The deep carbon cycle in the Archean is poorly constrained. Carbonate sedimentation only became an important reservoir for carbon from the late Archean onwards. It has been proposed that the transfer of carbon from the hydrosphere/atmosphere to the lithosphere mainly occurred during alteration of basalts of the oceanic crust [1]. While carbonation of ancient ultramafic rocks including komatiites has been described, it is often assumed that this carbonation occurred much later than the komatiite formation.In this contribution, we investigate the role of carbonation of komatiites for the Archean deep carbon cycle. The Barberton Greenstone Belt of the Kaapvaal Craton, South Africa, provides a well-preserved pre-3 Ga terrestrial record and hosts hydrated komatiites that erupted ~3.48 Ga ago. We present data from samples of the ICDP drill core BARB1, transecting ultramafic lavas of the Komati Formation at depths of 108.77 to 112.73 meters. These komatiites have remained shielded from surface alteration throughout geological history and thus have not been affected by carbonate formation related to recent weathering.The 3 m thick komatiite flow is covered by an andesitic volcaniclastic rock, where carbonate is intergrown with titanite in an albite-biotite-amphibole assemblage. A U-Pb age for titanite of 3266 ± 44 Ma demonstrates carbonate formation prior to this metamorphic overprint. In the uppermost 1.2 m of the komatiite flow only calcite is present, coexisting with chlorite, serpentine, tremolite, talc and magnetite. The volume of calcite decreases from 8-10 vol% in the first 40 cm to 2-6 vol% at 1 m depth where mainly spinifex textured komatiite is present. At 2-3 m depth, cumulate textures predominate, and the volume of calcite is always

Subduction processes regulate elemental cycling and consequently the composition of Earth's geochemical reservoirs - mantle, crust, atmosphere, and hydrosphere, exerting far-reaching effects for the evolution of life. Barium (Ba) is essential for understanding crust-mantle recycling, especially since over 90 % of Ba in arc lavas originates from subducted materials. While Ba enrichments in arc lavas have long been attributed to contributions from oceanic crust derived aqueous fluids, recent studies highlight hydrous sediment melts as a key carrier. Here, we present high-precision Ba isotope data (delta Ba-138/134) of input and output from the Tongan subduction zone. The results show that Ba is not predominantly supplied by the subducting oceanic crust. Instead, most Ba can be traced back to subducting sediments from which it is released in two stages. During early subduction, sedimentary barite dissolves and releases isotopically heavy Ba via aqueous fluids into the serpentinized mantle wedge, where this Ba is later remobilized during subsequent breakdown. With continued slab descent, remaining lithogenic phengite releases isotopically lighter Ba, which is recycled at subarc depths by hydrous melting. Both mechanisms can explain the entire range of Ba/Th and delta Ba-138/134 in Tongan arc lavas without requiring Ba isotope fractionation between fluids and solids. These results from the endmember setting of Tonga, with minimal subducting sediments, imply higher proportion of sediment-derived Ba in other arcs worldwide. The Forearc Serpentinite Signature (FSS), which is created by the interaction of the mantle wedge with sediment-derived fluids at low temperatures (<600 degrees C), represents an additional fluid endmember that must be considered in the genesis of arc lavas. Lithogenic Ba that is not completely released at subarc depths can be further subducted into the deeper mantle, where it may later enrich mantle domains that source mid-ocean ridge and hotspot lavas. The relative proportions of sedimentary components in subducted slabs have changed over Earth's history, highlighting the link between arc magma and mantle compositions and long-term shifts in Earth's surface (bio)geochemical cycles.

The Swiss Journal of Geosciences (SJG) is the scientific full Open-Access journal of the Swiss Geological Society. First published in 1888 as “Eclogae Geologicae Helvetiae”, it has a long tradition. From the 1920s until 2006, it was published in collaboration with “Birkhäuser”, a scientific publisher based in Basel which was sold to Springer in 1985. In 2006, the SJG merged with the «Schweizerische Mineralogische und Petrographische Mitteilungen (SMPM, first published in 1921), after which the journal was renamed to Swiss Journal of Geosciences (see Schmid, 2006; Schmid et al., 2007). From 2007 (Vol. 100) to 2019 the SJG was published with Springer, and since 2020 it has been published as a full Open Access Journal with SpringerOpen.

