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1.
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 × RA 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.  相似文献   

2.
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.  相似文献   

3.
The purpose of this work is to refine our knowledge about the nature of helium with a high abundance of the rare isotope3He(3He/4He= 10?5) discovered in terrestrial volcanic gases in 1968.We will discuss here the results of isotope analyses of helium released by step-wise heating of ultrabasic xenoliths and some volcanic rocks. On the basis of these results, possible sources of3He in the earth due to fission and nuclear reactions are considered critically. The most probable source of the high abundance of3He is shown to be due to the capture and trapping of primordial He by the earth during its formation (primordial helium3He/4He= 3 × 10?4), a small but significant fraction of which has been retained to the present time.  相似文献   

4.
3He/4He ratios in dissolved helium at GEOSECS stations 115, 117, 30, and 120 provide an east-west section across the Mid-Atlantic Ridge at 30°N. Below 1500 m depth, the3He/4He profiles show little structure and have values within a few percent of the atmospheric ratio, indicating that the Mid-Atlantic Ridge is not a significant source of injected3He at this latitude. Mean3He/4He ratios calculated for the deep water at each station show that the3He/4He ratio in the western Atlantic at 30°N is 2–3% higher than in the eastern basin, probably due to mixing between a3He-rich boundary current in the western basin and low-3He deep water to the east.  相似文献   

5.
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.  相似文献   

6.
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.  相似文献   

7.
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.  相似文献   

8.
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   

9.
Fifteen submarine glasses from the East Pacific Rise (CYAMEX), the Kyushu-Palau Ridge (DSDP Leg 59) and the Nauru Basin (DSDP Leg 61) were analysed for noble gas contents and isotopic ratios. Both the East Pacific Rise and Kyushu-Palau Ridge samples showed Ne excess relative to Ar and a monotonic decrease from Xe to Ar when compared with air noble gas abundance. This characteristic noble gas abundance pattern (type 2, classified by Ozima and Alexander) is interpreted to be due to a two-stage degassing from a noble gas reservoir with originally atmospheric abundance. In the Kyushu-Palau Ridge sample, noble gases are nearly ten times more abundant than in the East Pacific Rise samples. This may be attributed to an oceanic crust contamination in the former mantle source.There is no correlation between the He content and that of the other noble gas in the CYAMEX samples. This suggests that He was derived from a larger region, independent from the other noble gases.Except where radiogenic isotopes are involved, all other noble gas isotopic ratios were indistinguishable from air noble gas isotopic ratios. The3He/4He in the East Pacific Rise shows a remarkably uniform ratio of (1.21±0.07)×10?5, while the40Ar/36Ar ranges from 700 to 5600.  相似文献   

10.
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   

11.
The Marangudzi ring complex, Rhodesia, consists essentially of a gabbro mass intruded by ring dykes of quartz syenite and cone sheets of nepheline syenite. Five intrusive units (gabbro, two quartz syenite and two nepheline syenite units) have been studied using Rb-Sr and K-Ar methods. A total of 24 whole rock samples define a Rb-Sr isochron which gives an age of 186 ± 3m.y. and an initial (87Sr/86Sr)0 ratio of 0.70769 ± 0.00006 (±2sigma; based on λ = 1.42 × 10?11yr?1). K-Ar and Rb-Sr analyses on biotite and hastingsite separates are consistent with this age assignment. Whole rock Rb-Sr isochrons for the different units treated individually agree with the above age and initial Sr ratio within analytical uncertainties. This is believed to indicate that the different rock types are comagmatic forming by fractional crystallization of a parental, mantle-derived, K2O-rich basaltic magma, having an initial Sr ratio of 0.7077, without appreciable assimilation of the Precambrian country rock. The entire differentiation, emplacement and crystallization processes took place over a rather short time span.  相似文献   

12.
We have analysed volatiles (H2O, He, Ar, CO2) in differentiated (basaltic andesite, dacite) volcanic glasses dredged at a depth of ca. 2000 m in the eastern part of the Manus Basin between 151°20′ and 152°10′ E. These samples have Sr–O–B isotopic ratios that show that they most likely represent lavas evolved from a common magma source. Since these glasses are very fresh, they provide a unique opportunity to study the behaviour of magmatic volatiles during assimilation–fractional crystallisation–degassing (AFCD). The samples are highly vesicular (up to 18%) and the volatiles trapped in vesicles consist predominantly of H2O with minor amounts of CO2, and the concentration of water in the glasses indicates that H2O saturation was attained. Rare gases except helium are atmospheric in origin, and the 3He/4He ratios and the CO2/3He ratios are respectively lower and higher than those typical of Mid-Ocean Ridge Basalt (MORB), and appear to correlate with the degree of differentiation. AFCD allows efficient degassing of mantle-derived volatiles and contribution of crust-derived and atmosphere-derived volatiles. Given the widespread occurrence of differentiated magmatism at arcs, we suggest that AFCD is responsible for large-scale occurrence of 3He-rich crustal fluids and of atmospheric-like rare gases in arc emanations, and that most of the volatiles are lost continuously during fractional crystallisation, rather than catastrophically during eruptions.  相似文献   

