Mantle-derived CO2 Research Articles

The origin of methane in serpentinite-hosted hyperalkaline hot spring at Hakuba Happo, Japan: Radiocarbon, methane isotopologue and noble gas isotope approaches

Methane (CH4) associated with marine and terrestrial sites of serpentinization has been proposed to be abiotic in origin. However, the source of carbon and the depth and temperature of CH4 synthesis often remain inconclusive. We measured the radiocarbon (14C), bulk stable isotope ratios (δ13C and δD) and isotopologue abundance (Δ13CH3D) of CH4, and noble gas isotope composition of gas samples from two hyperalkaline geothermal wells (Happo #1 and Happo #3) at Hakuba Happo, Japan, to constrain the source of carbon and the CH4 generation processes. The CH4 samples from both sites were nearly 14C-free, whereas the carbonate precipitates inside the Happo #1 well contained 14C corresponding to 51 to 62 percent modern carbon, indicating that the majority of CH4 was not generated from the reduction of dissolved carbonate in the hot spring water. CH4 samples from Happo #1 and Happo #3 yielded Δ13CH3D-based apparent temperatures of 206−40+52°C and 323−85+143°C, respectively, which are much higher than the measured well water temperatures (∼50°C). Therefore, 14C and clumped isotopologue temperatures suggest that Hakuba Happo CH4 was generated below the depth where the shallow meteoric water circulated. The 3He/4He ratios were 4.10 and 4.47 Ratm for Happo #1 and Happo #3, respectively. The 3He/4He and 4He/20Ne ratios revealed that approximately 50% of He was of mantle origin, suggesting that the Hakuba Happo hot spring received volatiles, including CO2, from the mantle. However, the observed δ13CCH4 values (approximately −35‰) were significantly lower than those of typical mantle-derived carbon (–5‰), implying that mantle-derived CO2 is not the major carbon source for CH4 formation. High CH4 concentrations in the Hakuba Happo fluids, compared to those in hot springs in the area not associated with serpentinization, suggest that CH4 was generated from 13C-depleted fossil carbon sources and serpentinization-derived H2 at high temperatures (>200°C), and subsequently entrained into the cooler circulating meteoric water system.

CO2 Fluid Flow Patterns near Major Deep Faults: Geochemical and 3D Seismic Data from the Ying-Qiong Basin of the South China Sea

CO2 and hydrocarbon fluids typically migrate from deeper layers into the shallow crust via large deep faults. Consequently, CO2, hydrocarbon, and water reservoirs tend to occur in traps near deep and large faults. In this study, we use structural and stratigraphic data to identify and predict CO2 and hydrocarbon gas reservoirs near major deep faults. In order to investigate how CO2 accumulates in the major deep faults of the Ying-Qiong Basin (YQB), we quantify the carbon footprint of this area by analyzing the No. 1 fault, the No. 2 fault, and the adjacent gas reservoirs. Using 3D seismic data and geochemical data, we determine how the fault structure affects the ambient CO2 enrichment on a given fault. Our results indicate that the LD10 and BD19 gas reservoirs have high inorganic CO2 contents, while the HK29 gas reservoir has a low organic CO2 content. Based on our analyses, we conclude that the gas source, fault activity, and fault structure control the CO2 accumulation in subsurface layers. While mantle-derived volcanic inorganic CO2 disperses upward along the main fault when a given fault is independent (i.e., it lacks secondary faults), the absence of additional vertical migration channels largely prevents the CO2 from travelling upward through thick mudstone cap rocks and collecting in shallow traps. These shallow traps are typically filled by shallow organic CO2 sources. However, parallel forward fault-step structures, such as secondary faults, can transport gas that is produced at deeper sources (such as CO2 generated by basement limestone) to shallower depths. If the hanging wall of a deep fault has many branching secondary faults, then these intersecting faults act as conduits that enable mantle-derived CO2 to travel vertically into shallow layers.

Source rock potential, main depocenters, and CO2 occurrence in the pre-salt section of Santos Basin, southeast Brazil

