text: Xe is the heaviest noble gas in the Earth's atmosphere. It has seven stable isotopes (Xe,Xe,Xe,Xe,Xe, Xe, Xe) and two isotopes (Xe, Xe) with long-lived half-lives. Xe has four synthetic radioisotopes with very short half-lives, usually less than one month.
  Xenon-129 can be used to examine the early history of the Earth. Xe was derived from the extinct nuclide of iodine, iodine-129 or I (with a half-life of 15.7 Million years, or Myr), which can be used in iodine-xenon (I-Xe) dating. The production of Xe stopped within about 100 Myr after the start of the Solar System because I became extinct. In the modern atmosphere, about 6.8% of atmospheric Xe originated from the decay I in the first ~100 Myr of the Solar Systems history, i.e., during and immediately following Earths accretion.
  Fissiogenic Xe isotopes were generated mainly from the extinct nuclide, plutonium-244 or Pu (half-life of 80 Myr), and also the extant nuclide, uranium-238 or U (half-life of 4468 Myr). Spontaneous fission of U has generated ~5% as much fissiogenic Xe as Pu. Pu and U fission produce the four fissiogenic isotopes, Xe, Xe, Xe, and Xe in distinct proportions. A reservoir that remains an entirely closed system over Earth's history has a ratio of Pu- to U-derived fissiogenic Xe reaching to ~27. Accordingly, the isotopic composition of the fissiogenic Xe for a closed-system reservoir would largely resemble that produced from pure Pu fission. Loss of Xe from a reservoir after Pu becomes extinct (500 Myr) would lead to a greater contribution of U fission to the fissiogenic Xe.