He isotopes

Abstract. We recall the emergence of the 3He problem , its currently accepted solution, and we summarize the presently available constraints on models of stellar nucleosynthesis and studies of Galactic chemical evolution from observations of the He isotopic ratio in the Galaxy. 

For many years, meteorites have provided the only means to determine the abundance of 3He in protosolar material. The values obtained by mass spectroscopy techniques in the so-called planetary component of gas-rich meteorites have been critically examined by Geiss (1993) and Galli et al. (1995). The latter recommend the value 3He/4He= (1.5 0.1) x 10-4. The meteoritic value has been confirmed by in situ measurement of the He isotopic ratio in the atmosphere of Jupiter by the Galileo Probe Mass Spectrometer. The isotopic ratio obtained in this way, 3He/4He= (1.66 0.04) x 10 4 (Mahaffy et al. 1998), is slightly larger than, but consistent with, the ratio measured in meteorites, reflecting possible fractionation in the protosolar gas in favor of the the heavier isotope, or differential depletion in Jupiter s atmosphere. 

Fig. 1. Evolution of 3He/H in the solar neighborhood, computed without extra-mixing (upper curve) and with extra-mixing in 90% or 100% of stars M < 2.5 M (lower curves). The two arrows indicate the present epoch (assuming a Galactic age of 13.7 Gyr) and the time of formation of the solar system 4.55 Gyr ago. Symbols and errorbars show the 3He/H value measured in meteorites (empty squares) Jupiter s atmosphere (errorbar) the local ionized ISM (filled triangle) the local neutral ISM (filled circle) the sample of simple Hll regions (empty circles). Data points have been slightly displaced for clarity. The He isotopic ratios has been converted into abundances relative to hydrogen assuming a universal ratio He/H= 0.1. See text for references.

The solution to the problem, which was reported recently (5), is the use of a ( He,a) reaction which through the greater stopping power of the He isotopes provides enhanced depth resolution. The result is shown in Figure 9 which is a profile of a relatively shallow C implant in the Ti alloy. 

Whatever model is assumed for the mantle source of Aeolian magmas, Sr-Nd-Pb-He isotope variations call for the involvement of upper crustal components in magma genesis. It can be calculated that less than 10% of upper crust added to a pyrolite mantle could explain the entire range of radiogenic isotopic compositions encountered in the Aeolian mafic rocks. Sediments transported by the Ionian subducting plate represent the most likely candidates for such a mantle source contaminant (e.g. Ellam et al. 1988 Francalanci et al. 1993b). 

Fig. 10.4. Sr-Nd-Pb-Hf-He isotope diagrams for Italian Plio-Quatemary mafic rocks (MgO > 4 wt %) and for the main worldwide mantle compositions HIMU, DMM, EMI and EM2. Star represents the composition of European Astheno-spheric Reservoir.

Smooth trends of Sr-Nd-Pb-Hf-He isotopes connecting different mantle compositions (Fig. 10.4). These are suggested to derive from mixing among different mantle end-members (HIMU or FOZO, EMI and EM2), which occur worldwide and are believed to represent deep mantle plumes by several authors (e.g. Hofmann 1997) ... 

Other geochemical characteristics of Italian volcanism are also not easily explained by the plume hypotheses. For example, deep mantle plumes are commonly associated with high 3He/4He ratios (e.g. Farley and Neroda 1998). However, measurements carried out on fluid inclusions in olivine phenocrysts from mafic Italian rocks have yielded low He isotopic ratios with R/Ra < 7.5 (e.g. Sano et al. 1989 Graham et al. 1993 Marti et al. 1994 Di Liberto 2003 Martelli et al. 2004), which are much lower than compositions found for plume-related magmas. 

The custom for reporting He isotopic composition is to use a ratio anomaly <53He, usually in percent ... 

