Helium isotopes in the earth's interior and in the atmosphere: A degassing model of the earth

A first-order degassing model was applied to describe the evolution of helium content and isotope composition in the earth and in the atmosphere. The main events described by the model are: (1) the earth-trapped primordial rare gases at the moment of its accretion; (2) later, the solid earth lost primordial and radiogenic rare gases, and (3) they were accumulated in the atmosphere; (4) in addition,³He was formed in the atmosphere due to cosmic irradiation, accretion from solar wind, etc.; (5)³He and⁴He dissipated into space at different loss rates.Study of this model confirms the concept that some of primordial helium is retained in the interior of the earth; terrestrial helium (³He/⁴He～ 2 × 10^?5) was most probably formed as a mixture of primordial (³He/⁴He= 3 × 10^?4) and radiogenic (³He/⁴He～ 3 × 10^?8) helium. For achondritic concentrations of heavy radioactive elements (U= 2.25 × 10^?8g/g) the calculated⁴He flux from the earth is equal to 5.7 × 10⁶ at cm^?2 sec^?1. The corresponding³He flux is about 114 at cm^?2 sec^?1. In discussing the aeronomic problem of helium it is necessary to take into account that the earth is the main source of the light helium isotope.     

Excess 3He in oceanic basalts: Evidence for terrestrial primordial helium

Helium trapped in the chilled glass rims of Pacific Ocean basalts is highly enriched in ³He; the ³He/⁴He and ³He/Ne ratios are respectively 10 and 1000 times the atmospheric ratios. We interpret these large enrichments as further evidence that primordial ³He is still present in the interior of the earth. The ³He/⁴He ratio in basalt glass is the same as the isotope ratio of the “excess helium” in Pacific Ocean deep water, supporting the theory that the atmospheric escape rate of ³He is balanced by a flux of primordial ³He from the mantle.     

Excess 3He in deep water on the East Pacific Rise

Helium isotope measurements show that water on the crest and flanks of the East Pacific Rise has the highest enrichment in ³He so far observed in the oceans; the ³He/⁴He ratio anomaly relative to atmospheric helium is + 32% at the mid-depth maximum in the profiles. The corresponding ³He solubility anomaly relative to saturation with atmospheric helium is +50%. These data indicate that active sea-floor spreading sites on the crests of the mid-ocean rises are the sources of primordial helium injected into the ocean from the earth's interior. The ³He/⁴He ratio in this flux is approximately 1.6 × 10^?5, about 11 times the atmospheric ratio of 1.4 × 10^?6. The total flux of ³He into the atmosphere is 4.6 atoms cm^?2 earth-surface sec^?1, most of which (4.0 atoms cm^?2 sec^?1) is supplied by the oceanic flux. The corresponding atmospheric residence time for ³He is 10⁶ years, which, within the large uncertainties of supply and demand (thermal escape), is consistent with the requirement for a steady state.     

Helium-3 and manganese at the 21°N East Pacific Rise hydrothermal site

Water samples collected at the 21°N hydrothermal site on the East Pacific Rise crest, including Deep-Tow and hydrocast samples collected in 1977 and three hot vent water samples collected recently with the submersible “Alvin”, contain significant additions of³He,⁴He, and Mn. Although the vent water collections were at least 50-fold diluted with ambient seawater, they are up to 53 times enriched in³He and 7.4 times enriched in⁴He relative to saturated seawater, with concentrations of total dissolvable manganese (TDM) up to 310 μg/kg.³He and⁴He covary in the vent samples, with³He/⁴He about 8 times the atmospheric ratio, reflecting a mantle helium source. In contrast to the helium isotopes the Mn/³He ratio in the vent samples is variable, ranging from 4.3 × 10⁴ up to 1.0 × 10⁵ g/cm³. Profiles of³He/⁴He and TDM in the water column at 21°N show a sharp maximum ofδ(³He) = 47%and TDM= 0.69 μg/kg, much higher than the average values of 34% and 0.2 μg/kg for the deep water in this region. This spike in³He and Mn occurs at 2400 m depth, 200 m above the level of the 21°N vents, and 100 m higher than any local bathymetry, evidence for upward transport of the hydrothermal discharge via rising plumes of hot vent water. Two of the 21°N Deep-Tow samples associated with small (?0.010°C) temperature anomalies hadδ(³He) = 38%and TDM= 0.28 and 0.58 μg/kg, also slightly elevated relative to background. The Deep-Tow and hydrocast samples have lower Mn/³He ratios than average vent samples due to Mn removal by scavenging. Comparison of vent samples and water column measurements at 21°N indicate that the pure vent water could be detected using³He and Mn even when diluted ～10⁵ times with seawater, confirming that these two tracers are extremely sensitive indicators of submarine hydrothermal activity.     

