RADIOGENIC COMPONENTS IN THE BULK EARTH AND ATMOSPHERE

Parent nuclides BSE concentration Daughter nuclide Total BSE production (atoms) Fraction of total BSE production in atmosphere 

Notes 1. BSE is bulk silicate Earth, with 4.02x10 g. 

Production due to entire decay series, assuming secular equihbrium. 

The decay of U and Th series nuclides is the dominant source of He and Ne. There is little debate regarding the concentrations of U and Th, which are obtained from those in carbonaceous chondrites and by assuming that refractory elements (e.g., Ca, U, Th) are unfractionated in the bulk Earth (e g., O Nions et al. 1981). In this case, a BSE concentration of 21 ppb U is obtained (Rocholl and Jochum 1993). Pb isotope systematics indicate that Th/U = 3.8 (e.g., Doe and Zartman 1979). 

The production rate of He is well constrained, with 3.94x10 atoms produced in the BSE over 4.55 Ga, or 9.8x10 atoms/g in any undegassed mantle. However, the atmosphere is severely depleted from losses to space at long-term rates that cannot be easily quantified, so the fraction of He that has degassed is unknown. 

---