Noble Gas Isotopic Compositions Research Articles

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.

Source identification of methane in groundwater in shale gas development areas: A critical review of the state of the art, prospects, and future challenges

Shale gas exploration and development carry the risk of causing groundwater contamination and enhancing the greenhouse effect through methane leakage. Identifying the source of abnormal methane in groundwater of shale gas development areas is becoming a research hotspot in the fields of groundwater and climate change. This paper reviews the traditional methodology in identifying sources of methane and its deficiency in groundwater application. Then potential and advantages of using noble gases were discussed on how to overcome these limitations of the traditional method. Finally, based on noble gas, the current application status and future challenges of methane source identification in groundwater were analyzed. It can be summarized as: (1) due to chemical and/or microbial processes in the aquifer system, the traditional methodology for methane source identification, which utilizes molecular and isotopic compositions of hydrocarbon gas, has multiple interpretationsand large uncertainties; (2) the non-reactive nature and well-characterized isotopic compositions of noble gases in the atmosphere, hydrosphere, and crust, make noble gases ideal indicators of the sources of methane in groundwater. Moreover, the mechanism of formation and release of crustal noble gas prevent shale gas signatures from being interfered with by natural gas; (3) the key scientific tasks surrounding the use of noble gases for methane source identification include quantitatively separating the components of atmosphere-derived, mantle-derived, and crust-derived noble gases from the bulk noble gases in groundwater. It quantifies the solubility fractionation of noble gases induced by water-gas interaction during methane migration to the aquifer. The application of noble gases can bring a new perspective to tracing the source of methane in groundwater and is of great significance to the protection of groundwater quality in shale gas development areas and mitigation of climate change.

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.

Exposure history, petrology, and shock‐induced sulfidization reaction of Alatage Mountain 001 strewn field samples

AbstractSeveral hundred meteorites with a total mass of over 100 kg were collected as the Alatage Mountain (AM) strewn field located in the Kumtag desert, Xinjiang Province, China. Twelve AM meteorites were studied in this work. Petrography, mineralogy, bulk chemistry, bulk oxygen isotopic compositions, and light noble gas concentrations and isotopic compositions were determined for all or for a selection of the meteorites. The studied meteorites are L‐chondrites that suffered a very strong impact; impact melt veins and melt pockets are widely distributed. More than 50% of the troilite exists in the form of blebs and veins in olivine and pyroxene. Some of these meteorites are impact melt recrystallized rocks (e.g., AM 037). The strong impact caused the decomposition of troilite, which led in AM 034 to the sulfidization reaction of olivine. The metal in most meteorites is almost completely altered, and the troilite has been significantly oxidized. Weathering resulted in the depletion of Mg, Fe, Co, and Ni, and the enrichment of Sr, Ba, Pb, and U in these meteorites. The cosmic ray exposure (CRE) ages measured for these specimens range between 6.2 ± 1.9 Ma and 9.0 ± 2.7 Ma, depending on the cosmogenic nuclide used. The average CRE age is 7.6 ± 1.3 Ma. Both 4He and 40Ar gas retention ages indicate that the strong impact which caused the shock effects occurred about 320 Ma ago.

Analytical protocols for Phobos regolith samples returned by the Martian Moons eXploration (MMX) mission

