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.     

Isotope composition of helium in ultrabasic xenoliths from volcanic rocks of Kamchatka

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.     

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.     

Dissipation of the rare gases contained in the primordial Earth's atmosphere

If the Earth was formed by accumulation of rocky bodies in the presence of the gases of the primordial solar nebula, the Earth at this formation stage was surrounded by a massive primordial atmosphere (of about 1 × 10²⁶ g) composed mainly of H₂ and He. We suppose that the H₂ and He escaped from the Earth, owing to the effects of strong solar wind and EUV radiation, in stages after the solar nebula itself dissipated into the outer space.The primordial atmosphere also contained the rare gases Ne, Ar, Kr and Xe whose amounts were much greater than those contained in the present Earth's atmosphere. Thus, we have studied in this paper the dissipation of these rare gases due to the drag effect of outflowing hydrogen molecules. By means of the two-component gas kinetic theory and under the assumption of spherically symmetric flow, we have found that the outflow velocity of each rare gas relative to that of hydrogen is expressed in terms of only two parameters — the rate of hydrogen mass flow across the spherical surface under consideration and the temperature at this surface. According to this result, the rare gases were dissipated below the levels of their contents in the present atmosphere, when the mass loss rate of hydrogen was much greater than 1 × 10¹⁷ g/yr throughout the stages where the atmospheric mass decreased from 1 × 10²⁶ g to 4 × 10¹⁹ g.     

Condensation and the composition of iron meteorites

The pressure-temperature conditions in the primordial nebula which could produce the observed Ni, Ga and Ge abundances in the major iron meteorite groups have been calculated assuming equilibrium condensation. Included in these calculations are the effect on the metal composition of Fe oxidation and sulphide formation during accretion, GeS and GaCl in the nebula gases and pressure variations in the nebula. It was found that the IIAB irons had their abundances of these elements fixed at the low-pressure extreme of the range which gives the IAB irons, but at 50 ± 10K higher temperatures. IIIAB and IVA formed over the same temperature range as IAB (600–670_?40⁺⁶⁰ K) in regions where the pressure was lower by a factor of 20 and 10⁴ respectively. Group IVB accreted soon after condensation of the metal and at pressures of less than 10^?3 atmosphere. The distribution of sulphur and carbon are consistent with this. The abundance of carbon in group IAB suggests that this and group IIAB accreted at about 10^?4 atmosphere, so that IIIAB and IVA accreted where the pressure was 5 × 10^?6 and 10^?8 atmosphere, respectively.     

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.     

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.     

Characteristics and origins of primary fluids and noble gases in mantle-derived minerals from the Yishu area, Shandong Province, China

springerlink.com Studies of mantle fluids are currently one of the hot topics in the earth science, greatly contributing to re-vealing origins and evolutions of fluids. In general, the concept of mantle fluids refers to their active compo-nents, such as CO2, H2O, N2, etc., while the noble gases inert in chemical properties belong to another research system. Due to their marked differences in various fluid sources of the Earth[1], the isotopic sig-natures of He and Ar have been widely used a…     

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.     

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%.     

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.     

Isotope systematics of noble gases in the Earth's mantle: possible sources of primordial isotopes and implications for mantle structure

The Earth's mantle contains a mixture of primordial noble gases, in particular solar-type helium and neon, and radiogenic rare gases from long-lived U, ²³²Th, ⁴⁰K and short-lived ¹²⁹I, ²⁴⁴Pu. Rocks derived from deep mantle plume magmatism like on Hawaii or Iceland contain a higher proportion of primordial nuclides than rocks from the shallow upper mantle, e.g. mid ocean ridge basalts (MORBs). This is widely regarded as the key evidence for survival of a less degassed and more “primitive” reservoir within the lower mantle. We present an evaluation of noble gas composition showing the shallow mantle to have about five times more radiogenic (relative to primordial) isotopes than Hawaii/Iceland-type plume reservoirs, no matter if short- or long-lived decay systems are considered. This fundamental property suggests that both MORB and plume-type noble gases are mixtures of: (1) a homogeneous radiogenic component present throughout most of the mantle and (2) a uniform primordial noble gas component with very minor radiogenic ingrowth. This conclusion depends crucially on the observed excess of radiogenic Xe in plume-derived rocks, and is only valid if this Xe excess is inherent to the plume sources.Possible sources of the primordial component of mantle plume reservoirs—and possibly also the MORB mantle—could be mantle reservoirs that remained relatively isolated over most of Earth's history (“blobs”, a deep abyssal layer, or the D” layer), but these need a considerable concentration of primordial gases to compensate U, Th, K decay over 4.5 Ga. Earth's core is evaluated as an alternative viable source feeding primordial nuclides into mantle reservoirs: even low metal-silicate partitioning coefficients allow sufficient primordial noble gases to be incorporated into the early forming core, as the undifferentiated proto-Earth was initially gas-rich. Massive mantle degassing soon after core formation then provides the opposite concentration gradient that allows primordial noble gases reentering the mantle at the core-mantle boundary, probably via partial mantle melts. Another possible source of primordial noble gases in Earth's mantle are subducted sediments containing extraterrestrial dust with solar He and Ne, but this supply mechanism crucially depends on largely unconstrained parameters. The latter two scenarios do not require the preservation of a “primitive” mantle reservoir over 4.5 Ga, and can potentially better reconcile increasing geochemical evidence of recycled lithospheric components in mantle plumes and seismic evidence for whole mantle convection.     

