Mantle-derived CO2 Research Articles

Conceptual groundwater model of the north-eastern flanks of Mount Kenya

Despite serious concerns over declining river flows and prolonged dry spells in the north-eastern region of Mount Kenya and the Ewaso Ng’iro River watershed many aspects of the groundwater system remain unexplored. In particular, the recharge-discharge dynamics of the Ewaso Ng’iro River have not been studied, and no conceptual groundwater model currently links the recharge areas in the high-elevation humid regions to the drier lowlands. This study aims to address this significant knowledge gap by assessing the recharge-discharge dynamics of the Ewaso Ng’iro River and identifying the relevant groundwater subsystems and the main flow paths within the various lava layers constituting the aquifer system. Hydrochemical and stable isotope analyses revealed three distinct subsystems with slightly different chemistries and different recharge zones, all of recent meteoric origin. Groundwater from Mt. Kenya is of the HCO3-Na-Mg-Ca type with no dominant cations, whereas groundwater from the Nyambene Range, the other surrounding volcanic hills in the area, is of the HCO3-Na type. Groundwater in the third subsystem in between is of the HCO3-Mg type and is confined or semi-confined. In this area, carbon-13 analysis showed a strong influence of mantle-derived CO2 on the groundwater chemistry (very high electrical conductivity and left-shifted oxygen-18 ratios). Finally, major ions and stable isotopes (ẟ18O and ẟ2H) confirmed that during the dry season, the river is entirely groundwater-fed, with the Mt. Kenya subsystem contributing half of the maximum river flow rate of 800 l/s.

The impacts of CO2 on sandstone reservoirs in different fluid environments: insights from mantle-derived CO2 gas reservoirs in Dongying Sag, Bohai Bay Basin, China

The impacts of CO2 on sandstone reservoirs in different fluid environments: insights from mantle-derived CO2 gas reservoirs in Dongying Sag, Bohai Bay Basin, China

Groundwater–rock interactions and mixing in fault–controlled karstic aquifers: A structural, hydrogeochemical and multi-isotopic review of the Pontina Plain (Central Italy)

Karstic aquifers represent crucial water resources and are categorized as either stratigraphically or fault–controlled. This study investigates groundwater–rock interactions and mixing processes within one of the largest fault–controlled karstic aquifers in Central Italy, adjacent to the Pontina plain, which is a highly populated area where agricultural activities and climate change challenge the groundwater assessment of a complex aquifer. We conducted structural, hydrogeochemical, and multi-isotopic screening of ten selected springs with different degrees of mineralization (ranging from Ca–HCO3 to Na–Cl hydrofacies), incorporating new analyses and modeling of δ34S(SO4), δ18O(SO4), 87Sr/86Sr, and δ11B. Additionally, the reinterpretation of a seismic section provides a more detailed framework extending to depths of approximately 5–7 km that allows the identification of the geometry of normal faults, which act as pathways for upwelling fluids. Our findings reveal that hydrogeochemical compositions result from multiple interactions between karstic water and deeper fluids that have interacted with different rocks. Concentration (Na/Li) and isotope (SO4–H2O) geothermometers, coupled with geochemical modeling and trace element analysis, enabled the estimation of a water temperature equilibrium of approximately 95.5 °C, with Triassic evaporites generally corresponding to a depth of approximately 3 km and a temperature of 40 °C with magmatic rocks at approximately 1 km depth, which is likely associated with ongoing tectonics and the Quaternary tectonically controlled Volsci Volcanic Field. To obtain the latter estimate, we used a new geothermometer activity based on the equilibrium between analcime and pollucite. Furthermore, this multidisciplinary approach enhances the understanding of groundwater behavior in fault–controlled karstic aquifers, where mantle-derived CO2 dissolved in groundwater is the driving force behind water–rock interactions. Given the potential for further variations in mixing, which may worsen water quality and increase aquifer vulnerability, periodic monitoring of these processes is essential in a human-impacted environment amidst ongoing climate change.