Japan Aerospace Exploration Agency (JAXA) will launch a spacecraft in 2024 for a sample return mission from Phobos (Martian Moons eXploration: MMX). Touchdown operations are planned to be performed twice at different landing sites on the Phobos surface to collect > 10 g of the Phobos surface materials with coring and pneumatic sampling systems on board. The Sample Analysis Working Team (SAWT) of MMX is now designing analytical protocols of the returned Phobos samples to shed light on the origin of the Martian moons as well as the evolution of the Mars–moon system. Observations of petrology and mineralogy, and measurements of bulk chemical compositions and stable isotopic ratios of, e.g., O, Cr, Ti, and Zn can provide crucial information about the origin of Phobos. If Phobos is a captured asteroid composed of primitive chondritic materials, as inferred from its reflectance spectra, geochemical data including the nature of organic matter as well as bulk H and N isotopic compositions characterize the volatile materials in the samples and constrain the type of the captured asteroid. Cosmogenic and solar wind components, most pronounced in noble gas isotopic compositions, can reveal surface processes on Phobos. Long- and short-lived radionuclide chronometry such as 53Mn–53Cr and 87Rb–87Sr systematics can date pivotal events like impacts, thermal metamorphism, and aqueous alteration on Phobos. It should be noted that the Phobos regolith is expected to contain a small amount of materials delivered from Mars, which may be physically and chemically different from any Martian meteorites in our collection and thus are particularly precious. The analysis plan will be designed to detect such Martian materials, if any, from the returned samples dominated by the endogenous Phobos materials in curation procedures at JAXA before they are processed for further analyses.

Investigating Fracture Network Deformation Using Noble Gas Release

We investigate deformation mechanics of fracture networks in unsaturated fractured rocks from subsurface conventional detonation using dynamic noble gas measurements and changes in air permeability. We dynamically measured the noble gas isotopic composition and helium exhalation of downhole gas before and after a large subsurface conventional detonation. These noble gas measurements were combined with measurements of the subsurface permeability field from 64 discrete sampling intervals before and after the detonation and subsurface mapping of fractures in borehole walls before well completion. We saw no observable increase in radiogenic noble gas release from either an isotopic composition or a helium exhalation point of view. Large increases in permeability were observed in 13 of 64 discrete sampling intervals. Of the sampling intervals which saw large increases in flow, only two locations did not have preexisting fractures mapped at the site. Given the lack of noble gas release and a clear increase in permeability, we infer that most of the strain accommodation of the fractured media occurred along previously existing fractures, rather than the creation of new fractures, even for a high strain rate event. These results have significant implications for how we conceptualize the deformation of rocks with fracture networks above the percolation threshold, with application to a variety of geologic and geological engineering problems.

Microthermometry and noble gas isotope analysis of magmatic fluid inclusions in the Kerman porphyry Cu deposits, Iran: constraints on the source of ore-forming fluids

This paper reports microthermometric and noble gas isotope data for fluid inclusion assemblages (FIAs) with evidence of phase separation, i.e. coexisting vapor-rich and halite-saturated inclusions, hosted in the early-formed quartz stockwork veins and post-magmatic quartz eye crystals in two economic porphyry Cu deposits (PCDs; Sar Cheshmeh and Miduk) and two sub-economic prospects (Sar Kuh and Abdar) from the Kerman porphyry copper belt (KPCB), Iran. The multiphase halite-saturated inclusions (i.e., Type I) in all studied PCDs and prospects had the highest homogenization temperature (Th = 525–594 °C) and salinities (63–73 wt% NaClequiv), whereas vapor-rich inclusions (Type II) had lower Th (362–460 °C). Fluid inclusion data show that like economic PCDs, the sub-economic prospects were formed in a fertile hydrothermal system and benefited from a mineralizing fluid, which evolved from a primary hot (mostly > 400 °C), metal-rich and oxidized fluid (as evidenced by the presence of opaque- and hematite-bearing fluid inclusions) of unknown salinity, which underwent a phase separation process to form both brine and vapor phases in the early stage of mineralization. The helium abundance and its isotopic composition document a mantle-derived magmatic source for the primary ore fluid in the formation of the studied PCDs and prospects (3He/4He ratios ranging from 0.46 to 2.8 Ra, corresponding to a mantle He contribution in ore fluids between ~ 7 and 45%). However, subsequent hydrothermal processes, i.e., vapor–brine phase separation, fluid-rock interaction with crustal rocks, and mixing with meteoric pore water containing dissolved atmospheric (e.g., Ne and Xe) and some crustal noble gases (e.g., Ar), changed the initial noble gas composition of the magmatic ore fluid to predominantly atmospheric- and crustal-like compositions. A significant proportion of mantle-derived He (up to 45%) in high-temperature (513–594 °C) and high-salinity (61.5–73 wt% NaClequiv) FIAs may indicate the existence of buried, economic, porphyry Cu mineralization in the Abdar prospect; therefore, it is suggested to be a possible target for further exploration. Comparing the He and Ar noble gas isotope composition in porphyry copper systems of different size and economic importance in this study showed that the ore-forming fluids of the outsized PCD (i.e., Sar Cheshmeh) have higher contributions of crustal-derived fluids characterized by predominantly radiogenic noble gas signatures (4He and 40Ar) than the smaller PCDs. This could have been achieved by a prolonged hydrothermal circulation in a large volume of crustal rocks containing radiogenic noble gases under a long-lived heat regime resulting from a deeply emplaced and slowly cooled composite intrusive body.

