Gas Isotopic Compositions Research Articles

An FTIR method for accurate CO2 mole fraction measurements with correction for differences in isotopic composition of gases

The precision of optically based techniques for long term CO2 monitoring in the atmosphere is approaching the ±0.02 µmol mol−1 level, however biases of potentially up to fifteen times greater than this can arise due to differences in isotopic composition of sample and calibration gases used. We have developed and describe a Fourier transform infrared spectroscopy (FTIR) method for CO2 mole fraction measurements, using an instrument configured with a liquid N2-cooled mid-infrared indium antimonide (InSb) detector and a 10.01 m multipass Whitetype gas cell (0.75 l) and fitting of the 12C16O2 isotopologue absorption in the wavenumber region (3500–3800) cm−1 where the measurement bias resulting from isotopic variation between sample and calibration gases can be corrected for. The method was completed with a comprehensive uncertainty budget from which considerations on the importance of spectroscopic specific biases could be deduced. The method was validated using 29 standards produced by 15 National Metrology Institutes/Designated Institutes, including the Central and a Regional Calibration Laboratory (McClenny et al 2002 J. Air Waste Manag. Assoc. 52 542–62) from the World Meteorological Organization (2011 WMO/TD-No. 1560, Ed.: Hohenpeissenberg Metrological Observatory), with δ13C values in CO2 ranging from −0.4‰ to −67‰ versus VPDB. The standards were all produced with CO2 mole fractions typical of background atmosphere calibration gases (either 380 µmol mol−1 or 480 µmol mol−1), and reference values for each of them were established during a comparison performed with a GC-FID analyzer, insensitive to their isotopic composition. Measurements have confirmed that a Δδ13C of 1‰ between sample and calibration gases can bias the CO2 mole fraction measurements made by FTIR by 4 nmol mol−1 and a Δδ18O of 1‰ by 2 nmol mol−1. These biases can be corrected with the method described here, allowing FTIR based measurements of the CO2 mole fraction with standard uncertainties reaching 0.05 µmol mol−1 to be achieved, even if sample and calibration gases isotopic ratios differ. These findings have significant implications for the use of CO2 calibration mixtures in particular for the atmospheric community, when high accuracy measurements are required.

Genesis of the Wulong gold deposit, Liaoning Province, NE China: Constrains from noble gases, radiogenic and stable isotope studies

The Wulong lode deposit contains over 80 tonnes of gold with an average grade of 5.35 g/t. It is one of the largest deposits in Dandong City, Liaoning Province in northeast China. Previous studies on the deposit focused on its geological characteristics, geochemistry, fluid inclusions, and the timing of gold mineralization. However, controversy remains regarding the origin of the ore-forming fluids and metals, and the genesis of the gold deposit. This paper presents zircon U–Pb and pyrite Rb–Sr ages and S, Pb, He, and Ar isotopic results along with quartz H and O isotopic data for all litho-units associated with the deposit. Laser ablation inductively coupled mass spectrometry measurements yielded zircon U–Pb dates for samples of pre-mineralization rocks like granite porphyry dike, the Sanguliu granodiorite, fine-grained diorite, and syn-mineralization diorite, as well as post-mineralization dolerite, and lamprophyre; their emplacement ages are 126 ± 1 Ma, 124 ± 1 Ma, 123 ± 1 Ma, 120 ± 1 Ma, 119 ± 2 Ma, and 115 ± 2 Ma, respectively. The pyrite Rb–Sr isochron age is 119 ± 1 Ma, indicating that both magmatism and mineralization occurred during the Early Cretaceous. The δ18OH2O values of ore-forming hydrothermal fluids from the quartz–polymetallic sulfide vein stage vary from 4.8‰ to 6.5‰, and the δDV-SMOW values are between −67.7‰ and −75.9‰, indicating that the ore-forming fluids were primarily magmatic. The noble gas isotope compositions of fluid inclusions hosted in pyrite suggest that the ore-forming fluids were dominantly derived from crustal sources with minor mantle input. Sulfur isotopic values of pyrite vary between 0.2‰ and 3.5‰, suggesting that S was derived from a homogeneous magmatic source or possibly from fluids derived from the crust. The Pb isotopic compositions of sulfides (207Pb/204Pb = 15.51–15.71, 206Pb/204Pb = 17.35–18.75, 208Pb/204Pb = 38.27–40.03) indicate that the Pb of the Wulong gold deposit is a mixture of crust and mantle components. Geochronological and geochemical data, together with the regional geological history, indicate that Early Cretaceous magmatism and mineralization of the Wulong gold deposit occurred during the rollback of the subducting Paleo-Pacific Plate, which resulted in lithospheric thinning and the destruction of the North China Craton (NCC), which indicates that the deposit is of magmatic–hydrothermal origin.