Massive crustal carbon mobilization and emission driven by India underthrusting Asia

The active Himalayan-Tibetan orogen, where India underthrusts into Asia, is an important geological source of carbon dioxide (CO2) emission into Earth’s atmosphere. However, the extent to which Indian underthrusting could stimulate the mobilization of deeply-sourced carbon and its subsequent emission remains unknown. Here, we use a combination of field observations coupled with in-situ CO2 flux measurements and helium and carbon isotopic data, to study the controls on CO2 origins and fluxes in a 400-kilometre-long rift transecting northern Himalaya and southern Tibet. High diffuse CO2 fluxes sustained by pure crustal fluids are confined to rift segments in the northern Himalaya, while toward southern Tibet, CO2 fluxes become lower but mantle fluid inputs are identified. Such rift-related CO2 degassing profile suggests metamorphic decarbonation and release of carbon-bearing fluids enhanced by the underthrusting Indian lower crust, agreeing well with Himalayan metamorphism and orogen-parallel lithospheric extension. Deep CO2 fluxes from extensional tectonics in northern Himalaya and southern Tibet, primarily of crustal origins, are comparable to mantle CO2 fluxes from global mid-ocean ridges. Our findings demonstrate that geophysical and geo-tectonic responses to continental underthrusting could facilitate massive crustal carbon mobilization and emission, making active collisional orogens globally important carbon sources.

Insight into the mechanism of helium enrichment in natural gas from the Bohai Bay basin, China: Chemical and isotopic composition perspectives

Helium (He) is an important societal resource, yet its enrichment mechanism is unclear in the Bohai Bay Basin, despite the presence of He-rich (He > 0.1% v/v) gas. A comprehensive analysis of 112 sets of chemical and isotopic data from natural gases in the literature was conducted to further elucidate the source and accumulation mechanism of He in this basin. Natural gas is categorized into five groups based on CO2 and N2 abundances: N2 gas (CO2+N2 ≥ 95%, CO2 < N2), high-purity CO2 gas (CO2+N2 ≥ 95%, CO2 > N2), low-purity CO2 gas (50 ≤ CO2+N2 < 95%, CO2 > N2), low-purity HC gas (15 ≤ CO2+N2 < 50%) and high-purity HC gas (CO2+N2 < 15%). δ13C–CO2 (−8.3 to −3.0‰) and 3He/4He (1.66–6.45 Ra, where Ra is the atmospheric 3He/4He) suggest a mantle origin of CO2 in N2 and CO2 gases. The δ13C–CO2 of N2 gas (−8.3‰) is lighter than that of high-purity CO2 gas (−6.2 to −3.4‰), indicating CO2 dissolution and mineralization. A positive correlation between He and CH4 content in the N2 and high-purity CO2 gases suggests water-derived CH4 capture during CO2 migration. Similarly, the positive correlation between He and N2 content across all samples indicates water-derived N2 capture during gas migration. A mixture of mantle and crust-derived He is indicated by 3He/4He ratio (0.06–6.45 Ra). Two distinct types of He-rich gas were identified: He-rich N2 gas (3He/4He = 3.1–3.2 Ra) and He-rich HC gas (3He/4He = 0.36–0.48 Ra). He enrichment in N2 gas is attributed to water-derived He capture and concurrent loss of CO2 during mantle-derived CO2 migration. He-rich HC gas formation results from He-rich N2 gas dilution with hydrocarbons. The findings provide theoretical support for exploring He-rich natural gas in this basin and offer valuable insights into He enrichment mechanisms in comparable sedimentary basins globally.

Hydrothermal alteration mechanisms of an Archaean metamorphic buried hill and the models for reservoir zonation, Bozhong depression, Bohai Bay Basin, China

Multiple metamorphic buried-hill hydrocarbon reservoirs have been recently discovered in the Bozhong Depression, Bohai Bay Basin. Among them, the buried-hill reservoirs in the BZ13, BZ19 and BZ26 regions have the largest condensate gas reserve in eastern China. The hydrocarbon-bearing reservoir has a large thickness with a maximum of ∼1 km, making the oil-equivalent reserve exceed 5 × 108 tons. This study used petrography, isotopes, gas composition, zircon U–Pb geochronology, X-ray diffraction, electronic probe and hydrothermal dissolution experiments to discuss the formation mechanisms of the metamorphic buried-hill reservoirs. The results indicate that the reservoirs 500 m below the surface of the buried hill have been significantly altered by the mantle-derived CO2. This is supported by the carbon isotope of CO2, He isotope of the helium gas, strong negative excursions in δ18OPDB, positive excursions in δ13CPDB, Co/Ni ratio greater than 1 in pyrite and secondary characteristic minerals such as dawsonite. Hydrothermal dissolution experiments using CO2 result in the precipitation of kaolinite, ankerite and quartz cements, which is consistent with petrographic observation. The mantle-derived CO2 reacts with feldspars, hornblende, biotite and carbonates, creates secondary dissolution pores and significantly improves reservoir quality. Moreover, partial carbonate cements exhibit negative excursions in δ13CPDB and the depletion of light rare earth elements, indicating the influence of organic acids-bearing low-temperature hydrothermal fluids. Therefore, three development models illustrating the formation of metamorphic buried-hill reservoirs were proposed. This study may serve as a reference for the hydrocarbon exploration in metamorphic buried-hill reservoirs.

