Gas Isotopic Compositions Research Articles

Noble gas isotopic compositions and abundance ratios of mantle xenoliths from Honolulu series volcanism, Oahu, Hawaii

We report new noble gas isotopic compositions and abundance ratios (helium, neon, and argon) extracted from six lherzolite xenoliths on Oahu Island, Hawaii, by vacuum crushing. Considering the equilibrium temperature of the xenoliths and the fluid density of their fluid inclusions, they originated from a pressure of 0.98–1.04 GPa, corresponding to 33–34 km depth. The Moho depth beneath Oahu Island is around 15 km, suggesting that the xenoliths are part of the sub-oceanic lithospheric mantle (SOLM), 90 million years old, after generation of mid-oceanic ridge basalts (MORB). The samples have 3He/4He of 3.0–9.5 times atmosphere (Ra). The neon isotope compositions overlap with the data of previous studies for lava flow samples of the Hawaiian Islands, MORB, and air. The 40Ar/36Ar range from atmospheric to 3100. They show very low 4He/40Ar* and 4He/21Ne*, where asterisks denote correction for atmospheric contributions. The low 4He/40Ar* and 4He/21Ne* are characteristic of small fluid inclusions, based on step crushing experiments, indicating that they record kinetic fractionation that occurred in the SOLM most likely due to melt infiltration. Using 4He/40Ar* and 20Ne/22Ne respectively as indices of the kinetic fractionation and atmospheric contribution, we corrected for these effects. The corrected noble gas isotopic compositions are best explained by interaction between plume-derived melts and SOLM. The kinetic fractionation yields a decrease in 3He/238U in SOLM, which should result in a rapid decrease in 3He/4He with time. The slightly elevated 3He/4He (above 8 times atmospheric), as found in the present xenoliths cannot be maintained for more than one million years in the fractionated SOLM with 3He/U molar ratio of ∼1 × 10−8, which is calculated assuming that the 3He/U has the same degree of kinetic fractionation as 4He/40Ar* and 4He/21Ne*. Therefore, the fractionation event is likely to be recent, and may be related to recent magmatism in the SOLM beneath Oahu Island. Such igneous activity is limited to Honolulu series volcanism (0.35–0.80 Ma). Therefore, the xenoliths contain a record of magmas with plume-derived noble gases, most likely associated with the Honolulu series volcanism.

Geochemistry and fluxes of gases from hydrothermal features at Newberry Volcano, Oregon, USA

We present the chemical and isotopic compositions of gases and fluxes of CO2 from the hydrothermal features of Newberry Volcano, a large composite volcano located in Oregon's Cascade Range with a summit caldera that hosts two lakes, Paulina and East Lakes. Gas samples were collected from 1982 to 2021 from Paulina Hot Springs (PHS) on the shore of Paulina Lake, East Lake Hot Springs (ELHS) on the shore of East Lake, and Obsidian Flow Gas Seep (OFGS), an area of diffuse gas emissions. Surveys of CO2 flux were conducted in 2020 at OFGS (1400 m2) and East Lake (4.1 km2). Gases from all three sites were CO2-rich (≥79 mol% in dry gas) but showed considerable compositional variability over time due to interaction with ground and surface water. An increase in H2S concentrations and decline in CO2/H2S ratios in ELHS gases coincided with a drop in East Lake water level from 1999 to 2021. ELHS and OFGS gases were high in CH4 relative to PHS and the δ13C of CH4 values for ELHS gases (−72.2 and − 63.6 ‰) reflected a predominantly biogenic origin. The dominant source of N2 and Ar in PHS, ELHS, and OFGS samples was likely groundwater. Helium isotopic ratios (6.47 to 8.02 Rc/Ra) support a persistent source of magmatic He beneath Newberry caldera and consistently high values measured at OFGS and PHS relative to ELHS suggest distinct fluid flow paths from depth to the surface features. The δ13C of CO2 and CO2/3He values (−8.9 to −5.35 ‰ and 1.3 × 109 to 4.6 × 1010, respectively) measured in gases reflect contributions of CO2 from both mantle and crustal sources. Measured CO2 fluxes at OFGS and East Lake ranged from 1 to 8808 and < 1 to 364 g m−2 d−1, respectively. A CO2 emission rate of 0.5 t d−1 was calculated for OFGS. The CO2 emission rate estimated for East Lake was 30 t d−1 and when compared to prior estimates, reflects steady-state lake degassing. An enhanced geochemical monitoring plan, including annual sampling of gases at ELHS, OFGS, and PHS for geochemical analysis, installation of a continuous lake-level monitoring station at East Lake, and annual CO2 flux surveys at OFGS, would provide valuable background data and insights into any precursor volcanic activity. Integrating geochemical data with data from the real-time seismic and GPS network at Newberry Volcano could better resolve and interpret potential changes in its magma-hydrothermal system.

