The origin of rare gases on Earth: The noble gas `subduction barrier' revisited

Manuel Moreira (1) , Aude Raquin (1)

1 IPGP - Institut de Physique du Globe de Paris

The origin of the Earth's atmosphere can be constrained by the study of noble gases in oceanic basalts. If it is clear that the mantle is degassed and formed part of the present atmosphere, it has been proposed that an important subduction of atmospheric noble gases in the mantle occurred during Earth's history, altering the primordial signature of the solid Earth. This subduction process has been suggested on the basis of the measurements of light xenon isotopes in CO ₂ well gases. Moreover, the fact that the ³⁸Ar/ ³⁶Ar ratio is atmospheric in all oceanic basalts, even for uncontaminated samples (e.g. with high ²⁰Ne/ ²²Ne), may also suggest that a massive subduction of atmospheric argon occurred, if the primitive Earth had a solar-like ³⁸Ar/ ³⁶Ar. This also implies that the atmosphere suffered a massive gas loss accompanied by mass fractionation (e.g. hydrodynamic escape) after mantle degassing or that a late veneer with an atmospheric composition occurred. Such a hypothesis is explored for rare gases, by developing a model in which degassing and subduction of atmospheric noble gases started ∼4.4 Ga ago. In the model, both radiogenic and non-radiogenic isotopic ratios are used (e.g. ³⁸Ar/ ³⁶Ar and ⁴⁰Ar/ ³⁶Ar; ¹²⁴Xe/ ¹³⁰Xe and ¹²⁹Xe/ ¹³⁰Xe) to constrain the subduction flux and the degassing parameters. It is shown that subduction and massive contamination of the entire mantle is possible, but implies that the ⁴⁰Ar/ ³⁶Ar and the ¹²⁹Xe/ ¹³⁰Xe ratios were higher in the past than today, which is not observed in Archean samples. It also implies that the sediments and the altered oceanic crust did not loose their noble gases during subduction or that the contaminated mantle wedge is mixed by the convective mantle. Moreover, such a model has to apply to the oceanic island source, since this later shows the same signature of argon and xenon non-radiogenic isotopic ratios. A scenario where the isotopic compositions of the argon and xenon were settled before or during accretion is therefore preferred to the subduction.

Domaines: Planète et Univers [physics]

Manuel Moreira, Aude Raquin. The origin of rare gases on Earth: The noble gas `subduction barrier' revisited. Comptes Rendus. Géoscience, 2007, 339, pp.937-945. ⟨10.1016/j.crte.2007.09.009⟩. ⟨insu-03603218⟩