The absence of expressive hydrocarbon columns and high CO2 content observed in the pre-salt of Santos Basin reservoirs over the last decade reveal the risk of hydrocarbon charge and contamination to be a relevant concern in exploration activities in the basin. Therefore, this study was focused on elucidation of the source potential, basin depocenters, and CO2 occurrence in the pre-salt of the Santos Basin, aimed at reducing risk of upcoming exploration activities. Based on geochemical analysis, our study revealed that Late Barremian-Early Aptian fine-grained rock intervals, rich in organic matter, from the Itapema Formation and secondarily, from the Piçarras Formation, are likely the main source rocks of the Santos Basin. Seismic interpretation revealed five wide pre-salt depocenters. The Main Depocenter, followed by the Central and North depocenters, could be the major hydrocarbon sources for pre-salt reservoirs. Based on the CO2 distribution data, low amounts of the contaminant (under 5%) were detected in the post-salt and proximal pre-salt reservoirs, with intermediate values (from 10 to 50%) in the main pre-salt accumulations. High CO2 amounts (over 50%) were encountered mainly in a northeast to southwest trend, linking Peroba-Júpiter-Libra areas, and could be related to a deep-seated fault system associated with mantle contamination.Accordingly, the northern and central parts of the Central Santos sub-basin have a low risk of hydrocarbon charge and contamination, whereas its eastern portion has a high risk because of the absence of an expressive hydrocarbon column and elevated CO2 content. The Southwest Santos sub-basin presented a high risk for pre-salt discoveries because of salt windows which limit the top seal for pre-salt-sourced hydrocarbons. In the Southeast Santos sub-basin, the risk of hydrocarbon charge and contamination is significant because of the absence of mapped depocenters and its location on a crustal domain favorable to mantle CO2 access.

Deep CO2 and N2 emissions from Peruvian hot springs: Stable isotopic constraints on volatile cycling in a flat-slab subduction zone

Gas-rich hot springs throughout the Peruvian Andes contain a surprising contribution of mantle and crustal volatiles (CO2 and N2) despite being located along a volcanic gap associated with modern flat-slab subduction. Similar mantle and crustal volatile degassing is observed in springs from the backarc region of the northern Altiplano Plateau which experienced flat-slab subduction in the Oligocene. We constrain the sources of deeply-derived volatiles using C, N, O and H stable isotope analyses alongside previously reported helium isotope (3He/4He) and gas abundance data. δ18O and δ2H values of spring waters are consistent with regional meteoric water, with deviations towards higher values due to fluid mixing and high-temperature fluid-rock interaction. δ15N values of N2 gas range from −0.5 to +4.4‰ (vs AIR), indicating contributions of mantle and crustal (sedimentary) nitrogen. δ13C values of dissolved inorganic carbon (DIC) and CO2 gas from bubbling springs are used to model initial δ13C values before degassing, with resulting values ranging from −13.6 to −0.3‰ (vs VPDB) and an average value of −6.9‰. DIC concentrations range from 1 to 57 mmol/kg, of which carbonate dissolution accounts for only a small fraction (average of 15%). The remaining carbon (1 to 44 mmol/kg) is derived from a mixture of deep CO2 sources with endmember δ13C values of −7‰ and −14‰ which, in light of relationships with 3He/4He ratios, are interpreted to be the subcontinental lithospheric mantle (SCLM) and metamorphic crustal carbon, respectively. These results are consistent with previous interpretations that slab-to-lithosphere fluid transfer is mobilizing volatiles from the SCLM to the continental crust above the flat slab and confirms that mantle CO2 is migrating with mantle helium. Metamorphic CO2 and N2 are mobilized from the continental crust and entrained by mantle fluids on their way to the surface. In the backarc region, similar geochemical patterns suggest that volatiles are released by dehydration and/or partial melting of previously hydrated lithosphere. We estimate a CO2 flux of ~3 × 108 mol yr−1 (34 t d−1) from 45 springs with available discharge values. If scaled to all reported thermal springs in Peru, then the total CO2 emissions from Peruvian thermal springs may approach 2 × 109 mol yr−1 (203 t d−1), or ~ 0.2% of global emissions from subaerial volcanism. Regional CO2 emissions may be 10 to 100 times greater when considering CO2 diffusely lost along fault zones or temporarily dissolved near-surface aquifers. These results demonstrate that flat-slab subduction leads to an efficient and unexpected transfer of mantle and crustal volatiles to Earth's surface, and more generally, that deep volatile fluxes in subduction zones are not limited to active volcanism.

Reassessment of the Distribution of Mantle CO2 in the Bohai Sea, China: The Perspective from the Source and Pathway System

AbstractResearch on the distribution of mantle CO2 should involve comprehensive analysis from CO2 source to accumulation. The crust‐mantle pathway system is the key controlling factor of the distribution of mantle CO2, but has received little attention. The pathway system and controlling factors of CO2 distribution in the Bohai Sea are analyzed using data on fault styles and information on the mantle and lithosphere. The relation between volcanic rocks and the distribution of mantle CO2 is reassessed using age data for CO2 accumulations. The distribution of mantle CO2 is controlled by uplift of the asthenosphere and upper mantle, magma conduits in the mantle and fault systems in the crust. Uplifted regions of the asthenosphere are accumulation areas for CO2. The area with uplift of the Moho exhibits accumulation of mantle CO2 at depth. CO2 was mainly derived from vertical migration through the upper mantle and lower crust. The fault style in the upper crust controls the distance of horizontal migration and the locations of CO2 concentrations. The distribution of mantle CO2 and volcanic rocks are not the same, but both probably followed the same pathways sometimes. Mantle CO2 in the Bohai Sea is concentrated in the Bozhong sag and the surrounding area, particularly in a trap that formed before 5.1 Ma and is connected to crustal faults (the Bozhang faults) and lithospheric faults (the Tanlu faults).

