High 3He Research Articles

Liquid carbon dioxide of magmatic origin and its role in volcanic eruptions

Natural liquid carbon dioxide is produced commercially from a 2.5-km-deep well near the 4,500-yr-old maar volcano, Mount Gambier, South Australia. The carbon dioxide has accumulated in a dome that is located on the extension of a linear chain of volcanic activity. A magmatic origin for the fluid is suggested by the geological setting, δ13CPDB of –4.0‰, for the CO2 (where PDB represents the carbon-isotope standard), and a relatively high 3He component of the contained helium and high 3He/C ratio (6.4 x 10−10). The 3He/4He and He/Ne ratios are 3.0 and > 1,370 times those of air, respectively. The CO2, as collected at the Earth's surface at 29.5 °C and 75 bar, expands more than 300-fold to form a gas at 1 atm and 22 °C. We suggest that liquid CO2 or high-density CO2 fluid (the critical point is 31.1 °C, 73.9 bar) of volcanic origin that expands explosively from shallow levels in the Earth's crust may be a major contributor to 'phreatic' volcanic eruptions and maar formation. Less violent release of magmatic CO2 into crater lakes may cause gas bursts with equally disastrous consequences such as occurred at Lake Nyos, Cameroon, in August 1986.

Helium in soil gas: a method of mapping groundwater circulation systems in fractured plutonic rock

The Carnmenellis granite is one of seven Variscan plutons exposed in SW England which intrude a folded and thrusted succession of metasedimentary and metavolcanic rocks of Devonian and Carboniferous age. The granite contains higher than average radionuclide concentrations which give rise to enhanced heat flow and fracture-controlled, low enthalpy hydrothermal circulation. Measurements of 4He in soil gas have been made at 91 sites over the pluton and the surrounding host rocks, in order to assess variation of 4He emanation in relation to the distribution of radionuclides and the groundwater circulation system. The pattern of variation obtained shows little correlation with the results of an airborne total gamma-ray activity survey, but agrees well with the distribution of 4He in spring waters. Because the occurrence of zones of high 4He concentration in these spring waters shows a significant correlation with local variations of heat flow within the pluton, it is believed that the distribution of upwelling limbs of groundwater circulation cells controls the groundwater 4He flux. The spatial association of high values of 4He in soil gas with high values in spring waters thus suggests that fracture-controlled groundwater circulation systems can be mapped using the soil gas method, even in the absence of springs. However, the variation of 4He concentrations in soil gas is more complex than that given by spring waters. This is due to variation in soil moisture content affecting the pore space concentration of 4He. Nevertheless, resolution of this effect is possible by statistical analysis of the 4He distribution.

Noble gas studies on the host phase of high [formula omitted] ratios in deep-sea sediments

Elemental abundances and isotopic compositions of noble gases were determined for a magnetic separate and chemically leached samples from deep-sea sediments (14°29.61′S, 158°52.98′W). High 3He 4He ratios suggesting the presence of cosmic material were observed in all samples, and especially enriched in magnetic fractions. Ne isotopes and light noble gas abundances suggested that the high 3He 4He ratio was of solar wind origin and not due to cosmic ray spallation. However, heavy noble gases (Kr and Xe) were of atmospheric origin. Altogether the noble gases were well explained by a mixture of solar wind implanted on interplanetary dust particles (IDPs) and terrestrial noble gas fractionated from the atmosphere. The degree of contamination of IDPs in the magnetic fractions was estimated to be about 100 ppm.

Carbon isotope systematics of a mantle “hotspot”: a comparison of Loihi Seamount and MORB glasses

The carbon isotope geochemistry of glasses from Loihi Seamount has been compared with that of MORB glasses. Stepped heating shows two carbon components in both sample suites: (1) isotopically light carbon ( avg. δ 13C = −26.3‰ ) released < 600°C, ascribed to surficial contamination, and (2) isotopically heavy carbon released > 600°C, regarded as indigenous. The high-temperature component in MORB samples varied from 52 to 169 ppm C, average δ 13C = −6.6‰ , consistent with previous studies (overall MORD average δ 13C = −6.4 ± 0.9‰ ), and new results for Indian Ocean glasses are similar to Atlantic and Pacific Ocean samples. Carbon release profiles produced by stepped heating may be typical of locality, but there are no significant differences in δ 13C values between MORB samples from different areas. Lower yields (17–110 ppm C) correlated with depth in the Loihi samples suggest that they are partially degassed. This degassing has not affected δ 13C values significantly (avg. −5.8‰). Loihi tholeiites have higher δ 13C (avg. −5.6‰) than the alkali basalts (avg. −7.1‰). Carbon abundances correlate well with He concentration data. Comparison of the δ 13C values with trace element and He, Sr, Nd, and Pb isotope data from the literature suggests that the Loihi samples with highest δ 13C have high 3He/ 4He and possibly the least depleted 143Nd/ 144Nd and 87Sr/ 86Sr. The carbon isotope data are consistent with previous models for Loihi involving several mantle sources, lithospheric contamination, and mixing. The slightly higher δ 13C of Loihi tholeiites suggests that the undegassed “plume” component manifested by high 3He/ 4He values might have δ 13C about 1‰ higher than the MORB average.