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.     

Carbon monoxide on the primitive earth

The reaction of CO + OH^? in aqueous solution to give formate was studied as a carbon monoxide sink on the primitive earth and in the present ocean. The reaction is first order in OH^? and first order in the molar CO concentration. The second order rate constant is given by log k(M^?1hr^?1) = 15.83?4886/T between 25°C and 60°C. Using the solubility of CO in sea water, and assuming a pH of 8 for a primitive ocean of the present size, the halflife of CO in the atmosphere is calculated to be 12 × 10⁶ yr at 0°C and 5.5 × 10⁴ yr at 25°C.Three other CO sinks would have been important in the primitive atmosphere: CO + H₂ → H₂CO driven by various energy sources, CO + OH → CO₂ + H, and the Fischer-Tropsch reaction of CO + H₂ → hydrocarbons, etc. It is concluded that the lifetime of a CO atmosphere would have been very short on the geological time scale although the relative importance of these four CO sinks is difficult to estimate.The CO + OH^? reaction to give formate is a very minor CO sink on the earth at the present time.     

The relationship between vertical eddy diffusion and buoyancy gradient in the deep sea

Vertical eddy diffusivities (K_v's) have been estimated at fourteen widely separated locations from fourteen²²²Rn profiles and two²²⁸Ra profiles measured near the ocean floor as part of the Atlantic and Pacific GEOSECS programs. They show an inverse proportionality to the local buoyancy gradient [(g/?)(??_pot/?z)] calculated from hydrographic measurements. The negative of the constant of proportionality is the buoyancy flux [?K_v(g/?)(??_pot/?z)] which has a mean of ?4 × 10^?6 cm²/sec³. Our results suggest that the buoyancy flux varies very little near the ocean floor. K_v's for the interior of the deep Pacific calculated from the relationship K_v = (4 × 10^?6cm²/sec³)/[(g/?)(??_pot/?z)] agree well with published estimates. K_v's calculated for the pycnocline are one to two orders of magnitude smaller than upper limits estimated from tritium and⁷Be distributions.Heat fluxes calculated with the model K_v's obtained from the²²²Rn profiles average 31 μcal cm^?2 sec^?1 in the Atlantic Ocean and 8 μcal cm^?2 sec^?1 in the Pacific Ocean.     

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.     

Primordial gases in graphite-diamond-kamacite inclusions from the Haveröureilite

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.     

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.     

Helium,oxygen, strontium and neodymium isotopic relationships in Icelandic volcanics

³He/⁴He ratios have been obtained for basaltic, intermediate and acid volcanic glasses from Iceland. Basaltic glasses exhibit a wide range of ³He/⁴He ratios (4 < R/R_a < 24), which is consistent with the previously recorded range for Icelandic geothermal systems. In contrast the glasses with intermediate and acid compositions have ³He/⁴He values close to the atmospheric value (R_a) with the exception of a 13-Ma sample which has R/R_a= 0.07. ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd ratios and δ¹⁸O values are reported for the same samples.³He/⁴He does not correlate with either ⁸⁷Sr/⁸⁶Sr or ¹⁴³Nd/¹⁴⁴Nd ratio and radiogenic components of He, Sr and Nd have apparently been decoupled. Interaction of Icelandic magmas with hydrothermally altered and older Icelandic crust is the preferred explanation for variable and often low δ¹⁸O values. It is suggested that primary ³He/⁴He ratios may have been modified by incorporation of radiogenic helium developed within the Icelandic crust to impose a larger range of ³He/⁴He ratios on the erupted products than was actually inherited from the mantle beneath Iceland. Intermediate and acid samples have all been severely contaminated by atmospheric helium, most probably at very shallow levels within the crust.     

Noble gases in submarine pillow volcanic glasses

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.