Linking deep CO2 outgassing to cratonic destruction.

Outgassing of carbon dioxide from the Earth's interior regulates the surface climate through deep time. Here we examine the role of cratonic destruction in mantle CO2 outgassing via collating and presenting new data for Paleozoic kimberlites, Mesozoic basaltic rocks and their mantle xenoliths from the eastern North China Craton (NCC), which underwent extensive destruction in the early Cretaceous. High Ca/Al and low Ti/Eu and δ26Mg are widely observed in lamprophyres and mantle xenoliths, which demonstrates that the cratonic lithospheric mantle (CLM) was pervasively metasomatized by recycled carbonates. Raman analysis of bubble-bearing melt inclusions shows that redox melting of the C-rich CLM produced carbonated silicate melts with high CO2 content. The enormous quantities of CO2 in these magmas, together with substantial CO2 degassing from the carbonated melt–CLM reaction and crustal heating, indicate that destruction of the eastern NCC resulted in rapid and extensive mantle CO2 emission, which partly contributed to the early Cretaceous greenhouse climate episode.

Geomorphological evidence of tectonic activity of the Marianske Lazne Fault (Czech Republic) and its influence to stream network evolution

The Mariánské Lázně Fault (western Bohemia, CZE) is a morphologically, geologically and tectonically prominent structure with that is 150 km long with an NNW-SSE orientation. Its tectonic activity, especially in the NW part and in the neighbouring Cheb basin, is well known and has been proven by present-day earthquake swarms, mantle-derived CO2 emanations, geophysical and paleoseismological research. It seems that other parts of the MLF (especially segments of NNW-SSE and N-S orientation) might also have been active during the Pleistocene, possible even in the Holocene. This study provides a robust set of morphometric analyses – mountain-front sinuosity, basin asymmetry, longitudinal stream profiles, SL-index, which assesses the possibility of recent tectonic activity. The results suggest that the activity of the central and southern part of the MLF could have been very young. A reconstruction of the evolution of the stream network of the Mže River, as a result of different timing of the activity of particular segments of the MLF, is also put forward. The first ideas about the evolution of terrain morphology and the stream network are proposed by this study, however subsequent field research (geophysics, paleoseismology) could prove and date the tectonic activity. The delineation of segments with young activity may also have a great implication for seismic hazard assessment.

Genesis and mineralization style of gold occurrences of the Lower Lom Belt, Bétaré Oya district, eastern Cameroon

The Bétaré Oya district has a substrate composed of the Neoproterozoic metavolcanic-metasedimentary rocks of the Lom Belt in eastern Cameroon. The district is well-known for alluvial gold mining activities, however, the primary gold mineralization has received little attention. In the current study, we newly report geological, ore mineralogy, fluid inclusion microthermometric, laser Raman spectroscopy, and stable isotope data from auriferous veins of the Lom Belt to characterize the style of gold mineralization and to constrain the origin of the ore-forming fluids. The auriferous quartz veins are laminated, fractured, N- to NE-trending, and spatially associated with the Bétaré Oya Shear Zone. Fieldwork coupled with microscopic examination and the textural relationships of ore minerals revealed two stages of mineralization. The first stage is characterized by the presence of pyrite, sphalerite, galena, chalcopyrite, pyrrhotite, hematite, petzite, hessite, wolframite, electrum, and gold, while the second stage is characterized by the presence of a later deposition of galena and pyrite, as well as minor greenockite. The gangue minerals are quartz, sericite, muscovite, chlorite, calcite, ankerite, and barite, whereas the supergene assemblage (stage 3) includes goethite, hematite, covellite, and enargite. Two fluid inclusion assemblages containing three types of inclusions are identified in mineralized quartz veins. Assemblage 1 consists of gold-related types 1 and 2 with daughter minerals consisting of nahcolite, magnesite, and arsenolamprite (black native arsenic), whereas assemblage 2 consists of post-gold type 3 fluid inclusions. Type 1 has H2O-NaCl-CO2 ± N2 ± CH4 primary fluid inclusions (FI), with a TmCO2 ranging from −59.8 to −56.6 °C, salinities from 0.5 to 10.8 wt% NaCl eq., densities from 0.87 to 1.00 g.cm−3, and total homogenization temperatures between 280 and 360 °C. Type 2 contains CO2 (±H2O-NaCl) ± N2 ± CH4 FI that exhibit TmCO2 ranging between −60.0 and −56.7 °C, ThCO2 from 13 to 25 °C, and densities between 0.73 and 0.85 g.cm−3. Type 3 shows H2O-NaCl FI with salinities between 0.2 and 10.1 wt% NaCl eq., densities between 0.82 and 0.98 g.cm−3, and total homogenization temperatures from 160 to 235 °C. Measured δ18O for gold-bearing quartz (+11.5 to +16.0‰), δD from FI (−50.6 to −21.8‰), δ13C from FI (−5.8 to −5.5‰), and δ34S from galena and pyrite grains (+5.3‰ and +8.2‰, respectively) suggest a metamorphic source as most likely for the ore-forming fluids and sulfur, although a mantle CO2 contribution cannot be ruled out.The gold deposition probably took place by fluid-rock interaction and fluid unmixing at ∼310 °C and at a depth of about 6–9 km. The ore-forming fluid was a low salinity (∼6.2 wt% NaCl eq.) H2O-NaCl-CO2 ± N2 ± CH4 solution. These data revealed that the Lom Belt gold mineralization is best classified as a mesozonal orogenic gold deposit.