Noble gas geochemistry of phenocrysts from the Ciomadul volcanic dome field (Eastern Carpathians)

Noble gas isotopic composition of fluid inclusions was analyzed in amphibole, plagioclase and clinopyroxene phenocrysts from the shoshonitic and dacitic volcanic products of the Ciomadul volcanic dome field, the youngest volcanic system within the Carpathian-Pannonian Region. The highest Rc/RA ratios (3.0–3.8 RA) were obtained for high-mg clinopyroxene of the Malnaş shoshonite. High-Al amphiboles from the Bixad dacitic pumices have Rc/RA ratios between 1.16 and 2.11 RA. These values overlap with the noble gas signature of the present-day CO2 emission. Thus, our new results reinforce the conclusion that the mantle component of the Ciomadul primitive magmas has relatively lower Rc/RA signature compared to the nearby Perşani lithospheric mantle. This is likely due to the thorough metasomatic nature resulting in elevated large ion lithophile elements and high water content of the Ciomadul magmas. On the other hand, the Rc/RA ratios from plagioclase-hosted fluid inclusions and those from low-Al amphiboles are 0.06–0.12 RA and 0.39–0.77 RA, respectively defining a dominant crustal origin (>90%) for the trapped fluids. Noteworthy, these minerals represent a low-temperature crystal mush assemblage that existed for protracted time in the magma reservoir. We tested the fraction of mantle contribution for different mantle end-member values, considering also the effect of magma aging on the R/RA ratios due to longer (up to 50 kyr) residence time. This resulted in a maximum of ~50–60% mantle fluid contribution for the high-Al amphibole-hosted fluid inclusions and a lower, ~25% mantle fluid contribution, for the low-Al amphiboles. The elevated mantle fluid contribution in the case of the high-Al amphiboles can be explained by a fresh magma recharge event and shorter residence time before the eruption. The results of this study imply that fluid inclusion in primitive clinopyroxene and amphibole phenocrysts could reflect the magmatic end-member, which is in the case of Ciomadul a strongly metasomatized lithospheric mantle with relatively low Rc/RA values. Thus, the noble gas signature of the lithospheric mantle could be heterogeneous even in a restricted area.

Trace element geochemistry and O-S-Pb-He-Ar isotopic systematics of the Lishan Pb-Zn-Cu hydrothermal deposit, NE Hunan, South China