Helium accumulation in groundwater, III. Limits on helium transfer across the mantle-crust boundary beneath Australia and the magnitude of mantle degassing

The groundwaters of the Great Artesian Basin (Australia) have been previously shown to be accumulating in-situ production helium for groundwaters ages < 50 kyr and an external helium flux equivalent to whole crustal production for groundwater ages > 100 kyr [1,2]. New helium isotope measurements show that the observed in-situ production helium (³He/⁴He 1.6 × 10⁻⁸) is isotopically distinct from the crustal degassing helium flux (³He/⁴He 6.6 × 10⁻⁸). Furthermore, the crustal degassing helium isotope ratio is marginally in excess of the whole crustal production ratio (³He/⁴He= 3.5 × 10⁻⁸) and the production ratio in a variety of continental rock types. This suggests that the upper limit on volatile transport across the mantle-crust boundary beneath the (relatively) stable and “complacent” Australian continent can be characterized by a “conductive-diffusive” helium/heat flux ratio of 2.6 × 10⁶⁴He atoms mW⁻¹ s⁻¹ which is two orders of magnitude less than the “intrusive-volcanic” ratio of 2.9 × 10⁸⁴He atoms mW⁻¹ s⁻¹ measured at the Galapagos [16]. These results constrain the transcrustal mantle degassing fluxes of⁴He and⁴⁰Ar to be much less than the mid-ocean ridge degassing fluxes; which are much less than the degassing of⁴He and⁴⁰Ar from continental crust. Thus, the degassing of the Earth's interior is dominated by magmatic processes but the dominant fluxes of⁴He and⁴⁰Ar to the atmosphere must come from the continental crust.     

Primordial neon,helium, and hydrogen in oceanic basalts

A primordial neon component in neon from Kilauea Volcano and deep-sea tholeiite glass has been identified by the presence of excess²⁰Ne; relative to atmospheric neon the²⁰Ne enrichments are 5.4% in Kilauea neon and about 2.5% in the basalts. The²⁰Ne anomalies are associated with high³He/⁴He ratios; the ratio in Kilauea helium is 15 times the atmospheric ratio, while mid-ocean ridge basalts from the Atlantic, Pacific, and Red Sea have uniform ratios about 10 times atmospheric. Mantle neon and helium are quite different in isotopic composition from crustal gases, which are highly enriched in radiogenic²¹Ne and⁴He. The²¹Ne/⁴He ratios in crustal gases are consistent with calculated values based on G. Wetherill's¹⁸O (α,n) reaction; the lack of²⁰Ne enrichment in these gases shows that the mantle²⁰Ne anomalies are not radiogenic.²¹Ne enrichments in Kilauea neon and “high-³He” Pacific tholeiites are much less than in crustal neon, about 2 ± 2% vs. present atmospheric neon, as expected from the much lower⁴He/Ne ratios.Neon concentrations in two Atlantic tholeiites were found to be only 1–2% of the values obtained by Dymond and Hogan; helium concentrations are slightly greater and our He/Ne ratios are greater by a factor of 150. The large Ne excess relative to solar wind and meteoritic gases is thus not confirmed. Pacific and Atlantic basalts appear to be quite different in He/Ne ratios however, and He and Ne may be inversely correlated. He concentration variations due to diffusive loss can be distinguished from variations due to two-phase partitioning or mantle heterogeneity by the effects on³He/⁴He ratios. The He isotopic and concentration measurements on “low-³He” basalts are consistent with diffusive loss and dilution of the 3/4 ratio by in-situ radiogenic⁴He, and may provide a method for dating basalt glasses.Deuterium/hydrogen ratios in Atlantic and Pacific tholeiite glasses are 77% lower than the ratio in seawater. The inverse correlation between deuterium and water content observed by Friedman in erupting Kilauea basalts is consistent with a Rayleigh separation process in which magmatic water is separated from an initial melt with the same D/H ratio as observed in deep-sea tholeiites. The consistency of the D/H ratios in tholeiites containing primordial He and Ne components indicates that these ratios are probably characteristic of primordial or juvenile hydrogen in the mantle.     