Mantle-Derived Helium Distribution and Tectonic Implications in the Sichuan-Yunnan Block, China.

The geochemical characteristics of mantle degassing observed on the surface of the earth can indicate the origin and migration path of mantle fluids. Compared with the plate boundary tectonic environment, the intraplate tectonic environment does not have a large number of active volcanoes and active faults, and the observation of mantle volatiles in hot spring gas is relatively limited. We selected the Sichuan–Yunnan block to discuss mantle degassing based on the carbon and noble gas isotopes of the spring gases and previous studies on the fault slip rate and geophysical research. A total of five hot spring gas samples (including two free gases and three dissolved gases) were collected from the Sichuan–Yunnan block. Chemical and isotopic compositions were analyzed in N2-dominant hot spring gases. The 3He/4He ratio (0.068–0.541 Ra) indicates the occurrence of mantle-derived helium throughout the Sichuan–Yunnan block, which has been diluted by a crustal radiogenic 4He component. The occurrence of mantle-derived helium in the study areas ranges from 0.74 to 5.67%. The lower proportion of mantle-derived helium in YNWQ and HGWQ than that in other spring gases near the Jinghe-Qinghe fault may be caused by the smaller scale of fault around YNWQ and HGWQ than the Jinghe-Qinghe fault. The correlation between 4He, 20Ne, and N2 concentrations implies a common trapping mechanism for 4He, 20Ne, and N2 in hot spring gases. The 40Ar/36Ar ratios and N2/Ar ratios indicate that N2 and Ar are mostly meteoric, and YNWQ and HGWQ have more crustal-derived Ar contribution (40.56 and 51.49%, respectively). The δ13C(CO2)o values calculated by Rayleigh fractionation and CO2 concentration suggest that CO2 has inorganic and organic origins. The plot of Rc/Ra versus δ13C(CO2) indicates that the spring gas CO2 origin in the Sichuan–Yunnan block is mainly derived from mixing of limestone and organic sediments with minor mantle CO2. The δ13C(CH4) versus CH4/3He values indicate that the origin of methane is thermogenic and microbial oxidation. The low mantle-derived helium distribution pattern is most likely controlled by the weak fault activity rate, the small fault scale, and not obvious magmatic activity in the Sichuan–Yunnan block.

Metamorphic CO2 emissions from the southern Yadong-Gulu rift, Tibetan Plateau: Insights into deep carbon cycle in the India-Asia continental collision zone

Extensional rift systems provide important pathways for the release of large amounts of deeply-sourced CO2 into the atmosphere. Continental rifting zones (e.g., East African rift) are thus invoked to be important for understanding the links between CO2 outgassing and global climate change associated with continental breakup. However, deeply-sourced CO2 emissions from extensional rift systems in continental collision zones remain poorly understood. Here, we focus on hydrothermal CO2 emissions from the southern segment of the Yadong-Gulu rift (YGR), the largest extensional rift in southern Tibetan Plateau, aiming at delineating rift-related CO2 emissions from the India-Asia continental collision zone. In-situ measurements of diffuse soil CO2 emissions indicate that average soil CO2 fluxes from the Kangbu, Mengzha, and Chaduo hydrothermal fields are 40, 700, and 255 g m−2 d−1, respectively. Combined with average soil CO2 fluxes (20–437 g m−2 d−1) from central and northern YGR, we speculate a relatively steady-state CO2 degassing pattern for the entire rift. The magnitude of soil CO2 fluxes of the YGR is higher than that of representative areas of the East African rift system. Evidence from 3He/4He reveals a pure crustal origin for CO2-bearing fluids in southern YGR, while the involvement of mantle CO2 is recognized in the central and northern rift segments. We suggest that metamorphic decarbonation of crustal rocks at variable depths is the primary cause for CO2 origin in southern YGR, which differs from the magma‑carbonate interaction model of central and northern YGR. Ternary mixing calculation based on He-CO2 systematics of hydrothermal gases indicates dominant contributions from carbonate rocks to total carbon inventory, with mantle CO2 contributions absent in southern YGR but discernible in northern YGR. The characteristic high proportions of crustal CO2 (>90%) distinguish the YGR from extensional rifts fed primarily by mantle CO2 in continental rifting zones. Our results reveal a crustal-scale carbon cycle in accretionary wedge of the India-Asia continental collision zone, together with magmatic front setting of the central and northern YGR, outlining a transect of the rift-related CO2 emissions across continental collision zones. This would contribute to better understanding the role of continental assembly in deeply-sourced CO2 emissions and global carbon budget.

