Helium accumulation in groundwater. Pt. 3

The groundwaters of the Great Artesian Basin (Australia) have been previously shown to be accumulating in-situ production helium for groundwaters ages <50 kyr and an external helium flux equivalent to whole crustal production for groundwater ages >100 kyr. New helium isotope measurements show that the observed in-situ production helium (3He/4He ≅ 1.6x10-8) is isotopically distinct from the crustal degassing helium flux (3He/4He ≅ 6.6x10-8). Furthermore, the crustal degassing helium isotope ratio is marginally in excess of the whole crustal production ratio (3He/4He = 3.5x10-8) and the production ratio in a variety of continental rock types. This suggests that the upper limit on volatile transport across the mantle-crust boundary beneath the (relatively) stable and 'complacent' Australian continent can be characterized by a 'conductive-diffusive' helium/heat flux ratio of < or approx.2.6x1064He atoms mW-1s-1 which is two orders of magnitude less than the 'intrusive-volcanic' ratio of 2.9x1084He atoms mW-1s-1 measured at the Galapagos. These results constrain the transcrustal mantle degassing fluxes of 4He and 40Ar to be much less than the mid-ocean ridge degassing fluxes; which are much less than the degassing of 4He and 40Ar from continental crust. Thus, the degassing of the Earth's interior is dominated by magmatic processes but the dominant fluxes of 4He and 40Ar to the atosphere must come from the continental crust. (orig.)