13.
Vesicle characteristics (vesicularity, largest vesicle size, number of vesicles/cm2), CO2-δ13C and CO2-4He-40Ar-40Ar/36Ar in vesicles and CO2-δ13C in the glass have been measured in 19 tholeiitic basalt glasses from the Easter Microplate East Ridge (East Pacific Rise) collected at 3 different sites (26°S East Ridge, Pito Seamount and Pito Deep at 23°S).Carbon supersaturation values (Cmelt/Csolubility) vary from 1.3 to 4.3. Carbon supersaturation values are strongly correlated with the number of vesicles/cm2. There is also a correlation between number of vesicles/cm2 and vesicle size. At the Pito Seamount site, there is a negative correlation between carbon supersaturation values and observed carbon isotope fractionation between CO2 in vesicles and carbon dissolved in the glass (Δ13Cobserved). High 4He/40Ar* ratios in vesicles (from 49 to 190) are observed in both the most and least carbon supersaturated samples, while samples with intermediate carbon supersaturation have the lowest 4He/40Ar* ratios (16±1). These correlations show that most quenched melts record different disequilibrium to equilibrium states during closed-system degassing.The samples showing the highest carbon supersaturation (4.3) have the highest 4He/40Ar* (from 94 to 190). This observation shows for the first time that the 4He/40Ar* ratio can be kinetically fractionated during incomplete degassing of magmas from the magma chamber to the seafloor. This result implies that high 4He/40Ar* ratios are not a systematic indicator of open-system degassing (Rayleigh distillation) and that caution should be taken when using this ratio for any degassing correction.A two-stage degassing model, with the first stage being a closed-system degassing occurring between the source and the magma chamber, and the second stage of degassing (with a mode varying from open-system degassing to different degrees of kinetic closed-system degassing) taking place between the magma chamber and eruption on the seafloor, is the most appropriate to describe the degassing of MORB. Reconstructing initial carbon content of the magma prior to degassing and extrapolating the results to the entire ridge system results in a carbon flux of 1.6-0.3+0.6×1014 g/year. This value implies vigorous exchange of carbon between the mantle and the surface throughout geological times.  相似文献   

14.
Diffusion coefficients for Si, Ti, Al, Na, K, Ca, Mg and Fe between pairs of glasses of basaltic, rhyolitic or phonolitic compositions have been determinated experimentally. This method involves the heating of coaxial cylinders of paired glasses under atmospheric conditions, over a range of temperatures from 900 to 1300°C, followed by microprobe analysis determination of the concentration gradients across the interface.The measured diffusivities are similar for all cations and range from 10?13 cm2/s at 900°C to 5 × 10?9 cm2/s at 1300°C. Depending mainly on the composition contrast, the diffusion is characterized by asymmetrical concentration profiles. This peculiar feature increases with temperature and chemical gradients across the contact surface of the glasses and leads to higher diffusion coefficients (D) in the more “basic” glass of a given pair. In the case of the rhyolite-basalt couple, this variation increases by a factor of about 10 at 1300°C. Diffusion dependence on temperature follows an Arrhenius equation which gives activation energies ranging from 65 to 85 kcal/mole. Assuming a constant and overall D for the two glasses we have attempted to apply our results to some geological examples such as exchanges between molten enclaves and liquids of contrasting composition.  相似文献   

15.
Relaxation experiments on vitreous SiO2, densified at 900°C, 20 kbar to a permanent density of 2.36 g cm?3 yield an activation energy of69 ± 5kcal mol?1 in the temperature range 700–800°C. The data can be used to estimate minimum cooling rates, maximum depositional temperatures or maximum thicknesses of shock-metamorphosed rocks containing diaplectic glasses, if relaxation of a glass densified under dynamic conditions is assumed to be similar to that of statically densified glass.  相似文献   

16.
Stepwise heating experiments on separated graphite-diamond-kamacite aggregates have revealed a pronounced difference in the release patterns of spallogenic3He and trapped gases. About half the3He is released at T ? 920°C, without being accompanied by significant amounts of primordial gases; the latter, together with the remaining3He, is given off only at T ? 1200°C. Acid treatment of an aliquant dissolved about 2/3 of the total Fe in the sample but did not cause a significant change in the gas concentrations. It is concluded that (a) there is no evidence for a loss of spallogenic3He from the graphite-diamond-kamacite aggregates, (b) one major constituent of the aggregates - graphite - is almost void of trapped gases, (c) kamacite is not a main carrier of the gases. This leaves diamond as the most probable site of the primordial gases.The elemental abundance pattern in the noble gases is essentially as reported previously. In particular, the excellent correlation between relative depletion factors, normalized to the cosmic abundance ratios, and the respective ionisation energies is confirmed. Other important features of the trapped gases are a20Ne/22Ne ratio of 12.3 ± 0.6, intermediate between solar wind and solar flare implanted Ne,36Ar/38Ar = 5.20 ± 0.06 and a measured40Ar/36Ar ratio (before blank correction) of 0.0076.Possible modes of trapping of the noble gases are discussed.  相似文献   

