánchez-Vizcaíno, V., Padrón-Navarta, J. A., Laborda-López, C., Gómez-Pugnaire, M. T., Menzel, M. D., and Garrido, C. J.: Metaserpentinite carbonation and decarbonation reactions during subduction metamorphism and subsequent tectonic exhumation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9996, https://doi.org/10.5194/egusphere-egu22-9996, 2022.

It has been recently proposed that high-pressure genesis of abiotic hydrocarbon can lead to strain localization in subducted carbonate rocks¹. However, the mechanical effects of the migration of these hydrocarbon-bearing fluids on the infiltrated rocks still need to be constrained.

In this study, we investigate omphacitite (i.e. omphacite-rich rock) adjacent to an high-pressure methane source from the Western Italian Alps (Italy) using a multiscale and analytical approach including petrographic, microstructural, X-ray compositional mapping and electron backscatter diffraction analyses (EBSD). In the field, omphacitite bands are 1-5 metres thick and tens of metres long and are adjacent to carbonate rocks affected by high-pressure reduction and methane production.

Hand specimens and thin sections display a brecciated structure, with omphacitite fragments ranging in size from a few microns to several centimetres, surrounded by a matrix of jadeite, omphacite, grossular, titanite, and graphite. X-ray compositional maps and cathodoluminescence images highlight oscillatory zoning and skeletal (jackstraw) textures in jadeite, omphacite and garnet in the matrix, suggesting a fast matrix precipitation under plausible disequilibrium conditions. CH₄ and H₂ are found in fluid inclusions in the jadeite grains. This feature suggests a potential link between the genesis of CH₄ in the adjacent carbonate rocks and the brecciation event.

EBSD analysis was performed on omphacitite clasts close to their borders, where omphacite grain size varies between a few microns and a maximum of 100 microns. Those omphacite grains display no crystallographic preferred orientation, abundant low angle boundaries and low (< 5°) internal lattice distortion. We interpret these textures as formed by pervasive and diffuse micro-fracturing related to the brecciation occurring at high pore fluid pressure, reaching sub-lithostatic values. This study suggests that at high-pressure conditions in subduction zones, the genesis and migration of hydrocarbon-bearing fluids can trigger fracturing in adjacent lithotypes.

¹Giuntoli, F., Vitale Brovarone, A. & Menegon, L. Feedback between high-pressure genesis of abiotic methane and strain localization in subducted carbonate rocks. Sci. Rep. 10, 9848 (2020).

This work is part of project that has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 864045).

How to cite: Giuntoli, F., Vitale Brovarone, A., and Menegon, L.: Hydrocarbon-bearing fluid migration produces brecciation at high pressure condition in subduction, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11350, https://doi.org/10.5194/egusphere-egu22-11350, 2022.

The role of subduction zones has been considered critical to understand carbon fluxes among the Earth’s reservoirs. At plate margins, most of the carbon is stored in carbonate sediments. Nevertheless, the past decade saw an increasing focus also on reduced carbon - kerogen and graphite – to understand its role in the deep carbon cycle. Most of reduced carbon derive from seafloor organic-rich sediments, even if, a little portion can form by decarbonation during metamorphism.

In the Palaeoproterozoic supracrustal rocks of the Lewisian Complex, graphitic marbles were found in a mixed succession of metasediments at Gott Bay, Island of Tiree (Scotland). Such marbles show bedding-parallel slip surfaces associated with chlorite that are absent in other marbles on the island that are devoid of graphite. Marbles and schists-hosted graphite were analysed showing marked differences in carbon isotopic composition and structural ordering measured by means of Raman spectroscopy.

Petrographic and chemical evidence support the hypothesis of an abiotic origin of the marble-hosted graphite and the mechanisms that led to its formation could explain the heavy isotopic composition of many Proterozoic marbles in the world.

How to cite: Schito, A., Parnell, J., Muirhead, D., and Boyce, A.: Evidence of abiotic graphite formation in Proterozoic marbles of the Lewisian Complex: mechanisms and consequences for the deep carbon cycle, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5014, https://doi.org/10.5194/egusphere-egu22-5014, 2022.

High-pressure COH fluids have a fundamental role in a variety of geological processes. Their composition in terms of volatile species can control the solidus temperature, carbonation/decarbonation reactions, and influences the amount of solutes generated during fluid-rock interaction at depth. Over the last decades, several systems have been experimentally investigated to unravel the effect of COH fluids at upper mantle conditions. However, fluid composition is rarely tackled as a quantitative issue, and rather infrequently fluids are analyzed in the same way as the associated solid phases in the experimental assemblage. A comprehensive characterization of carbon-bearing aqueous fluids in terms of composition is hampered by experimental difficulties in synthetizing and analyzing high-pressure fluids, without altering their composition upon quench.

Recently, improved ex situ techniques have been proposed for the analyses of experimental COH fluids, leading to significant advancement in synthetic fluids characterization. The development of customized techniques in order to investigate these fluids, in terms of volatile speciation and dissolved solute load, allowed to elucidate some of the processes involving carbon at high-pressure conditions and to assess its influence in the mantle wedge.

Some of the recently developed techniques employed for ex situ quantitative analyses of carbon-saturated COH fluids will be presented, such as the capsule piercing QMS technique (Tiraboschi et al., 2016) and the cryogenic LA-ICP-MS technique (Kessel et al., 2004; Tiraboschi et al., 2018). The capsule piercing QMS technique allow to measure the main uncharged volatile species in the COH system (i.e., H₂O, CO₂, CH₄, H₂, O₂, CO), while the cryogenic LA-ICP-MS technique permits to measure the amount solutes generated by mineral dissolution in COH fluids, in terms of mol/kg.