Comparison on Rare Gas Geochemical Characteristics and Gas Originations of Kuche and Southwestern Depressions in Tarim Basin, China

The Kuche Depression is considered as the most important gas resource potential and gas exploring area with great gas resource potential and prospect in the Tarim Basin. Based on geochemical experimental analyses and comprehensive geological studies, the general geochemical characteristics of molecular and isotope compositions of rare gases as well as hydrocarbon gases and nonhydrocarbon gases are comparatively studied in the Kuche and Southwestern Depressions. Then, their genetic types are separately identified and gas originations are comprehensively discussed. The main results are as follows. (1) Gas fields in the Kuche Depression have a higher methane abundance, accompanied with low N2and CO2abundances, but the Akemomu gas field in the Southwestern Depression has a relatively lower average methane abundance, accompanied with high average N2and CO2abundances. The helium abundance of natural gases in gas fields from the Kuche Depression general has 1 order of magnitude higher than the air value. Comparatively, it has more than 2 orders of magnitude higher than the atmospheric value in the Akemomu gas field from the Southwestern Depression. The neon, argon, krypton, and xenon abundances in both Kuche and Southwestern Depressions are lower than the corresponding air values. (2) Natural gases from gas fields in the Kuche Depression and the Southwestern Depressions are generally typical coal-formed gases. The rare gases in the Kuche Depression have typical crustal genesis, mainly deriving from the radioactive decay of elements in the crust, while in the Akemomu gas field from the Southwestern Depression, the rare gases have main crustal genesis with a proportion of 92.5%, probably accompanied with a little mantled genetic contribution. (3) Natural gases in the Kuche Depression are generally derived from coal measure source rocks of Jurassic and Triassic, which principally originated from Jurassic in strata period and coals in source rock types. The Jurassic source rocks account for 55%-75% and the Triassic source rocks account for 25%-45% approximately, while coals occupy 68% and mudstones occupy 32% separately. Natural gases from the Akemomu gas field in the Southwestern Depression mainly originated from humic mudstones of marine and continental transitional source rocks of Carboniferous to Permian.

Isotopic-geochemical peculiarities of distribution of gas hydrocarbons in the Bazhenov formation section (South part of the Western Siberia)

In this work we describe the distribution of stable carbon isotopes in hydrocarbon gases from four wells located in the southern and central parts of the Western Siberia. The main goal was to understand the influence of the lithological composition and geological settings on isotopic composition of gases and their formation. Two genetically different groups of hydrocarbon gases were identified based on the author’s research, as well as analysis of archival and literature materials on the isotope composition of hydrocarbons. We estimated the close relationship between the main factors of the migration, degree of metamorphism of organic matter and the isotope composition of hydrocarbon gases produced by the Bazhenov formation.

ИССЛЕДОВАНИЕ РАСПРЕДЕЛЕНИЯ ВЫХОДОВ УГЛЕВОДОРОДНЫХ ГАЗОВ В ЗАВИСИМОСТИ ОТ ГЕОЛОГИЧЕСКОГО СТРОЕНИЯ ЮЖНОГО КАСПИЯ ПО ДАННЫМ ГАЗОВОЙ СЪЕМКИ