Sano (1986) and Sano et al. (1986) found He isotopic variations with depth, 3He/4He decreasing toward the surface, in two natural gas wells in northern Taiwan. This relation is interpreted as a mantle flux to the bottom of the well, progressively diluted by radiogenic He released from the surrounding sediment as the gas migrates upward. With a simple mixing model, they obtained mantle He fluxes close to the mean oceanic value (Table 6.4), but the situation in a gas well is rather complicated, and it remains to be seen whether or not the coincidence with the oceanic value is accidental. 

To set an abundance scale for listing the abundances of the elements, astronomers usually set H = 1012 atoms. Other elemental abundances in stars are then given by their numbers per thousand billion H atoms. In the Sun the ratio H/He = 10. For 3He more reliable information about relative He isotopic abundances comes from the primitive classes of meteorites, which are dominated by silicon. Thus the scale frequently used for geochemistry and for stellar nucleosynthesis takes a sample containing one million Si atoms, so that abundances of the elements are then their numbers per million Si atoms. Since helium in the Sun is observed by astronomers to be 2720 times more abundant than silicon, the He total solar abundance is therefore... 

The -decay involves the spontaneous emission of an energetic electron (j5 particle) and of an antineutrino from the radioactive nucleus, whose positive charge increases of one unit. The )3-particles are emitted with all energies, from zero to a maximum value characteristic of the particular transition, the difference between the maximum and actual energy being carried by the antineutrino. The process is illustrated by the decay of tritium, which leads to the formation of a stable nuclide, the He isotope of mass 3, through the emission of )3-particles with energies up to 18 keV ... 

Primordial isotopes are those trapped during Earth formation and are not nucleogenic. Only globally significant other sources are included. See Ballentine and Bumard (2002) for production rates and other nuclear production mechanisms. Various mechanisms supply He isotopes from space see Torgersen (1989) for compilation. The source of variations is within the solid Earth, but has not contributed significantly to the atmosphere. 

Marcantonio F., Anderson R., Higgins S., Fleisher M., Stute M., and Schlosser P. (2001a) Abrupt intensification of the SW Indian Ocean monsoon during the last delaciation constraints from Th, Pa and He isotopes. Earth Planet. Sci. Lett. 184, 505-514. 

The types of information that can be extracted are illustrated by a typical experiment involving, say, transitions between the 1 25 state and the fine structure levels of the l52p Poq,2 manifold of states. The total transition frequency gives the QED shift, for which theory is not yet fully developed and experimental checks are very valuable. However, if one measures the He - He isotope shift for the same transition, then the QED uncertainty largely cancels, allowing the differential nuclear radius to be accurately determined. If one measures... 

Moreover, the tunability of the developed green laser and the rich density of I2 absorptions give a grid of frequency references at 1083 nm as we show in Fig. 9. It can be used for other frequency difference measurements, such as He hyperfine splittings and He- He isotope shifts. This shift is large enough to allow a precise determination of the relative nuclear radii of these atoms, and it is also used to test QED corrections [23]. But, perhaps, the most important advantage of the new reference, is the possibility of absolute frequency measurements of... 

Investigations into these possible applications of helium surveys have been conducted in North America, the former USSR, Australia and northern Europe over the last three decades. Initial results were commonly quite encouraging, but further studies have demonstrated that their potential in mineral exploration is limited and there has been little research and few publications since about 1987. Nevertheless, applications in hydrocarbon exploration and earthquake prediction remain possible. Total He analysis is ineffective for dating groundwaters but He isotope ratios are routinely applied to distinguishing mantle-derived gases. In this chapter, the occurrence and properties of helium are briefly outlined, followed by a description of appropriate sampling and analytical techniques and reviews and assessments of the possible uses of helium surveys. 

The two He isotopes have approximately the same abundance throughout the universe. Some primordial helium was trapped during the accretion of the Earth, with an estimated He/ He ratio of 10 . This is still retained in the mantle and, to a lesser extent, the core, and is now outgassing along deep faults and fissures and during volcanic eruptions. The... 

H2 storage in graphite nanofibers [60], the feasibility of using carbon nanotubes to separate hydrogen isotopes [61] and phase behavior of H2 and He isotopes in nanotubes [62]. 

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