Rutile is a nominally anhydrous mineral that can incorporate significant amounts of hydrogen through different trace element-related H+-defects. The hydrogen content in rutile has been proposed to be pressure- and redox-dependent. To evaluate the influence of temperature, pressure, oxygen fugacity, and rutile trace element composition on hydrogen solubility in rutile, we performed experiments on rutile with three different compositions (two "pure" and one Cr-doped) at 600 and 950 degrees C, 0.8 and 2 GPa, and two different oxygen fugacity conditions (one set unbuffered close to the nickel-nickel oxide buffer and the other buffered at graphite-COH). Under the experimental conditions, only Ti3+-related H+-defects formed, suggesting a kinetic barrier hindering the formation of trace element-related H+-defects coupled with divalent and trivalent cations substituting for Ti4+. H2O contents in rutile from experimental run products range from 60-500 mu g/g and increase with increasing pressure and decreasing oxygen fugacity. Contrary to previous studies, a significant temperature dependence was not observed. Impurities in rutile, which result in high oxygen-vacancy defect densities, enhance hydrogen incorporation, leading to higher H+ contents in rutile with greater di- and trivalent impurity concentrations. The amount of oxygen vacancies largely controls the H+ solubility in rutile, making it important to constrain the trace element chemistry of rutile before hydrogenation experiments. Natural rutile from low-temperature eclogite facies conditions (2 GPa, 600 degrees C) has similar (or higher) H+ contents (200-900 mu g/g H2O) to those observed in representative experimental run products (150-500 mu g/g H2O). Thus, H+ retention in natural systems is possible at much higher temperatures (similar to 600-700 degrees C) than suggested by previous experimental studies on hydrogen diffusion in rutile. High-temperature experiments show higher H+ contents in rutile compared to natural high-T rutile, in agreement with diffusive hydrogen loss above 700 degrees C in natural rocks. This suggests that rutile is a viable, qualitative barometer and potential oxybarometer in medium- to low-temperature (<650 degrees C) metamorphic rocks.

The evolution of plate tectonics and subduction conditions on Earth is widely debated in geosciences. Modernstyle cold subduction is characterized by the formation of low-temperature - high-pressure (low-T-high-P) and ultra-high-pressure (UHP) metamorphic mafic rocks from subducted oceanic crust. However, such low-T eclogites and UHP metamorphic rocks are largely absent in the Precambrian geological record, either because such conditions were not reached or because these rocks have been eroded. Rutile is a common metamorphic accessory mineral that can form in subduction zone conditions, is stable during sedimentary cycling and is therefore a target mineral to screen for subduction-related signatures in sediments. Detrital rutile carried by rivers draining cold subduction-related mafic rocks in the Internal Western Alps has a distinct fingerprint, characterized by high H2O/Zr ratios and low total Nb+W+Sn contents, which is consistent with a cold subduction-signature. Thus, detrital rutile can be used to evaluate cold subduction signatures in Precambrian sediments. A case study from the Hebridean Terrane, NW Scotland, was employed to search for a potential cold subduction signature prior to the Neoproterozoic. Detrital rutile ages range between 1.9 and 1.0 Ga, and while high Zr-in-rutile temperatures and trace element systematics indicative of other sources are predominant, a few rutile grains have an unequivocal mafic cold subduction signature, indicating local cold subduction in the Paleoand Mesoproterozoic in East Laurentia.