Актуальность исследования заключается в изучении газов верхней части разреза Южного Каспия, в особенности ее глубоководной части, представляющей значимый практический интерес при поисках и оценках углеводородного потенциала зон газогидратов. Анализ ранее проведенных работ в пределах Южного Каспия показал, что все исследования в основном сконцентрированы в пределах разбуренной зоны Абшеронского, Бакинского и Нижнекуринского нефтегазоносных районов и не охватывают глубоководную зону Южного Каспия. Цель исследования заключается в изучении распределения интенсивности проявления углеводородных газов по площади, выявлении зон газогенерации, а также проведении работ по определению и анализу путей миграции углеводородов и типов формирования газогидратов в пределах глубоководной зоны Южного Каспия. Объектом исследования послужили 1282 пробы газа, отобранные в Южном Каспии на месторождениях Шах-дениз, Бахар, Пираллахи, Хали, в глубоководной части Южного Каспия и др., а также данные геолого-геофизических исследований. Использованы данные изучения химического и изотопного состава углеводородных газов Южно-Каспийской впадины по 147 пробам газа, отобранным из нефтегазовых месторождений, 153 пробам из грязевых вулканов, 29 пробам донных осадков и 4 пробам газогидратов. Эти данные позволили более обосновано подойти к анализу генетических вопросов происхождения газов в верхней части разреза. Методы исследования основаны на интерпретации данных газовой съемки и изучении изотопного и химического состава углеводородных газов Южного Каспия. Наличие точных значений координат, а также данных по структурным картам в координатной сетке позволили сопоставить имеющиеся данные с геологическим строением. В результате исследования построены карты изменения содержания метана, этана, пропана, бутана, изо-, нормального пентана в пределах верхней части разреза по изучаемым месторождениям. Сопоставлены данные результатов анализа газовой съемки с данными структурных карт и других геолого-геофизических материалов. Установлено, что структура Бахар характеризуется относительно пониженной интенсивностью газопровялений по метану от 5,37•10–4 до 152944•10–4 % (в среднем 9109,0354•10–4 %), в то время как месторождение Шах-дениз характеризуется максимальными значениями от 4,6200•10–4 до 367840•10–4 %, в среднем составляя 85572,3•10–4 %. Небольшие значения интенсивности проявления газа, установленные в зоне структур Пираллахи, Хали, свидетельствуют о незначительном газовом потенциале глубокопогруженных отложений изучаемой зоны. Детально рассмотрены результаты газовой съемки в пределах глубоководной части Южного Каспия и установлено, что для этой зоны характерна газогенерация с преобладанием двух компонентов – метана и этана. Исходя из анализа изменения интенсивности проявления газа по площади и в зависимости от геологических условий установлено, что грязевые вулканы и разломы являются прекрасными путями для миграции углеводородов и отражают геологическую обстановку, генерационный потенциал погруженных отложений Южного Каспия. Установлено, что углеводородные газы в донных осадках и отложениях верхней части разреза Южной части Каспийского моря находятся в тесной зависимости от источников формирования углеводородов, миграции и других процессов, протекающих в глубокопогруженных отложениях, а также в верхней части разреза. Изучены данные химического и изотопного состава газов газогидратов, нефтяных и газовых месторождений Южного Каспия. На базе данных изотопно-геохимических исследований описаны механизмы образования гигантских месторождений нефти Балаханы-Сабунчи-Романы и газоконденсата Шах-дениз, расположенных на одной антиклинальной зоне Абшеронского нефтегазоносного района, а также аномалии, выявленные по данным газовой съемки в пределах изучаемых структур Южного Каспия.

Optical clumped isotope thermometry of carbon dioxide

Simultaneous analysis of carbon dioxide isotopologues involved in the isotope exchange between the doubly substituted 13C16O18O molecule and 12C16O2 has become an exciting new tool for geochemical, atmospheric and paleoclimatic research with applications ranging from stratospheric chemistry to carbonate-based geothermometry studies. Full exploitation of this isotope proxy and thermometer is limited due to time consuming and costly analysis using mass spectrometric instrumentation. Here, we present an all optical clumped CO2 isotopologue thermometer with capability for rapid analysis and simplified sample preparation. The current development also provides the option for analysis of additional multiply-substituted isotopologues, such as 12C18O2. Since the instrument unambiguously measures all isotopologues of the 12C16O2 + 13C16O18O rightleftharpoons 13C16O2 + 12C16O18O exchange, its equilibrium constant and the corresponding temperature are measured directly. Being essentially independent of the isotope composition of the calibration gas, an uncalibrated working reference is sufficient and usage of international calibration standards is obsolete. Other isotopologues and molecules can be accessed using the methodology, opening up new avenues in isotope research. Here we demonstrate the high-precision performance of the instrument with first gas temperature measurements of carbon dioxide samples from geothermal sources.

Evidence for vapor transport of the base and precious metals in the Panormos Bay Ag-Au-Te deposit, Tinos Island, Cyclades

Tellurium-bearing, base and precious metal sulfides from the Au-Ag-Te mineralization at Panormos Bay, Tinos Island, Cyclades, preceded and followed the main telluride-forming event. Ore petrography and chemistry of the sulfides suggest that they have precipitated under supersaturation conditions, i.e., grown with hopper habit, enriched in Zn, Fe, Ag, Cd and Pb, and Te in stage II, III and VI sulfides and incorporate nanoparticles with variable Ag/(Pb + Ag + Cu + Fe + Zn)% values. The noble gas, i.e., He, Ar and Ne isotope compositions obtained from these sulfides form two distinct populations, the “Liquids” and “Vapors”. All these features constitute overwhelming evidence that favors vapor transport for the base and precious metals by the vapor-like Panormos ore fluid.The widespread occurrence of CO2-effervescence and boiling at the Panormos Bay Au-Ag-Te ores provoked degassing of the CO2, HCl(g), H2S(g), H2Te(g), Te2(g), light noble gas isotopes (3He, 36Ar, 20Ne, and 21Ne), base- and precious-bearing laden vapors from the mineralizing fluid and transported a fraction of the base and precious metals to vapor. Geochemical simulation suggests that supersaturation in the vapor phase, rapid up-vapor transport, and subsequent condensation in the Panormos Bay ore solution of H2Te(g), Te2(g), and base and precious-bearing vapors led to the precipitation of sulfide (stages II, III and VI) and telluride (stage V) mineral assemblages.