Isotopic and kinetic constraints on methane origins in Icelandic hydrothermal fluids

The origin of methane in hydrothermal fluids has long been a subject of debate – whether it is abiotic or biotic. In this study, we aim to unravel and quantify the sources of CH4 in active hydrothermal systems by adopting a holistic approach analyzing well characterized high-temperature hydrothermal fluids (∼230–310 °C) in Iceland. We employ a broad variety of geochemical and isotope indicators, encompassing chemical and isotope compositions of the targeted fluids. These signatures are then compared with results from chemical and isotope kinetic models and data from sedimentary-hosted hydrothermal systems. Carbon species in these fluids include CO2 (2.60–184 mmol/kg), CH4 (2.39·10−4–0.325 mmol/kg), dissolved organic carbon (4.78·10−3–0.112 mmol/kg), and CO (1.89·10−6–4.16·10−4 mmol/kg). Carbon and helium isotopes suggest a relatively uniform mantle-derived source of CO2 (δ13C-CO2: −4.80 to −1.50 ‰, CO2/3He: 1.49·109–4.14·1010 14C-CO2: 0.11–2.42 pMC). Methane, in contrast, has multiple sources. Overall, chemical equilibria among carbon species (CO2, CH4, CO) is not attained, suggesting kinetic controls. Tritium content (<0.8–1.42 TU) and hydrologic constraints indicate relatively short hydrothermal fluid residence times (∼5–200 years), with occasional inputs from older water components. Within this short timeframe, CH4 concentrations vary from lower, to significantly higher than those calculated using CO2 reduction kinetics. The isotope composition (δD-CH4: −172 to −138 ‰, δ13C-CH4: −32.0 to −24.6 ‰; 14C-CH4: 0.36–11.54 pMC) and geochemical and isotope modeling suggest that the majority (>80–90 %) of CH4 originates from a radiocarbon inactive source, i.e. mantle CH4, reduction of mantle CO2 and/or old organic matter, with relatively small contributions from both marine (<20 %) and terrestrial (<10 %) dissolved organic carbon. Measured isotopic compositions of CH4 do not match those expected for mantle-derived CH4 as well as values generated from reduction of mantle-derived CO2. Instead, differences in δD-CH4 and δ13C-CH4 values exist between systems fed by meteoric water and those fed by seawater, challenging the assumption of a uniform CO2 source and invariable reaction mechanisms. Differences between systems are best explained by variable extent of thermal decomposition and primary variations in the isotope composition of marine and terrestrial organic matter. Also, δD-CH4 and δ13C-CH4 values in meteoric water-fed systems closely resemble those in the Öxarfjördur sedimentary-hosted systems. In summary, our data supports a predominant thermogenic origin of CH4 in both seawater and terrestrial hydrothermal fluids in Iceland. The source of organic matter appears to be a combination of modern dissolved organic carbon and older sedimentary deposits. In addition, some of the hydrothermal systems studied (Krafla, Reykjanes, Theistareykir) which are characterized by low CH4 concentrations, may contain a significant portion of CH4 that may originate from CO2 reduction.

Geochemistry of Geothermal Fluids in the Three Rivers Lateral Collision Zone in Northwest Yunnan, China: Relevance for Tectonic Structure and Seismic Activity