Dieser Band bietet eine Einführung in die Entstehung und Geschichte der Erde. Im Zentrum steht die Plattentektonik als treibende Kraft einer dynamischen Erde, und die Prozesse, die zur Bildung von magmatischen, metamorphen und sedimentären Gesteinen führen. Dargestellt wird, wie Interaktionen zwischen der Lithosphäre, der Hydrosphäre und der Atmosphäre den globalen Wasserhaushalt und Klimawandel über geologische Zeiträume beeinflussen. basics – Lehrbücher mit einem klaren Konzept: - Definitionen, Beispiele und Zusammenfassungen erleichtern den Überblick - Zahlreiche Abbildungen veranschaulichen dynamische geologische Zusammenhänge und Prozesse - ideal für die Prüfungsvorbereitung

Fault healing (i.e., strength recovery) is a crucial process impacting the magnitude and timescale of fault failure within the earthquake cycle. Detailed knowledge of the processes and their timescale during fault healing occurring after coseismic faulting in the lower crust remain elusive. Earthquakes are frequently recorded in silicate rocks as pseudotachylytes (quenched coseismic-derived frictional melts). We examine the pristinely preserved microstructures of a lower-crustal pseudotachylyte vein formed in the minutes after a lower-crustal earthquake. Garnet-clinopyroxene geothermometry, major and trace element mapping, electron backscatter diffraction, and transmission electron microscopy combined with a 1D cooling model demonstrate that these microstructures crystallised rapidly within an hour following the earthquake, from >1200 °C to the ambient temperature of ~700 °C. This pseudotachylyte vein captured a geologically instantaneous temporal sequence of phase nucleation and growth, resulting from ultrafast fault healing during the incipient postseismic transition. This transition is marked by the rapid growth and sintering of orthopyroxene and garnet from comminuted grains. Preservation of primary vein-scale garnet compositional zoning further demonstrates that garnet crystallisation from the frictional melt was faster than chemical homogenisation in melt. These microstructures provide an exceptional record of the processes occurring immediately after an earthquake because of the dry ambient conditions.

Mid- to lower-crustal shear zones accommodate large strains by high-temperature viscous flow. Yet, strain localization is strongly modulated by transient thermal and chemical perturbations such as the presence of syn-kinematic melts or fluids.We aim at unravelling the general strain localization behavior, the role of fluid/melt presence on the rheology of polymineralic shear zones, with focus on potential changes from mid- to lower-crustal levels.For this purpose, we use the Cossato-Mergozzo-Brissago (CMB) and the Pogallo shear zone systems in the Southern Alps, Northern Italy (Handy, 1987) as a natural laboratory. Field observations and targeted sampling were combined with quantitative microstructural analysis of polymineralic mylonites and ultramylonites. Quartz paleopiezometry (monomineralic quartz bands) provides differential stress and Ti-in-biotite (Henry et al., 2005) provides temperature for felsic lithologies. Such data from natural mylonites are used as input for granitoid shear-zone flow laws (Nevskaya et al., 2025b) to derive strain rates and compare rheology across crustal depths.Field and microstructural observations indicate two endmember microfabric types. Type I (CMB) fabrics occur within a broad (~2-3 km) belt of felsic mylonites (grain size ~50-100 µm). Inside the mylonites, many dykes developed with episodic pulses of melt injection and syn-kinematic back veining (Handy and Streit, 1999). These mylonites commonly contain quartz-feldspars-mica domains with steady-state grain sizes stabilized by pinning and dissolution-precipitation processes. Ti-in-biotite thermometry indicates lower-crustal temperatures of ~680-730 °C.Type II (Pogallo) fabrics also represent microstructural steady states characterized by fine- to ultrafine-grained ultramylonites (grain size