Realization of a high sensitivity neutron rem counter

Abstract Two types of high sensitivity rem counters were realized by using large volume and high pressure 3He spherical counters: one filled with 10 atm 3He having a diameter of 5.08 cm, and one filled with 5 atm 3He having a diameter of 12.7 cm. The responses to neutron energy from thermal to 15 MeV were experimentally determined by using the national neutron standard field at the Electrotechnical Laboratory. It was confirmed that these two rem counters have about 10 and 70 times higher sensitivities, respectively, than the commercial rem counter Studsvik 2202D, and their neutron responses are well fitted to the ICRP-21 rem response and are almost isotropic to the incident neutron direction.

Helium-3 and deuterium in back-arc basalts: Lau Basin and the Mariana Trough

Samples of fresh basalt glass from the Mariana Trough and the Lau Basin were analyzed for their isotopic composition of water and helium in order to identify the sources of the volatiles in back-arc basin basalts. In the Mariana Trough basalts, the concentration (0.64–2.1 wt.%) and D/H ratio ( δD = −46 to −32‰) of the water provide important evidence for a water-rich component from the subducting lithosphere. Extrapolation to infinite water content gives an end-member D/H ratio of δD = −25‰. 3He/ 4He ratios are, in general, similar to MORB values ( 8 R A ) and indicate that Mariana Trough basalts represent a mixture of a MORB-type mantle and an alkali and water-rich component from the descending slab. In contrast, the Lau Basin produces both hydrous (1.3 wt.% H 2O) and relatively anhydrous (0.12–0.35 wt.%) basalts. The D/H ratio ( δD = −43‰) in the hydrous Lau Basin basalt resembles those of Mariana Trough lavas. The low water contents (0.12–0.35 wt.%) and MORB-like D/H ratios ( δD = −63 to −70‰) in three of five Lau Basin samples show that the water-rich component, observed in all Mariana Trough lavas, is absent in some Lau Basin lavas and not essential for the production of back-arc basalts. The high 3He/ 4He ratio of Rochambeau Bank in the northern Lau Basin ( 11 R A ), confirms an earlier analysis by Lupton and Craig and provides evidence for an enriched mantle source region beneath the Lau Basin which is perhaps related to the high- 3He Samoan “hotspot”.

SrNd and ArHe isotopic relationships: Comparison, geotectonic implications and approximation by model computing

SrNd and ArHe isotope relationships are used as the principal data for a five-reservoir model of the Earth evolution and, correspondingly, contemporary results obtained for SmNd, RbSr, KAr and UHe systematics are briefly discussed. The UHe system seems to be the only one which enables one to make a choice between evolution and mixing models. He escape accompanied by isotope fractionation provides an extremely low content of both isotopes, especially the light one, 3He, in the atmosphere, which is one of the end-member terrestrial reservoirs. So the mixing of He from three end-member reservoirs (that is, the depleted mantle, the crust and the atmosphere) cannot be responsible for high 3He 4He ratios in helium which are observed in some rocks and fluids of hot-spot areas and are interpreted as undepleted mantle emanations. A good agreement between the observed SrNd and ArHe data and the calculated results supports the model described in the paper. Several preliminary results inferred from this mode, viz. some peculiarities of Ar and Sr evolution in fractionated reservoirs, calculated K Rb ratios in the Earth, etc., are discussed.