The Three Rivers Lateral Collision Zone (TRLCZ), situated at the southeastern margin of the Tibetan Plateau, is a crucial frontier where materials from the plateau flow southeastward. This study extensively investigated the hydrochemical characteristics and origin of helium and carbon isotopes in 73 thermal springs within the TRLCZ. The analysis revealed dominant processes, including carbonate and silicate interactions, resulting in elevated concentrations of HCO3− and Na+. The impact of Ca/Mg-rich minerals, particularly dolomite, influenced the cation composition. Additionally, gypsum dissolution, notably in the Lancangjiang Fault and Weixi–Qiaohou Fault, was highlighted through Ca/SO4 ratios. The positive correlation between SO42− and Cl− indicated dilution by shallow cold water, explaining the lower SO42− content in the Jingshajiang–Zhongdian Fault and Nujiang Fault compared to the Weixi–Qiaohou Fault and Lancangjiang Fault. The circulation depth of thermal spring water varied, with the northern Weixi–Qiaohou Fault exhibiting the shallowest circulation depth (~3 km), while the Jingshajiang–Zhongdian Fault and southern segments of the Nujiang Fault displayed deeper depths—ranging from 4 to 7 km. A positive correlation between the circulation depth and fault activity was also observed. The Rc/Ra ratios of free gas samples, predominantly indicating crustal origin, varied from 0.01 Ra to 0.53 Ra. Elevated Rc/Ra ratios in the research area suggested potential minor additions of mantle helium through faults and fractures. Crustal limestone was identified as the primary source of CO2-rich samples, with δ13CCO2 values ranging from −1.6‰ to −7.2‰, while trace amounts of mantle CO2 were found. The spatial distribution of the H2 concentration, CO2 concentration, He concentration, and mantle He proportions in gases indicated that higher values of He concentration and mantle He% always occur near sampling points with deeper circulation depths. However, no similar correlation was observed for H2 and CO2. Most earthquakes of magnitude 5 or greater occurred near the regions with high values of mantle source He release, highlighting the critical role of mantle fluids in the occurrence of earthquakes in the region. In this study, a fluid circulation model was developed to describe the process of fluid (water and gas) circulation migration and earthquake generation in the TRLCZ.

Deglaciation-enhanced mantle CO2 fluxes at Yellowstone imply positive climate feedback

Mantle melt generation in response to glacial unloading has been linked to enhanced magmatic volatile release in Iceland and global eruptive records. It is unclear whether this process is important in systems lacking evidence of enhanced eruptions. The deglaciation of the Yellowstone ice cap did not observably enhance volcanism, yet Yellowstone emits large volumes of CO2 due to melt crystallization at depth. Here we model mantle melting and CO2 release during the deglaciation of Yellowstone (using Iceland as a benchmark). We find mantle melting is enhanced 19-fold during deglaciation, generating an additional 250–620 km3. These melts segregate an additional 18–79 Gt of CO2 from the mantle, representing a ~3–15% increase in the global volcanic CO2 flux (if degassed immediately). We suggest deglaciation-enhanced mantle melting is important in continental settings with partially molten mantle – including Greenland and West Antarctica – potentially implying positive feedbacks between deglaciation and climate warming.

The Origin and Differentiation of CO2-Rich Primary Melts in Ocean Island Volcanoes: Integrating 3D X-Ray Tomography with Chemical Microanalysis of Olivine-Hosted Melt Inclusions from Pico (Azores).

ABSTRACT Constraining the initial differentiation of primary mantle melts is vital for understanding magmatic systems as a whole. Chemical compositions of olivine-hosted melt inclusions preserve unique information about the mantle sources, crystallisation behaviour and volatile budgets of such melts. Crucially, melt inclusion CO2 contents can be linked to mantle CO2 budgets and inform us on Earth's carbon fluxes and cycles. However, determining total inclusion CO2 contents is not straightforward, as they often need to be reconstructed from CO2 dissolved in melts and CO2 stored in a vapour bubble. Here, we improve upon existing reconstruction methods by combining 3D X-ray computed tomography (CT) with geochemical microanalyses of major, trace and volatile elements. We show that in comparison to CT data, traditional reconstruction methods using 2D photomicrographs can underestimate CO2 budgets by more than 40%. We applied our improved methods to basaltic olivine-hosted melt inclusions from Pico volcano (Azores) in order constrain the formation and differentiation of volatile-rich primary melts in the context of a mantle plume. Results for these inclusions yielded 1935 to 9275 μg/g reconstructed total CO2, some of the highest values reported for ocean island volcanoes to date. Using these CO2 concentrations, we calculate entrapment pressures of 105 to 754 MPa that indicate a magma reservoir comprising stacked sills straddling the crust–mantle boundary. In the magma reservoir, crystallisation of volatile saturated melts drives extensive degassing, leading to fractionated CO2/Ba ratios of 3.5 to 62.2 and a loss of over 79% of primary mantle-derived CO2. Variabilities in trace elements (La, Y) show that differentiation occurred by concurrent mixing and crystallisation of two endmember melts, respectively depleted and enriched in trace elements. Geochemical models show that enriched endmember melts constitute 33 wt % of all melts supplied to the crust at Pico and that primary melts underwent 60% crystallisation prior to eruption. Mantle melting models indicate that the enriched and depleted primary melt endmembers are low- and high-degree melts of carbon-poor lherzolite and carbon-rich pyroxenite, respectively. Moreover, since deep magmas at Pico island are dominantly pyroxenite derived, their CO2-enrichement is mainly controlled by mantle source carbon content. Overall, our study illustrates that by combining 3D imaging, geochemical microanalyses and numerical modelling, melt inclusions provide a unique record of differentiation and storage of deep magmas, as well as mantle melting.