Magmatic-hydrothermal ore deposits, such as pegmatites, are an increasingly important source of metals and critical elements for the development of green-energy resources. The geochemical processes at the magmatic-hydrothermal transition influence the degree of element enrichment in these ores. Quartz is a mineral that grows throughout the complete crystallization sequence of granitic pegmatites. Tracking the systematics of trace element incorporation into the quartz crystal structure throughout the magmatic-hydrothermal pegmatite evolution may offer unprecedented insights into pegmatite genesis.Quartz crystals from the Rosina pegmatite, Elba, Italy, and from the Misox pegmatite, Ticino, Switzerland, were mapped using scanning electron microscopy (SEM) charge contrast imaging and Fourier transform infrared (FTIR) spectroscopy to determine the distribution of OH coupled to Li, B, and Al. Trace element laser ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS) spot measurements were done on the same quartz crystals, navigated by the spatial distribution of zonation observed via FTIR maps. In Rosina quartz, total Li, B, Al, and Ti mass fractions are higher in the cores of the zoned crystals when compared to the rims, whereas in Misox quartz, the total Li, B, and Al increase from core to rim. In both Rosina and Misox quartz, OH coupled to Li, B, and Al closely follows the zonation of the total trace element contents. Lithium, B, and Al coupled to OH represent 5 %-30 % of the coupled substitutions in quartz. Thus, OH-related point defects provide another tool for provenance and ore body prospecting studies. In Rosina quartz, the LiOH defect is dominant in the FTIR spectra, which is rare for quartz and only characteristic for evolved pegmatitic quartz crystals.Temperature estimates of quartz formation were constrained by Ti-in-quartz geothermometry, linking the observed geochemical processes to the pressure-temperature conditions at which they took place. Rosina quartz formed at pressures of 2.3 kbar and temperatures between 590 (core) and 330 (rim) degrees C, while Misox quartz formed at pressures of 5-6 kbar and temperatures between 520 (core) and 300 (rim) degrees C (calculated at TiO2 activity of 0.5). Relative temperatures consistently decrease from core to rim, and absolute temperatures are uncertain due to the difficulty of constraining the activity of Ti during quartz crystallization. The trace element evolution during quartz crystallization was spatially resolved and tracked through the complete quartz crystal growth period of the pegmatites. Trace elements in quartz crystals from Rosina record a prominent change in incorporation at the core-rim transition, while the interpretation of quartz trace element patterns in Misox quartz is further complicated by twinning patterns. These results show that a combination of quartz FTIR and LA-ICP-MS analyses successfully constrains the changes in the geochemical environment during ore body formation. In particular, Li enrichment in combination with H2O contents in quartz might be useful in the study of detrital quartz when prospecting for Li-rich pegmatites or other economically significant magmatic-hydrothermal ore deposits.

To understand the onset and evolution of cold subduction on Earth, detrital sedimentary rocks of Precambrian age, potentially derived from exposed high-P low-T metamorphic rocks can be investigated. This requires the estimation of peak metamorphic pressure and temperature, time of formation, and source lithology (P-T-t-X) of detrital single grains. Rutile is a common accessory mineral in subducted oceanic crust and one of the most likely minerals from subducted rocks to survive sedimentation processes. As single grain T-t-X estimates on rutile are possible, it is a prime candidate for the investigation of subduction processes through time.We developed a method to identify rutile formed in modern cold subduction conditions, by combining in-situ polarised Fourier Transform Infrared Spectroscopy (FTIR) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS).Our study reveals a pressure-dependent variation in hydrogen content within metamorphic rutile, ranging from less than 10 to 2500 μg/g H2O. Higher peak metamorphic pressures correspond to elevated H2O contents, particularly noticeable in mafic low-temperature eclogite facies rutile, suggesting H-in-rutile can be used as a pressure indicator. Using Zr in rutile as a temperature indicator, H2O/Zr ratios act as proxies for thermal gradients (P/T) in metamorphic rutile. When combined with low Nb, W, and Sn contents, typical of mafic protoliths, it is possible to identify modern-style cold subduction of mafic crust using trace element signatures in detrital rutile.Therefore, detrital rutile can serve as a tracer for subduction conditions over time, as modern-style cold subduction signatures are preserved in rutile during weathering and sedimentary processes. In this study, we test our novel approach on detrital rutile grains of sandstones and arkoses from the Torridon and Ardvreck Groups, Hebridean in NW Scotland. Our analysis reveals that some grains of the Torridon Group of late Proterozoic age (detrital ages ranging from 1.0 to 1.9 Ga; Pereira et al., 2020) exhibit high H2O/Zr ratios and low total Nb+W+Sn contents, typical of low-T eclogite facies rutile. This implies that low-T eclogites which formed during cold subduction were likely exposed and eroded in the catchment of the sedimentary basins, indicating modern-style cold subduction during the Mesoproterozoic. We propose that the combined measurement of H2O and trace elements in detrital rutile is a powerful tool to search for remnants of cold subduction through the Earth’s history. Pereira, I., Storey, C.D., Strachan, R.A., Bento dos Santos, T., Darling, J.R., 2020. Detrital rutile ages can deduce the tectonic setting of sedimentary basins. Earth Planet. Sci. Lett. 537, 116193. https://doi.org/10.1016/j.epsl.2020.116193

