Isotopic Composition Of He Research Articles

Tracking the interaction between injected CO2 and reservoir fluids using noble gas isotopes in an analogue of large-scale carbon capture and storage

Industrial scale carbon capture and storage technology relies on the secure long term storage of CO2 in the subsurface. The engineering and safety of a geological storage site is critically dependent on how and where CO2 will be stored over the lifetime of the site. Hence, there is a need to determine how injected CO2 is stored and identify how injected CO2 interacts with sub-surface fluids. Since July 2008 ∼1 Mt of CO2 has been injected into the Cranfield enhanced oil recovery (EOR) field (MS, USA), sourced from a portion of the natural CO2 produced from the nearby Jackson Dome CO2 reservoir. Monitoring and tracking of the amount of recycled CO2 shows that a portion of the injected CO2 has been retained in the reservoir. Here, we show that the noble gases (20Ne, 36Ar, 84Kr, 132Xe) that are intrinsic to the injected CO2 can be combined with CO2/3He and δ13CCO2 measurements to trace both the dissolution of the CO2 into the formation water, and the interaction of CO2 with the residual oil. Samples collected 18 months after CO2 injection commenced show that the CO2 has stripped the noble gases from the formation water. The isotopic composition of He suggests that ∼0.2%, some 7 kt, of the injected CO2 has dissolved into formation water. The CO2/3He and δ13CCO2 values imply that dissolution is occurring at pH = 5.8, consistent with the previous determinations. δ13CCO2 measurements and geochemical modelling rule out significant carbonate precipitation and we determine that the undissolved CO2 after 18 months of injection (1.5 Mt) is stored by stratigraphic or residual trapping. After 45 months of CO2 injection, the noble gas concentrations appear to be affected by CO2-oil interaction, overprinting the signature of the formation water.

Geochemical characterisation of gases along the dead sea rift: Evidences of mantle-co2 degassing

The Dead Sea Transform (DST) fault system, where a lateral displacement between the African and Arabian plates occurs, is characterised by anomalous heat flux in the Israeli area close to the border with Syria and Jordan. The concentration of He and CO2, and isotopic composition of He and total dissolved inorganic carbon were studied in cold and thermal waters collected along the DST, in order to investigate the source of volatiles and their relationship with the tectonic framework of the DST. The waters with higher temperature (up to 57.2°C) are characterised by higher amounts of CO2 and helium (up to 55.72 and 1.91∗10−2 cc l−1, respectively). Helium isotopic data (R/Ra from 0.11 to 2.14) and 4He/20Ne ratios (0.41–106.86) show the presence of deep-deriving fluids consisting of a variable mixture of mantle and crust end-members, with the former reaching up to 35%. Carbon isotope signature of total dissolved carbon from hot waters falls within the range of magmatic values, suggesting the delivery of deep-seated CO2. The geographical distribution of helium isotopic data and isotopic carbon (CO2) values coupled with (CO2/3He ratios) indicate a larger contribution of mantle-derived fluids affecting the northern part of the investigated area, where the waters reach the highest temperature. These evidences suggest the occurrence of a favourable tectonic framework, including a Moho discontinuity up-rise and/or the presence of a deep fault system coupled with the recent magmatic activity recognised in the northern part of Israel.

