Constraints on the origin of the 129Xe on Earth using the tellurium double beta decay

Abstract

It is admitted that 129I ( T 1/2 = 17 Ma) existed at the moment of the Earth's formation. Mid Oceanic Ridge basalts and some oceanic island basalts present anomalies of 129Xe compared to air. This is generally attributed to the 129I decay and to an early degassing of the mantle since the 129Xe/ 130Xe ratio reflects the 129I/ 130Xe ratio. Some rocks, such as Archean cherts, diamonds or samples from the continental crust present also 129Xe anomalies compared to air. This can be attributed to a mantle derived component or in some cases, it has been evoked a possible 128Te( n, γ ) 129Xe origin (neutron capture) to explain these 129Xe anomalies. We demonstrate that the 128Te( n, γ) 129Xe is not the source of the 129Xe because if some Te is present, the double beta decay of the 130Te will decrease the 129Xe/ 130Xe ratio rather than to increase it. Moreover, we propose a model of production of xenon isotopes due to the different neutron captures on I, Ba and Te, to the spontaneous fission of 238U and to the different double beta decays. We apply this model to different environments such as recycled carbonates or manganese crusts, the continental crust or Archean cherts. Our model shows that, clear 128Xe excesses will be the main consequence of a high neutron flux due to the presence of Iodine. Furthermore, the 129Xe anomalies observed in rocks can only be interpreted in term of a mantle contribution.