Hydrocarbon Gases of the Gorevoi Utes Underwater Oil-Gas Seep (Lake Baikal, Russia)1

Abstract —We investigated the molecular and stable isotope compositions of hydrocarbon gases of the Gorevoi Utes oil–gas seep (Lake Baikal) characterized by a simultaneous discharge of oil and gas from the lake floor. It has been found that these hydrocarbon gases are mostly thermogenic methane (δ13C–C1 = –53.9 to –38.8‰; δ13C–C2 = –23.4 to –33.3‰). At the same time, the gases have a minor amount of C2+, seldom reaching 10%. The C1/C2+ value varies from 9 to 2700, with the average value being 807, which is atypical of petroleum hydrocarbons. We suggest that degassing of oil leaking to the lake floor affects the molecular composition of such gases. Some secondary processes, primarily molecular fractionation of gas during its migration into the subsurface sediments and anaerobic biodegradation of methane homologues, contribute to the decrease in C2+ content.

The geochemistry of water and gas phases from high pCO2 sparkling springs within the northern Sikhote-Alin ridge region (Russian Far East)

Two types of cold CO2 rich groundwaters are located within the northern part of the Sikhote-Alin ridge: the first type is a Ca–Mg–HCO3 water with low TDS content (≤ 1.7 g/l) and high concentrations of Fe2+, Mn2+, Ba2+, and SiO2, whilst the second type is a Na–HCO3 water with a high TDS content (≈ 14 g/l) and elevated concentrations of Li+, Btot, Sr2+, Br–, and I–. A notable feature of these waters is a predominance of CO2 in the gas phase (up to 99 vol. %) and low contents of other gases (CH4, N2, O2, etc.). The origins of both water types can be identified on the basis of water (δD, δ18O) and gas (δ13C, 3He/4He, 4He/20Ne) isotopic compositions considered in the context of the geology and hydrology of the region.

Method for Analyzing the Molecular and Carbon Isotope Composition of Volatile Hydrocarbons (C1-C9) in Natural Gas.

Solid-phase microextraction (SPME) coupled with gas chromatography-isotope ratio mass spectrometry (GC-IRMS) has already been applied to collect and identify volatile light hydrocarbons in oil and source rocks. However, this technology has not yet been used to analyze volatile light hydrocarbons in dry gas (natural gas with C1/C2+ > 95%). In this study, we developed a method to measure the molecular and carbon isotope composition of natural gas using divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) fiber. This fiber proved to be suitable for extracting C1–C9 hydrocarbons from natural gas without inducing carbon isotopic fractionation. Notably, the extraction coefficients of the analytes were not the same but rather increased with the increasing carbon number of the hydrocarbons. Nevertheless, we successfully identified 24 hydrocarbons from the in-lab standard natural gas, while also obtaining the carbon isotope composition of C1 to C9 hydrocarbons with satisfying repeatability. The relative standard deviation (RSD) of the molecular composition data was in the range of 0.06–0.74%, with the RSDs of the carbon isotope composition data not exceeding 1‰. Finally, seven natural gas samples, collected from different sedimentary basins, were successfully analyzed and the stable carbon isotope compositions of C1–C9 hydrocarbons present in these were determined through this method. Overall, the new approach provides a simple but useful technique to obtain more geochemical information about the source and evolution of natural gas.

Carbon Isotope Fractionation in Coal and Marine Source Rocks and Implications for Exploration