ISOTOPIC MASS FRACTIONATION OF SOLAR WIND: EVIDENCE FROM FAST AND SLOW SOLAR WIND COLLECTED BY THEGENESISMISSION

NASA's Genesis space mission returned samples of solar wind collected over ~2.3 years. We present elemental and isotopic compositions of He, Ne, and Ar analyzed in diamond-like carbon targets from the slow and fast solar wind collectors to investigate isotopic fractionation processes during solar wind formation. The solar wind provides information on the isotopic composition for most volatile elements for the solar atmosphere, the bulk Sun and hence, on the solar nebula from which it formed 4.6 Ga ago. Our data reveal a heavy isotope depletion in the slow solar wind compared to the fast wind composition by 63.1 ± 2.1‰ for He, 4.2 ± 0.5‰ amu–1 for Ne and 2.6 ± 0.5‰ amu–1 for Ar. The three Ne isotopes suggest that isotopic fractionation processes between fast and slow solar wind are mass dependent. The He/H ratios of the collected slow and fast solar wind samples are 0.0344 and 0.0406, respectively. The inefficient Coulomb drag model reproduces the measured isotopic fractionation between fast and slow wind. Therefore, we apply this model to infer the photospheric isotopic composition of He, Ne, and Ar from our solar wind data. We also compare the isotopic composition of oxygen and nitrogen measured in the solar wind with values of early solar system condensates, probably representing solar nebula composition. We interpret the differences between these samples as being due to isotopic fractionation during solar wind formation. For both elements, the magnitude and sign of the observed differences are in good agreement with the values predicted by the inefficient Coulomb drag model.

Sm-Nd and Rb-Sr isotopic systems and captured He and hydrocarbon gases as markers of melt sources and fluid regime under which the oceanic crust of the Mid-Atlantic Ridge was formed at 5°–6° N

The paper presents characteristics of the Nd and Sr isotopic systems of ultrabasic rocks, gabbroids, plagiogranites, and their minerals and data on He and hydrocarbons in fluid inclusions in the same samples. The materials presented in this publication were obtained by studying samples dredged from the MAR crest zone at 5°–6° N (U/Pb zircon dating, geochemical and petrological-mineralogical studies). It was demonstrated that the variations in the isotopic composition of He entrapped in the rocks and minerals were controlled by variable degrees of mixing of juvenile He, which is typical of basaltic glass of MAR (DM source) and atmospheric He. An increase in the fraction of atmospheric He in the plutonic rocks and, to a lesser degree, in their minerals reflects the involvement of seawater or the hydrated material of the oceanic crust in the magmatic and postmagmatic processes. This conclusion finds further support in a positive correlation between the fraction of mantle He (R) and the 87Sr/86Sr ratio. The high-temperature hydration of ultrabasic rocks (amphibolization) was associated with an increase in the fraction of mantle He, while their low-temperature hydration (serpentinization) was accompanied by a drastic decrease in this fraction and a significant increase in the 87Sr/86Sr ratio. The insignificant variations in the 143Nd/144Nd (close to 0.5130) and 87Sr/86Sr (0.7035) in most of the gabbroids and plagiogranites, as well as the fraction of mantle He in these rocks, amphibolites, and their ore minerals, indicate that the melts were derived from the depleted mantle. SimilarɛNd values of the gabbroids, plagiogranites, and fresh harzburgites (6.77–8.39) suggest that these rocks were genetically related to a single mantle source. The value of ɛNd = 2.62 of the serpentinized lherzolites likely reflects relations of these relatively weakly depleted mantle residues to another source. The aforementioned characteristics of the rocks generally reflect the various degrees of mixing of components of the depleted mantle with crustal components (seawater) during the metamorphic and hydrothermal processes that accompanied the formation of the oceanic crust.

Noble gases in ten Nullarbor chondrites: Exposure ages, terrestrial ages, and weathering effects

Abstract— We present concentration and isotopic composition of He, Ne, and Ar in ten chondrites from the Nullarbor region in Western Australia as well as the concentrations of 84Ke, 129Xe, and 132Xe. From the measured cosmogenic 14C concentrations (Jull et al. 1995), shielding-corrected production rates of 14C are deduced using cosmogenic 22Ne/21Ne ratios. For shielding conditions characterized by 22Ne/21Ne >1.10, this correction becomes significant and results in shorter terrestrial ages. The exposure ages of the ten Nullarbor chondrites are in the range of values usually observed in ordinary chondrites. Some of the meteorites have lost radiogenic gases as well as cosmogenic 3He. Most of the analyzed specimens show additional trapped Ar, Kr, and Xe of terrestrial origin. The incorporation of these gases into weathering products is common in chondrites from hot deserts.

