The Mantle He Flux

Direct observation of present-day mantle degassing was first reported by Clark et al. (1969) and Mamyrin et al. (1969) for the case of 3He a 3He degassing rate was subsequently determined by Craig et al. (1975). The degassing rate reported by Craig et al. is still the best available estimate of this important geochemical parameter and 

The flux estimate is actually based on 4He supersaturation. The governing relation is 

Craig et al. (1975) cite minimal 20% uncertainties in both (Z EHe) and w and assign a 50% uncertainty to the overall result. With due allowance for thin geographic and temporal sampling, an error of a factor of two would not be surprising. This is an important geochemical parameter that clearly warrants more attention. 

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 

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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. 

Noble gases have been at the forefront of studies defining volatile fluxes between the mantle and other terrestrial reservoirs. This stems from the fact that in the case of He there is no question or ambiguity regarding its origin the mantle He... 

It should be emphasized that in many locations, the high He/ He source component comprises a small fraction of the sample source. A very gas-rich He source need only contribute a very small mass fraction of the source region to dominate the He budget. For example, gas-rich mantle with He/" He = 40 Ra mixed with MORB-source mantle could generate a source that has He/" He = 25 Ra and a He concentration that is at most double that of the MORB source. Therefore, in many cases (Fig. 4), source concentrations may be closer to that of MORB. In this case, the overall He flux relative to that from MORE is proportional to the relative melt production rate, or 1-12% that of MORB. It is not clear to what extent more gas-rich OIB augment this flux. 

The global He flux at mid-ocean ridges is equal to production in the upper mantle, suggesting that the upper mantle He concentration is in steady state and so requiring transfer of He from the lower mantle (O Nions and Oxburgh 1983). 

Bulk transfer of core material into the mantle (e.g., Macpherson et al. 1998). This provides the most straightforward method for transporting He. Mass transfer may occur into the mantle by exsolution of oxides from the core (Walker 2000), although the noble gas flux cannot be calculated without knowledge of the relevant partitioning behavior during exsolution. For simple entrainment of material from the core, the mass flux is ffc =. Using the OIB He flux of F = 5 x 10 " ... 

Both approaches are likely to lead to errors. On the one hand, it has been shown that there are continental regions where the mantle heat flux into the crust is lower than that of the oceans (Torgersen et al. 1992b, 1995). On the other, it is clear that the different transport properties of helium and heat through both mantle and crust will result in transfer of volatiles and heat to the crust with He/heat ratios different from those of the mantle. 

Farley KA, Maier-Reimer E, Schlosser P, Broecker WS (1995) Constraints on mantle He fluxes and deep-sea circulation from an oceanic general circulation model. J Geophys Res 100(B3) 3829-3839 Farley KA, Love SG, Patterson DB (1997) Atmospheric entry heating arrd hehum retentivity of interplanetary dust particles. Geochim Cosmochim Acta 61 2309-2316 Farley KA, Montanari A, Shoemaker EM, Shoemaker CS (1998) Geochemical evidence for a comet shower in the Late Eocene. Science 280 1250-1253... 

Mantle He also emanates in continental and island arc regions, but flux estimates are more difficult than for the world ocean. Simply by comparing areas, we do not expect that continental or arc fluxes will make a major difference to the global flux calculated for the world ocean, but the subject is of keen interest because He serves as a tracer for mantle influences. It is expected, and observed, that flux varies according to tectonic regional setting. Summary data are exhibited in Table 6.4. 

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