Stable isotope composition of gas is widely used in hydrocarbon exploration to determine the composition and thermal maturity of source rocks. The analyses of Australian coal and marine shale samples indicate that during gas desorption both molecular and isotopic compositions change with time. Therefore, a detailed understanding of the mechanism of isotope fractionation is required to improve our ability to better characterize source rocks and fluids. δ13C of Permian coals range between -22 and -26‰ (VPDB) and that of thermogenic methane generated from these range from -25 to -40‰. δ13C of gas desorbed from coal varies with time according to molecular weight and sorption properties. For example, in a set of deep Bowen Basin coals the difference in δ13C-CH4 between early and late desorbed gas varies from 2‰ to 29‰. For higher hydrocarbons this fractionation is lower, where for ethane it is <8‰ and for propane <3‰. Similar isotope fractionation happens during desorption from marine source rocks in the Beetaloo Basin. δ13C for an immature kerogen from the Velkerri shale is -33‰. Where the Velkerri shale is gas mature, during desorption, δ13C-CH4 shows fractionation of up to 28‰. δ13C-C2H6 and δ13C-C3H6 show lower isotope fractionation of 1.6‰ and 0.9‰, respectively. Many published classification systems relating isotope composition of gas to source rock and thermal maturity do not consider the effects of such isotope fractionation. The fractionation mechanism may also have an impact on the so-called “isotope reversal” behaviour in some shale reservoirs which, at present, is poorly understood.

Data Report: Molecular and Isotopic Compositions of the Extracted Gas from China’s First Offshore Natural Gas Hydrate Production Test in South China Sea

Three hundred gas samples recovered from SHSC-4 during China’s first gas hydrate production test in the South China Sea were examined for gas component and isotopic composition. According to the gas chromatography analysis, all the gas samples from SHSC-4 are predominated by CH4, with minor N2 + O2, as well as trace amounts of CO2, C2H6, and C3H8. No H2S was detected. The molecular and isotopic data of the gas samples fall into the region of “mixed origin” on the plot of C1/(C2 + C3) − δ13C1, which is close to the microbial origin. The discrimination diagram of δ13C1 − δDCH4 shows that the methane in all of the samples is of microbial origin, and is derived from the CO2 reduction.

Evolution of noble gas and water isotopes along the regional groundwater flow path of the Konya Closed Basin, Turkey

Noble gas and water isotope compositions of regional groundwater were investigated along two transects in the Konya Closed Basin (KCB) of central Turkey. According to the 3He/4He versus Ne/He plot of samples, crust (up to 86%) and mantle (up to 26%) appear to be the primary and secondary sources of dissolved He in groundwater, respectively. After the beginning of both transects where the flow domain is confined, both 3He and 4He accumulate steadily in groundwater. Thereafter, the intermediate recharge from the surface in the unconfined part of regional flow system disrupts the steady accumulation trend of 3He and 4He. Effect of intermediate recharge on regional groundwater is also indicated by the spatial variations in specific conductance, temperature, water isotopes, and tritium signals. At the last part of the flow domain where the system becomes confined again 3He and 4He start to accumulate in the eastern transect whereas they continue to decline in the western transect probably because of degassing to atmosphere through thinner confining unit. The 4He accumulation rates based on radiocarbon ages and 4He concentrations along both transects between the mountain flank and Obruk Plateau are in agreement with literature values but differ from each other. The accumulation rate along the eastern and western transects are 4.93 × 10-11 cm3 STP/g•year and 1.99 × 10-10 cm3 STP/g•year, respectively. These values reveal that the 4He accumulation rates may differ at least four times at different parts of a single aquifer, which are located 30 to 60 km apart. It is understood that any groundwater age-dating attempt by 4He requires a firm pre-assessment of spatial 4He accumulation rates. He isotope signal in the groundwater of KCB proves the mantle and/or crustal gas input into the carbonate aquifer in which an on-going hypogenic karst development has been suspected.

Compositional variations and carbon isotope reversal in coal and shale gas reservoirs of the Bowen and Beetaloo basins, Australia