The Swiss Journal of Geosciences (SJG) is the scientific full Open-Access journal of the Swiss Geological Society. First published in 1888 as “Eclogae Geologicae Helvetiae”, it has a long tradition. From the 1920s until 2006, it was published in collaboration with “Birkhäuser”, a scientific publisher based in Basel which was sold to Springer in 1985. In 2006, the SJG merged with the «Schweizerische Mineralogische und Petrographische Mitteilungen (SMPM, first published in 1921), after which the journal was renamed to Swiss Journal of Geosciences (see Schmid, 2006; Schmid et al., 2007). From 2007 (Vol. 100) to 2019 the SJG was published with Springer, and since 2020 it has been published as a full Open Access Journal with SpringerOpen.

Present-day continental lithospheric mantle (CLM) heat production estimates vary considerably and likely overestimate heat generation due to the infiltration of the host magma (i.e., kimberlite), mantle metasomatism or variable heat-producing element (HPE) ratios. We present estimates of heat production in the CLM beneath Jagersfontein, from bulk rock reconstruction of 11 peridotitic xenoliths based on in-situ analyses of primary mineralogy, to avoid kimberlite contamination. Higher concentrations of Th and U are observed in the reconstructed bulk rocks at shallower depths (< 5 GPa) and decrease towards the deepest parts of the CLM (Th: 0.5–26 versus 1–5 ppb; U: 0.4–19 versus 1–3 ppb). Moreover, the reconstructed samples have a broad range of bulk K/U ( 70-16500) and Th/U ratios ( 0.2–3.8), outside the expected range of the modern convecting mantle. A crucial factor is garnet, as it can control the U budget, has Th/U < 1 and is present across the CLM in the garnet stability field. The differences of the CLM with the convecting mantle challenge the use of assumedly constant HPE ratios to calculate the heat production. Our estimates of present-day heat generation from reconstructed bulk data yield 0.0002–0.008 µW/m3 at shallow depths, decreasing down to 0.0005 µW/m3 near the lithosphere-asthenosphere boundary, lower than typical heat generation values used in most previous models. The variable heat production in the CLM derives from the metasomatism and re-fertilization near the base caused by rising asthenospheric melts, which react and fractionate as they ascend, potentially carrying most of the HPE in a fluid phase to shallower depths.

The estimates of the chemical composition of the lower continental crust ranges from predominantly mafic to felsic. The Ivrea Zone in the Southern European Alps provides insight into this variability, featuring a pre-Permian mostly felsic lower crust modulated by additions of mafic rocks during Permian underplating. The Ivrea zone is an ideal location to examine major, trace, and volatile elements over the full range of proposed lower crustal compositions. Our study presents whole-rock data derived from a drill core of the first hole (DT-1B) of the ICDP-funded project DIVE (Drilling the Ivrea-Verbano Zone). The drilled section spans nearly 600 m, representing an upper part of the Ivrea lower continental crust. Logging of the drill core showed that biotite-gneisses (Qtz + Pl + Bt ± Gt ± Kfs ± Sil – 75 vol%) and metamafic rocks (Amp + Pl + Qtz ± Px ± Bt ± Gt – 21 vol%) are the main rock types with minor calcsilicate rocks (Cc + Gt + Px + Ttn + Qtz + Pl ± Amp – 2 vol%), and some minor pegmatites (2 vol%). Both targeted and grid sampling strategies aimed to minimize sampling bias, providing a reliable basis for understanding the Ivrea lower continental crustal composition and extrapolating the results toward a realistic assessments of the LCC composition in general.Amphibolite facies metasediments (34 samples) range from calc-silicates to pelites and psammites, exhibiting a wide range of major element compositions (32 - 89 wt.% SiO2; 0.5 - 5.8 wt.% K2O; 0.35 - 0.54 Mg#). Metamafic rocks (16 samples) cover a more restricted compositional range (43 - 57 wt.% SiO2; 0.1 - 5 wt.% K2O; 0.3 - 3 wt.%; 0.36 - 0.61 Mg#). Most mafic rocks are LREE enriched, but a few resemble MORB-like compositions. A preliminary comparison of the bulk rock estimate of the entire drill core relative to the integrated composition derived from geological maps indicates that deviations between the two approaches are considerable, ranging from