Monitoring of magmatic–hydrothermal system by noble gas and carbon isotopic compositions of fumarolic gases

We repeatedly measured isotopic compositions of noble gases and CO2 in volcanic gases sampled at six fumaroles around the Kusatsu-Shirane volcano (Japan) between 2014 and 2021 to detect variations reflecting recent volcanic activity. The synchronous increases in 3He/4He at some fumaroles suggest an increase in magmatic gas supply since 2018. The increase in magmatic gas supply is also supported by the temporal variations in 3He/CO2 ratios and carbon isotopic ratios of CO2. The 3He/40Ar* ratios (40Ar*: magmatic 40Ar) show significant increases in the period of high 3He/4He ratios. The temporal variation in 3He/40Ar* ratios may reflect changes in magma vesicularity. Therefore, the 3He/40Ar* ratio of fumarolic gases is a useful parameter to monitor the current state of degassing magma, which is essential for understanding the deep process of volcanic unrest and may contribute to identifying precursors of a future eruption. These results provide additional validation for the use of noble gas and carbon isotopic compositions of fumarolic gases for monitoring magmatic–hydrothermal systems.

The effect of thermochemical sulfate reduction of n-pentane and condensate on the chemical and isotopic compositions of gases: Implications for the organic reactant and reaction extent

Evaluating the composition of organic reactants initially involved in thermochemical sulfate reduction (TSR) reaction and better understanding the chemical and isotopic compositions of gases are helpful to minimize the risks involved in exploration of natural sour gas reservoirs. Variations in the chemical and carbon isotopic compositions of the gas produced during high pressure (50Mpa), isothermal (360 °C) hydro-pyrolysis experiments of the TSR reaction and thermal cracking involving n-pentane (n-C5) and condensate were investigated using a high-pressure gold tube system. The results show that the yields of CH4, CO2 and H2S increased with reaction time, and that the yields of C2–C4 firstly increased and then decreased. The δ13C value of alkane gases increased with reaction time. The δ13C value of CO2 produced from the TSR reaction involving n-C5 became less negative. The yields of alkane gas, CO2 and H2S from the TSR reaction of n-C5 were up to two times higher than that those from the TSR reaction of condensate. This indicates that lighter liquid hydrocarbons with higher hydrogen content produce more hydrocarbon and non-hydrocarbon gases. The relationship between the dryness, δ13C values of alkane gases and sourness could help to estimate whether the organic reactant was liquid or gaseous hydrocarbons. The value of ln(C2/C3) at the early stage of TSR reaction can be used to roughly estimate the composition of the liquid hydrocarbons involved in the TSR reaction. Our analysis also shows that the value of ln(C2/C3) firstly increased and then decreased with increasing gas dryness during the TSR reaction, which could be an effective index to assess the TSR reaction extent and provide new clues for identifying gas origin. The δ13C values of methane and ethane produced in the TSR reaction involving n-C5 reversed to δ13C1 > δ13C2 during the 48–60h period, which could be related to the oxidation of alkanes by HSO4−. This could help explain the δ13C1 > δ13C2 in TSR influenced gas reservoirs with low H2S content.