Isotopic, chemical and dissolved gas constraints on spring water from Popocatepetl volcano (Mexico): evidence of gas–water interaction between magmatic component and shallow fluids

Geochemical research was carried out on cold and hot springs at Popocatepetl (Popo) volcano (Mexico) in 1999 to identify a possible relationship with magmatic activity. The chemical and isotopic composition of the fluids is compatible with strong gas–water interaction between deep and shallow fluids. In fact, the isotopic composition of He and dissolved carbon species is consistent with a magmatic origin. The presence of a geothermal system having a temperature of 80–100° C was estimated on the basis of liquid geothermometers. A large amount of dissolved CO 2 in the springs was also detected and associated with high CO 2 degassing.

Geochemistry of fluids from submarine hot springs at Punta de Mita, Nayarit, Mexico

Thermal springs with a maximum measured temperature of 89°C discharge hot water and gas from a depth of 11 m, 400 m offshore of Punta Pantoque, located in the northern part of Bahı́a de Banderas, near Puerto Vallarta, Mexico. The composition of all water samples collected from the sea bottom is close to that of sea water. Nevertheless, it was possible to estimate the thermal endmember composition by extrapolating the sulfate concentration to zero. This endmember is similar in chemical composition both to waters of the Rio Purificacion and La Tuna thermal springs, located to the South along the Pacific coast of the Jalisco Block, and to pore waters from the deep-sea drilling cores from some accretionary complexes. Gas composition as well as isotopic composition of He and carbon from CO2, CH4 and C2H6 suggests an essentially thermo-biogenic origin for the gas and the presence of a high proportion of radiogenic, crustal helium. Isotopic composition of He in the Punta de Mita gas (0.4 Ra) is the lowest ever measured in Mexican hydrothermal gases. These findings do not support the idea that there exists a direct connection between the Punta de Mita springs and the last volcanic events which occurred in this area at ∼3 Ma. Rather, this hydrothermal activity is related to deep active faulting and the existence of a deep regional aquifer or local aquifers of connate waters underlying the granites of the Jalisco Block.

Measurement of the Isotopic Composition of Cosmic‐Ray Helium, Lithium, Beryllium, and Boron up to 1700 MeV per Atomic Mass Unit

We present data from the second flight of the superconducting magnet instrument for light isotopes (SMILI), which took place on 1991 July 24. This instrument was optimized to determine the isotopic composition of He, Li, Be, and B in the Galactic cosmic rays, up to an energy of 2 GeV amu-1. The abundances of He, Li, and B are found to be consistent with standard models of cosmic-ray propagation. Our measurement of the abundances of the beryllium isotopes suggests an enhancement of the fraction of the isotope 10Be over that found at low energy. Of 26 beryllium events, nine are found to be 10Be. Monte Carlo calculations based on this observation imply the mean lifetime of cosmic rays to be less than 6 Myr at the 97.5% confidence level.

Chemical (He, H2, CH4, Ne, Ar, N2) and isotopic (He, Ne, Ar, C) variations at the solfatara crater (southern italy): mixing of different sources in relation to seismic activity

Here we describe temporal variations of noble gas (He, Ne and Ar), carbon isotopes and chemical data of fluids collected at the Bocca Grande fumarole (Solfatara crater) from 1990 to 1994, before the occurrence of a seismic swarm. After more than seven years of quiescence, a sudden vertical ground deformation and a seismic swarm of 100 quakes with an Mmax = 1.5 with epicenter located beneath the Solfatara crater, was recorded from August 23 to 25 1994. Isotopic composition of He, Ne, Ar and C (from CO2) and abundances of He, Ne, Ar, H2, CH4 and N2 have been analysed from fumarolic fluids collected at the Solfatara crater. To date these isotopic ratios have not been used as precursors of earthquakes, although several cases have been reported in which the helium isotopic ratio was used to interpret volcanic unrest. All the analyzed isotopic ratios, chemical species and a calculated equilibrium temperature show a significant change a few months before the occurrence of the seismic activity and ground deformation, followed by a sudden return to values recorded prior the swarm. Migration towards the surface of deep (and hotter) gas phase related to crustal and magmatic sources with the following decrease of the surficial atmospheric component is the cause of such wide variations.

