The noble gas "subduction barrier" revisited
Résumé
It has been proposed that an important subduction of atmospheric noble gases in the mantle occurred during Earth's history, on the basis of the measurements of light xenon isotopes in CO2 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 air subduction started ~4.4Ga ago. In the model, both radiogenic and non-radiogenic isotopic ratios are used 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 do not lose 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 OIB source since it shows the same isotopic signature of argon and xenon. A scenario where the isotopic composition of the argon and xenon were acquired before or during accretion is therefore preferred to the subduction hypothesis (e.g. irradiation by solar wind for argon).