Where is the Earth's Radiogenic Helium?

AT the estimated 2 × 10²⁰ gm. uranium and 5 × 10²⁰ gm. thorium in the lithosphere, helium should be generated radiogenically at a rate of about 3 × 10⁹ gm./year. Moreover, the (secondary) cosmic-ray source of helium has been estimated to be of comparable magnitude. Apparently nearly all the helium from sedimentary rocks and, according to Keevil¹ and Hurley², about 0.8 of the radiogenic helium from igneous rocks, have been released into the atmosphere during geological times (currently taken to be about 5 × 10⁹ yr.). Hence more than 10²⁰ gm. of helium should have passed into the atmosphere since the `beginning'. Because the atmosphere contains only 3.5 × 10¹⁵ gm. helium-4, the common assumption is therefore that about 10²⁰ gm. of helium-4 must also have passed out through the exosphere, and that its present rate of loss through the exosphere balances the rate of exudation from the lithosphere.