The Jiangnan Orogenic Belt, as an important polymetallic metallogenic belt in South China, hosts numerous Au-Sb, W, Cu, Co, and Pb-Zn deposits. The newly discovered Lishan Pb-Zn-Cu deposit (> 0.64 Mt at 1.64% Pb, 2.54% Zn, and 0.42% Cu) in the northeastern Hunan Province is mainly accommodated by the Late Jurassic to Early Cretaceous Mufushan pluton, with vein-type Pb-Zn-Cu orebodies controlled by nearly N-S-trending faults. However, ore genesis, mineralization process, and the contribution of magmatism are unclear despite its closely spatial association with the Mufushan pluton. In this contribution, in-situ trace element, O and S isotopic analyses, as well as Pb, He and Ar isotopic analyses were conducted to determine the sources of ore-forming materials and fluids and potential mineralization process in the Lishan Pb-Zn-Cu deposit. Four stages of hydrothermal processes, from early to late, are recognized: (1) quartz; (2) quartz + fluorite + chlorite + chalcopyrite + pyrite; (3) quartz + fluorite + chlorite + sphalerite + galena + chalcopyrite + pyrite; and (4) quartz + calcite + chalcopyrite + pyrite. The trace element compositions reveal several important substitutions, including 4Zn2+ ↔ 2Sb3+ + Pb2+ + □ (vacancy) in sphalerite, and Ag+ + Sb3+ ↔ 2Pb2+ and (Ag, Cu, Tl)+ + (Bi, Sb)3+ ↔ 2Pb2+ in galena. The Lishan sphalerite is characterized by high contents of Mn (44.3–244 ppm), Fe (5379–19037 ppm), Co (30.0–186 ppm), and Ga (3.81–381 ppm) and low contents of Ag (1.04–20.7 ppm) and Sb (0.03–1.88 ppm). The sphalerite geothermometer combined with Ga/In (1.32–9716) ratios suggest moderate to low mineralization temperatures of 174–251 °C. The δ34S values of sphalerite, galena, chalcopyrite, and pyrite vary between −5.1 and −0.4‰, consistent with those of the magmatic sulfur source. The Pb isotopic compositions (206Pb/204Pb = 18.140–18.378, 207Pb/204Pb = 15.666–15.819 and 208Pb/204Pb = 38.574–39.207) of sulfides demonstrate that the ore metals were most likely derived from the Mufushan granitoids, but contaminated by the Lengjiaxi Group rocks. The oxygen isotopic values of quartz and noble gas isotopic compositions of sulfides indicate that the ore-forming fluids were mainly formed by the mixture of crust-derived magmatic hydrothermal fluid and meteoric water. Combined with principal component analysis, the Lishan sphalerite is different from the trace element characteristics of sphalerite in other types of deposits. In combination with the available data, we propose that the Lishan deposit belongs to a moderate to low temperature hydrothermal vein-type deposit.

Helium and argon isotope geochemistry of the Tibetan Qulong porphyry Cu-Mo deposit, China

The Qulong porphyry Cu–Mo deposit, generated in the Miocene post-collisional extension environment of the Gangdese Copper (Molybdenum) Metallogenic Belt, is one of the largest porphyry Cu deposits in China. This study reports the noble gas isotopic compositions of volatiles released from fluid inclusion reserved in pyrite from the Qulong deposit. 3He/4He and 40Ar/36Ar ratios range from 0.54 to 1.015 Ra and 300–359, respectively. Concentrations of 4He and 40Ar range from 1.77 to 2.62 × 10−8 cm3 STP and 1.7–34 × 10−8 cm3 STP, respectively. The isotopic composition of noble gases indicates that the ore-forming fluids of the Qulong Cu–Mo deposit were a mixture of fluid containing mantle component, which is exsolved from the porphyry magma, and crustal fluid characterized by atmospheric Ar and crustal radiogenic He. The δ34S values of pyrite and molybdenite range from − 0.52‰ to 0.31‰, with an average of − 0.12‰, indicating a magmatic origin. More mantle components were involved in the Cu–Mo deposit than in the Mo–Cu deposit in the Qulong-Jiama ore-district.

Heavy noble gas signatures of the North Atlantic Popping Rock 2ΠD43: Implications for mantle noble gas heterogeneity