The results obtained by employing these analytical strategies indicate that a quantitative approach to COH fluid analyses is a fundamental step to understand the effect of carbon-bearing fluids at upper mantle conditions and to ultimately unravel the deep cycling of carbon.

Kessel, R., Ulmer, P., Pettke, T., Schmidt, M. W. and Thompson, A. B. (2004) A novel approach to determine high-pressure high-temperature fluid and melt compositions using diamond-trap experiments, Am. Mineral., 89(7), 1078–1086.

Tiraboschi, C., Tumiati, S., Recchia, S., Miozzi, F. and Poli, S. (2016) Quantitative analysis of COH fluids synthesized at HP–HT conditions: an optimized methodology to measure volatiles in experimental capsules, Geofluids, 16(5), 841–855.

Tiraboschi, C., Tumiati, S., Sverjensky, D., Pettke, T., Ulmer, P. and Poli, S. (2018) Experimental determination of magnesia and silica solubilities in graphite-saturated and redox-buffered high-pressure COH fluids in equilibrium with forsterite + enstatite and magnesite + enstatite, Contrib. to Mineral. Petrol., 173(1), 1–17.

How to cite: Tiraboschi, C.: Carbon-saturated COH fluids in the upper mantle: what ex situ experiments tell us about carbon at high-pressure and high-temperature conditions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11725, https://doi.org/10.5194/egusphere-egu22-11725, 2022.

The garnet in the ultra-high pressure (UHP) eclogites of the Erzgebirge (Bohemian Massif, Germany) trapped primary inclusions of metasomatic melt originated by the partial melting of the continental crust. The study of these inclusions alow us to estimate the contribution of the subducted continental crust to mantle metasomatism and deep carbon fluxes. The inclusions are randomly distributed in the inner part of the garnet, they are micrometric and occur as both polycrystalline, i.e. nanogranitoids, and glassy, often with a shrinkage bubble. Nanogranitoids consist of kumdykolite, quartz, kokchetavite, biotite, white mica, calcite and rare graphite. The inclusions share their microstructural position in the garnet with inclusions of polycrystalline quartz interpreted as quartz pseudomorph after coesite that indicate the entrapment at UHP conditions. The melt composition, measured on glassy inclusions and rehomogenized nanogranitoids, is granitic. The melt is also hydrous, slightly peraluminous and the trace element enrichments observed are consistent with an origin from the continental crust, testified by the high amount of incompatible elements such as Cs, Pb, Th, U, Li and B. Similar signatures were also reported elsewhere in the Bohemian Massif, e.g. in other metasomatic melts hosted in HP mantle eclogites, in metasomatized mantle rocks and in post-collisional ultrapotassic magmatic rocks, suggesting that mantle metasomatism from melts originated in the continental crust is widespread in the orogen.

The melt H₂O and CO₂ contents were measured with the NanoSIMS. The CO₂ values in particular were corrected reintegrating the vapor contained in the shrinkage bubble and are in average 19552 ± 772 ppm, the highest content of CO₂ measured so far in crustal melt inclusions. The modelled endogenic carbon flux associated with the subduction of the continental crust of the Variscan Orogenic Cycle is 22 ± 8 Mt C yr^-1. This flux within error is similar to the endogenic carbon fluxes in the serpentinized mantle (~ 14 Mt C yr^-1) and to the exogenic fluxes in mid-oceanic ridges (~ 16 Mt C yr^-1) and arc volcanoes (~ 24 Mt C yr^-1). Hence, in collisional settings, deeply subducted continental crust carried a large amount of volatiles to the mantle and the lower crust. Due to the absence of post collisional arc volcanism, most of these volatiles remained trapped in the root of mountain belts. This long-term storage of the carbon in the orogen roots prevents ultimately the closure of the carbon cycle.

How to cite: Borghini, A., Nicoli, G., Ferrero, S., O'Brien, P. J., Laurent, O., Remusat, L., Borghini, G., and Milani, S.: Deep subduction of continental crust contributes to mantle metasomatism and deep carbon cycle, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5782, https://doi.org/10.5194/egusphere-egu22-5782, 2022.

Earth’s carbon cycle is strongly influenced by subduction of sedimentary material into the mantle. The composition of the sedimentary subduction flux has changed considerably over Earth’s history, but the impact of these changes on the mantle carbon cycle is unclear. Here we show that the carbon isotopes of kimberlite magmas record a fundamental change in their deep-mantle source compositions during the Phanerozoic Eon. The ¹³C/¹²C of kimberlites prior to ~250 Myr preserves typical mantle values, whereas younger kimberlites exhibit lower and more variable ratios – a switch coincident with a recognised surge in kimberlite magmatism. We attribute these changes to increased deep subduction of organic carbon with low ¹³C/¹²C following the Cambrian Explosion when organic carbon deposition in marine sediments increased significantly. These observations demonstrate that biogeochemical processes at Earth’s surface have a profound influence on the deep mantle, revealing an integral link between the deep and shallow carbon cycles.

How to cite: Giuliani, A., Drysdale, R. N., Woodhead, J. D., Planavsky, N. J., Phillips, D., Hergt, J., Griffin, W. L., Oesch, S., Dalton, H., and Davies, G. R.: Perturbation of the deep-Earth carbon cycle in response to the Cambrian Explosion, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3611, https://doi.org/10.5194/egusphere-egu22-3611, 2022.