He flux

See Figure 6.7(b). The tnaehine now operates in a constitnt h.p. region. The frequency is raised but the voltage is kept constant at its rated value (as it should not be raised beyond rated). I he flux will diminish while / and also /,r will remain almost the same. The torque therefore reduces so that the h.p. developed remains almost a constant (h.p. T.N). This is also known as the field-... 

This is the principle on which a DR works. The advantage of this process over the 3He evaporation is evident for example, at 0.28 K, the 3He vapour pressure is about 10-3 torr, whereas the osmotic pressure from the rich to the diluted phase is about 10 torr for T->0. Thus, it is always possible to force a 3 He flux from the concentrated phase to the diluted one. 

An independent estimation of the mantle He-flux would be possible if we know the He escape rate from the Earth s upper atmosphere (e.g., Kockart, 1973). The present atmosphere contains about 0.7 ppm of He. Because He is too light to be gravitationally bound to the Earth, the present He concentration in the atmosphere can be concluded to represent a stationary value in balance between the mantle He influx and its outflow from the upper atmosphere. Therefore, if we know the outflow flux, we can equate it to the mantle He flux, or vice versa. However, the former estimation is even more difficult, and the present best estimate of the He escape flux is still based on the mantle He flux. 

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... 

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. 

In the case of subducting area, however, Van Soest, Hilton, and Kreulen (1998) called attention to the importance of the arc crust carbon in the C/He flux the crustal... 

The world average is calculated from the average heat flow 0.082 W/m2 and the He flux 4 x 109 atoms m 2/s (Craig Lupton, 1976). Note that both the world average and the observed values are significantly below the production ratio. L. Mashu, Igarashi et al. 

In Table 7.2, we summarize major He fluxes in the atmosphere. The He budget seems to be in balance, at least within a factor of two, although there is certainly room for modification of some of the terms or introduction of new ones. However, there is no reason to require the He budget to be in balance. On the 106 year scale of the mean residence time, variations in fluxes will be smoothed out, and the present epoch may not be typical. The geological and cosmic ray sources are probably fairly steady on this time scale, but the thermal loss is very sensitive to solar activity, the nonthermal loss is sensitive to the geomagnetic field, and precipitation is sensitive to both. At an extreme, the fluxes may be highly irregular Sheldon and Kern (1972),... 

It is in the non-pulse component (N ), which initially develops a peak flux which becomes negative Before finally approaching zero, that the effect of the pressure gradient is seen. A positive flux in Ng would not be predicted by the isobaric equations, since it must be equal and opposite to the positive He flux. The explanation is apparent by examining the total pressure gradient, where it can be seen that bulk motion of the fluid can result in a net positive flux of Ng. Similar results are obtained with longer pulses, and for these cases the differences between isobaric and nonisobaric diffusivity becomes larger. 

It is important to point out that neither approach attempts a direct measurement of the arc He flux. To estimate magma production rates, scaling is used in the first instance, whereas the second methodology relies on knowledge of an absolute flux of some chemical species from volcanoes together with a measurement of the ratio of that species to He. The most widely used species to derive absolute chemical fluxes from subaerial volcanoes is SO2 using the correlation spectrometer technique (COSPEC) (Stoiber et al., 1983). Carbon dioxide (see Brantley and Koepenick, 1995) and Po (Marty and LeCloarec, 1992) have also been used in an analogous manner. 

Farley K. A., Maier-Reimer E., Schlosser P., and Broecker W. S. (1995) Constraints on mantle He fluxes and deep-sea circulation from an oceanic general circulation model. J. Geophys. Res. 100, 3829-3839. 

The flux of He from intraplate volcanic systems is dominantly subaerial and so it is not possible to obtain directly time-integrated flux values for even a short geological period. While the Loihi hotspot in the Pacific is submarine, calculation of He fluxes into the ocean using ocean circulation models have not required a large flux from this location that is comparable to the He plumes seen over ridges (Gamo et al., 1987 Farley et al., 1995), although recent data has seen an extensive He plume from Loihi (Lupton, 1996). 

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