Fault associated with the 1967 M 6.3 Koyna earthquake, India: A review of recent studies and perspectives for further probing

The Koyna region in western India, characterized by recurrent seismic activity over more than five decades and confined largely within a 20 km × 30 km area and ∼10 km in depth, is a classical site to study earthquake processes. The largest earthquake in the region, the M 6.3 Koyna earthquake of December 1967, formed a ∼NNE-SSW trending surface rupture, known as Donichawadi fissure zone. The fissure zone, mapped originally over a length of ∼4 km and width of ∼200 m between Nanel and Kadoli by the Geological Survey of India (GSI) in the wake of the earthquake, comprises en-echelon fractures, near-vertical fissures, oblique or diagonal tensional cracks, mole tracks, soil lumps and laterite boulders in paddy cultivated land. We review the Donichawadi fissure zone in the light of extensive seismological data acquired in the past five decades, surface mapping studies, recent geophysical studies, soil-gas helium studies over the fissure zone during 1996–1997, chemical and noble gas isotope compositions of formation gases, and subsurface fracture data from downhole image logs in a 3 km - deep scientific borehole drilled in 2017. Integration of the datasets provide clinching evidence that the Donichawadi fissure zone is the surface manifestation of a deeper crustal fault (referred as Donichawadi fault) that has been active for more than 55 years and yield critical new insights on the geometry, distribution and possible subsurface disposition of the fissures and the associated fractures in the subsurface. We conclude that the Donichawadi fault offers a potential target for further deep probing through scientific drilling, downhole measurements and long-term monitoring to gain new insights into the genesis of triggered earthquakes in the area.

Choice of Gas Isotope Composition for Neutral Beam Injectors of the FNS-ST Compact Fusion Neutron Source

The dependence of the neutron yield of the FNS-ST (spherical tokamak) fusion neutron source on the fraction of tritium in the core D+T plasma is analyzed for the case of using tritium neutral beam injectors with 200-keV energy and 6-MW power. The FNS-ST operating regimes are explored using the SOLPS4.3 and ASTRA codes for different values of core plasma density ne, T fraction in the plasma, and particle diffusivity. The FC-FNS code is used to estimate the fluxes of the fuel components in the fuel cycle (FC), which are produced by different injection systems: gas puffing, pellet injection, and neutral beam (T) injection. It is shown that in the case of the Т beam injection, in the operating range of parameters, the neutron yield can reach 6.0 × 1017 s−1, which is the value comparable to that obtained for the scenario of D-beam injection into the balanced D+T plasma. In the case of the T-beam injection, in the range of parameters, for which the neutron yield is close to its maximum, the amount of tritium in the FC is lower than in the case of the D-beam injection. The neutron yield can be increased to 6.5 × 1017 n/s if full separation of the D and T is introduced for the gas pumped out from the divertor and puffed back into the torus. With this approach, in the case of the tritium beam, the amount of tritium in the FC is Tinv of ~170 g. If this approach is used in the case of the deuterium beam, the neutron yield can reach 7.0 × 1017 n/s. However, in this case, the amount of tritium contained in the FC increases to 215 g. The results of the analysis performed are used for optimizing the FC of the FNS-C (compact) fusion neutron source, which is planned for construction in the framework of the comprehensive program of the State Corporation Rosatom “Development of Engineering, Technology and Scientific Research in the Field of Using Atomic Energy in the Russian Federation for the Time Period up to 2030.”

Pre-subduction mantle noble gas elemental pattern reveals larger missing xenon in the deep interior compared to the atmosphere