Time-dependent models of U–Th–He and K–Ar evolution and the layering of mantle convection

High-resolution seismic tomography indicates that lithospheric plates penetrate the 670 km discontinuity. A common view holds that the isotopic composition of He and Ar is incompatible with geophysical evidence because (a) the inventory of terrestrial radiogenic argon requires that the lower mantle is largely undegassed, and (b) recycling of outgassed lithospheric material should destroy the primitive isotopic composition of helium and argon in the lower mantle. It is first shown, however, that the `argon problem' results from the assumption that the terrestrial K/U ratio is ≥1.2×10 4, a value that cannot be taken for granted. In addition, earlier geochemical models of steady-state He and Ar cycles are shown to be inadequate, because common concepts of residence time do not apply to multiple reservoir systems. A time-dependent model with four reservoirs (lower and upper mantle, crust, atmosphere) is evaluated in which elemental fractionation upon transfer and variable growth rate for crustal growth, atmosphere extraction, and lithosphere production are taken into account. Vigorous lithosphere production early in the history of the Earth enhances the degassing of the upper mantle in agreement with the very radiogenic character of 4He/ 3He and 40Ar/ 36Ar ratio in mid-ocean ridge basalts. Recycling of oceanic lithosphere stripped of its rare gases, but also of its U, Th, and K, into the lower mantle preserves the primitive isotopic signature of the rare-gas inventory of this reservoir. Accumulation in the lower mantle of ancient plates essentially barren of U, Th, and K, but nevertheless showing isotopic evidence of plate-tectonic processes in the less incompatible elements (Nd, Hf, Sr), can account for the isotopic composition of rare gases in hot-spot basalts. The outgassed character of the upper mantle was probably acquired during the early Archaean when mantle activity was intense and is still being enhanced by the fluxing of K, U, and Th from the descending lithospheric plates. The view of the mantle offered by seismic tomography is therefore not incompatible with the isotopic evidence provided by terrestrial rare gases.

Helium and Argon in Ore Fluids: So What?

The isotopic composition of He and Ar trapped in crustal fluids and fluid inclusions carries a signature of the source of the fluids and the processes which have affected them. There are three distinct sources of the noble gases; atmosphere, crust and mantle. Atmospheric gases carried in groundwaters are characterised by the presence of 36Ar and 4~ and minor amounts of helium (3He/4He = 1.39 x 10 _6 = Ra). The crust is a source of radiogenic 4~ and 4He, very low levels of3He (-0.01-0.05 Ra) and no 36Ar. The prime signature of the mantle is helium with a high 3He content (e.g. 3He/4HeMoRB ~ 8 Ra), together with 4~ and, in most situations, negligible 36Ar (4~ ~ 40,000). Fluids trapped in ore minerals frequently carry all three components (Stuart et al., 1995). We have investigated He and Ar isotopes, sometimes in combination with halogens, trapped in ore fluids from a variety of areas and types of mineralisation. Our aims have been to see what qualitative aspects of the mineralisation processes are reflected in noble gas patterns and to what extent these can be used to resolve conflicting interpretations, what aspects can be understood in a quantitative way, and whether these can be used, for example, to model the large scale features of specific ore deposits. In this report we contrast the He and Ar isotope observations from two widely separated porphyry-Cu deposits (PCD) with those from an MVT deposit. The Machangqing PCD deposit in West Yunnan province, China, is part of the Ailaoshan-Jinshajiang alkali-porphyry intrusive zone of Eocene age formed in an extensional setting. The mantle derived magmas intrude lower Ordovician microelastic rocks and Devonian limestone and appear to have assimilated some crustal material. The Sungun PCD is located in NW Iran and is associated with a series of calc-alkaline hypabyssal igneous intrusions of Oligo-Miocene age. The magma was emplaced in a continental margin tectonic setting related to the subduction and assimilation of the Persian plate during the major Alpine orogenic phase. The Sungun porphyries occur as stocks and dykes intruding a series of Oligocene dacitic breccias, tufts and trachyandesitic lavas, Eocene arenaceous-argiUaceous rocks, and UpperCretaceous limestones. The He and Ar compositions of gases released by crushing sulphides from the two deposits are shown in Figs. 1 and 2. 4~ is the only isotope present in high abundance in all components and consequently this is the only method of representing all the data, end members and processes in a single figure. Mixtures of three end members define a mixing plane, while elemental fractionation of He and Ar drive compositions radially from, and perpendicular to, the ASW axis. In both figures the data lie on 3component mixing planes. The three end members are fluids enriched respectively in atmosphere (ASW), mantle helium and argon ( label led EARLY) and crustal helium (labelled LATE). The 3He/4He ratios, 1.2 Ra (Machangqing) and 2.25 Ra