Heavy noble gas (Ne, Ar, Xe) isotopic compositions provide powerful constraints on the nature of mantle heterogeneities. The North Atlantic popping rock sample 2ΠD43 is unusually gas-rich and has long formed the basis of our understanding of the noble gas composition of the mid-ocean ridge basalt (MORB) mantle source. Here we present new high-precision He, Ne, Ar and Xe isotopic compositions as well as He, Ne, Ar and Xe abundances measured in a split of the 2ΠD43 sample. After correcting the measured values for syn- to post-eruptive atmospheric contamination, we estimate a mantle source 21Ne/22Ne of 0.0590 ± 0.0002, 40Ar/36Ar of 23,900 ± 100 and 129Xe/130Xe of 7.41 ± 0.02 for 2ΠD43. Significant regassing of atmospheric Xe into the North Atlantic popping rock mantle source is evident, though differential regassing cannot explain systematic variations between MORB and plume-derived signatures. Rather, the fissiogenic Xe isotope signature of the 2ΠD43 mantle source indicates a greater extent of long-term degassing of the upper mantle relative to the plume mantle to explain heavy noble gas isotopic variations. The popping rock data also add to growing evidence that the MORB mantle differentiated from the reservoir supplying noble gases to plume sources within the first 100 Myr of Earth history, and that the two reservoirs have not been homogenized though 4.45 Ga of mantle convection.

Factors Affecting Shale Gas Chemistry and Stable Isotope and Noble Gas Isotope Composition and Distribution: A Case Study of Lower Silurian Longmaxi Shale Gas, Sichuan Basin

The Weiyuan (WY) and Changning (CN) fields are the largest shale gas fields in the Sichuan Basin. Though the shale gases in both fields are sourced from the Longmaxi Formation, this study found notable differences between them in molecular composition, carbon isotopic composition, and noble gas abundance and isotopic composition. CO2 (av. 0.52%) and N2 (av. 0.94%) were higher in Weiyuan than in Changning by an average of 0.45% and 0.70%, respectively. The δ13C1 (−26.9% to −29.7%) and δ13C2 (−32.0% to −34.9%) ratios in the Changning shale gases were about 8% and 6% heavier than those in Weiyuan, respectively. Both shale gases had similar 3He/4He ratios but different 40Ar/36Ar ratios. These geochemical differences indicated complex geological conditions and shed light on the evolution of the Lonmaxi shale gas in the Sichuan Basin. In this study, we highlight the possible impacts on the geochemical characteristics of gas due to tectonic activity, thermal evolution, and migration. By combining previous gas geochemical data and the geological background of these natural gas fields, we concluded that four factors account for the differences in the Longmaxi Formation shale gas in the Sichuan Basin: a) A different ratio of oil cracking gas and kerogen cracking gas mixed in the closed system at the high over-mature stage. b) The Longmaxi shales in WY and CN have had differential geothermal histories, especially in terms of the effects from the Emeishan Large Igneous Province (LIP), which have led to the discrepancy in evolution of the shales in the two areas. c) The heterogeneity of the Lower Silurian Longmaxi shales is another important factor, according to the noble gas data. d) Although shale gas is generated in closed systems, natural gas loss throughout geological history cannot be avoided, which also accounts for gas geochemical differences. This research offers some useful information regarding the theory of shale gas generation and evolution.

Isotopic Composition of Noble Gases, Nitrogen, and Carbon in the Ozerki New L Chondrite

The isotopic composition of noble gases, nitrogen, and carbon in two samples of the Ozerki L chondrite, which differ in the degree of impact metamorphism, analyzed by the methods of stepwise oxidation and crushing, is reported. The data obtained indicate that the meteorite contains gases trapped on the asteroid during the impact events. The isotopic composition of trapped argon, studied by the stepwise crushing method, is dominated by radiogenic 40Ar (the average 40Ar/36Ar values are 846 in the chondrite material and 1908 in the melt with fine chondrite fragments). Most of the trapped 36Ar is located in positions inaccessible for crushing. The isotopic composition of Ne is a mixture of the solar-wind neon, cosmogenic, and most likely planetary (Q) components. The elemental composition of the trapped noble gases is formed by mixing of the solar, planetary (Q), and cosmogenic components in different proportions. Diffusion processes caused by impact events most likely influenced the elemental abundance of noble gases, primarily helium. Almost all carbon and nitrogen are chemically bound in the rock. In general, their isotopic composition corresponds to that of ordinary chondrites; however, an atypically light carbon isotopic composition with a bulk value δ13C = –47.6 ± 4.8 (‰) was detected in a sample of the chondrite material. The nitrogen released during crushing is isotopically lighter than that released during oxidation. This may indicate that in the course of impact processes, solar nitrogen is more easily mobilized and redistributed into voids than organic nitrogen enriched in the heavy isotope.