17.
Helium isotope compositions of the mantle xenoliths and megacrysts in the Cenozoic basalts in the eastern China were measured. The samples were collected from Ludao of Heilongjiang, Huinan and Jiaohe of Jilin, Kuandian of Liaoning, Hannuoba of Hebei, Nüshan of Anhui, Dingan of Hainan. The3He/4He ratios of the mantle xenoliths and megacrysts from the most areas were about 1 × 10-5, and were similar to those of the MORB, thus reflecting the characteristics of the MORB-typed depleted mantle. The3He/4He ratios of the mantle xenoliths from Jiaohe were 4.8×10-6 and the3He/4He ratios of xenoliths from Hannuoba vary from 0.15× 10-6 to 7.4 ×10-6, obviously lower than those of the MORB, and even lower than the atmospheric helium isotope ratios, indicating that the continental mantle was strongly replaced in Jiaohe and Hannuoba areas. The helium isotope compositions of the mantle xenoliths and megacrysts in the same region vary in a very wide range. It is inferred that the mantle xenoliths and megacrysts were from different parts of the continental mantle. There were not necessary origin relations between the mantle xenoliths, megacrysts and their host basalts. An extremely high3He/4He ratio of garnet megacryst from Hannuoba, Hebei was found.  相似文献   

18.
In situ Terrestrial Cosmogenic Nuclides (hereafter TCNs) are increasingly important for absolutely dating terrestrial events and processes. This study aimed at improving our knowledge of the production rate of Terrestrial Cosmogenic 3He formed in situ in rock surfaces at low latitude and sea level as well as re-evaluation of the Canary Islands as a calibration site for TCNs. For this purpose, we sampled basaltic lava flows from some of the youngest and yet undated volcanic sites and used the 40Ar/39Ar incremental heating method on groundmass samples and in situ cosmogenic 3He on olivine and clinopyroxene phenocrysts. 40Ar/39Ar analysis was done on a Hiden HAL Series 1000 triple filter quadrupole mass spectrometer with extraction furnace. Incremental heating data shows ages in the Late Pleistocene from 52.7 ± 21.6 ka to 398.6 ± 27.6 ka.We measured cosmogenic 3He concentrations in olivine and clinopyroxene phenocrysts from flow top samples on a MAP 215-50 sector mass spectrometer with a crushing device and a diode laser extraction system. Exposure age calculations yielded ages in the range 38.9 ± 4.0 ka to 62.3 ± 6.7 ka for the youngest lava flow and the data series is in broad agreement with the argon data up to 250 ka and reveals a more continuous time line of volcanism during the late Pleistocene on the island. However, the dataset was not sufficient for calculation of production rates for in situ Terrestrial Cosmogenic 3He as many samples showed signs of erosion. Calculated erosion rates range from none to as high as 7.3 mm/kyr assuming a rock density of 2.9 g/cm2. This finding puts a constraint on the use of Fuerteventura as a calibration site for exposure histories older than 50–100 ka. A comparison with cosmogenic 36Cl data supports these findings and indicates substantial weathering.  相似文献   

19.
A positive water temperature anomaly of 0.11°C and an inverse gradient of potential temperature of 1.5 × 10?2°C/m has been measured at the TAG hydrothermal field in the rift valley of the Mid-Atlantic Ridge at latitude 26°N by means of a thermistor array towed between 2 and 20 m above the seafloor. This anomaly appears to be associated with hydrothermal discharge from the oceanic crust. The temperature data are interpreted in terms of (1) a steady, turbulent thermal plume rising in a homogeneous, neutrally buoyant medium, and (2) turbulent diffusion in the ocean-bottom boundary layer. The calculations indicate that the thermal output of the TAG anomaly area is of the order of several megawatts, which is of the same order of magnitude as some continental geothermal systems. The thermal output from the TAG anomaly area represents a significant fraction of the total heat loss resulting from the generation of new lithosphere at the Mid-Atlantic Ridge at 26°N.  相似文献   

20.
Lu-Hf total-rock data for the Amîtsoq gneisses of West Greenland yield an age of 3.55±0.22Gy(2σ), based on the decay constant λ176Lu=1.96×10?11y?1, and an initial176Hf/177Hf ratio of 0.280482±33. The result is in good agreement with Rb-Sr total-rock and U-Pb zircon ages. In spite of severe metamorphism of the area at 2.9 Gy, zircons from two of the samples have remained on the total-rock line, and define points close to the initial Hf ratio. The initial176Hf/177Hf lies close to a chondritic Hf isotopic evolution curve from 4.55 Gy to present. This is consistent with the igneous precursors to the Amîtsoq gneisses having been derived from the mantle at or shortly before 3.6 Gy. Anomalous relationships between Hf concentration and the176Lu/177Hf ratio may suggest that trace element abundances in the Amîtsoq gneisses are partly controlled by processes related to metamorphism.  相似文献   

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