Reconciling Mantle Rare Gas Geochemistry with Tomographic Evidence of Whole Mantle Convection

High-resolution seismic tomography clearly shows that lithospheric plates penetrate the 670 km discontinuity (e.g. van der Hilst et al, 1997). A common view defended by geochemists (Turekian, 1959; Allbgre et al. , 1996) holds that the isotopic composition of He and Ar is incompatible with geophysical evidence because (a) the inventory of terrestrial radiogenic argon requi-res that the lower mantle is largely undegassed and (b) recycling of outgassed lithospheric material should destroy the primitive isotopic composition of helium and argon in the lower mantle. It is first shown, however, that the 'argon problem' is largely a 'potassium problem' that results from the assumption that the terrestrial K/U ratio is 12,500 (Jochum et al., 1983). In view of the large variability of this ratio amongst telluric objects (from 3,000 in the Moon to 60,000 in chondrites), of the volatile character of alkalis demonstrated by the Sr isotope geochemistry of chondrites and terrestrial material, and of the uncertainties on the K/U fractionation during melting, such a value value cannot be taken for granted. In addition, earlier geochemical models of steady-state He and Ar cycles (e.g. O'Nions and Tolstikhin, 1996) are shown to be inadequate. The common concept of residence time is inappropriate for multiple reservoir systems. An unwanted artifact resulting from the assumption that an isotopically heterogeneous system is time-invariant, is actually a strong reduction of mass fluxes between reservoirs, with, in particular, a small exchange rate between the lower mantle and the upper mantle. The view that tomography reflects a present-day snapshot while the Earth's mantle was essentially layered until recently, requires some unsubstantiated serendipity. A regime of intermittent layering (avalanches) is geochemically indistinguishable from whole mantle convection and, as such, does not solve the dilemma. A time-dependent model with four reservoirs (lower and upper mantle, crust, atmo-sphere) is evaluated, in which elemental fractionation upon transfer and variable rate of crustal growth, atmosphere extraction, and lithosphere production are taken into account. Vigorous lithosphere production early in the history of the Earth enhances the degassing of the upper mantle in agreement with the very radiogenic character of 4He/3He and 4~ ratio in MORB. The preferential reinjection into the lower mantle of lithospheric material that lost its component rich in lithophile elements to the continental crust has been underestimated by previous modelers. Recycling of oceanic lithosphere stripped of its rare gases, but more importantly of its U, Th, and K, into the lower mantle preserves the primitive isotopic signature of the rare-gas inventory of this reservoir. Accumulation in the lower mantle of ancient plates essentially barren of U, Th, and K, but nevertheless showing isotopic evidence of platetectonic processes in the less incompatible elements (Nd, Hf, St), can account for the isotopic composition of rare gases in hot-spot basalts. A prominent effect of plate tectonics and whole mantle convection is to clutter and dilute the lower mantle with old barren lithospheric debris. The geochemistry of the lower mantle is therefore expected to show characters that somehow conflict with conventional views. Although the incompatible element content of the lower mantle has been diluted with respect to the bulk silicate Earth (BSE), the fractionation of elements, and in particular parentdaughter pairs such as Sm/Nd and Rb/Sr, is still intermediate between that of BSE and that of the upper mantle. For such a model to be acceptable, melting should reverse the relative level of enrichment of melts relative to that of their mantle source. OIB should be produced by very small degrees of melting of relatively barren lower mantle material and MORB by large degrees of melting of a relatively fertile upper mantle, regardless of the apparent 'depletion' or 'enrichment' indicated by the isotopic systems and trace element patterns. The outgassed character of the upper mantle was probably acquired during the early Archaean when mantle activity was intense and is still being enhanced by the fluxing of K, U, and Th from the descending lithospheric plates. The view of the mantle offered by seismic