Abstract Stable isotope composition of gas is widely used in hydrocarbon exploration to determine the composition and thermal maturity of source rocks. Many isotope classification systems used for gas to source rock correlation and thermal maturity determination are primarily based on empirical observations made in conventional reservoirs and the kinetic isotope effects observed during pyrolysis experiments performed on source rocks. However, such relationships may not be readily applicable to onshore unconventional reservoirs due to the strong molecular and isotope fractionation that occur during extensive gas expulsion associated with basin uplift and depressurization. Degassing studies of freshly recovered core samples can provide useful insight into the behaviour of gas molecules in unconventional reservoirs during basin uplift. The analyses of Australian coal and marine shale samples demonstrate that during desorption both molecular and isotopic compositions of gas change at variable rates. Gas initially desorbed from the samples is mostly CH 4 , whereas later desorbed gas becomes increasingly enriched in C 2 H 6 and higher hydrocarbons. Hydrocarbon molecules also fractionate according to their isotopic composition, where the early released gas is enriched in 12 C causing the remaining gas in the reservoir to be enriched in the heavier 13 C isotope. During the release of gas from the Bowen Basin coals the C isotope ratio of CH 4 ( δ 13 C 1 ) changes by up to 21‰ (VPDB), whereas that for C 2 H 6 ( δ 13 C 2 ) and C 3 H 8 ( δ 13 C 3 ) changes by &lt;6‰. Similar changes in the isotope composition can be seen during the release of gas from marine source rocks of the Beetaloo Sub-basin. In a fully gas-mature middle Velkerri shale sample, δ 13 C 1 changes by up to 28‰ and δ 13 C 2 by up to 3‰ with no appreciable change occurring in δ 13 C 3 . The extent of molecular fractionation during gas flow through carbonaceous rocks is primarily related to the adsorption–desorption properties of organic matter and diffusivity through the overall rock matrix. Using the current dataset, the magnitude of the contributions exerted by the desorption and diffusion processes cannot be readily distinguished. However, both Bowen Basin coals and Beetaloo Sub-basin shale show similar fractionation effects during gas flow, where the heavier alkane molecules, including those containing more 13 C, desorb and move slowly compared with the lighter components, in particular CH 4 . Different rates of isotope fractionation between hydrocarbon molecules during gas flow cause the shape of compound-specific-isotope (CSI) curve to change with time. Early released gas is characterized by a normal CSI trend where the short-chain hydrocarbons are isotopically lighter compared with the longer-chain hydrocarbons. Because CH 4 and C 2 H 6 molecules enriched in 12 C desorb and diffuse more readily than the heavier hydrocarbons (including those enriched 13 C), the gas remaining in the coal and shale samples after extensive desorption shows a reversed CSI trend where CH 4 and C 2 H 6 are isotopically heavier compared with the longer chain hydrocarbons. Reversed isotope trends may also develop over geological time, particularly where the source rock is fully gas-mature and has expelled hydrocarbons due to prolonged degassing. As seen in the Beetaloo Sub-basin, the CSI trend in the dry-gas-mature Velkerri shale is reversed, possibly due to the loss of a large proportion of originally generated CH 4 during post-Cambrian basin uplift.

Age and metal source of orogenic gold deposits in Southeast Guizhou Province, China: Constraints from Re–Os and He–Ar isotopic evidence

The orogenic gold deposits in Southeast Guizhou are an important component of the Xuefeng polymetallic ore belt and have significant exploration potential, but geochronology research on these gold deposits is scarce. Therefore, the ore genetic models are poorly constrained and remain unclear. In the present study, two important deposits (Pingqiu and Jinjing) are investigated, including combined Re–Os dating and the He–Ar isotope study of auriferous arsenopyrites. It is found that the arsenopyrites from the Pingqiu gold deposit yielded an isochron age of 400 ± 24 Ma, with an initial 187Os/188Os ratio of 1.24 ± 0.57 (MSWD = 0.96). An identical isochron age of 400 ± 11 Ma with an initial 187Os/188Os ratio of 1.55 ± 0.14 (MSWD = 0.34) was obtained from the Jinjing deposit. These ages correspond to the regional Caledonian orogeny and are interpreted to represent the age of the main stage ore. Both initial 187Os ratios suggest that the Os was derived from crustal rocks. Combined with previous rare earth element (REE), trace elements, Nd–Sr–S–Pb isotope studies on scheelite, inclusion fluids with other residues of gangue quartz, and sulfides from other gold deposits in the region, it is suggested that the ore metals from Pingqiu and Jinjing were sourced from the Xiajiang Group. The He and Ar isotopes of arsenopyrites are characterized by 3He/4He ratios ranging from 5.3 × 10−4 Ra to 2.5 × 10−2 Ra (Ra = 1.4 × 10−6, the 3He/4He ratio of air), 40Ar*/4He ratios from 0.64 × 10−2 to 15.39 × 10−2, and 40Ar/36Ar ratios from 633.2 to 6582.0. Those noble gas isotopic compositions of fluid inclusions also support a crustal source origin, evidenced by the Os isotope. Meanwhile, recent noble gas studies suggest that the amount of in situ radiogenic 4He generated should not be ignored, even when Th and U are present at levels of only a few ppm in host minerals.