Total diffuse CO2 flux from Yellowstone caldera incorporating high CO2 emissions from cold degassing sites

Previous calculations of total diffuse carbon dioxide (CO2) emissions from Yellowstone National Park have been based on flux measurements from thermally active, altered basins. However, our recent measurements show that thermally inactive, cold degassing sites can emit more diffuse CO2 per km2 than thermally active basins. The contribution of these cold degassing sites must be included in calculations of total diffuse CO2 flux for adequate quantification of Yellowstone's role as a major contributor within the global CO2 budget.We acquired 1342 diffuse CO2 flux point measurements across 19 different sites in Yellowstone National Park during summer 2018 and 2019. Eleven of the measured sites are thermally active with heat flux elevated above background, four sites are vegetation sites, and four sites are thermally inactive, cold degassing sites outside the caldera. Our measurements of diffuse CO2 flux agree with previous findings that the highest average flux occurs in acid sulfate soils (706 ± 79 gm−2 day−1, average for both thermally active and cold degassing sites) with lower diffuse CO2 flux in travertine (189 ± 98 gm−2 day−1) and neutral-chloride (19 ± 8 gm−2 day−1) soils. All cold degassing sites measured were in acid sulfate dominated soil chemistries. The average diffuse CO2 flux for these cold degassing sites was higher (840 ± 163 gm−2 day−1) than the average flux through acid sulfate soils in thermally active basins (625 ± 79 gm−2 day−1). We also measured the carbon isotope composition of diffuse CO2 soil gas samples from most sites to characterize the potential source(s) of these emissions. The majority of carbon isotope values from cold degassing sites are within the typical range of values for Yellowstone fumaroles. This suggests that cold degassing sites have a similar magmatic source for diffuse CO2.Previous calculations of the total daily diffuse CO2 emitted through Yellowstone National Park use the arithmetic average flux per soil chemistry and extrapolate that value across the entire park using only thermally active altered soils. Instead, we calculate the total CO2 flux per site in tons CO2 per km2 per day using Gaussian Geostatistical Simulations through the Geostatistical Analyst toolbox in ArcGIS. To account for the large component of cold degassing CO2, we extrapolate the average emission rate per soil chemistry to all hydrothermally altered soils rather than only thermally active soils across the entire area of Yellowstone National Park. We calculate a total deeply sourced (biogenic flux removed) CO2 flux of 24 ± 12 kt CO2 per day, with nearly two-thirds of that flux estimated to be from cold degassing acid sulfate soils. Cold diffuse degassing sites can be significant sources of magmatic carbon emissions and this has implications for the relationship between the flux of CO2 and heat as well as for global contributions from mantle CO2 emissions.

Phase Behavior Identification and Formation Mechanisms of the BZ19-6 Condensate Gas Reservoir in the Deep Bozhong Sag, Bohai Bay Basin, Eastern China

Significant developments have been observed in recent years, in the field of deep part exploration in the Bozhong Sag, Bohai Bay Basin in eastern China. The BZ19-6 large condensate gas field, the largest gas field in the Bohai Bay Basin, was discovered for the first time in a typical oil-type basin. The proven oil and gas geological reserves in the deeply buried hills of the Archean metamorphic rocks, amount to approximately 3 × 10 8 tons of oil equivalent. However, the phase behavior and genetic mechanisms of hydrocarbon fluids are still unclear. In this study, the phase diagram identification method and various empirical statistical methods, such as the block diagram method, φ 1 parameter method, rank number method, and Z -factor method were implemented to comprehensively identify the phase behavior types of the BZ19-6 condensate gas reservoir. The genetic mechanism of the BZ19-6 condensate gas reservoir was investigated in detail through analyses of physical properties of the fluid at high temperatures and pressures, organic geochemical characteristics of condensate oil and gas, and regional tectonic background. Consequently, this study is shown as follows: (1) The BZ19-6 condensate gas reservoir is a part of a secondary condensate gas reservoir with oil rings, formed synthetically since the Neogene period due to multiple factors, such as retrograde evaporation from deep burial and high temperature, inorganic CO2 charging from the deep mantle, and late natural gas invasion. (2) The hydrocarbon accumulation process of the BZ19-6 condensate gas reservoir is as follows: First, a large amount of oil is accumulated at the end of the lower Minghuazhen deposition (5 Ma BP); second, a large amount of natural gas is generated in the deep-source kitchen and mantle-derived inorganic CO2 charged into the early formed oil reservoirs at the end of the upper Minghuazhen deposition (2 Ma BP). As a result, the content of gaseous hydrocarbons in the hydrocarbon system of the reservoir increased, which led to large amounts of liquid hydrocarbons dissolved in gaseous hydrocarbons and significantly reduced the critical temperature of the hydrocarbon system. Therefore, existing secondary condensate gas reservoirs are formed when the critical temperature is lower than the formation temperature and it enters the critical condensate temperature range.