Understanding volatile evolution on Earth provides critical information on the processes that shape the Earth, and hence the solar system. The noble gas elemental and isotopic composition of the Earth's mantle traces the sources of Earth's volatiles as well as evolution processes, such as mantle degassing and regassing via subduction. However, ubiquitous shallow-level atmospheric contamination of mantle-derived samples has hampered determining the deep mantle heavy noble gas (Kr, Xe) isotopic and elemental composition. Moreover, the present-day elemental composition of the mantle reflects mixing of initial volatiles with atmospheric noble gases recycled through subduction that makes understanding the elemental signatures acquired during accretion difficult. Using a recently-developed protocol, we previously measured the krypton and xenon isotopic and elemental compositions of the Galápagos and Iceland plume sources, which have among the most primitive helium and neon isotopic signatures, sampling one of the least degassed, most primordial mantle reservoirs. Based on these measurements, here we introduce a new approach to correct for recycling and hence determine the initial noble gas elemental ratios of the deep mantle. Our study leaves room for a substantial proportion of 36Ar in the mantle to be primordial in origin. Most importantly, we show that the deep mantle, prior to the injection of any atmospheric volatiles, was depleted in Xe relative to Kr by about two orders of magnitude when compared to chondritic compositions, and the Xe depletion was larger in the mantle than in the modern-day atmosphere. Unlike the atmosphere that has protracted history of Xe depletion through the Archean, the deep mantle xenon depletion was acquired very early in Earth's history, most likely during accretion. Hence, missing xenon in the Earth's interior and in the atmosphere appears to be two distinct problems. Three possible scenarios, potentially acting in sync, may have led to the mantle xenon depletion: Xe partitioning into the core, magma ocean outgassing and a xenon deficit in the Earth's parent bodies, such as comets.

Characterization of chemical and carbon isotopic compositions of gases during thermochemical sulfate reduction and implications for gas origin and content

For identifying the occurrence and extent of thermochemical sulfate reduction (TSR) reaction of natural gas and better understanding the chemical and carbon isotopic variations in natural gas reservoirs, high-pressure hydro-pyrolysis with a special designed apparatus was performed using natural gas and various amounts of MgSO4·7H2O at up to 360 °C. The yields, chemical and isotopic compositions of the gases produced during TSR and thermal cracking were measured. As the extent of TSR reaction increased, the concentrations of CH4, CO2 and H2S increased in a nonlinear way, while those of C2H6 and C3H8 decreased. According to the variation of gas content, the TSR reaction of alkane gases can be divided into an uncatalyzed and a catalyzed stage, which is different from previous studies that treated the TSR reaction of alkane gases as a non-autocatalytic reduction process. As the concentration of MgSO4·7H2O increased, the rate of TSR reaction with hydrocarbon gases increased. The concentrations of HSO4− and volume of aqueous phase could be responsible for the different TSR reaction rates in the catalyzed stage. The co-variation of ln(C1/C2) and ln(C2/C3) could be related to the TSR reaction of alkane gases. Our study provides clues for understanding the compositional variations in natural conditions.

Integrated geochemical approach to determine the source of methane in gas hydrate from Green Canyon Block 955 in the Gulf of Mexico

Massive volumes of gas are sequestered within gas hydrate in subsurface marine sediments in the Gulf of Mexico. Methane associated with gas hydrate is a potentially important economic resource and a significant reservoir of carbon within the global carbon cycle. Nevertheless, uncertainties remain about the genetic source (e.g., microbial, thermogenic) and possible migration history of natural gas incorporated into hydrate. Previous studies have primarily used the hydrocarbon molecular (CH4/C2H6+) and isotopic (δ13C-CH4, δ2H-CH4) compositions of natural gas to address these uncertainties. However, hydrocarbon tracers are altered by mixing, oxidation, secondary methanogenesis, or fluid migration, which presents challenges when deciphering the mechanisms responsible for methane formation and accumulation. To evaluate the genetic source of natural gases from Green Canyon Block 955 (GC 955), east of the Sigsbee escarpment, we collected and analyzed samples from the first pressurized hydrate-bearing sediment cores collected from a coarse-grained hydrate reservoir in the Gulf of Mexico. Gas samples were analyzed for hydrocarbon (C1- C5), major gas (e.g., N2, CO2), and noble gas (He-Xe) abundance and isotopic (e.g., δ13C-CH4, δ2H-CH4, δ13C-CO2, δ15N-N2, 3He/4He, 4He/20Ne) compositions. We determined that natural gas in hydrates from this location are predominantly of primary microbial origin (conservatively at least 76%) and are formed by the hydrogenotrophic (CO2 reduction) methanogenesis pathway. We also note increased thermogenic proportions (~6%) in a hydrate-bearing layer below the main hydrate-bearing interval (separated by a 5 m water-bearing layer). Our results suggest that microbial methane may be abundant below the base of gas hydrate stability at GC 955.