Performance of CRONUS-P – A pyroxene reference material for helium isotope analysis

Helium isotope analyses are central to modern earth science and measured by many noble gas laboratories around the globe (Burnard, 2013; Wieler et al., 2002), spanning a wide spectrum of fundamental research – from identifying primordial reservoirs in the Earth mantle to paleoclimate reconstructions. The CRONUS-Earth initiative included the manufacturing, distribution and analysis of a pyroxene reference material (CRONUS-P) that was designed to be useful for internal reliability control of ³He measurements within a few percent and potentially for ⁴He on a higher level of uncertainty.This short paper describes the CRONUS-P material and its performance as ³He and ⁴He reference sample for noble gas laboratories. The companion paper by Blard et al. 2015 describes in depth the inter-laboratory helium isotope experiment within CRONUS-Earth.We show normalized helium isotope data of CRONUS-P measured at three different noble gas laboratories. Data from all three laboratories show no relation between helium isotope concentrations and sample mass, implying that the material is homogeneous. The data show that CRONUS-P is useful as an internal standard for ³He within better 2% (1σ) and for ⁴He within better 10%.     

An important source ofHe (andHe) in diamonds

A large data base has recently accumulated on the concentrations of helium isotopes in diamonds mined from various regions. It was noted earlier (Ozima et al. (1985) [1]; Lal et al. (1989) [2]) that the frequency distribution of the⁴He concentrations is a fairly narrow one, whereas that of³He concentrations is a broad one with no pronounced peaks. The ratios ³He/⁴He, on the other hand show a broad maximum around 2 R_a (R_a equals atmospheric ³He/⁴He ratio, = 1.40 × 10⁻⁶) with a slow decrease over two orders of magnitude on either side. Does this imply that the diamonds sample a wide variety of helium reservoirs having a range of ³He/⁴He ratios but somehow attain similar⁴He concentrations? We propose that in a majority of the diamonds studied,⁴He is primarily due to implantation of radiogenic alpha particles from the host material after emplacement in the crust, usually kimberlite, and that the concentrations of⁴He in diamonds often get appreciably altered by this process. Thus the⁴He trapped in the diamond at the time of its crystallization is usually overwhelmed by the implanted helium and the measured ³He/⁴He ratios do not generally correspond to any “sources” in the mantle. However, the implanted⁴He resides in the outer 16 μm of the diamond, and the intrinsic⁴He and ³He/⁴He ratios in the diamond can be studied if its outer layers are removed.The wider implications of diamond being the “target” material for nuclear reaction products from the host material are discussed. Radiogenic³He produced in the host material is also implanted in the diamond, but this contribution is small on a gross basis. However, since the depth of implantation of³He is greater than that of⁴He, some of the very high ³He/⁴He ratios observed in diamonds could be due to the “implantation” of radiogenic³He. The radiogenic reactions in the host material can also contribute to appreciable²¹Ne in diamonds.     

Investigations of cosmic-ray-produced nuclides in iron meteorites, 4. Identification of noble gas abundance anomalies

The literature on cosmic-ray-produced nuclides in iron meteorites occasionally reports unusual (“anomalous”) abundance proportions for the associated noble gases. The anomalies are in some cases ascribed to excesses of⁴He, caused by the presence of primordial or radiogenic components; in other cases to abundance deficiencies of³He, caused by partial loss of cosmogenic tritium. The arguments and data used previously for the recognition, identification and determination of anomalies are, however, imperfect or incorrect. New procedures are here proposed. They are based on more reliable data on the abundance patterns of the cosmogenic component and on a novel system of test correlations which describes these patterns. In most cases in which anomalies are recognised, the system allows an unequivocal identification of the nuclide which is the cause of the anomaly. This is a prerequisite for the quantitative determination of excesses and deficiencies. The procedures are applied to evaluate anomalous noble gas data reported in the literature for about 15 samples of various iron meteorites. In some cases, previous identifications of³He deficiencies and of⁴He excesses prove to be correct. However, guesses that⁴He excesses were present in certain specimens from Arispe, Cranbourne, El Taco, Hoba, Pin?on, Pitts and Sandia Mountains are invalidated by the present investigation.⁴He excesses are more exceptional than heretofore believed.     

