The iodine-plutonium-xenon age of the Moon-Earth system revisited

Abstract

Iodine-plutonium-xenon isotope systematics have been used to re-evaluate time constraints on the early evolution of the Earth-atmosphere system and, by inference, on the Moon-forming event. Two extinct radionuclides ((129)I, T1/2=15.6 Ma and (244)Pu, T1/2=80 Ma) have produced radiogenic (129)Xe and fissiogenic (131-136)Xe, respectively, within the Earth, the related isotope fingerprints of which are seen in the compositions of mantle and atmospheric Xe. Recent studies of Archaean rocks suggest that xenon atoms have been lost from the Earth's atmosphere and isotopically fractionated during long periods of geological time, until at least the end of the Archaean eon. Here, we build a model that takes into account these results. Correction for Xe loss permits the computation of new closure ages for the Earth's atmosphere that are in agreement with those computed for mantle Xe. The corrected Xe formation interval for the Earth-atmosphere system is [Formula: see text] Ma after the beginning of Solar System formation. This time interval may represent a lower limit for the age of the Moon-forming impact.

Keywords: Moon; age; atmosphere; xenon.

Figure 1.

Relationship between the evolution of the solar EUV flux with time and the progressive isotopic fractionation of atmospheric xenon. (a) Evolution of the solar flux with time. Figure and data are from [39]. The wavelength of ionization of Xe atoms corresponds to the range 920–1200 Å. (b) Progressive isotopic fractionation of atmospheric Xe from cosmochemical components [10] to modern atmosphere [31]. Ancient rocks record intermediate isotopic compositions: 3.5 Ga-old barites [35,36]; quartz samples [37,38]; Proterozoic deep fracture fluids [40].

Figure 2.

(a) Schematic explanation of the model and evolution of the budget of atmospheric Xe over time. The model is built with three boxes: solid Earth, atmosphere and space. Some Xe isotopes are produced by radioactive decay of ¹²⁹I, ²⁴⁴Pu and ²³⁸U (see text). After a time interval Δt (40 Ma, one outcome of the model), the Earth begins to retain its volatile elements and accumulates xenon degassed from the solid Earth to the atmosphere without isotopic fractionation. Xenon atoms escape from the atmosphere to outer space with isotopic fractionation. The evolution of the budget of atmospheric xenon shows the progressive escape of Xe atoms with time. The escape lasts until the end of the Archaean eon (t=2 Ga). At this time, the abundance has almost reached the current abundance of xenon in the atmosphere. The process is completed by further degassing of the xenon atoms. (b) Isotopic spectrum of xenon relative to the current isotopic composition of the Earth's atmosphere using ¹³⁰Xe as a reference isotope. Xe–U is the starting isotopic composition (circles). The fractionated Archaean atmosphere (around 1% amu⁻¹) is shown with squares and the ‘artificial’ current isotopic composition of the reference solution is shown with stars. The current isotopic composition is reproduced within 0.7% or better.

Figure 3.

(a) Evolution of the atmospheric content of ¹²⁹Xe derived from the decay of ¹²⁹I with time. The non-corrected amount gave a closure age of 98 Ma for the I–Xe system. After correction for subsequent loss, the age becomes 41 Ma. (b) Evolution with time of the ratio of radioactive products in the atmosphere (¹²⁹Xe(I) and ¹³⁶Xe(Pu)). The non-corrected ratio gave a closure age of 66 Ma for the I–Pu–Xe system. After correction, the age becomes 34 Ma, in agreement with the time of closure of the I–Xe system and with the closure age of the mantle given by the mantle samples [4,32].