Noble gas and carbon isotopic compositions of petit-spot lavas from southeast of Marcus Island

We measured noble gas isotopic compositions of quenched lavas sampled from seamounts, so-called petit-spot volcanoes, on the 160-million-year-old northwestern Pacific Plate. The samples 3He/4He and 40Ar/36Ar ratios were, respectively, 2.5–8.3 Ra and up to 1735, where Ra stands for atmospheric 3He/4He, which are analogous to or lower than those of MORB. Considering narrow sampling regions, a secondary effect might be responsible for variation of the data.During ascent and subsequent cooling of magma in the oceanic lithosphere, chemical components in the magma will be assimilated with those in the lithosphere. Correlation between CO2/3He ratios and carbon isotopic ratios suggests that carbon was affected by the incorporation of seafloor carbonate. The same would be true of noble gases. The mixing of noble gases among a mantle source, an atmospheric component dissolved in seawater and a radiogenic component can explain the data distribution. No 3He/4He ratio exceeds the MORB-like value. The mantle source of the petit-spot magma was likely to have had a MORB-like 3He/4He ratio originally. The eruption of petit-spot magma shows a close relation with the bending of subducting oceanic plates. The MORB-like 3He/4He ratio supports the hypothesis that the petit-spot magma is derived from the lithosphere–asthenosphere boundary.

Formation of epithermal Sn-Ag-(Zn) vein-type mineralization at the Pirquitas deposit, NW Argentina: Fluid inclusion and noble gas isotopic constraints

The Pirquitas Sn-Ag-(Zn) deposit in northwestern Argentina is thought to be an analogue to the Miocene polymetallic epithermal Sn-Ag deposits of the southern Bolivian Tin Belt, but little is known in detail about the origin and evolution of ore-forming fluids at Pirquitas. This paper reports on a microthermometric study of fluid inclusions in quartz, sphalerite, Ag-Sn sulfides, and Ag-rich sulfosalts using transmitted near infrared and visible light, combined with noble gas isotope analyses of fluids released from mineral separates. The study focused on the vein-hosted mineralization, which formed during two major mineralization events, whereby the first event I comprises two stages (I-1 and I-2). All studied minerals exclusively contain aqueous two-phase inclusions, indicating that the ore-forming fluids did not undergo two-phase phase separation (boiling). Salinity of fluid inclusions in I-1 quartz that precipitated along with pyrite and pyrrhotite ranges between 0 and 7.5 wt% NaCl equiv. and homogenization temperatures (Th) are between 233 and 370 °C. Stage I-2 is characterized by abundant Sn-Ag-Pb-Zn-sulfides and a variety of Ag-rich sulfosalts. Fluid inclusions in stage I-2 Ag-Sn sulfides have salinities up to 10.6 wt% NaCl equiv. and Th between 213 and 274 °C. The deposition of stage I-2 ore is likely related to a new pulse of saline magmatic fluids to the hydrothermal system. The mineralization event II deposited the richest Ag ores at Pirquitas. Colloform sphalerite and pyrargyrite deposited during event II contain two-phase aqueous fluid inclusions with homogenization temperatures between 190 and 252 °C and salinities between 0.9 and 4.3 wt% NaCl equiv.Noble gas concentrations and isotopic compositions of ore-hosted fluid inclusions were determined from crushing hand-picked ore minerals from both mineralization events. With one exception, all samples yielded 3He/4He ratios between 1.9 and 4.1 Ra, which is within the range of published data from the volcanic arc and somewhat higher than typical values of meteoric water-derived hot-springs in the region. This demonstrates a significant contribution of magmatic fluids to the Pirquitas mineralization although no intrusive rocks are exposed in the mine region. Taking the noble gas evidence for a magmatic fluid source, we interpret the trends of decreasing Th and salinity values in fluid inclusions from events I and II to represent waning of the magmatic-hydrothermal system and/or increased admixing of meteoric water to the magmatic fluids.

The Kumtag 016 L5 strewn field, Xinjiang Province, China

AbstractThe Kumtag 016 strewn field was found in the eastern part of the Kumtag desert, Xinjiang Province, China. In this study, 24 recovered meteorites have been characterized by a suite of different analytical techniques to investigate their petrography, mineralogy, bulk trace elements, noble gas isotopic composition, density, and porosity. We attribute to the strewn field 22 L5 chondrites with shock stage S4 and weathering grade W2–W3. Two different meteorites, Kumtag 021, an L4 chondrite and Kumtag 032, an L6 chondrite, were recognized within the strewn field area. Moreover, Kumtag 003, an H5 chondrite, was previously found in the same area. We infer that the Kumtag 016 strewn field most likely consists of at least four distinct meteorite falls. The effects of terrestrial weathering on the studied meteorites involve sulfide/metal alteration, chemical changes (Sr, Ba, Pb, and U enrichments and depletion in Cr, Co, Ni, and Cs abundances), and physical modifications (decrease of grain density and porosity). Measurements of the light noble gases indicate that the analyzed Kumtag L5 samples contain solar wind‐implanted noble gases with a 20Ne/22Ne ratio of ~12.345. The cosmic‐ray exposure (CRE) ages of the L5 chondrites are in a narrow range (3.6 ± 1.4 Ma to 5.2 ± 0.4 Ma). For L4 chondrite Kumtag 021 and L6 chondrite Kumtag 032, the CRE ages are 5.9 ± 0.4 Ma and 4.7 ± 0.8 Ma, respectively.

