Juvenile 3He

Because of its low natural abundance and, in normal practice, severe interferences in mass spectrometric analysis, in the early noble gas analyses of sea water 3He was either ignored or used only as the spike in isotopic dilution. In the past decade, however, 3He data have assumed a position among the most important in terrestrial noble gas geochemistry. 

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

Here (3He/4He)s is the observed sample ratio. Note that the normalization is the air ratio, not an expected ratio. The distinction is significant for He dissolved in water, for which there is a perceptible isotopic fractionation (Weiss, 1970b), so that the null value, for solubility equilibrium with air, is 53He -1.4%. 

In the expectation of using this effect to verify and more accurately quantify the presence of juvenile radiogenic 4He, Clarke et al. (1969) measured 3He/4He ratios in South Pacific (Kermadec Trench) deep waters. Instead of negative anomalies, they 

Since radiogenic He has a 3He/4He ratio less than the air value, the only serious candidate for an alternative to primordial 3He is production by decay of tritium (3H). Cosmic ray interactions in the atmosphere are a well-known source of 3He (Section 5.5), and some of this is channeled through tritium, which will enter surface water 

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More detailed examination and sampling allows association of hydrothermal circulation with specific vent fields. In such waters samples in the Galapagos Rift by the Alvin deep submersible, Jenkins, Edmond, and Corliss (1978) report juvenile He enrichments which dwarf the normal saturation concentrations by factors up to 11 for 4He and 60 for 3He (Figure 4.6). A particularly significant feature of this report is that added He occurs roughly in proportion to added heat AT up to 12°C in sampled water), corresponding to about 7.6 x 10 xcal/atom of 3He (Figure 4.7). Jenkins et al. note that if this value is representative, hydrothermal circulation may indeed account for the depression of conductive heat flow relative to models for total heat flux. As... 

Figure 4.7 Relationship between 3He and water temperature in hydrothermal waters sampled in specific vent fields in the Galapagos Rift (cf. Figure 4.5). The correlation suggests that both heat and juvenile He are brought to the surface by the same volcanic process, in the ratio 7.6 x l(T8cal/atom (of 3He). Reproduced from Jenkins et al. (1978).

The composition of juvenile He is obviously a parameter of considerable interest. In normal waters, the limitation on the calculation is identification of the excess 4He, since the fractional 4He excess is smaller than that of 3He, and must be identified by... 

Farley et al. (1995) recently applied a global circulation model (GCM) for the world ocean to the He flux problem, assuming a source function that injects juvenile He only along ridge axes at a rate proportional to the spreading rate. They iterated the Hamburg Large-Scale GCM (Meier-Reimer, Mikolajewicz Hasselmann, 1993) until steady-state 3He distribution was obtained and concluded that the reasonable... 

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