Collection of interstellar gas on the Moon

The isotopic composition of He and Ne in the local interstellar medium (LISM) is of great scientific interest, since it will help us to understand the chemical evolution of our galaxy since the formation of the solar system 4.6 × 10 9 years ago. In addition, the He isotope ratio is of cosmological interest as it is linked to primordial (Big Bang) nucleosynthesis and gives us a clue to the baryon density in the universe. To measure the abundance of He and Ne and their isotopes in situ in the interstellar gas is very ambitious due to the low density. Remote sensing, however, gives us only little information on the isotopic composition. Currently, most of the information on He and Ne isotopes comes from the measurement of pick-up ions and of the anomalous component of the cosmic rays, however, the measurement errors are still too high to reach firm conclusions. Recently, a novel measurement method has been proposed, using the foil collection technique and the Moon as a collection platform (Reber et al., 1995). In this notice a detailed description of the experiment is given. The foil collection technique uses thin metallic foils to trap the atoms which impinge on the foil. After retrieval of the foil the trapped particles are extracted by stepwise heating and the released atoms can be analysed using laboratory mass spectrometers. This technique was applied for the first time to successfully measure the noble gas composition of the solar wind (Geiss et al., 1970, 1972). (Interestingly enough, this experiment has been carried out on the Moon as well.) It has been tried before to measure the composition of the interstellar gas applying the foil collection method on an Earth orbiting satellite. However, in this case the measurements might be contaminated by the high fluxes of magnetospheric particles. It is shown that by using the Moon as a platform to collect the interstellar atoms one can overcome these difficulties. The experiment has to be carried out during the lunar night to shield off the solar wind and during times when the Moon is not in the magneto-sheath. An evaluation of the measurement conditions for January 1997, as an example, shows that it is possible to collect the interstellar gas for a consecutive period of about 6 days. Compared to known backgrounds of the mass spectrometric foil analysis a signal-to-background ratio higher than 20 for 3He, 4He, 20Ne, and 22Ne is obtained. A rough estimate shows that neither retrapped lunar ions nor bow-shock ions pose a background problem.

Helium isotope compositions in sedimentary basins in China

The abundance and isotopic composition of He have been determined for a variety of natural gases (oil-field gases) in sedimentary basins in the continent of China. The 3He/ 4He and 4He/ 20Ne ratios range from 0.004 to 4.99 Ra (where Ra denotes atmospheric 3He/ 4He ratio = 1.4 × 10 −6), and from 100-39,000, respectively. Helium in the gas sample is composed of mantle-derived He and crustal radiogenic He. The high 3He/ 4He ratios were observed in the sedimentary basins which underwent Cenozoic extension in the eastern domain, such as the Songliao, Liaohe, North China, Subei, Sanshui and South China Sea basins. The basins where mantle-derived He is recognized are characterized by the presence of surface and subsurface volcanic rocks, high terrestrial heat flow, deep faults and thinned crust. On the other hand, regions which underwent pre-Tertiary extension such as the Jungar, Tarim, Tuba, Caidam and Yumen basins in the NW and tectonic loading such as the Ordos and Sichuan basins in the central domain are marked by almost pure radiogenic 4He of crustal origin and thus by an apparent absence of mantle-derived He, which coincides with complete lack of Cenozoic volcanics, low terrestrial heat flow and thickened crust. Such a 3He/ 4He distribution pattern clearly indicates that tectonic environment control the distribution of He isotopes in the continent of China.