Geochemical evolution of geothermal waters in the Pearl River Delta region, South China: Insights from water chemistry and isotope geochemistry

The Pearl River Delta (PRD) region in South China is abundant of geothermal water resources whereas geochemical evolution of the geothermal water is still poorly understood. In this study, geothermal water is sampled to investigate the provenance of CO2 and sulphate, residence time, recharge elevation, circulation depth, and water-interaction processes by analyzing chemistry of major ions and isotope geochemistry. The range of δ13C and content of bicarbonate reveal multiple sources of soil CO2, carbonate rocks, and small amounts of mantle CO2. The δ34S ratio and sulphate concentration are attributed to evaporites dissolution, input of atmospheric sulphur, seawater intrusion, and oxidation of pyrite. The residence time of up to 21.01 ka for geothermal water likely suggests the earliest precipitation recharge during the Late Pleistocene. Mountains or/and hills at an altitude of 300–760 m in north PRD region are currently the major recharge areas for the geothermal water which can circulate at the depths of up to 3000–4800 m along regional structures. The isotopic and hydrochemical analysis confirms that extensive water interaction with silicate rock, carbonate rock and evaporite, evaporation-crystallization process, and cation exchange are the key mechanisms that control the geochemical evolution of geothermal water in the PRD geothermal system.

Structural controls of the migration of mantle-derived CO2 offshore in the Santos Basin (Southeastern Brazil)

We present a multi-scale conceptual model based on structural controls of the migration of mantle-derived CO2 offshore in the Santos Basin (Southeastern Brazil). We assembled the model from a regional 2D seismic reflection line integrated with potential gravimetric field data and a local 3D seismic reflection volume integrated with well data (lithologies and in situ stress). (i) The geochemical isotope range of δ13CCO2 falls mostly within −7‰ and −5‰ and shows relatively high values for 3He/4He represented by an R/Ra rate of up to 5.60, indicating CO2 mantle generation and degassing. (ii) Seismic interpretation feasibly validated by potential gravimetric responses of the crustal structure (Moho discontinuity) show CO2 migration through deep-seated faults in a region of highly stretched continental crust with oceanward mantle uprising. (iii) Early Cretaceous basement highs generated in an obliquely syn-rift faulting system control CO2 accumulation in thermogenic travertines (hydrothermal carbonate reservoirs of continental lakes), and Aptian evaporites subsequently trap it.

Discovery and genesis of helium-rich geothermal fluids along the India–Asia continental convergent margin

