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1.
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 Ra (Ra 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.  相似文献   

2.
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.  相似文献   

3.
Gas concentrations and isotopic compositions of water have been measured in hydrothermal waters from 13°N on the East Pacific Rise. In the most Mg-depleted samples ( 5 × 10−3 moles/kg) the gas concentrations are: 3–4.5 × 10−5 cm3 STP/kg helium, 0.62–1.24 cm3 STP/kg CH4, 10.80–16.71 × 10−3 moles/kg CO2. The samples contain large quantities (95–126 cm3/kg) of H2 and some carbon monoxide (0.26–0.36 cm3/kg) which result from reaction with the titanium sampling bottles. δ13C in methane and CO2 (−16.6 to −19.5 and −4.1 to −5.5 respectively) indicate temperatures between 475 and 550°C, whereas δ13CCO is compatible with formation by reduction of CO2 on Ti at 350°C close to the sampling temperature.3He/4He are very homogeneous at (7.5 ± 0.1)RA(3He/4He = 1.0 × 10−5) and very similar to already published data as well as CH4/3He ratios between 1.4 and 2.1 × 106.18O and D in water show enrichments from 0.39 to 0.69‰ and from 0.62 to 1.49‰ respectively. These values correspond to W/R ratios of 0.4–7. The distinct18O enrichments indicate that the isotopic composition of the oceans is not completely buffered by the hydrothermal circulations. The3He-enthalpy relationship is discussed in terms of both hydrothermal heat flux and3He mantle flux.  相似文献   

4.
We have undertaken helium, neon and argon step-heating, isotopic analyses of eleven polycrystalline diamonds of known peridotite/eclogite paragenesis from the Jwaneng kimberlite pipe, Botswana. In contrast to the findings of crustal noble gases in framesites from the same kimberlite pipe (Honda et al., 2004. Unusual noble gas compositions in polycrystalline diamonds: preliminary results from the Jwaneng kimberlite, Botswana. Chem. Geol. 203, 347–358.), the Jwaneng polycrystalline diamonds appear to contain similar noble gas isotopic compositions (particularly Ne) to those representing a mantle source for MORBs. This implies that the Jwaneng polycrystalline diamonds may have formed in recent times, possibly close to the time of kimberlite emplacement at ~ 235 Ma. In contrast, Jwaneng framesites could be as old as gem diamonds (mineral inclusion ages of ~ 2.9 Ga). Furthermore, the data indicate that the sub-continental mantle lithosphere in the region has heterogeneous Ne isotopic compositions, or that these compositions changed over time from crustal Ne (as observed in the framesites) to MORB-like (as observed in the polycrystalline diamonds).  相似文献   

5.
In an attempt to constrain the origin of polycrystalline diamond, combined analyses of rare gases and carbon and nitrogen isotopes were performed on six such diamonds from Orapa (Botswana). Helium shows radiogenic isotopic ratios of R/Ra = 0.14–1.29, while the neon ratios (21Ne/22Ne of up to 0.0534) reflect a component from mantle, nucleogenic and atmospheric sources. 40Ar/36Ar ratios of between 477 and 6056 are consistent with this interpretation. The (129Xe/130Xe) isotopic ratios range between 6.54 and 6.91 and the lower values indicate an atmospheric component. The He, Ne, Ar and Xe isotopic compositions and the Xe isotopic pattern are clear evidence for a mantle component rather than a crustal one in the source of the polycrystalline diamonds from Orapa. The δ13C and δ15N isotopic values of − 1.04 to − 9.79‰ and + 4.5 to + 15.5‰ respectively, lie within the range of values obtained from the monocrystalline diamonds at that mine. Additionally, this work reveals that polycrystalline diamonds may not be the most appropriate samples to study if the aim is to consider the compositional evolution of rare gases through time. Our data shows that after crystallization, the polycrystalline diamonds undergo both gas loss (that is more significant for the lighter rare gases such as He and Ne) and secondary processes (such as radiogenic, nucleogenic and fissiogenic, as well as atmospheric contamination). Finally, if polycrystalline diamonds sampled an old mantle (1–3.2 Ga), the determined Xe isotopic signatures, which are similar to present MORB mantle – no fissiogenic Xe from fission of 238U being detectable – imply either that Xe isotopic ratios have not evolved within the convective mantle since diamond crystallization, or that these diamonds are actually much younger.  相似文献   