Noble gas evidence for two fluids in the Baca (Valles Caldera) geothermal reservoir

Noble gas elemental and isotopic abundances were measured in steam from four wells in the Baca geothermal reservoir located in the Valles Caldera, New Mexico. The 40Ar 36Ar ratio and noble gas elemental abundances relative to 36Ar are all strongly correlated with 1/ 36Ar, the inverse of the argon content. Ratios of ( α, n)-produced 21Ne∗ and radiogenic 40Ar∗ to total 4He (dominantly radiogenic) are nearly constant at 2.1 × 10−8 and 0.20, respectively. The 3He 4He ratio covers a restricted range of 3.9 to 4.8 times atmospheric. The high 3He content of the gas indicates the presence of a helium component ultimately derived from the mantle. Kr and Xe isotopic compositions are close to atmospheric; excess 129Xe∗ is <0.25% of the total 129Xe. The high degree of linear correlation among the various noble gas results strongly suggests that the Baca reservoir contains two distinct fluids that are produced in varying proportions from individual wells. The noble gases in fluid A (~2900 mg/1 C1) are air-like, but with lighter gases and isotopes preferentially enriched. The fluid A 36Ar content is low, only 13% that of 10°C air-saturated water (ASW). The second fluid, B (~ 1700 mg/1 C1), is the dominant carrier of the radiogenic and mantle-derived gases. The heavier non-radiogenic gases are preferentially enriched in fluid B, and its 36Ar content is very low, only 5–7% ASW. The source of the noble gases in fluid A is tentatively ascribed to leaching of the relatively young (<1.4 m.y.) volcanic Bandelier Tuff. The radiogenic gases and mantle-derived helium in fluid B suggest a deeper source, possibly including gases escaping from a magma.

Rare gases, water, and carbon in kaersutites

Kaersutites from Kakanui, New Zealand and from three localities in the southwestern United States have been analyzed for rare gases, water and carbon to investigate the volatile signature of the sub-continental mantle. This study does not confirm the high 3He/ 4He and 21Ne/ 22Ne ratios reported by Saito et al. [1] for the Kakanui kaersutite. Instead, a 3He/ 4He ratio of 6 R A and atmospheric 21Ne/ 22Ne ratios were measured which are consistent with our current knowledge of the earth's mantle. A low 40Ar/ 36Ar of 320 and more than 10 −8 cm 3/g of 36Ar confirms the argon results of Saito et al. and indicates that significant quantities of 36Ar reside in this portion of the mantle. Kaersutites from the southwestern United States (Arizona) have a heterogeneous helium isotope signature, ranging from 8.8 R A at San Carlos to 0.46 at Hoover Dam. All D/H ratios for the water in kaersutites (−56‰ to −78‰) represent typical mantle values with no apparent correlation with 3He/ 4He. The correlation of increasing carbon content (140–400 ppm) with increasing δ 13C (−24.5‰ to −16.7‰) may reflect differences in the proportions of oxidized and reduced carbon in these samples.

A fast neutron spectrometer for D-D fusion neutron measurements at the Alcator C tokamak

A neutron spectrometer using a high pressure 3He ionization chamber has been designed and used to measure the neutron spectrum from an ohmically heated deuterium plasma. The resolution of the spectrometer at 2.45 MeV is determine to be 46 keV full width at half-maximum (fwhm). Particular attention has been paid to optimizing the detector shielding and collimation to reject thermal and epithermal neutrons scattered from the tokamak structure. As a result, measurements indicate that the ratio of the number of counts in the 2.45 MeV peak to the total number of detected neutron events is 1 67 . For the 8 μs amplifier time constant used, a count rate as high as 44 counts per second has been achieved in the thermonuclear peak. The observed spectra have been compared with calculated spectra using the MCNP Monte Carlo Neutral Particle Transport code and they show good agreement. There is little evidence of neutrons produced from photoneutron reactions or electrodisintegration. It has been possible to confirm that the shape of the thermonuclear peak is consistent with the Gaussian shape predicted and that the ion temperature as determined from the line width is consistent with other Alcator C ion temperature diagnostics, and follows the trends predicted by the theory of Doppler line broadening.

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 × 10 6-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.

Formula omitted] ratio, noble gas abundance and K-Ar dating of diamonds—An attempt to search for the records of early terrestrial history

The 3He 4He ratios measured in 27 Southern Africa diamond stones, four from Premier Mine and the rest of unidentified origin, range from 4.2 × 10 −8 to 3.2 × 10 −4, with three stones above 1 × 10 −4. We conclude that the initial helium isotopic ratio ( 3He 4He) 0 in the earth was significantly higher than that of the planetary helium-A ( 3He 4He = 1.42 × 10 −4 ), but close to the solar helium ( 3He 4He − 4 × 10 −4 ). The apparent K-Ar ages for the twelve diamonds of unidentified origin show enormously old age, indicating excess argon-40. 3He 4He evolution in diamonds suggests that the diamonds with the high 3He 4He ratio (>2 × 10 −4) may be as old as the earth. Noble gas elemental abundance in the diamonds relative to the air noble gas abundance shows monotonie decrease with a decreasing mass number. This paper discusses the implications of these observations on the early solar system and the origin of diamonds.