A large body of work has challenged the paradigm of carbonate stability at the forearc and subarc (> 80 km) conditions in subducted slabs and revealed a variety of complex processes that play an important role in the so-called slow C cycle. Serpentinite-hosted carbonate rocks (i.e., ophicarbonates) are an important rock type for the deep C cycle because they can occur either in the slab or in the mantle wedge. The question revolves around phase stability and metamorphic reactions upon subduction that can lead to a change in carbonate phase assemblage and fluid composition. Moreover, the phase relation between carbonates, silicates, oxide and sulfide minerals in ophicarbonates can be informative about the redox conditions during prograde metamorphism. We present a case study of ophicarbonate rocks from the Zermatt-Saas unit, Western Alps, that were subducted up to eclogite facies conditions at 2.5 GPa, 560° C. In the study area, ophicarbonates overlie a large body of partially dehydrated serpentinites. This allows us to understand whether fluids released from the serpentinites infiltrated the ophicarbonates or not, and to what extent decarbonation reactions occurred in an open or closed system. We investigated three carbonate-bearing rock types: ophicarbonates, olivine-carbonate veins, and a talc-magnesite reaction rind at the contact between ultramafic and mafic/felsic lithologies. Our petrological and geochemical investigation, as well as thermodynamic modelling, reveal that the metamorphic evolution of the ophicarbonate was in a closed system, where calcite/aragonite was replaced by metamorphic dolomite and diopside, and that this reaction is nearly CO2 conservative, with the released fluid composition close to pure water. In situ LA-ICP-MS trace element analyses also show that carbonate in olivine-carbonate veins was most likely sourced from the ophicarbonates. Our thermodynamic modelling indicates that the talc-magnesite reaction zone was most likely formed during early exhumation between 9-13 kbar and 530-460° C, at XCO2 between 0.007 and 0.009. Lastly, we will discuss how the silicate-oxide-sulfide redox buffering assemblage indicates that all three rock types were equilibrated at redox conditions < FMQ. In conclusion, our study demonstrates that in the absence of external fluid infiltration, carbonates in ultramafic lithologies are stable at subduction conditions. This suggests that ophicarbonate have a potential important role in the deep, long term, carbon cycle.

Serpentinite-hosted carbonate rocks (i.e., ophicarbonates) are an important rock type for the deep C cycle because they can occur either in the slab or in the mantle wedge. We present a case study of ophicarbonate rocks from the Zermatt-Saas unit, Western Alps, that were subducted up to eclogite facies conditions at 2.5 GPa, 560 °C. In the study area, ophicarbonates overlie a large body of partially dehydrated serpentinites. This allows us to understand whether fluids released from the serpentinites infiltrated the ophicarbonates or not, and to what extent decarbonation reactions occurred in an open or closed system. We investigated three carbonate-bearing rock types: ophicarbonates, olivine-carbonate veins, and a talc-magnesite reaction rind at the contact between ultramafic and mafic/felsic lithologies. Our petrological and geochemical investigation, as well as thermodynamic modeling, reveal that the metamorphic evolution of the ophicarbonate was in a closed system, where calcite/aragonite was replaced by metamorphic dolomite and diopside, and that this reaction is nearly CO2 conservative, with the released fluid composition close to pure water. Limited carbonate mobility is indicated by the occurrence of minor olivine-carbonate veins. In situ LA-ICP-MS trace element analysis shows that carbonate in veins is most likely sourced from the ophicarbonates suggesting CO2 transport on the 10 m scale. The silicate-oxide-sulfide redox buffering assemblage indicates that both ophicarbonates and olivine-carbonate veins are equilibrated at redox conditions at or below FMQ. Field and thermodynamic modeling show that C-rich fluids circulated during exhumation along major structures and/or lithological interfaces. This leads to the formation of metasomatic talc-magnesite rocks during early exhumation between 9 and 13 kbar and 530–460 °C, at XCO2 between 0.007 and 0.009. Our study demonstrates that in the absence of external fluid infiltration, carbonates in ultramafic lithologies are stable at subduction zone conditions and can efficiently return C to the deep mantle.