Geographical Distribution of 3He/4He Ratios in the Chugoku District, Southwestern Japan

We have collected 34 hot spring and mineral spring gases and waters in the Chugoku and Kansai districts, Southwestern Japan and measured the ³He/⁴He and ⁴He/²⁰Ne ratios by using a noble gas mass spectrometer. Observed ³He/⁴He and ⁴He/²⁰Ne ratios range from 0.054 R_atm to 5.04 R_atm (where R_atm is the atmospheric ³He/⁴He ratio of 1.39 × 10⁻⁶) and from 0.25 to 36.8, respectively. They are well explained by a mixing of three components, mantle-derived, radiogenic, and atmospheric helium dissolved in water. The ³He/⁴He ratios corrected for air contamination are low in the frontal arc and high in the volcanic arc regions, which are consistent with data of subduction zones in the literature. The geographical contrast may provide a constraint on the position of the volcanic front in the Chugoku district where it was not well defined by previous works. Taking into account the magma aging effect, we cannot explain the high ³He/⁴He ratios of the volcanic arc region by the slab melting of the subducting Philippine Sea plate. The other source with pristine mantle material may be required. More precisely, the highest and average ³He/⁴He ratios of 5.88 R_atm and 3.8±1.6 R_atm, respectively, in the narrow regions near the volcanic front of the Chugoku district are lower than those in Kyushu and Kinki Spot in Southwestern Japan, but close to those in NE Japan. This suggests that the magma source of the former may be related to the subduction of the Pacific plate, in addition to a slight component of melting of the Philippine Sea slab.     

Helium isotopic variability within single diamonds from the Orapa kimberlite pipe

The distribution and isotopic composition of helium has been measured in a suite of well-characterized one-carat diamonds from the Orapa kimberlite, Botswana. Crushing of the diamonds in vacuo indicates that most of the helium is contained by the matrix (generally greater than 90%), rather than by the inclusions. Step-heating experiments, performed on inclusion-free fragments remaining after crushing, indicate that the³He/⁴He ratio is variablewithin individual diamonds. The fragments, as small as 10 mg, were heated in two timed steps, both at 2000°C. In every case, lower³He/⁴He ratios are observed in the first graphitization step (0.05–3 × atmospheric), while the last heating step releases helium with systematically higher³He/⁴He ratio (30–80 × atmospheric). We suggest that this internal isotopic variability is the result of stepwise graphitization: the first heating step initiates graphitization, which nucleates around defects, and the second heating step graphitizes the relatively defect-free regions of the diamond. The³He/⁴He ratio measured, using the partial graphitization technique, differs by up to a factor of 100 within a single specimen. The inclusion-free fragments release small quantities of helium below 2000°C, which suggests that helium release is obtained only by graphitization. The³He contents of the monocrystalline diamonds are relatively constant (at 3 × 10⁻¹³ cm³ STP/gram) and indicate that most of the isotopic variability is due to radiogenic⁴He. The variations in⁴He content are either related to zoning of Th and U in the diamonds (i.e., in-situ decay), to zoning of inherited⁴He, or to implantation of α-particles from a Th and U rich environment (i.e., kimberlite). Because the Orapa diamonds were mined from roughly 40 m depth in the kimberlite, spallation reactions from cosmic ray interactions are not a significant source of³He. However, calculations based on the age of the kimberlite (90 m.y.) and reasonable Th and U abundances suggest that most of the³He in the Orapa diamonds could be produced by⁶Li(n, α)T in the diamond. Although this may not be true of all diamonds, nuclear reactions in the crust and mantle (including spallation reactions at the surface) can explain many of the high³He/⁴He ratios previously reported for diamonds.     

Helium isotopes and mantle volatiles in Loihi Seamount and Hawaiian Island basalts and xenoliths