Stable isotopic composition of coal bed gas and associated formation water samples from Raniganj Basin, West Bengal, India

This study was carried out to document the source and stable isotopic characteristics of coal bed gas and associated coal bed water samples of the Raniganj Basin, West Bengal, India. The Raniganj Basin is one of the largest coal bed gas-producing sites in India. However, the gas isotopic composition of this basin was not well investigated until now. The exploration has been focused on mature coals with a high content of assumed thermogenic methane. Interestingly, the present study has revealed that although the gas is thermogenic, there is a significant effect of microbial activities in gas generation. The gas explored from this Basin contains almost 98–99% methane (CH4) in general with very little portions of other hydrocarbons and thus, this gas can be considered as “dry gas”. It is consistent also with late-stage microbial alteration of thermogenic gases. The δ13C and δD values of CH4 range from −47.4 to −49.7‰ and −207 to −211‰ respectively across the three wells as well as from −48.8 to −50.1‰ and −209 to −211‰ respectively in the gas samples collected from gas gathering station. The Whiticar style cross plot of these two parameters reveals that the gas samples have mixed thermogenic and biogenic sources and the microbial gas was generated through the CO2 reduction pathway rather than the acetate fermentation pathway. The difference in δD values of water and methane samples [δD (H2O-CH4)], that varies from 173 to 175‰, may also exhibit that the biogenic methane was produced through reduction of CO2. The stable hydrogen and oxygen isotopic signatures of formation water samples (δD and δ18O-H2O) stand with the mixed origin of the gas. The cross-plot of these two parameters reveals that the water samples lie almost along the Global Meteoric Water Line (GMWL), which may suggest the combined effect of methanogenesis due to mixing of coal seam formation water with modern meteoric water. In summary, the produced coal bed gas from the Raniganj Basin, although being largely thermogenic in origin, has been influenced by secondary microbial alteration.

Geology and Isotope Geochemistry of the Dongzhongla Pb–Zn Deposit in Tibet: Implications for the Origin of the Ore‐Forming Fluids and Storage Condition of Certain Metals

AbstractAs a newly discovered medium‐sized deposit (proven Pb + Zn resources of 0.23 Mt, 9.43% Pb and 8.73% Zn), the Dongzhongla skarn Pb–Zn deposit is located in the northern margin of the eastern Gangdese, central Lhasa block. Based on the geological conditions in this deposit of ore‐forming fluids, H, O, C, S, Pb, Sr, and noble gas isotopic compositions were analyzed. Results show that δ18OSMOW of quartz and calcite ranged from −9.85 to 4.17‰, and δDSMOW ranged from −124.7 to −99.6‰ (where SMOW is the standard mean ocean water), indicating magma fluids mixed with meteoric water in ore‐forming fluids. The δ13CPDB and δ18OSMOW values of calcite range from −1.4 to −1.1‰ and from 5.3 to 15.90‰, respectively, show compositions consistent with the carbonate limestone in the surrounding rocks, implying that the carbon was primarily sourced from the dissolution of carbonate strata in the Luobadui Formation. The ore δ34S composition varied in a narrow range of 2.8 to 5.7‰, mostly between 4‰ and 5‰. The total sulfur isotopic value δ34S was 4.7‰ with characteristics of magmatic sulfur. The 3He/4He values of pyrite and galena ranged from 0.101 to 5.7 Ra, lower than those of mantle‐derived fluids (6 ± 1 Ra), but higher than those of the crust (0.01–0.05 Ra), and therefore classified as a crust–mantle mixed source. The Pb isotopic composition for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of the ores were in the ranges of 18.628–18.746, 15.698–15.802, and 39.077–39.430, respectively, consistent with the Pb isotopic composition of magmatic rocks in the deposit, classified as upper‐crust lead. The ore lead was likely sourced partially from the crustal basement of the Lhasa Terrane. The initial (87Sr/86Sr)i value from five sulfide samples ranged from 0.71732 to 0.72767, and associated ore‐forming fluids were mainly sourced from the partial melting of the upper‐crust materials. Pb isotopic compositions of ore sulfides from the Dongzhongla deposit are similar to that of the Yuiguila and Mengya'a deposit, indicating that they have similar sources of metal‐rich ore‐forming solution. According to basic skarn mineralogy, the economic metals, and the origin of the ore‐forming fluids, the Dongzhongla deposit was classified as a skarn‐type Pb–Zn deposit.