Noble gas isotopic compositions of seamount lavas from the central Chile trench: Implications for petit-spot volcanism and the lithosphere asthenosphere boundary

We report new noble gas isotopic compositions of submarine basaltic glasses sampled from two seamounts discovered offshore of Chile, and inferred to have erupted as petit-spot volcanoes near the Juan Fernández hotspot. The samples have 3He/4He of 1–15 times atmosphere (Ra). Their neon isotope compositions are similar to those of the Hawaiian Islands and Réunion Island. Their 40Ar/36Ar range from atmospheric to 2300. Although the lavas are likely to be influenced by a hotspot-related component, the cause of the 3He/4He variation must be clarified to ascertain the mantle source. Variations in 3He/4He are not attributable to processes occurring at the Earth's surface such as degassing fractionation, mixing with atmosphere dissolved in seawater, or in-situ post-eruptive addition of 4He. A combination of the Ne-Ar isotope ratios corrected for atmospheric influence and He isotope ratios indicates that the noble gas isotopes of the lavas are a mixture of a hotspot magma, MORB-source, and radiogenic components. The lower 3He/4He are attributed to assimilation with the oceanic lithosphere, suggesting that the pristine 3He/4He of the lavas is hotspot-like. These features can be interpreted as indicating that part of the Juan Fernández plume infiltrated the lithosphere–asthenosphere boundary (LAB), and that the ponding magma has erupted as petit-spot volcanoes because of plate bending. The noble gas data indicate that LAB can be a reservoir for exotic melts, which might have lubricated plate tectonics.

Origin and Evolution of Gas in Salt Beds of a Potash Mine

Abstract. In order to better understand both the fixation and migration of gases in evaporites, investigations were performed in five horizontal boreholes drilled in an underground potash seam. One of the five boreholes was pressurised with Ar and the pressure signal and chemical gas composition were then monitored in the other holes. A further gas sample from a separate borehole was characterised for the chemical composition and for noble gas and carbon isotopic compositions to conclude on the origin and evolution of the gas in the salt rocks. Additionally, in order to determine the total gas amount in the salt rocks, a potash-bearing salt sample was dissolved in water and from the mass of 1 kg salt sample, 9 cm(STP)3 gas was liberated. Due to the relatively large permeability of the disturbed salt rocks (4×10-17 to 4×10-18 m2), which is about 3–4 orders of magnitude higher than in undisturbed salt rocks, we assume that the migration of injected Ar most likely takes place along micro-cracks produced during the mining process. The geogenic gas concentrations found in the observation holes correlate directly to the Ar concentration, suggesting that they were stripped from the rocks in between the holes. According to the He-isotopes (0.092 Ra), a small contribution of mantle gas can be found in the geogenic salt gas. The δ13CCO2-isotopic composition (−7.8 ‰ to 6.7 ‰) indicates a magmatic source, whereas 13C∕12C of CH4 (−22.2 ‰ to −21.3 ‰) is typical for a thermogenic gas. We assume that CO2 and CH4 are related to volcanic activity, where they isotopically equilibrated at temperatures of 513 to 519 ∘C about 15–16 Ma ago.

