The Accretionary Origin of Noble Gases in the Earth’s Mantle

Introduction: An outstanding problem in the Earth and planetary sciences is the source and mechanism by which the Earth-like planets accreted their highly volatile elements. Unlike highly refractory elements, which preserve close to solar proportions in even quite evolved meteorites and the inner planets, moderately refractory and more volatile elements are depleted relative to the highly refractory elements. While the degree of depletion of moderately refractory and even volatile elements all appear to be related to degree of parent body processing [1], the origin of the planetary ice-forming elements and inert noble gases is more complex. This is because gravitational capture of the solar nebula as an early atmosphere and subsequent equilibration with a magma ocean has the potential to source these elements in considerable excess to that found trapped in accreting material [2–5]. Well gases – a unique noble gas resource: Early studies of terrestrial noble gases in erupted basalts found isotopic anomalies in 3 He/ 4 He ratio when compared to air, a 3 He excess implying a previously undegassed primordial component within the mantle [6]. It is still the case today that most studies of terrestrial volatiles focus on xenoliths and basalts. However, the eruptive origin of these samples may introduce elemental fractionation during magmatic degassing and the low abundances of noble gases retained in erupted samples often leads to substantial air contamination [7]. In contrast, noble gas data from magmatic CO2 natural gases have proved to be an additional and invaluable resource in these investigations. Measurements from a single well head yielded 3 He/ 4 He, 20,