Large-scale helium accumulations are rarely reported in tectonically active continental subduction zones. This study shows geochemical variations in helium and other volatiles (N2, CO2, Ne and Ar) in hot springs from the India–Asia continental convergent margin (Indus–Yarlung suture, IYS) to the intracontinental extensional region in southern Tibet and identifies a 1200-km-long helium enrichment zone (He > 5000 ppm) within 50 km north of the IYS. In total, 144 gas samples were categorized into four groups: Group A, with dominant N2 species and He concentrations (0.07–2.31 vol%) 1.4–23 times higher than industrial-grade He resources (0.05–0.1 vol%); Group B, with dominant CO2 species and high He concentrations (0.05–1.57 vol%); and Groups C and D, depleted in He (<0.05 vol%) and dominated by CO2 and N2, respectively. The gas mixtures exhibit systematic variations from the extensional zone to the continental convergent margin, changing from deep-origin, He-depleted, CO2-dominated fluids (Group C) to relatively shallow crust-derived, He-rich, N2-dominated gases (Group A). The former exhibits small but detectable amounts of mantle-derived He (2.3 ± 2.6%) and CO2 (<3.2%), primarily atmosphere-derived N2 (δ15N = 0.0 ± 0.6‰) and Ar (40Ar/36Ar = 307 ± 25), and crustal CO2 (82 ± 9% from carbonate and 17 ± 9% from sediments/metasediments) from the subducted Indian slab-carbonate and Tibetan crust. The low He flux of Group C matches the steady release of He from the 70-km-thick crust into the geothermal waters during 0.02–43.5 ka. Shallow calcite precipitation and dissolution reactions cause CO2 loss and slight He enrichment in the extensional zone. Group A samples at the convergent margin show pure crustal He, more radiogenic 40Ar (40Ar/36Ar = 330 ± 67), and higher sedimentary contributions of 28 ± 23% for N2 and 37 ± 11% for CO2. A high He flux near the IYS requires an external source with high 4He/N2 (∼0.05) and 4He/20Ne (103–105) to provide >62% of the total He flux. This finding is best explained by He production in the south-dipping Gangdese granitic belt and deep crust, accumulation below the accretionary wedge, and episodic liberation along Great Counter thrust structures since 0.7 Ma by Tibetan Plateau uplifts and intense hydrothermal activity. The He-rich fluids mix with the sedimentary and atmosphere-derived N2 from the accretionary wedge, forming He-rich N2-type gases. This study systematically reports large-scale He enrichment along the continental convergent margin for the first time and demonstrates that complex crustal-scale structures in the convergent margin significantly controlling gas geochemistry. These results have profound implications for potential helium resource exploration and He-CO2-N2 recycling in continental subduction zones.

Effects of inorganic CO2 intrusion on diagenesis and reservoir quality of lacustrine conglomerate sandstones: Implications for geological carbon sequestration

Mantle-sourced fluids are common in rift basins and often migrate into the reservoir along deep faults. They have a significant role in hydrocarbon evolution and reservoir quality modifications. In this study, we selected Chagan Sag in China as an example and applied reactive transport modeling to explore the influence of mantle-derived CO2 on diagenesis and reservoir quality evolution in the Lower Cretaceous Bayingebi conglomerate reservoirs. The geochemical modeling constrained by natural CO2 reservoirs provides beneficial information on the underground physical processes and chemical reactions during CO2 injection. This is also an efficient means of validating geological carbon sequestration models. Petrographic, geochemical, and modeling data reveal that CO2 intrusion could reduce the reservoir porosity and permeability via significant CO2-water-mineral interactions. Ankerite and illite are the dominant diagenetic minerals responsible for reservoir quality reduction. Among the different lithologies, the higher reservoir quality conglomerate is mainly caused by its high initial porosity and small quantities of clay minerals, with relatively weak carbonate cementation. The lower reservoir quality finer-grained conglomeratic siltstone is caused by intensive albitization, carbonation, and illitization. In the long-term CO2-water-rock interaction process, CO2 can be sequestered through the precipitation of secondary carbonate minerals such as ankerite and dolomite.

Enrichment of rare methanogenic Archaea shows their important ecological role in natural high-CO2 terrestrial subsurface environments.

Long-term stability of underground CO2 storage is partially affected by microbial activity but our knowledge of these effects is limited, mainly due to a lack of sites. A consistently high flux of mantle-derived CO2 makes the Eger Rift in the Czech Republic a natural analogue to underground CO2 storage. The Eger Rift is a seismically active region and H2 is produced abiotically during earthquakes, providing energy to indigenous microbial communities. To investigate the response of a microbial ecosystem to high levels of CO2 and H2, we enriched microorganisms from samples from a 239.5 m long drill core from the Eger Rift. Microbial abundance, diversity and community structure were assessed using qPCR and 16S rRNA gene sequencing. Enrichment cultures were set up with minimal mineral media and H2/CO2 headspace to simulate a seismically active period with elevated H2. Methane headspace concentrations in the enrichments indicated that active methanogens were almost exclusively restricted to enrichment cultures from Miocene lacustrine deposits (50-60 m), for which we observed the most significant growth. Taxonomic assessment showed microbial communities in these enrichments to be less diverse than those with little or no growth. Active enrichments were especially abundant in methanogens of the taxa Methanobacterium and Methanosphaerula. Concurrent to the emergence of methanogenic archaea, we also observed sulfate reducers with the metabolic ability to utilize H2 and CO2, specifically the genus Desulfosporosinus, which were able to outcompete methanogens in several enrichments. Low microbial abundance and a diverse non-CO2 driven microbial community, similar to that in drill core samples, also reflect the inactivity in these cultures. Significant growth of sulfate reducing and methanogenic microbial taxa, which make up only a small fraction of the total microbial community, emphasize the need to account for rare biosphere taxa when assessing the metabolic potential of microbial subsurface populations. The observation that CO2 and H2-utilizing microorganisms could only be enriched from a narrow depth interval suggests that factors such as sediment heterogeneity may also be important. This study provides new insight on subsurface microbes under the influence of high CO2 concentrations, similar to those found in CCS sites.