Tracing the Origins of the Ice Giants Through Noble Gas Isotopic Composition

The current composition of giant planet atmospheres provides information on how such planets formed, and on the origin of the solid building blocks that contributed to their formation. Noble gas abundances and their isotope ratios are among the most valuable pieces of evidence for tracing the origin of the materials from which the giant planets formed. In this review we first outline the current state of knowledge for heavy element abundances in the giant planets and explain what is currently understood about the reservoirs of icy building blocks that could have contributed to the formation of the Ice Giants. We then outline how noble gas isotope ratios have provided details on the original sources of noble gases in various materials throughout the solar system. We follow this with a discussion on how noble gases are trapped in ice and rock that later became the building blocks for the giant planets and how the heavy element abundances could have been locally enriched in the protosolar nebula. We then provide a review of the current state of knowledge of noble gas abundances and isotope ratios in various solar system reservoirs, and discuss measurements needed to understand the origin of the ice giants. Finally, we outline how formation and interior evolution will influence the noble gas abundances and isotope ratios observed in the ice giants today. Measurements that a future atmospheric probe will need to make include (1) the 3He/4He isotope ratio to help constrain the protosolar D/H and 3He/4He; (2) the 20Ne/22Ne and 21Ne/22Ne to separate primordial noble gas reservoirs similar to the approach used in studying meteorites; (3) the Kr/Ar and Xe/Ar to determine if the building blocks were Jupiter-like or similar to 67P/C-G and Chondrites; (4) the krypton isotope ratios for the first giant planet observations of these isotopes; and (5) the xenon isotopes for comparison with the wide range of values represented by solar system reservoirs.

Exposure Ages, Noble Gases and Nitrogen in the Ordinary Chondrite Karimati (L5)

Noble gas and nitrogen isotopic compositions of Karimati ordinary (L5) chondrite are presented. Aliquots of the meteorite were studied in two noble gas mass spectrometers. Its cosmic ray exposure (CRE) history, trapped noble gases and nitrogen isotopic systematic are examined. The compositions of Ne and Kr in this meteorite indicate presence of mixture of solar wind and Q trapped components. In addition to the primordial components, radiogenic 129Xe (from the decay of short-lived radioactive 129I) is observed in the two aliquots (129Xe/132Xe ranges between 1.054 and 1.311). The U/Th-4He and K-40Ar ages are discordant. U/Th-4He ages are younger than the K-40Ar ages, indicating loss of helium. The trapped N component is isotopically light analogous to Q gas/solar wind. The cosmic-ray exposure ages of the two aliquots are 16.1 ± 2.7 Ma and 16.6 ± 2.0 Ma based on the cosmogenic 21Nec and 38Arc concentrations.

Tracing the sources and evolution processes of shale gas by coupling stable (C, H) and noble gas isotopic compositions: Cases from Weiyuan and Changning in Sichuan Basin, China

The source and thermal evolution history of organic matter for the Longmaxi shale are still debated. This study analyzed the molecular and stable carbon isotopic compositions of hydrocarbons (CH4, C2H6, and C3H8) and CO2 as well as the stable hydrogen isotopic compositions of methane, ethane, and noble gases (He, Ne, Ar, Kr, and Xe). Shale gases in the WY and CN areas show an extremely-low-wetness with CH4 concentrations range from 93.41% to 99.01%. Non-hydrocarbon gases are mainly N2 (0.22%–2.81%) and CO2 (0.03%–1.35%). H2S have not been detected. Different δ13C1 and δ13C2 values in WY and CN shale gases (WY: −37.3‰ to −35.0‰ and −40.3‰ to −38.3‰, CN: −29.8‰ to −26.3‰ and −35.3‰ to −32.7‰) and various carbon isotope-composition distribution patterns (δ13C1>δ13C2<δ13C3 and δ13C1>δ13C2>δ13C3) of hydrocarbons indicate a complex evolution process. WY shale gases include more oil-cracking gas than CN shale gases, suggesting WY shale gases more like come from Type I-II organic matter. In shale gas systems, methane content and δ13C1 ratios vary with the degree of thermal evolution, so the origin of shale gas cannot be determined using carbon isotope data alone. The wide range of δ13CCO2 values (−8.9‰ to −0.8‰) and N2/40Ar ratios (20.8–165.1) suggests multiple origins of the gases. Emeishan mantle plume provides the source of heat for some thermo-genic gas. Noble gas isotopic compositions (3He/4He: 0.001Ra to 0.019Ra) indicate air and crustal origins with no significant contribution from the mantle. 40Ar/36Ar ratios (1194.3–4604.5) are consistent with the age of Longmaxi strata calculated by accumulative effect of Ar isotope. The shale gas humidity, carbon isotope ratios, and the carbon isotope-composition distribution patterns may contain information indicating the shale gas sweet spot.