6.
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 0.027RA to 6.2RA (RA 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 RA. 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 0.063RA 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 (6.2RA) 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.  相似文献   

7.
The concentrations of helium and carbon in fluorite associated with Cretaceous to Neogene (90–13 Ma) granitic magmatism in the Japanese arc have been measured. Concentrations of Li, U, Th and Gd were measured to correct for secondary generated 3He. The CO2/3He of fluorites are almost uniform (1.5×1010–4×1010) and in fair agreement with the range of present island arc volcanic gases. The calculated mantle C contribution in the Mesozoic subduction zone appear to have been identical to the present one (7–19%) indicating that the C flux from the mantle in supra-subduction zone environments has remained fairly constant during the past 70 million years.  相似文献   

8.
The internal structure and growth history of diamonds have been investigated by the study of carbon isotopes and the abundance, isotope composition and aggregation state of nitrogen. Polished plates of zoned diamonds (“coated stones”) were laser sectioned for infrared mapping and isotope profiling. Nine small cubic and three small octahedral diamonds from Zaïre and two coated stones and five small cubic diamonds specifically from the Mbuji Mayi area of that country were also investigated. The isotope composition and infrared characteristics of the diamonds of cubic habit, including coats, were found to be indistinguishable with δ13C in the range −5.9 to −7.5‰ and all the nitrogen being present as the paired structure (IaA) with a δ15N of between −3.1 and −8.0‰. The diamonds of octahedral habit, including the cores of the coated stones, were more variable: δ13C varied from −4.9 to −9.8‰. Infrared spectra of the octahedral diamond exhibited either strong IaA characteristics or a combination of IaA and IaB. Platelets were present in most cases. δ15N values from octahedral diamonds varied from −5.8 to +13.4‰. The data suggest that the octahedral diamonds represent a variety of growth events separated in time and/or space and that the diamonds resided in the lithosphere for a period sufficient for thermal maturation. In the case of the coated stones the octahedral diamonds were later involved in an event which added a component from a single relatively homogeneous reservoir; cubic habit diamonds were formed at the same time. This latter event is believed to be related to the kimberlite eruption. Comparison of the data with carbon isotope analyses of cubic diamond from other sources suggests that this reservoir could be widespread.  相似文献   

9.
Helium isotopic ratios ranging from 20 to 32 times the atmospheric 3He/4He(RA) 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 RA), 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 RA) 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 RA) 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.  相似文献   

10.
High 4He/3He ratios of 100 000 to 160 000 found at HIMU ocean islands (“high μ,” where μ is the U/Pb ratio) are usually attributed to the presence of recycled oceanic crust in the HIMU mantle source. However, significantly higher 4He/3He ratios are expected in recycled crust after residence in the mantle for periods greater than 1 Ga. In order to better understand the helium isotopic signatures in HIMU basalts, we have measured helium and neon isotopic compositions in a suite of geochemically well-characterized basalts from the Cook–Austral Islands. We observe 4He/3He ratios ranging from 56 000 to 141 000, suggesting the involvement of mantle reservoirs both more and less radiogenic than the mantle source for mid-ocean ridge basalts (MORBs). In addition, we find that the neon isotopic compositions of HIMU lavas extend from the MORB range to compositions less nucleogenic than MORBs. The Cook-Austral HIMU He–Ne isotopic compositions, along with Sr, Nd, Pb, and Os isotopic compositions, indicate that in addition to recycled crust, a relatively undegassed mantle end-member (e.g., FOZO) is involved in the genesis of these basalts. The association of relatively undegassed mantle material with recycled crust provides an explanation for the close geographical association between HIMU lavas and EM (enriched mantle)-type lavas from this island chain: EM-type signatures represent a higher mixing proportion of relatively undegassed mantle material. Mixing between recycled material and relatively undegassed mantle material may be a natural result of entrainment processes and convective stirring in deep mantle.  相似文献   

11.
The concentration and isotopic composition of nitrogen, measured in large diamonds (gram size) from the Mbuji Mayi kimberlite district (Zaïre) show a large range of variation (100<N<2100 ppm, ?11.2<δ15N< +6.0). The15N-depleted nitrogen is associated with the higher nitrogen concentrations. The large diamonds are individually rather homogeneous in13C (range ofδ13C < 0.9‰) while variations occur within small octahedral diamonds from the same district (range up to 5.8‰). The total range ofδ13C variation is about the same for the large diamonds (?10.5 <δ13C < ?5.5), the small octahedral diamonds (?10 <δ13C < ?4.6), and the carbonates from local kimberlites (?11.8 < δ13C < ?5.5). The diamond carbon isotopic data could indicate a complex story of crystallization within a deep-seated system fractionating its carbon isotopes.The nitrogen results indicate that nitrogen in diamonds is, on the average, markedly depleted in15N (weighted average ?5.15‰) relative to atmosphere, sediments and upper mantle.  相似文献   