Noble gas systematics for coexisting glass and olivine crystals in basalts and dunite xenoliths from Loihi Seamount

Noble gas isotopes including 3He/ 4He, 40Ar/ 36Ar and Xe isotope ratios were determined for coexisting glass and olivine crystals in tholeiitic and alkalic basalts and dunite xenoliths from Loihi Seamount. Glass and coexisting olivine crystals have similar 3He/ 4He ratios (2.8–3.4) × 10 −5, 20 to 24 times the atmospheric ratio ( R A), but different 40Ar/ 36Ar ratios (400–1000). Based on the results of noble gas isotope ratios and microscopic observation, some olivine crystals are xenocrysts. We conclude that He is equilibrated between glass and olivine xenocrysts, but Ar is not. The apparent high 3He/ 4He ratio (3 × 10 −5; = 21 R A) coupled with a relatively high 40Ar/ 36Ar ratio (4200) for dunite xenoliths (KK 17-5) may be explained by equilibration of He between MORB-type cumulates and the host magma. Except for the dunite xenoliths, noble gas data for these Loihi samples are compatible with a model in which samples from hot spot areas may be explained by mixing between P (plume)-type and M (MORB)-type components with the addition of A (atmosphere)-type component. Excess 129Xe has not been observed due to apparent large mass fractionation among Xe isotopes.

Methane and seismicity: A reply

In a recent Forum article in Eos (“Methane in Association With Seismic Activity,” June 14, 1983, p. 410), R. S. Oremland presents observations which he claims contradict the deep methane gas hypothesis. His principal case rests on observations of one M 5.7 earthquake near the volcanic area of Mammouth Lakes, California, which did not result in any increase in methane content of gases in four local seeps.In our published discussions of the deep gas hypothesis [Gold and Soter, 1980, 1982], we proposed (1) that outgassing from mantle depths is an ongoing process both in volcanic and nonvolcanic regions; (2) that the gases CO2 and CH4 are the principal carriers of the surface excess carbon; (3) that chemical equilibrium between CO2 and CH4 in the presence of hot or liquid rock is strongly shifted towards CO2, especially in the low pressure domain, and that therefore active volcanic or high heat flow regions would be less likely to exhibit CH4; and (4) that faultlines, particularly those which are seismically active, are locations where outgassing in cool regions can be sampled. The evidence there is that flames from the ground are often seen in association with major earthquakes. (Just as in many mud volcano eruptions, ignition of combustible gases can be attributed to electrostatic effects.) Methane is also observed in many of the major crustal rifts, together with helium having the high 3He to 4He ratio indicative of deep origin [Lupton, 1983].

Helium isotopic variations in the mantle beneath the central North Atlantic Ocean

We have determined the helium concentrations and helium isotopic ratios for a set of 35 basaltic glass samples dredged from the Mid-Atlantic Ridge between 28° and 53°N latitude. Total helium concentrations vary between < 1 × 10 −8cm 3STP/g and 1.4 × 10 −5cm 3STP/g . Most of this variation can be accounted for by gas loss on the sea floor via vesiculation, since the samples with the lowest concentrations have extremely high vesicularities. Analysis of the plagioclase phenocrysts from one of the samples suggests that helium behaves as an incompatible element ( K D ≪ 0.01 ). Results of in vacuo crushing and melting experiments confirm that there is no helium isotopic fractionation during melt-vesicle or melt-phenocryst partitioning. The variations in 3He/ 4He ratio along the ridge therefore must be interpreted to be a result of heterogeneity within the oceanic mantle. The 3He/ 4He ratios for this set of samples range from 6.5 to 11.1 times the atmospheric ratio. These data, coupled with strontium isotopic analyses performed on the same samples by White and Schilling (1978), show the existence of two distinct geochemical provinces: the regions between 50–52.5°N and 27–33°N, which are characterized by a trend toward high 3He/ 4He ratios and high 87Sr/ 86Sr ratios, and the region between 33° and 50°N, characterized by a trend toward low 3He/ 4He ratios and high 87Sr/ 86Sr ratios. Since primitive mantle should retain more primordial 3He, the low 3He/ 4He trend requires the presence of a mantle reservoir with lower 3He/(Th + U) and higher Rb/Sr. The generation of this component requires separation between He and Th + U, either by selective loss of He (degassing), or the addition of Th + U. We believe that the most reasonable source for this component is subducted oceanic crust that is recycled back into the mantle.