We examine in this paper the use of helium isotope ratios for the study of hotspot volcanism along age-progressive island volcanic chains. The Hawaiian Islands are the original “high ³He” hotspot, with ³He/⁴He ratios as high as 32 × the atmospheric ratio; in the Pacific they stand out against the surrounding sea of MORB (rather uniformly 8 × atmospheric) which fills the entire Pacific with the exception of the Macdonald-Mehetia-Samoa axis in the South Pacific. The recent availability of a variety of alkalic and tholeiitic glasses from the U.S. Geological Survey and our own dredge hauls has prompted us to look first at isotopic variability within a single fresh and new volcano which is probably sitting directly atop a mantle plume. Thus we have looked in some detail at the total helium in glass pillow rims, at He in the enclosed vesicles, and at He in the glass itself, in both tholeiitic and alkalic lavas, and also at helium in associated phenocrysts and xenoliths. The measured ³He/⁴He ratios range from atmospheric to 30 × atmospheric, but we see clear evidence that the highly vesiculated lavas suffer exchange of He between the thin glass walls of vesicles and ambient seawater, so that we observe a post-eruptive isotopic disequilibrium between glass and gas phases. The primary effect is the very large loss of initial He content during eruptive vesiculation, which results in quite large isotopic effects from small additions of ambient He (of the order of 0.02 μcc He per gram of basalt; corresponding to a “water/rock ratio” of 0.5). Phenocrystic He in olivines verifies that the gas-phase He is not affected by vesicularities up to about 5%. Alkali basalt He appears to be independent of vesicularity up to values as high as 35%; this He is somewhat lower in ³He/⁴He ratio, but matches precisely the associated xenolithic He. However, from the present data we cannot exclude the possibility that diffusive exchange with seawater has affected the He ratio in alkalic vesicles.On the large scale, along the 10% of the Hawaiian chain available for subaerial sampling, we find high ³He/⁴He ratios (24 × atmospheric) in 5.5 × 10⁶-year-old lavas on Kauai. Maximum values of the ratio so far observed are in the pre-erosional Kula basalts on Maui, confirming the previous results of Kaneoka and Takaoka. Hawaii, where these high values were first observed is now seen to range from MORB ratios at Mauna Loa to only 15 × R_A at Kilauea fumaroles. Most xenolithic He so far measured is MORB He, but Loihi xenoliths have high values and are quite different in this respect. Finally, we discuss also the hydrogen and carbon isotope results on Loihi lavas, and show that these elements resemble MORB and appear not to show a distinctive plume signature.     

Helium isotopic variations in volcanic rocks from Loihi Seamount and the Island of Hawaii

Helium isotopic ratios ranging from 20 to 32 times the atmospheric ³He/⁴He(R_A) have been observed in a suite of 15 basaltic glasses from the Loihi Seamount. These ratios, which are up to four times higher than those of MORB glasses and more than twice those of nearby Kilauea, are strongly suggestive of a primitive source of volatiles supplying this volcanism. The Loihi glasses measured span a broad compositional range, and the ³He/⁴He ratios were found to be generally lower for the alkali basalts than for the tholeiites. The component with a lower ³He/⁴He ratio appears to be associated with olivine xenocrysts, within which fluid inclusions are probably the carrier of contaminant helium. One Loihi sample has a much lower isotopic ratio (<5 R_A), but a combination of low He concentration, high vesicularity, and presence of cracks lined with clay minerals suggests that the low ratio is due to gas loss and contamination by atmospheric helium.Crushing and melting experiments show that for modest vesicularities (<5% by volume) the Loihi glasses obey a MORB-type partitioning trend, but at higher vesicularities the data show considerably more scatter due to volatile mobilization. The high vesicularities, low extrusion pressure and generally low helium concentrations are consistent with a considerable degree of degassing. Analyses of dunites, plus a correlation between total helium concentrations with xenocryst abundances also suggest that xenocrysts are a significant carrier of contaminating (low ³He/⁴He) helium.³He/⁴He ratios from samples of other Hawaiian volcanoes (Kilauea, Mauna Loa, Hualalai, and Mauna Kea) show a smooth decrease in ³He/⁴He with increasing volcano age and volume. We interpret this to be a synoptic picture of the time evolution of a hot-spot diapir: the earliest stage is characterized by primitive (> 30 R_A) helium with some (variable) component of lithospheric contamination added during “breakthrough”, while the later stages are characterized by a relaxation toward lithospheric ³He/⁴He ratios (～ 8 R_A) due to isolation of the diapir from the mantle below (as the plate moves on), and subsequent mining of the inherited helium and contamination from the surrounding lithosphere. The abrupt contrast in ³He/⁴He ratios between Kilauea and Loihi, despite their close proximity, is indicative of the small lateral extent of the plume.     