Global carbon dioxide output of volcanic lakes in the Azores archipelago, Portugal

The Azores archipelago (Portugal) contains 88 lakes, two located inside caves spread across six of the nine volcanic islands of the archipelago; the formation of the lakes is associated with various types of volcanic structures. Surveys were performed on 45 water bodies corresponding to a representative data set and omitting only small ephemeral ponds to extensively research CO2 emissions from the volcanic lakes.The modified accumulation chamber method was used to measure the CO2 flux at the surface of the lakes; 16,119 measurements were performed with flux values ranging from 0 to 20,960 g m−2 d−1. Graphical statistical approaches identified different geochemical populations from the CO2 flux data. Multiple populations associated with limnological and biogenic processes or volcanic/hydrothermal sources were identified in some cases. The influence of these sources was also confirmed by the determined δ13C content of the lakes.The lakes of the Azores emit approximately ~171 × 103 t yr−1 of CO2 into the atmosphere; much of this is released by the Furnas and Furna do Enxofre lakes, both showing mantle-derived CO2 origin. Deep-seated CO2 accounts for approximately 42% of the total degassing.

The composition of fluids stored in the central Mexican lithospheric mantle: Inferences from noble gases and CO2 in mantle xenoliths

We present the first isotopic (noble gases and CO2) characterization of fluid inclusions coupled to Raman microspectroscopy analyses in mantle xenoliths from Central Mexico, a geodynamically complex area where the Basin and Range extension was superimposed on the Farallon subduction (terminated at 28 Ma). To characterize the isotopic signature of the Central Mexican lithospheric mantle, we focus on fluid inclusions entrapped in mantle xenoliths found in deposits of the Joya Honda maar (JH), a Quaternary monogenetic volcano belonging to the Ventura Espiritu Santo Volcanic Field (VESVF) in the state of San Luis Potosí (central Mexico). Thirteen ultramafic plagioclase-free xenoliths were selected, all exhibiting a paragenesis Ol > Opx > Cpx > > Sp, and being classified as spinel-lherzolites and harzburgites. All xenoliths bring textural evidence of interstitial glass veins bearing dendritic trails of secondary melt and fluid inclusions (composed of silicate glass ± CO2 ± Mg-Ca carbonates ± pyrite). These are related to pervasive mantle metasomatism driven by carbonate-rich silicate melt. The Ar and Ne systematics reflect mixing between MORB-like upper mantle and atmospheric fluids, the latter interpreted as reflecting a recycled air component possibly inherited from the Farallon plate subduction. The 3He/4He ratios vary between 7.13 and 7.68 Ra, within the MORB range (7–9 Ra), and the 4He/40Ar* ratios (0.4–3.11) are similarly close to the expected range of the fertile mantle (1–5). Taken together, these pieces of evidence suggest that (i) either the mantle He budget was scarcely modified by the Farallon plate subduction, and/or (ii) that any (large) crustal contribution was masked by a later metasomatism/refertilization episode, possibly during the subsequent Basin and Range extension. A silicate melt-driven metasomatism/refertilization (revealed by the association between glass veins and fluid inclusions) is consistent with calculated helium residence time for the Mexican lithospheric mantle (20 to 60 Ma) that overlaps the timing of the above geodynamic events. We propose that, after the refertilization event (e.g., over the last ~20 Ma), the lithospheric mantle has evolved in a steady-state, becoming slightly more radiogenic. We also estimated 3He fluxes (0.027–0.080 mol/g), 4He production rates (340–1000 mol/yr), and mantle CO2 fluxes (3.93 × 107 mol/yr to 1.18 × 108 mol/yr) using the helium isotopic values measured in JH mantle xenoliths. Finally, the JH xenoliths exhibit CO2/3He ratios comparable to those of the upper mantle (from 3.38 × 108 to 3.82 × 109) but more positive δ13C values (between −1.0 and − 2.7‰), supporting the involvement of a crustal carbonate component. We propose that the metasomatic silicate melts recycled a crustal carbonate component, inherited by the Farallon plate subduction.

Microbial Signatures in Deep CO2-Saturated Miocene Sediments of the Active Hartoušov Mofette System (NW Czech Republic).