Deep magma degassing and volatile fluxes through volcanic hydrothermal systems: Insights from the Askja and Kverkfjöll volcanoes, Iceland

Mantle volatiles are transported to Earth's crust and surface by basaltic volcanism. During subaerial eruptions, vast amounts of carbon, sulfur and halogens can be released to the atmosphere during a short time-interval, with impacts ranging in scale from the local environment to the global climate. By contrast, passive volatile release at the surface originating from magmatic intrusions is characterized by much lower flux, yet may outsize eruptive volatile quantities over long timescales. Volcanic hydrothermal systems (VHSs) act as conduits for such volatile release from degassing intrusions and can be used to gauge the contribution of intrusive magmatism to global volatile cycles. Here, we present new compositional and isotopic (δD and δ18O-H2O, 3He/4He, δ13C-CO2, Δ33S-δ34S-H2S and SO4) data for thermal waters and fumarole gases from the Askja and Kverkfjöll volcanoes in central Iceland. We use the data together with magma degassing modelling and mass balance calculations to constrain the sources of volatiles in VHSs and to assess the role of intrusive magmatism to the volcanic volatile emission budgets in Iceland.The CO2/ΣS (10−30), 3He/4He (8.3–10.5 RA; 3He/4He relative to air), δ13C-CO2 (−4.1 to −0.2 ‰) and Δ33S-δ34S-H2S (−0.031 to 0.003 ‰ and −1.5 to +3.6‰) values in high-gas flux fumaroles (CO2 > 10 mmol/mol) are consistent with an intrusive magmatic origin for CO2 and S at Askja and Kverkfjöll. We demonstrate that deep (0.5–5 kbar, equivalent to ∼2–18 km crustal depth) decompression degassing of basaltic intrusions in Iceland results in CO2 and S fluxes of 330–5060 and 6–210 kt/yr, respectively, which is sufficient to account for the estimated CO2 flux of Icelandic VHSs (3365–6730 kt/yr), but not the VHS S flux (220–440 kt/yr). Secondary, crystallization-driven degassing from maturing intrusions and leaching of crustal rocks are suggested as additional sources of S. Only a minor proportion of the mantle flux of Cl is channeled via VHSs whereas the H2O flux remains poorly constrained, because magmatic signals in Icelandic VHSs are masked by a dominant shallow groundwater component of meteoric water origin. These results suggest that the bulk of the mantle CO2 and S flux to the atmosphere in Iceland is supplied by intrusive, not eruptive magmatism, and is largely vented via hydrothermal fields.

Experimental Modeling of the Interaction between Garnets of Mantle Parageneses and CO2 Fluid at 6.3 GPa and 950–1550 °C

Abstract —Experimental modeling of the interaction of eclogitic and lherzolitic garnets with CO2 fluid was carried out on a multianvil high-pressure apparatus of the “split-sphere” type (BARS) in platinum ampoules with inner graphite capsules, using a buffered high-pressure cell with a hematite container, at a pressure of 6.3 GPa and in the temperature range 950–1550 °C. It has been established that the main interaction processes at 6.3 GPa and 950–1250 °C are partial dissolution, recrystallization, and carbonation of garnet which lead to the formation of magnesian carbonate, kyanite, and coesite, a decrease in Mg contents in the recrystallized garnet, and the formation of carbonate, silicate, and oxide inclusions in it. Under these conditions, crystallization of metastable graphite and growth of diamond on the seed at ≥1250 °C were observed. In the temperature range 1350–1550 °C, the garnet underwent partial dissolution and recrystallization in CO2 fluid; no carbonation took place. These processes were accompanied by a decrease in the portion of the grossular component in the garnet and by the enrichment of the fluid phase with calcium. We have established the indicative characteristics of garnet that interacted with CO2 fluid: zoning, with low contents of CaO and MgO in the rims of crystals relative to the cores, and the presence of carbonate, kyanite, coesite, and CO2 inclusions. The compositions of the produced garnet and carbonates are consistent with the data on these minerals in mantle peridotite and eclogite parageneses and in inclusions in diamonds, which suggests a significant role of metasomatism with the participation of CO2 fluid in the evolution of deep-seated rocks and in the diamond formation. In this experimental research, we have first studied the processes of diamond crystallization and determined the boundary conditions for diamond growth in the system silicate–carbonate–CO2, which simulates natural diamond formation media. In general, the established regularities can be regarded as potential indicators of mantle metasomatism and mineral formation with the participation of CO2 fluid.