12.
Mantle-derived volatiles in continental crust: the Massif Central of France   总被引：1，自引：0，他引：1  
CO2-rich gases and groundwaters from springs and boreholes originating within the basement of the Massif Central have variable3He/4He ratios with correspondingR/Ra 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.  相似文献   

13.
Geothermal gases from submarine and subaerial hot springs in Ensenada, Baja California Norte, Mexico, were sampled for determination of gas chemistry and helium, nitrogen and stable carbon isotope composition. The submarine hot spring gas is primarily nitrogen (56.1% by volume) and methane (43.5% by volume), whereas nearby subaerial hot spring gases are predominantly nitrogen (95–99% by volume). The N2/Ar ratios and σ 15N values of the subaerial hot spring gas indicate that it is atmospheric air, depleted in oxygen and enriched in helium. The submarine hot spring gas is most probably derived from marine sediments of Cretaceous age rich in organic matter. CH4 is a major component of the gas mixture (σ 13C = −44.05%0), with only minor amounts of CO2 (σ13C= −10.46%0). The σ15N of N2 is + 0.2%0 with a very high N2/Ar ratio of 160. The calculated isotopic equilibra tion temperature for CH4---CO2 carbon exchange at depth in the Punta Banda submarine geothermal field is approximately 200°C in agreement with other geothermometry estimates. The 3He/4He ratios of the hot spring gases range from 0.3 to 0.6 times the atmospheric ratio, indicating that helium is predominantly derived from the radioactive decay of U and Th within the continental crust. Thus, not all submarine hydrothermal systems are effective vehicles for mantle degassing of primordial helium.  相似文献   

14.
The ophiolites from the Yarlung Zangbo River (Tibet),Southwestern China,were analysed for the con-tents of helium and neon and their isotopic compositions by stepwise heating. The serpentinites from Bainang showed a high 3He/4He value of 32.66Ra (Ra is referred to the 3He/4He ratio in the present air) in 700 ℃ fraction. At lower temperature,all of the dolerites displayed as very high 3He/4He ratios as ones investigated for hotspots. It was clear that the high 3He/4He ratio was one of immanent characterics in the magma source formed the dolerites,suggesting that there was a large amount of deep mantle fluids in these rocks. In the three-isotope diagram of neon,the data points from the ophiolites of the Yarlung Zangbo River were arranged along the Loihi Line. This is in agreement with the characteristics of he-lium isotopes,revealing that the high-3He plume from deep mantle had played an important role in the formation of the Neo-Tethyan Ocean. The helium isotopic compositions in the basalts were far higher than atomospheric value but lower than the average value of MORB,although there were various de-grees of alteration. The possible reasons were that basaltic magmas  相似文献   

15.
3He/4He ratios in lavas erupted during the last 360 years at Mt. Vesuvius are between 2.2 and 2.7 RA (RA = atmospheric ratio of 1.39 × 10−6), and are among the lowest values measured in young volcanic rocks. They are also identical to values measured in summit crater fumaroles sampled during 1987–1991. This agreement indicates that the 3He/4He ratio in the crater fumaroles faithfully tracks the magmatic value. The relatively low and uniform 3He/4He ratio in the lavas reflects either a mantle source enriched in (U + Th)/3He, or a mixture of magmatic and crustal components.  相似文献   