He, Ar, O, Sr and Nd isotope constraints on the origin and evolution of Mount Etna magmatism

The 0.5 Ma history of the origin and evolution of Mount Etna, Sicily has been investigated by analysing the isotopic composition of He, Ar, O, Sr and Nd in 21 selected lava samples. The near constancy of the isotopic compositions of oxygen ( δ 18 O = 5.4 ± 0.3‰ ) and of helium trapped in olivine phenocrysts (6.7 ± 0.4 R a) is interpreted as evidence of a single mantle source, despite drastic petrological changes during the volcano's history. He analyses performed by crushing cogenetic pyroxene and olivine phenocrysts show a tendency to lower 3 He 4 He ratios in pyroxenes. This is best explained by crystallization of pyroxenes at a depth shallower than that of olivines and/or by exchange of helium trapped in pyroxenes with atmospheric or radiogenic He before eruption. 87 Sr 86 Sr ratios of recent lavas tend to increase with time and to correlate with Rb Th ratios, and, for historical lavas, these variations are tentatively attributed to shallow selective contamination from underlying sediments. Based on the similarity of the 3 He 4 He ratios at Etna to those of European mantle xenoliths [1], we propose that the ‘baseline’ geochemical signature of isotopic tracers at Etna reflects the composition of the subcontinental mantle. Comparison to other southern Italian active volcanoes (Etna, Vulcano Ischia, Campi Flegrei, Vesuvius) shows gradual dilution of the predominantly mantle Etnean end member by more radiogenic Sr and He and material with higher δ 18O and C 3 He , which is reasonably explained by the progressively important influence of subducted continental crust.

Significance of the Isotopic Composition of He and Ar in Natural Gases

To identify the oil-gas source rocks is very important for the prospection of oil and gas, and it is a key method to detect the isotopic composition of He and Ar in natural gases for retracing source rocks. The detection of Ar isotopic composition was started in 1973 by Gavrigov et al., then the time-accumulating effect of radiogenic Ar was recognized by Chinese researchers after the discovery of the fact that the content of radiogenic Ar (i. e. ~(40)Ar) increases with the increase of the age of source rocks, and the formula to estimate the age of source rocks was established with the consideration of the K content

Carbon, helium, neon and argon isotopes in a Po basin (northern Italy) natural gas field

The abundance and isotopic composition of He, Ne and Ar have been determined in 19 gas samples taken from the Dosso degli Angeli hydrocarbon gas field, between 3000- and 3750-m depth in the Po Basin. Carbon isotope ratios in these samples vary between δ 13C(CH 4) = −67.5 and −73.4%, indicating a predominantly biogenic source for the methane. C 2+ [= Σ(C 2H 6 + C 3H 8 + …)] concentrations increase systematically from 0.04% to 0.17% by volume with depth, suggesting that there is a small additional thermogenic or diagenetic hydrocarbon contribution to these gases. CH 4/ 36Ar ratios in the Dosso degli Angeli gases are in the range predicted for a groundwater which has derived its rare gas by equilibration with the atmosphere, and is CH 4-saturated under reservoir conditions. There is also up to 10% excess of CH 4 which is not associated with the groundwater. These results are consistent with > 90% of the CH 4 in the Dosso degli Angeli reservoir originating from CH 4-saturated groundwater. Measured 20Ne/ 36Ar ratios depart from the predicted ratio in air-equilibrated groundwater. This fractionation is consistent with water/gas phase equilibration under reservoir conditions, and is similar to that observed in gas reservoirs in the Vienna and Pannonian Basins, Austria and Hungary, respectively. The He in the Dosso degli Angeli gases is almost entirely radiogenic and derived from the crust. The concentration of 4He in the reservoir gases requires that He is mostly sourced from outside the reservoir volume. Radiogenic 4He/ 40Ar ratios input into the reservoir gas must be much higher than averaged crustal production ratios, as no shift from the atmospheric 40Ar/ 36Ar ratio is observed. Preferential release of 4He over 40Ar from the sites of their production is the preferred explanation. This observation contrasts with previous studies from active extensional basins, in which radiogenic 4He and 40Ar appear to have been stored, released and transported at near production ratio on a regional scale. The most obvious difference between these studies is the comparatively low thermal gradient of the Po Basin.

Composition of solar flare noble gases preserved in meteorite parent body regolith.