The lower continental crust is a critical component of the deep carbon cycle, serving as a long-term reservoir for carbon (C) in the form of residual carbonates and graphite. Yet, the extent of C storage remains poorly understood. The Ivrea-Verbano Zone in northern Italy exposes lower crustal mafic and metasedimentary lithologies, providing a unique natural laboratory to investigate C retention. As part of the ICDP-funded DIVE project (Drilling the Ivrea-Verbano Zone), this study focuses on quantifying the C budget and exploring the isotopic composition of C phases.The upper portion of the lower continental crust (borehole 5071-1_B, Ornavasso) consists primarily of felsic metasedimentary rocks (kinzigites, 73 vol-%) alongside amphibolites (13 vol-%) and calcsilicate rocks (11 vol-%), metamorphosed under upper amphibolite facies conditions (~750 ± 50°C, 7.5 ± 1.5 kbar). C is hosted in the form of graphite (Gr) and calcite (Cc). Gr occurs as inclusions in garnet and in the matrix of kinzigites, while Cc is observed in calcsilicate rocks and occasionally in amphibolites and leucosomes. Notably, a single marble layer has been identified.The isotopic composition of C (Gr) and C-O (Cc) is being investigated to provide insights into the origin and evolution of C. Preliminary results range from -11.8 ‰ to -13.8 ‰ δ13CGr in the kinzigites and -0.7 to ‰ to -5.6 ‰ δ13CCc as well as 11.0-15.0 ‰ δ18OCc in the calcsilicate rocks.The marble layer from the borehole exhibits δ¹³C- and δ¹⁸O-values of -8.22 ‰ and 12.5‰, respectively, while marbles from nearby outcrops show a broader range of -0.7 to 1.19 ‰ δ13CCc and 13.6 to 22.6 δ18OCc. The isotope data supports the field observations suggesting that the sequence formed at the surface before burial and metamorphism.Two sampling approaches were employed to determine the average and the local variability of C concentrations. (i) A broad approach, where samples of 6-12 cm length were taken from each rock type at approximately 10-meter intervals throughout the entire borehole, providing a comprehensive overview of the C distribution across different lithologies. Carbon-Nitrogen-Sulfur analyses from this approach revealed that kinzigites contain an average of 0.26 wt.-% C, while amphibolites and calcsilicate rocks average 0.07 and 0.73 wt.-% C, respectively. For the 578.7m deep borehole, the overall carbon concentration reaches an average of 0.23 wt.-%. (ii) A complementary microbulk sampling approach was specifically designed compare the variability across different scales, which is especially relevant in heterogeneous rock types. This method involves extracting core segments perpendicular to the foliation and subdividing them into centimetre-scale slices to capture fine-scale heterogeneities. While detailed results from the microbulk approach are pending, preliminary observations reveal notable intra-rock variability in C content. For example, a single kinzigite segment analysed using the microbulk approach covers the range in which 63% of the total number of kinzigite samples from the more extensive broad approach dataset are contained (n=27).Our findings underline the importance of metasedimentary rocks at lower crustal depths (~25 km, ~750°C) as C reservoirs which enhances our understanding of the carbon cycle in deep crustal environments.