Factors controlling the noble gas abundance patterns of deep-sea basalts

A number of processes may modify the noble gas composition of silicate liquids so that the composition of noble gases observed in glassy margins of deep-sea basalts is not that of the upper mantle. Differential solubility enhances the light noble gases relative to the heavier gases; however, we demonstrate that the observed abundance pattern cannot be attributed to solubility of noble gases with atmospheric proportions. Partial melting and fractional crystallization increase the noble gas content of all species relative to mantle concentrations, but do not fractionate their relative abundances. Noble gases may be lost from an ascending magma in various ways, the most important, however, may be exclusion of gas from crystals forming at the time of solidification, which is shown to result in marked loss of gas from the basalt. Small amounts of low-temperature alteration of solidified basalt can produce dramatic changes in the noble gas abundance pattern, since the adsorption coefficients for the different noble gas favor uptake of heavy species relative to the light species. Atmospheric contamination can account for observed variations in the ⁴⁰Ar/³⁶Ar ratio of oceanic basalts. The degree of crystallinity of glassy margins of deep-sea basalts may control the helium abundance of these samples; however, the uniform ³He/⁴He values reported apparently reflect a relatively constant proportion of radiogenic and primordial helium in the mantle.     

The distribution of helium in oceanic basalt glasses

We have determined the concentrations and isotopic composition of helium in oceanic basaltic glass both by melting and by crushing in vacuo. A significant fraction of the helium is released by crushing, confirming that it resides within the vesicles. Comparison of volume percent vesicles to the fraction of helium contained in the vesicles gives qualitative agreement with experimental gas-melt partitioning studies. Measured concentrations are therefore, a function of original helium content, magmatic history, vesicle size and quantity, and grain size analyzed. Helium released by crushing is isotopically indistinguishable from that contained in the glass. Diffusion rates for helium in basaltic glass (in the temperature range 125–400°C) determined using the method of stepwise heating, yielded an activation energy of 19.9 ± 1 kcal/mole andlnD₀ = ?2.7 ± 0.6 (cgs units). Extrapolation of these results to ocean floor temperatures (0°C) gives a diffusivity of 1.0 ± 0.6 × 10^?17 cm²/s, indicating that diffusion is an insignificant mechanism for helium loss from fresh basaltic glasses. The³He/⁴He ratios are remarkably constant (at 1.10 ± 0.03 × 10^?5) for samples from the Mid-Atlantic Ridge (FAMOUS area and 23°N), the Juan de Fuca Ridge, the Galapagos spreading center, the Mid-Cayman Rise, and the Central Indian Ocean Ridge. This result is interpreted in terms of similar geochemical histories within the source regions for these samples.     

Variation in noble gas isotopic composition of gas samples from the Aegean arc, Greece

In contrast to most other arcs with oceanic plate subduction, the Aegean arc is characterized by continent–continent subduction. Noble gas abundances and isotopic compositions of 45 gas samples have been determined from 6 volcanoes along the arc, 2 islands in the back-arc region and 7 sites in the surrounding areas. The ³He/⁴He ratios of the samples ranged from 0.027R_A to 6.2R_A (R_A denotes the atmospheric ³He/⁴He ratio of 1.4×10⁻⁶), demonstrating that even the maximum ³He/⁴He ratio in the region is significantly lower than the maximum ratios of most oceanic subduction systems, which are equal to the MORB value of 8±1 R_A. Regional variations in the ³He/⁴He ratio were observed both along and across the arc. The maximum ³He/⁴He ratio was obtained from Nisyros volcano located in the eastern end of the arc, and the ratio decreased westward possibly reflecting the difference in potential degree of crustal assimilation or the present magmatic activity in each volcano. Across the volcanic arc, the ³He/⁴He ratio decreased with an increasing distance from the arc front, reaching a low ratio of 0.063R_A in Macedonia, which suggested a major contribution of radiogenic helium derived from the continental crust. At Nisyros, a temporal increase in ³He/⁴He ratio due to ascending subsurface magma was observed after the seismic crisis of 1995–1998 and mantle neon was possibly detected. The maximum ³He/⁴He ratio (6.2R_A) in the Aegean region, which is significantly lower than the MORB value, is not probably due to crustal assimilation at shallow depth or addition of slab-derived helium to MORB-like mantle wedge, but inherent characteristics of the subcontinental lithospheric mantle (SCLM) beneath the Aegean arc.     

