Helium isotopes in the earth's interior and in the atmosphere: A degassing model of the earth

Abstract

A first-order degassing model was applied to describe the evolution of helium content and isotope composition in the earth and in the atmosphere. The main events described by the model are: (1) the earth-trapped primordial rare gases at the moment of its accretion; (2) later, the solid earth lost primordial and radiogenic rare gases, and (3) they were accumulated in the atmosphere; (4) in addition, 3He was formed in the atmosphere due to cosmic irradiation, accretion from solar wind, etc.; (5) 3He and 4He dissipated into space at different loss rates. Study of this model confirms the concept that some of primordial helium is retained in the interior of the earth; terrestrial helium ( 3He/ 4He∼ 2 × 10 −5 ) was most probably formed as a mixture of primordial ( 3He/ 4He= 3 × 10 −4 ) and radiogenic ( 3He/ 4He∼ 3 × 10 −8 ) helium. For achondritic concentrations of heavy radioactive elements ( U= 2.25 × 10 −8g/g ) the calculated 4He flux from the earth is equal to 5.7 × 10 6 at cm −2 sec −1. The corresponding 3He flux is about 114 at cm −2 sec −1. In discussing the aeronomic problem of helium it is necessary to take into account that the earth is the main source of the light helium isotope.