16.
Cores and coats of five coated diamonds, one from Botswana and four from Zaire, were separately analyzed for their noble gases. Noble gases in the diamonds are essentially of a trapped origin, including radio- and nucleogenic components such as4He, 40Ar, 21Neexcess and excesses in Xe isotopes (129, 131–136). The fairly precise elemental and isotopic abundances allow us to infer the noble gas state in the ancient mantle. 20Ne/22Ne ratios are fairly constant (11.8 ± 0.4), and very close to that of SEP (solar energetic particle)-Ne, but distinctly different from the atmospheric ratio. 21Ne/22Ne ratios range from 0.028 to 0.06, which is attributed to nucleogenic 21Ne from 18O(α, n)21Ne and 24Mg(n, α)21Ne reactions. The difference in 20Ne/22Ne between atmosphere and mantle can be attributed to the hydrodynamic escape of hydrogen from the primitive atmosphere during the very early stage in the Earth's history. 38Ar/36Ar and Kr isotopic ratios are identical to the atmospheric values within 1%. After correction for 238U- or 244Pu-fission Xe, the 131–136Xe abundance ratios are indistinguishable from atmospheric ratios. Lighter Xe isotopes (124–128Xe) are also likely to be atmospheric, but a final conclusion must wait until better data are obtained.In a 136Xe/130Xe−129Xe/130Xe diagram, diamond data lie on the same line as defined for MORB. The observed identical correlation for both diamonds and MORB's appears to suggest that the progenitor of the excess131–136Xe is 244Pu, but not238U, though the direct Xe isotopic measurements was not precies enough to decide unanimously the progenitor.  相似文献   

17.
Measurements are reported of K, Rb and Sr levels and Sr isotopic compositions in silicate inclusions in diamonds, and of U, Th and Pb levels and Pb isotopic compositions in sulphide inclusions in diamonds. Samples are from the Premier Mine (Transvaal), the Finsch Mine (northern Cape Province) and the Kimberley mines (northern Cape Province). The isotopic composition of Pb in sulphide inclusions indicates that the diamonds containing these inclusions are xenocrysts in the host kimberlite. Model Pb ages for the sulphide inclusions in the diamonds from Finsch and Kimberley are in excess of 2 b.y., although the host kimberlites are Cretaceous. The sulphide inclusions from Premier, however, have a model age similar to the emplacement age of the Premier kimberlites (approximately 1.2 b.y.).In addition, K, Rb, Sr, U and Pb concentrations and Sr and Pb isotopic compositions in clinopyroxenes from 14 mantle-derived xenoliths were measured. Samples were eclogites from the Roberts Victor Mine (Orange Free State), peridotites from the Matsoku Pipe (Lesotho) and diopside megacrysts from the four mines in Kimberley. The samples from the Roberts Victor Mine define a large spread in Sr and Pb isotopic composition. The slope of a regression line through the Pb data in the 207/204 vs. 206/204 plot defines an age of roughly 2.5 b.y. The Matsoku samples have87Sr/86Sr ratios around 0.704. One group of samples (“fertile peridotites”) shows uniform, anomalously radiogenic Pb isotopic compositions, and bears evidence of a young event involving isotopic homogenisation. Lead isotopic heterogeneity, indicating an old age, has persisted in another group (“common peridotites”). The megacrysts from the four Kimberley mines have87Sr/86Sr ratios around 0.704 and uniform, anomalously radiogenic Pb isotopic compositions, indicating a young age. Their Pb isotopic uniformity contrasts with the spread shown by kimberlites in Kimberley, and shows that the megacrysts did not crystallise in equilibrium with their host kimberlites.The similarity between the Pb isotopic data obtained on clinopyroxenes from xenoliths and the data obtained on sulphide inclusions in diamonds from Finsch and Kimberley suggests that these diamonds might be cogenetic with old rock provinces in the subcontinental mantle.  相似文献   

18.
Primordial neon,helium, and hydrogen in oceanic basalts   总被引：3，自引：0，他引：3  
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.  相似文献   

19.
We have collected 14 water and gas samples from 9 thermal springs and gas vents near Nevado del Ruiz volcano, Colombia. The 3He/4He and 4He/20Ne ratios vary significantly from 0.98 Ratm (where Ratm is the atmospheric 3He/4He ratio of 1.4 × 10−6) to 6.30 Ratm, and from 0.37 to 7.0, respectively. The 3He/4He ratio (corrected for air contamination) decreases with increasing distance from the central crater of the volcano to the sampling site. The trend is very similar to that observed at Ontake volcano, Japan. A hydrodynamic porous-media dispersion model can explain the 3He/4He trend. The temporal variations in the 3He/4He ratio at four sites provide useful information on the apparent velocity of the magmatic fluid flow brought on by a volcanic eruption. The estimated value of several tens m day−1 agrees well with the inferred velocity of flow in Oshima volcano, Japan and is comparable to the largest rate of groundwater movement in a deep sedimentary basin.  相似文献   

20.
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.  相似文献   

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