Carbon Dioxide and its Carbon Isotopic Composition of Natural Gas in the Sichuan Basin, SW China

The Sichuan Basin, covering an area of 180 × 103 km2, has the following advantages in natural gas geology: The sedimentary rocks are 6,000–12,000 m thick with high maturity of source rocks, and nine sets of primary gas source rocks are developed in the basin with a gas–oil ratio of 80:1, and thus it is a gas basin. The remaining recoverable reserves of conventional and unconventional natural gas are up to 13.6404 × 1012 m3. Multiple gas-bearing systems are developed with 25 conventional and tight oil and gas producing layers and 135 discovered gas fields, and the total proved geological reserves and cumulative production of natural gas by the end of 2019 were 5.7966 × 1012 m3 and 648.8 × 109 m3, respectively. The CO2 components and the correlation with relevant parameters for 243 samples from 22 gas fields indicate that CO2 in the Sichuan Basin display the following two characteristics: (1) Relatively low CO2 content of 0.02%–22.90% with an average of 2.96%, which guaranteed the commerciality of natural gas exploration and production; (2) cratonic CO2, which is characterized by low CO2 contents (&amp;lt;5%) and low R/Ra ratios (&amp;lt;0.24). According to the δ13CCO2 values and the relationship with R/Ra, δ13C1, CO2 contents, and wetness coefficient (W) for 263 gas samples, the δ13CCO2 values display three characteristics: (1) The highest δ13CCO2 value (10.4‰) in China is found in the Fuling shale gas field, which extends the interval values from previous −39‰–7‰ to −39‰–10.4‰. (2) The δ13CCO2 values can be applied to identify the CO2 origin of natural gas in the Sichuan Basin: type A, organic origin from thermal decomposition of organic matter, with an average δ13CCO2 value of −12.8‰ and average wetness coefficient of 7.8% for 44 samples; type B, organic origin from thermal cracking of organic matter, with an average δ13CCO2 value of −15.7‰ and average wetness coefficient of 1.30% for 34 samples; type C, inorganic origin from thermal decomposition or organic acid dissolution of carbonate rocks or minerals, with an average δ13CCO2 value of −1.8‰ and average wetness coefficient of 0.85% for 175 samples. (3) δ13CCO2&amp;gt;δ13CCH4. This is a common characteristic shared by all geological age (from Z2dn to J2s) gas reservoirs and various gas types (coal-derived gas, oil-associated gas, and shale gas).

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.

Dual-comb cavity ring-down spectroscopy

Cavity ring-down spectroscopy is a ubiquitous optical method used to study light-matter interactions with high resolution, sensitivity and accuracy. However, it has never been performed with the multiplexing advantages of direct frequency comb spectroscopy without significantly compromising spectral resolution. We present dual-comb cavity ring-down spectroscopy (DC-CRDS) based on the parallel heterodyne detection of ring-down signals with a local oscillator comb to yield absorption and dispersion spectra. These spectra are obtained from widths and positions of cavity modes. We present two approaches which leverage the dynamic cavity response to coherently or randomly driven changes in the amplitude or frequency of the probe field. Both techniques yield accurate spectra of methane—an important greenhouse gas and breath biomarker. When combined with broadband frequency combs, the high sensitivity, spectral resolution and accuracy of our DC-CRDS technique shows promise for applications like studies of the structure and dynamics of large molecules, multispecies trace gas detection and isotopic composition.