Risk assessment of mantle-derived CO2 in the East China Sea basins, China

Risk assessment of mantle-derived CO2 in the East China Sea basins, China

ICDP drilling of the Eger Rift observatory: magmatic fluids driving the earthquake swarms and deep biosphere

Abstract. The new in situ geodynamic laboratory established in the framework of the ICDP Eger project aims to develop the most modern, comprehensive, multiparameter laboratory at depth for studying earthquake swarms, crustal fluid flow, mantle-derived CO2 and helium degassing, and processes of the deep biosphere. In order to reach a new level of high-frequency, near-source and multiparameter observation of earthquake swarms and related phenomena, such a laboratory comprises a set of shallow boreholes with high-frequency 3-D seismic arrays as well as modern continuous real-time fluid monitoring at depth and the study of the deep biosphere. This laboratory is located in the western part of the Eger Rift at the border of the Czech Republic and Germany (in the West Bohemia–Vogtland geodynamic region) and comprises a set of five boreholes around the seismoactive zone. To date, all monitoring boreholes have been drilled. This includes the seismic monitoring boreholes S1, S2 and S3 in the crystalline units north and east of the major Nový Kostel seismogenic zone, borehole F3 in the Hartoušov mofette field and borehole S4 in the newly discovered Bažina maar near Libá. Supplementary borehole P1 is being prepared in the Neualbenreuth maar for paleoclimate and biological research. At each of these sites, a borehole broadband seismometer will be installed, and sites S1, S2 and S3 will also host a 3-D seismic array composed of a vertical geophone chain and surface seismic array. Seismic instrumenting has been completed in the S1 borehole and is in preparation in the remaining four monitoring boreholes. The continuous fluid monitoring site of Hartoušov includes three boreholes, F1, F2 and F3, and a pilot monitoring phase is underway. The laboratory also enables one to analyze microbial activity at CO2 mofettes and maar structures in the context of changes in habitats. The drillings into the maar volcanoes contribute to a better understanding of the Quaternary paleoclimate and volcanic activity.

Tectonically driven carbonation of serpentinite by mantle CO2: Genesis of the Castiglioncello magnesite deposit in the Ligurian ophiolite of central Tuscany (Italy)

Carbonation of ultramafic rocks is a key piece of the global carbon cycle taking place from the seafloor to the surficial environment and becoming particularly efficient in the genesis of ultramafic-hosted magnesite deposits in the shallow crust. Even though these are exceptional occurrences for the study of carbonation reactions and the implementation of engineered CO2 storage solutions, their genetic model, in particular the role of tectonics and source of CO2, is still debated. In this study, we focus on the Castiglioncello magnesite deposit hosted in the Ligurian ophiolite of central Tuscany (Italy). Textural, mineralogical, and isotopic data indicate that here carbonation is the result of a serpentinite-hosted epithermal system where mantle CO2 mixed with meteoric water, became enriched in Mg2+ through serpentine alteration and precipitated magnesite in response to tectonic activity. More specifically, carbonation was driven by subsequent tectonic events during the Apennine orogenesis which: i) developed structural traps that allowed the concentration of CO2 into serpentinite lenses; ii) created crustal-scale pathways for the rising of mantle CO2 into the shallow crust; and iii) opened dilatational fault jogs where magnesite precipitated. Cyclical fracturation guaranteed the necessary permeability for the ingress of multiple batches of fluids, prolonged alteration, and repeated precipitation of magnesite, dolomite, and opal-CT/chalcedony. Our results highlight a fundamental control of tectonics on carbonation of ultramafic rocks in the shallow crust and the importance of appropriate crustal architecture for developing epithermal carbonation systems. Syn-tectonic epithermal carbonation systems provide an alternative view on CO2 storage in ultramafic rocks compared to the better-known post-tectonic supergene systems. The mantle origin of CO2 indicates that in extensional settings, transfer zones can be conveyors of CO2, and possibly other volatiles, from the Earth’s mantle to the surface.