The isotopic composition (long-term average) of solar flare (SF) Ne has been determined by three isotope correlation techniques applied to data measured on chemically etched pyroxene separates prepared from the Kapoeta meteorite, which is known to contain implanted solar gases. The SF 20Ne/22Ne ratio obtained is 11.6 +/- 0.2 and confirms previous determinations of this SF ratio in lunar and meteoritic samples. The same SF Ne composition is also obtained by applying an ordinate intercept technique to the same data set. The ordinate intercept technique was also applied to the Ar and He data, on which the three-isotope correlation technique cannot be applied. The isotopic composition of SF Ar and SF He so obtained are SF 36Ar/38Ar = 4.9 +/- 0.1 and SF 4He/3He = 3800 +/- 200, which are significantly different from the solar wind (SW) Ar and SW He values of approximately 5.35 and approximately 2500, respectively. Correlation between 20Ne/22Ne and 36Ar/38Ar for the same data set also gives a similar SF 36Ar/38Ar ratio of 4.8 +/- 0.2. The determined SF He, Ne and Ar isotopic ratios differ from those in SW by 52%, 17% and 9%, respectively, but the elemental compositions of 4He/36Ar and 20Ne/36Ar do not show obvious differences between SF and SW. The concentration of the SF component in Kapoeta pyroxenes is approximately 20% that of the SW component, which is orders of magnitude higher than expected from SW and SF proton flux measurements. Variations in elemental and isotopic composition of He, Ne and Ar in SF relative to SW are found to correlate well with a (Z/A)2 dependence, indicating a rigidity-dependent particle spectrum in solar flares.

The role of element distribution in production and release of radiogenic helium: the Carnmenellis Granite, southwest England

The abundance and isotopic composition of He has been measured in whole-rock samples and mineral separates from the Carnmenellis Granite and compared with circulation fluids from the experimental hot dry rock (HDR) project. These fluids are not now in He isotopic equilibrium with the rock through which they flow. Their 3 He 4 He ratios ( R) relative to an atmospheric standard ( R a) average R R a =0.025±0.003 , whereas the whole-rock and mineral separate values are R R a =0.0007–0.0033 , a factor of 8–36 times lower. These differences may be explained by dissimilar formation mechanisms for the two isotopes, and the consequent difference in release kinetics. 4He contents of whole-rock samples show that ⩾60% of He produced over the entire age of the granite is still retained. The bulk of U (∼95%) and Th (∼77–91%) and therefore 4He production is concentrated in uraninite and monazite. Analysis of a single grain of uraninite (1.6 μg) showed 4He retention of 71±9%. 3He on the other hand is formed by neutron-induced fission of Li, mainly in biotite. 3He escapes from this mineral relatively easily. In a rock where U and Th are not uniformly distributed and concentrated into accessory phases, the neutron production may be lower (by up to a factor of ∼5) than predicted by simple calculations based on the homogeneous situation. A model which allows for this geometry-dependent neutron production has been constructed which accounts for the low measured values of 3 He 4 He in the Carnmenellis Granite ( 0.0015⩽ R R a ⩽0.025 ) despite its high Li concentration. These results have two important implications for fluid tracing using He isotopes and for crustal degassing: (1) the difference in release mechanism of 3He and 4He may lead to a 3 He 4 He ratio in scavenging fluids either higher or lower than the production ratio; and (2) 4He accumulation in the crust may occur where U and Th are concentrated in accessory phases, but probably only at relatively low temperatures in the upper part of the continental crust. This may in some cases cause crustal degassing to be episodic.

Cosmic-ray records in the LL-chondrite Dhurmsala

The radionuclides 10Be, 26Al and 53Mn, the noble gas isotopic composition of He, Ne and Xe and cosmic-ray particle track densities were determined in six documented samples of a large fragment of the Dhurmsala meteorite fall. Using the 22Ne21Ne ratios and the 21Ne concentrations, the cosmic-ray exposure age of Dhurmsala is deduced to be 13.0 ± 1.0 My. The results suggest a simple exposure history. The six samples covering a distance of ~ 13 cm do not show any significant isotopic variations due to depth effects, as expected in a large meteorite. Consistent with the model calculations of Reedy (1985) and Bhandari and Potdar (1982), the results obtained in this study indicate that the Dhurmsala samples investigated here originate at a depth of ~20 to 24 cm in a meteoroid with a preatmospheric radius of R − 50 cm.