Probing the hidden magmatic evolution of El Misti volcano (Peru) with the Pb isotope composition of fumaroles

This work proposes a new method to probe the hidden magmatic evolution of quiescent Andean volcanoes from the Pb isotope composition of gases. The method is based on an assimilation-fractional crystallisation-degassing model linking the Pb isotope composition of gases with the SiO2 content of their magmatic source. The model is applied to El Misti volcano that threatens Arequipa, the second most densely populated city of Peru. Gas condensates and Pb-rich solid deposits (PbS, PbCl2, PbSO4) collected in 2018 in the bottom of El Misti crater at 260–150°C fumarole vents were used to reconstruct the mean composition of degassing magmas (60.8–61.8 wt% SiO2). These compositions are slightly more evolved than the lavas from the last AD 1440–1470 eruption, suggesting either the secular differentiation of the main magma reservoir, or the contribution of more evolved magmas to volcanic gases. On the other hand, the slight but significant difference between the instantaneous composition recorded in gas condensates and the time-integrated composition recorded in solid deposits points to the degassing of less evolved magmas over the last decades. This trend is ascribed to a recent recharge of El Misti reservoir with hot mafic magmas, in agreement with the evolution of fumarolic deposit mineralogy in the last half a century. The Pb isotope composition of gas appears to be a promising tool for probing the hidden magmatic evolution of quiescent volcanoes where assimilation-fractional crystallisation operates.

Geochemical studies of gases from oil and gas fields in the southern part of the caspian basin and their correlation with the results of oil geochemistry

This article presents the results and interpretation of the compositional and isotopic analysis of carbon in 14 gas samples derived from oil and gas fields and structures (Liman, East Makat, North Kotyrtas, Zholamanov, S. Nurzhanov and West Prorva) of the southern part of Pre-Caspian basin. According to the results of this study, all gases have a thermogenic source and their organic matters were deposited in the marine environment, which is also consistent with the results of biomarker analysis of oils from these studied fields. A sharp enrichment of propane with heavy carbon isotope in gas №55 of East Makat field suggests its biodegradation, which is also confirmed by the results of gas chromatographic analysis of oil from this well. A star diagram of gases was plotted based on normalized values of carbon isotopic composition of C1-C5, according to the results of which the studied samples can be divided into 5 genetically different groups. The identified groups based on results of this gas analysis are also consistent with the results of «fingerprinting» of oil from these fields. Keywords: gas composition; gas isotopic composition; Rayleigh fractionation; Bernard diagram; Clayton diagram; Laurent diagram; Chung diagram; enrichment in heavy isotope.

Comparing the results of lineament analysis with isotope geochemistry data

This article presents the results of a geochemical survey carried out in the southwestern part of the Siberian platform, within the Sayan-Yenisei (Angara) syneclise (a superorder Riphean-Middle Paleozoic structure). The object of research was hydrocarbon gases contained in the subsoil rocks (clays). The subsoil samples were taken from the bottom of boreholes (40 mm in diameter) made with an electric drill. The sampling depth was 0.6–1 m. Further laboratory studies included chromatographic and isotope analysis. Lineament analysis of the digital elevation model was carried out as a complementary study. One of the lineament analysis results was a lineament density map, which reflects the permeability (macro-fracture density) of the sedimentary cover. This allowed a comparison of the macro-fracture density with the gas content and isotopic composition. The study revealed that gases with a high content of heavy isotopes tend to gather in the low permeability areas. This can be explained by the fact that the gases disperse quickly within fractured zones, and deep gases should be expected only in the areas with strong cap rocks, i.e. in the areas with low macrofracture density where stable hydrocarbon deposits have already formed. Keywords: hydrocarbons; geochemical survey; isotope geochemistry; lineament analysis.