Planetary-type rare gases in an upper mantle-derived amphibole

All twenty-three stable rare gas isotopes have been measured in a mantle-derived amphibole, kaersutite. The elemental abundance pattern of the rare gases is similar to the “planetary” rare gas pattern as defined by carbonaceous chondrites. The³He/⁴He ratio, (4.9 ± 0.6) × 10^?5, is suggestive of primordial He degassing from the mantle. Excess²¹Ne is present. The measured⁴⁰Ar/³⁶Ar ratio,400 ± 5, may represent a mantle⁴⁰Ar/³⁶Ar ratio <240 when corrected for radiogenic⁴⁰Ar. The heavy isotopes of Kr and t0he Xe isotopes are within error of the atmosphere values.     

Geochemistry on mantle-derived volatiles in natural gases from eastern China oil/gas provinces (I)

Commercial accumulation of mantle-derived helium in the sedimentary shell is discussed. Generally speaking, a commercial helium pool is formed by accumulated⁴He that comes from uranium and thorium via α-decay; therefore, it has a very low³He/⁴He value in the magnitude of 10-⁸. The helium concentration in some gas wells of eastern China oil/gas provinces is about or over 0.05% —0. 1%, consequently forming commercial helium wells (pools), such as the Wangjinta Gas Pool in Songliao Basin, Huangqiao Gas Pool in North Jiangsu Basin and some gas wells in Sanshui Basin. Studies have proved that when the³He/⁴He value of a helium gas pool is about 3.7 × 10^-6 -7.2×10^-6 namely mantle-derived helium in its total helium concentration accounts for 33.5%—65.4%, it is a crust-mantle dual-source or dominantly mantle-derived helium gas pool, which is a novel helium resource and its formation is mainly related to the distribution of megafractures.     

He, Ne and Ar isotope compositions of fluid inclusions in hydrothermal sulfides from the TAG hydrothermal field Mid-Atlantic Ridge

Helium, neon and argon isotope compositions of fluid inclusions have been measured in hydrothermal sulfide samples from the TAG hydrothermal field at the Mid-Atlantic Ridge. Fluid-inclusion³He/⁴He ratios are 2.2—13.3 times the air value (Ra), and with a mean of 7.2 Ra. Comparison with the local vent fluids (³He/⁴He=7.5—8.2 Ra) and mid-ocean ridge basalt values (³He/⁴He=6—11 Ra) shows that the variation range of³He/⁴He ratios from sulfide-hosted fluid inclusions is significantly large. Values for²⁰Ne/²²Ne are from 10.2 to 11.4, which are significantly higher than the atmospheric ratio (9.8). And fluid-inclusion⁴⁰Ar/³⁶Ar ratios range from 287 to 359, which are close to the atmospheric values (295.5). These results indicate that the noble gases of fluid inclusions in hydrothermal sulfides are a mixture of mantle- and seawater-derived noble gases; the partial mantle-derived components of trapped hydrothermal fluids may be from the lower mantle; the helium of fluid inclusions is mainly from upper mantle; and the Ne and Ar components are mainly from seawater.     

Mantle-derived volatiles in continental crust: the Massif Central of France

CO₂-rich gases and groundwaters from springs and boreholes originating within the basement of the Massif Central have variable³He/⁴He ratios with correspondingR/R_a values ranging from 0.8 to 5.5 and 0.3 to 2.8 respectively, indicating the presence of a significant component of mantle helium. Molar concentrations of rare gases in the CO₂-rich gases are approximately 5 orders of magnitude greater than in the waters and suggest that a near-surface Henry's Law fractionation has occurred between exsolving CO₂ and water.δ¹³C values of the CO₂-rich gases are in the range −4.2 to −6.1‰, i.e. in that range normally attributed to mantle carbon, but which could also represent an average crustal composition and therefore do not discriminate between mantle and crustal sources.C/³He ratios show 4 orders of magnitude variation from 1.4 × 10¹² to 5 × 10⁸ and, compared to a mantleC/³He ratio of 10⁹, indicate that either a complex fractionation has occurred between mantle helium and mantle CO₂ or more likely that mantle rare gases have been diluted by large quantities of CO₂ with an average crustal carbon isotope composition. The regional distribution of³He and C does not show any obvious relationship to age or proximity of volcanic centres or major faults, suggesting that mantle-derived C and He components decoupled from their silicate melt sources at some depth.The results from this area of active fluid circulation suggest that C-isotope data derived from metamorphic terrains should be interpreted with great caution, but that input of some mantle-derived carbon is expected to accompany crustal extension.