Rare gas isotopic compositions in natural gases of Japan

Isotopic and elemental compositions of rare gases in various types of gas samples collected in the Japanese Islands were investigated. Excess 3He was found in most samples. Many samples showed a regionally uniform high 3He/ 4He ratio of about 7 times the atmospheric ratio. The He concentrations varied from 0.6 to 1800 ppm, and they were low in CO 2-rich gases and high in N 2-rich gases. Ne isotopic deviations from the atmospheric Ne were detected in most volcanic gases. The deviations and the elemental abundance patterns in volcanic gases can be explained by a mixing between two components, one is mass fractionated rare gases and the other is isotopically atmospheric and is enriched in heavy rare gas elements. Ar was a mixture of mass fractionated Ar, atmospheric Ar and radiogenic Ar, and the contribution of radiogenic 40Ar was small in all samples. Except for He, elemental abundance patterns were progressively enriched in the heavier rare gases relative to the atmosphere. Several samples were highly enriched in Kr and Xe relative to the abundance pattern of dissolution equilibrium of atmospheric rare gases in water. The component which is highly enriched in heavy rare gases may be released from sedimentary materials in the crust.

Improved resolving time in n - γ coincidence measurements with high pressure 3He ionization chambers

Resolving times in n - γ coincidence measurements with high-pressure, gridded 3He ionization chambers are shown to be reduced from about 5 us to less than 0.4 us when grid signals are utilized for timing of neutron events. Compared to that found in the normal case when anode signals are utilized, the coincidence efficiency is reduced to about 70%. Lack of compensation for variable pulse amplitude and rise time is a major factor limiting further improvement in the resolving time.

Radiation induced detrapping of implanted deuterium in BeO by high energy 3He and proton irradiation

BeO layers formed on Be are implanted with 5 keV deuterium ions and the amount of gas retained is determined using the D( 3He, H) 4He nuclear reaction. Detrapping of the retained deuterium by 790 keV 3He and 2.2 MeV H bombardment is measured at temperatures between 140 and 470 K and detrapping cross sections are determined. The maximum detrapping yields found are 17 D/ 3He and 0.8 D/H. The results indicate that the observed radiation induced detrapping is caused by electronic excitation.

Investigation of beta strength functions by neutron and gamma-ray spectroscopy: (I). The decay of 87Br, 137I, 85As and 135Sb

Neutron spectra from the decay of the β − delayed neutron precursors 55.6 sec 87Br, 24.5 sec 137I, 2.05 sec 85As and 1.71 sec 135Sb have been studied with high resolution 3He ionization chambers. By γ-ray measurements, the partial neutron emission probabilities to excited states in 84Se and 134Te have been determined independently. For the neutron-emitter nuclei 87Kr and 137Xe, from which neutron decay proceeds only to the ground states of the final nuclei, 86Kr and 136Xe, unique information on the shape of the β − strength function S β ( E) is obtained. For the emitter nuclei 85Se and 135Te, correlations between neutron transition energies and differences in level energies in 84Se and 134Te, have defined some levels in 85Se and 135Te which are strongly fed by β − decay. It is shown that the experimental shape of the β − strength function cannot be explained by the widely used assumptions S β ( E) = const or S β ( E) ∼ ϱ( E) or by the “gross theory” of β-decay, but that it is the result of structures in the low-energy tail of the Gamow-Teller giant resonance (GTGR) expected from general nuclear structure considerations.

Gas chemistry and helium isotopes at Cerro Prieto

Seven producing wells and seven hot springs in the Cerro Prieto geothermal field were sampled during April and September 1977, for determination of gas chemistry and helium isotope ratios. Well gases are remarkably uniform in gas chemistry and helium isotope ratio, showing high 3He/ 4He ratios characteristic of mantle-derived helium, and higher than expected N 2/Ar ratios. Comparison with the hot spring data suggests that the deep gas component observed in wells is modified during transit to the hot springs by addition of crustal helium, dissolved air and possibly organic methane; alternatively, chemical re-equilibration at lower temperatures may be responsible for increasing the CH 4/H 2 ratio.