The Hartoušov mofette system is a natural CO2 degassing site in the central Cheb Basin (Eger Rift, Central Europe). In early 2016 a 108 m deep core was obtained from this system to investigate the impact of ascending mantle-derived CO2 on indigenous deep microbial communities and their surrounding life habitat. During drilling, a CO2 blow out occurred at a depth of 78.5 meter below surface (mbs) suggesting a CO2 reservoir associated with a deep low-permeable CO2-saturated saline aquifer at the transition from Early Miocene terrestrial to lacustrine sediments. Past microbial communities were investigated by hopanoids and glycerol dialkyl glycerol tetraethers (GDGTs) reflecting the environmental conditions during the time of deposition rather than showing a signal of the current deep biosphere. The composition and distribution of the deep microbial community potentially stimulated by the upward migration of CO2 starting during Mid Pleistocene time was investigated by intact polar lipids (IPLs), quantitative polymerase chain reaction (qPCR), and deoxyribonucleic acid (DNA) analysis. The deep biosphere is characterized by microorganisms that are linked to the distribution and migration of the ascending CO2-saturated groundwater and the availability of organic matter instead of being linked to single lithological units of the investigated rock profile. Our findings revealed high relative abundances of common soil and water bacteria, in particular the facultative, anaerobic and potential iron-oxidizing Acidovorax and other members of the family Comamonadaceae across the whole recovered core. The results also highlighted the frequent detection of the putative sulfate-oxidizing and CO2-fixating genus Sulfuricurvum at certain depths. A set of new IPLs are suggested to be indicative for microorganisms associated to CO2 accumulation in the mofette system.

A review of the geology and origin of CO2 in mineral water springs in east Belgium

Naturally CO2-rich mineral water springs (pouhons) in east Belgium occur in the context of the Rhenohercynian domain of the Variscan fold-and-thrust belt, mostly within the Cambro-Ordovician Stavelot-Venn Massif. The origin of the CO2 is still unclear, although different hypotheses exist. In this review study, we show pouhon waters are of the calcium bicarbonate type (~310 mg/l HCO3- on average), with notable Fe (~15 mg/l) and some Ca (~43 mg/l). Pouhon waters are primarily meteoric waters, as evidenced by H and O isotopic signature. The δ13C of CO2 varies from -7.8 to +0.8‰ and contains up to ~15% He from magmatic origin, reflecting a combination of carbonate rocks and mantle as CO2 sources at depth. Dinantian and Middle Devonian carbonates at 2–6 km depth could be potential sources, with CO2 generated by dissolution. However, carbonates below the Stavelot-Venn Massif are only predicted by structural models that assume in-sequence thrusting, not by the more generally accepted out-of-sequence thrust models. The mantle CO2 might originate from degassing of the Eifel magmatic plume or an unknown shallower magmatic reservoir. Deep rooted faults are thought to act as preferential pathways. Overall low temperatures of pouhons (~10 °C) and short estimated residence times (up to 60 years) suggest magmatic CO2 is transported upwards to meet infiltrating groundwater at shallower depths, with partial to full isotopic exchange with carbonate rocks along its path, resulting in mixed magmatic-carbonate signature. Although the precise role and interaction of the involved subsurface processes remains debatable, this review study provides a baseline for future investigations.

Characteristics and implication of multilayer dawsonite in heterogeneity reservoir of the Honggang anticline, southern Songliao Basin, NE China

Reservoir heterogeneity is one of the key factors regarding the long-term fate of the injected CO2. Natural CO2 reservoir and ancient CO2-bearing reservoir is a natural analog to investigate the influence of reservoir heterogeneity on the migration of CO2 and CO2-rock interaction with geological timescale. Dawsonite cements are detected in both sandstone and mudstone of the Upper Cretaceous Qingshankou reservoir in Honggang anticline of the southern Songliao Basin, China. Here, we present results of a petrographic characterization of this reservoir based on polarizing microscope, X-ray diffraction, and fluid inclusion data. These data were used to identify the vertical distribution characteristic of dawsonite and to identify the migration characteristics for the supercritical CO2 in heterogeneity reservoir. Our analytical results show that as the “CO2 trace mineral,” dawsonite appears as multilayered zones of cementation that are separated by mudstone interlayers. These multilayered dawsonite could be one of the geological produces for supercritical CO2 flooding through the heterogeneous rock, acting as the lateral migration and upward diffusion. Supercritical CO2 could move through the thinner mudstone interlayer, with the product of dawsonite developing in the mudstone as its “footprint,” although the vertical distance of diffusion in low-permeability caprock is limited. Combined with the truth that most of the present CO2 gas reservoirs (in K1q4) are in the deeper layers than the dawsonite-bearing sandstone (K1q4-K2y1) (developed in non-CO2 reservoir), we can deduce that CO2 was at some time abundant in the Honggang anticline but had now part of CO2 been migrated, with the other part consumed by the reaction with the primary rock and captured as new carbonate minerals. The dawsonite-bearing sandstones also record a sequence of hydrocarbon filling events. Combined with the truth that the injection time of CO2 is later than that of hydrocarbon, the early hydrocarbon should be deasphalted by the injection and migration of mantle CO2. Therefore, CO2 could also be stored as long-term carbonate minerals after the termination of a CO2-EOR project.