Tracing interaction between hydrocarbon and groundwater systems with isotope signatures preserved in the Anyue gas field, central Sichuan Basin, China

Anyue gas field is a large gas field located in the central Sichuan Basin, China. Although many studies have been carried out previously, the formation mechanism of this field is unclear and currently under debate. To better understand the accumulation history, the role that groundwater plays in transporting hydrocarbons within sedimentary basins and water-gas interactions, stable and noble gas isotopes were measured in thirteen free gas samples from the Anyue gas field. In addition, nine formation water samples and five reservoir bitumen samples were analyzed for stable carbon isotopes. δ13C(CH4) values in the gas samples range from −35.0 to −32.6‰, showing evidence of thermogenic origin. δ13C values among three different types of samples (free gases, water-dissolved gases and reservoir bitumen) show a pattern that cannot be explained by oil cracking followed by free gas accumulation. It suggests the occurrence of gas-groundwater interaction in the Anyue field. Free gas samples can be divided into 2 distinct groups by their geographical locations and stratigraphical source formations. 3He/4He ratios (R/Ra) in group 1 and group 2 samples range from 0.0118 to 0.0132 and 0.0115 to 0.0256, respectively, indicating He is mainly derived from the crust. 20Ne/22Ne and 21Ne/22Ne ratios suggest a mixing between the air and crust sources. 40Ar/36Ar ratios ranging from 1658 to 2109 and 2168 to 5973 in group 1 and group 2 samples, respectively, are significantly higher than the air value of 298.6. In comparison, heavier noble gas (Kr and Xe) isotopic compositions are predominantly air-like. The relative enrichment of 4He and 21Ne* in group 1 samples can be possibly explained by preferential release of light noble gases in a low temperature environment. Samples in group 2 show a good fit to the solubility-controlled Rayleigh fractionation model, suggesting the presence of an open system degassing of gases from the groundwater. The excess heavy noble gases in natural gas samples can be attributed to the addition of sedimentary components from the source rocks during geological evolution. 4He groundwater ages considering in-situ production and external flux indicate the addition of young groundwater into the Anyue gas field. Low gas-groundwater ratios and high CH4/36Ar ratios suggest that only a small portion of the gases in the current Longwangmiao reservoir of Anyue gas field has been in contact with the relatively young groundwater. Based on the noble gas and stable carbon isotope results in all samples, we propose a two-stage gas and groundwater interaction process during the gas preservation and accumulation history in the Anyue gas field in China.

Nitrogen and noble gas isotopic compositions of mantle xenoliths from Far Eastern Russia: Implications for nitrogen isotopic characteristics of mantle wedge fluid

To investigate nitrogen circulation via subduction, we measured isotopic compositions of nitrogen and noble gases of mantle-derived xenoliths from Far Eastern Russia, where the tectonic setting is a plate convergence margin in the Mesozoic to Cenozoic era. Kinetic fractionation accompanies the extraction of volatiles from a subducting plate, engendering the infiltration of fluids with light isotopic compositions into mantle wedge. The δ15N values are −13.26 to +0.19‰, some of which are lower than those of air (0‰) and upper mantle (−5 ± 2‰). The 3He/4He and 40Ar/36Ar ratios are 0.2–5.6 Ra and 304–1156, respectively, which are common for the mantle xenoliths in eastern Asia, a representative subduction setting. The N2/36Ar are 1.5 × 104 – 4.5 × 104, which are much lower than the mantle value (6.7 × 106) and which are rather more similar to atmospheric (2.5 × 104) and deep seawater (1.1 × 104) values. Kinetic fractionation can lower the δ15N value to −15‰ without marked change of the N2/36Ar ratio. Therefore, the low δ15N values of the present xenoliths might be regarded as values of the fluid experiencing fractionation during infiltration of the fluid from the subducting oceanic plate into the mantle wedge.