Mantle degassing and atmospheric evolution: Noble gas view.

Abstract

On the basis of recent high quality noble gas experimental data, we propose a scenario for the origin of terrestrial noble gases. The isotopic compositions of non-radiogenic, terrestrial noble gases (except for atmospheric neon) were established, prior to the Earth's accretion, in planetesimals by gravitational capture of solar-type noble gases from the solar nebula. Gravitational capture by planetesimals produces a pattern in which the strongest fractionations occur in the heaviest elements. After the formation of the Earth, neon was further fractionated during hydrodynamic escape of hydrogen from the primitive atmosphere to give rise to the present atmospheric neon isotopic composition. Therefore, except for neon, isotopic compositions of nonradiogenic noble gases both in the atmosphere and the solid earth are the same. Noble gas degassing from the solid earth occurred during the magma ocean stage through bubble (gas)/melt partition. Subsequent magma production and eruption throughout geological time has contributed little to atmospheric noble gases except for radiogenic components. From the comparision of 3He/4He ratios between the upper and lower mantles, we estimate that the 40Ar/36Ar ratio in the lower mantle is more than 6, 000. Hence, we argue that the lower mantle has also undergone some degree of degassing.