Sediment-derived origin of the putative Munnar carbonatite, South India

Metacarbonate assemblages in high-grade metamorphic terranes often pose challenges when trying to distinguish between mantle-derived carbonatite and sedimentary carbonate protoliths. We present a study of granulite-facies metacarbonate samples of the putative Munnar carbonatite described as decimeter-thick dikes and veins, and layers of a meter-thick metacarbonate and calc-silicate assemblage, respectively. Thin sections of the metacarbonate dike samples show absence of pyrochlore and ubiquitous scapolite, titanite, wollastonite, and detrital zircons are compatible with impure limestone protoliths. Nd and Sr isotope compositions indicate protoliths with Paleoproterozoic crustal residence times which contrast the mantle sources of Indian and global carbonatites. Trace-element patterns display the characteristics of upper crust, and Ce- and Y-anomalies in a number of samples suggest protolith formation under marine conditions. Carbon and oxygen isotope compositions of the metacarbonate samples interlayered with calc-silicate rocks are similar to those in marine limestone. The metacarbonate dikes, however, show mantle-like compositions which are interpreted as reflecting equilibration with mantle-derived CO2 during granulite-facies metamorphism. The dikes yielded a U-Pb zircon crystallization age of 1020 ± 70 Ma and a cross-cutting quartz syenite, thought to be cogenetic, a magmatic age of 620 ± 35 Ma; the hosting gneiss provided a magmatic age of 2452 ± 14 Ma. We conclude that the layered metacarbonate and calc-silicate rocks represent a former marine limestone and marl sequence and the metacarbonate dikes and veins small-volume melts of crust-derived carbonate-rich sediment.

Displaced cratonic mantle concentrates deep carbon during continental rifting.

Continental rifts are important sources of mantle carbon dioxide (CO2) emission into Earth's atmosphere1-3. Because deep carbon is stored for long periods in the lithospheric mantle4-6, rift CO2 flux depends on lithospheric processes that control melt and volatile transport1,3,7. The influence of compositional and thickness differences between Archaean and Proterozoic lithosphere on deep-carbon fluxes remains untested. Here we propose that displacement of carbon-enriched Tanzanian cratonic mantle concentrates deep carbon below parts of the East African Rift System. Sources and fluxes of CO2 and helium are examined over a 350-kilometre-long transect crossing the boundary between orogenic (Natron and Magadi basins) and cratonic (Balangida and Manyara basins) lithosphere from north to south. Areas of diffuse CO2 degassing exhibit increasing mantle CO2 flux and 3He/4He ratios as the rift transitions from Archaean (cratonic) to Proterozoic (orogenic) lithosphere. Active carbonatite magmatism also occurs near the craton edge. These data indicate that advection of the root of thick Archaean lithosphere laterally to the base of the much thinner adjacent Proterozoic lithosphere creates a zone of highly concentrated deep carbon. This mode of deep-carbon extraction may increase CO2 fluxes in some continental rifts, helping to control the production and location of carbonate-rich magmas.

Roles of fault structures on the distribution of mantle-derived CO2 in the Bohai Bay basin, NE China

Based on studies on the contents, origin and filling history of CO2, and the seismic interpretation of the Bohai Sea, the sea area of the Bohai Bay basin, the distribution of mantle-derived CO2 and roles of fault structures have been examined in this study. Mantle-derived CO2 is mainly distributed in the structures within the Bozhong area, the center of the Bohai Sea. Laser Raman spectrum shows that CO2 inclusions are symbiotic with hydrocarbon inclusions with a charging time no earlier than 5.1 Ma. Late-stage (5.1–0 Ma) active basement faults are the major pathways for the migration of mantle-derived CO2. The intensity of the activity influences the CO2 content. Abundant CO2 occurs in the structures with basement faults that have relatively high fault activity rate (FAR) values. Along the basement fault, high CO2 content appears near the segments with high FAR values. Fault combination patterns affect the accumulation layers of mantle-derived CO2. Structures with fault combination patterns that resemble a “flower” or “Y” commonly accumulate CO2 in Cenozoic shallow layers as well. The branch faults that intersect with basement faults facilitate the adjustment of CO2 from deep to shallow layers. The uplifted mantle is the deep tectonic context, and the upwelling mantle magma is the source of the development of mantle-derived CO2. Below the middle crust, mantle-derived CO2 may be vertically transported along the lithosphere-scale strike–slip faults. Within the middle and upper crust, the late-stage active basement faults are the major conduits for mantle-derived CO2.