Noble gas variation during partial crustal melting and magma ascent processes

Noble gas isotopes, although present in trace amounts, are generally more reliable and less ambiguous recorders of their source than the major volatile species. In volcanic settings in particular, this advantage derives from their chemical inertness, as noble gas isotopic and elemental fractionations are strongly coupled to their source and modified only by physical processes during magma ascent and eruption. The Neogene volcano El Hoyazo (Betic Cordillera, SE Spain) is a highly favourable natural laboratory to study the links between partial crustal melting processes occurring at depth and the eruptive products at the surface, because partially melted crustal xenoliths are preserved in silicic lavas. Comparing the noble gas isotopic compositions of xenoliths and lavas has the potential to yield new insights into volatile behaviour during melting processes at inaccessible depths in the crust.At El Hoyazo, noble gases trapped in lava glasses, and the fluid/melt inclusions within xeno- and phenocrysts, provide novel information on: (i) their response to the crustal melting process including mechanisms such as magma mixing (and crustal assimilation) of two endmembers: i.e. the extracted felsic melt from the country metapelitic crust, and the basic-intermediate magma from the underplating in the region. The results reveal significant modification of magmatic noble gases by the interaction with the partially melted crust; (ii) noble gas variations during degassing and magma ascent, showing higher atmospheric influence in the lava samples from shallower depths than in the deeper lavas and minerals; and (iii) higher magmatic influence in crystals of garnet from deeper lava than in both shallower crystals of amphibole, and garnet crystals within the crustal xenoliths. In addition, we find that noble gases in melt inclusions are also likely accumulating in their shrinkage bubbles, and not only remaining dissolved in the melt.

Constraints on the sulfur subduction cycle in Central America from sulfur isotope compositions of volcanic gases

The sulfur cycle at convergent margins remains poorly constrained yet is fundamentally important for understanding the redox state of Earth's reservoirs and the formation of ore deposits. In this study we investigate the sulfur isotope composition of high temperature volcanic gases emitted from the Nicaraguan (average of +4.8 ± 1.3‰) and Costa Rican (average of +2.3 ± 1.3‰) arc segments contributing to emissions from the Southern Central American Volcanic Arc (SCAVA; average of +3.8 ± 1.7‰). Along-arc variations in geochemical tracers at SCAVA are widely accepted to reflect variations in subduction parameters and deep fluid sources and correlations between these parameters and gas S isotope compositions are observed. These correlations suggest that gas emissions are sourced from a mixture of mantle S with δ34S ~ 0‰ and isotopically heavy slab-derived sulfur with δ34S ≥ ~ +8‰. We employ Monte Carlo mass balance modeling to constrain S inputs to the subduction zone and relative contributions from mantle and slab to arc sulfur emissions. The models indicate that bulk subduction input in Nicaragua has a S isotope composition of +1.4 ± 0.5‰ compared to −0.2 ± 0.4‰ in Costa Rica, requiring preferential release of isotopically heavy oxidized S from the slab to explain the relatively high δ34S observed in arc outputs. We show that the flux of S from the slab is sufficient to oxidize the entire mantle wedge within the lifetime of the arc, indicating that S is a primary oxidizing agent in subduction zones. Furthermore, the preferential removal of heavy S from the slab requires retention of isotopically light S in the residual slab. Subduction-scale fractionation of S isotopes is fundamentally important in explaining why Earth's bulk surface reservoirs are isotopically positive.

Isotopic and hydrogeochemical evidence for biogenic gas in Cuervos Formation coal beds, Cesar Rancheria Basin, Colombia

Coal bed methane (CBM) has recently been produced from sub-bituminous to high volatile bituminous low rank coals (vitrinite reflectance 0.39 to 0.53%) from the Paleocene Cuervos Formation in the Cesar Sub-basin, Colombia. Understanding CBM gas origin is vital for designing exploration and production strategies, in order to target either shallow economic accumulations of microbial gas or deeper and thermally mature coal seams. In this context, this study aims to determine whether thermogenic gas, biogenic gas, or a mixture of both gas types have contributed to gas yield in the Cesar Sub-basin; it also discusses the relationship between gas origin and tectonics. For the first time, co-produced waters and gas samples from CBM multi-seam production wells and shallow aquifer water-wells were collected in the Boqueron compartment (Cesar Sub-basin) and were analysed via stable isotope composition and molecular geochemistry. Structural compartmentalisation in the Sub-basin as a result of interplate shortening during the Mid-Paleogene has resulted in slightly different coal ranks, saturation, and gas contents between compartments of the Paleocene Cuervos Formation. This event could have interrupted early thermogenesis and allowed the introduction of meteoric waters carrying bacteria consortia responsible for biogenesis.The produced waters from the CBM wells were HCO3 > Na+ > Cl− type, and ranged in TDS (Total dissolved solids) from 2268 to 6602 mg/L (avg. 3887 mg/L). Recently revised genetic diagrams of δ13C-CH4 versus δ2H-CH4 and additional parameters, such as gas dryness ratio and water chemistry, corroborated biogenesis as the main gas origin. Carbon isotopic differences between carbon dioxide and methane (Δ13CH2O–CH4), as well as those of hydrogen isotopes in water and methane (Δ2HH2O–CH4), also indicated a typical microbial CO2 reduction pathway. The positive carbon isotope composition of dissolved inorganic carbon (δ13C-DIC) not only showed a clear differentiation between waters from CBM wells and those from shallow aquifers, but also confirmed the occurrence of methanogenesis when paired with high alkalinity. In addition, the water quality analysis showed increasing sodium and bicarbonate concentrations with depth, which is typical of CBM production basins. Although biogenic gas was identified, tectonic settings do not seem to explain gas origin occurrence, since secondary biogenesis was expected in an uplifted basin such as Cesar Rancheria. Isotopic data was not decisive in differentiating primary versus secondary biogenesis, since the gas isotope compositions plot close to the boundary between the primary and secondary biogenic gas fields. Further studies on isotopic chemistry are required to refine this interpretation and confirm gas generation in the other two compartments.

Carbon Emissions From the Edge of the Greenland Ice Sheet Reveal Subglacial Processes of Methane and Carbon Dioxide Turnover

AbstractDirect gaseous emissions of methane (CH4) and carbon dioxide (CO2) from the subglacial environment under Greenland Ice Sheet (GrIS) were only recently discovered and it is yet to be determined how important it is for the panarctic carbon budget. We measured in situ net gaseous emissions of subglacial CH4 and CO2, dissolved concentrations and isotopic composition of gases (13C and 2H) at the onset, near maximum, and at the end of the melt season in 2018 and 2019. We found a tight relation between gaseous and dissolved CH4 and CO2, respectively, indicating that degassing from the subglacial meltwater is the main source of these gases in the subglacial air. The diurnal variability of in situ mole fractions of CH4 and CO2 in subglacial air was related to meltwater runoff showing that the net emission magnitude is directly related to glacial hydrology. We observed that maximum in situ mole fractions of CH4 and CO2 appeared at the onset of the melt season and decreased over the melt season. The isotopic signature of CH4 in the subglacial air indicated that it likely originated from microbial methanogenesis which remained constant during the season. Isotopic signatures of subglacial CO2 indicate mixed sources from microbial oxidation of CH4, remineralization of sedimentary organic carbon, and possibly influenced by removal of CO2 by weathering. Our study indicate large emissions of both CO2 and CH4, but continuous studies over entire melt seasons are needed to determine the origin and emission magnitudes and their relation to the glacial dynamics.

Noble gases and nitrogen in CV3 chondrite Bukhara

We investigated noble gas (He, Ne, Ar, Kr and Xe) and nitrogen isotopic composition in the carbonaceous chondrite Bukhara (CV3). This is the first report of N and noble gases measured in the same aliquot of bulk sample among the CV3 chondrites. Isotopic compositions of noble gases are mixtures of solar wind and Q-gases. We derive cosmic ray exposure (CRE) age of 22.6 ​± ​4.0 ​Ma from cosmogenic 21Ne and 38Ar for Bukhara. Nitrogen abundance observed in Bukhara is lowest compared with the data from CV3 chondrites. We estimate the trapped nitrogen composition of δ15N ​= ​+ 10.9 ​± ​1.6‰ for Bukhara, distinct from average (δ15N ​= ​- 16.3 ​± ​1.5‰) in the bulk CV3 chondrites. Bukhara shows signature of heavy nitrogen, which is distinct than the solar wind (SW) and Q-gas. Trapped nitrogen is mixture of the components, SW, Q and comet. It is observed that there is no correlation of nitrogen composition with N content in the CV3 chondrites.