CHEMICALLY FRACTIONATED FISSION-XENON IN METEORITES AND ON THE EARTH

This is a report on the nature of isotopically anomalous xenon, which has been detected in two Ca-Al-rich inclusions of the Allende carbonac eous chondrite. It is extremely enriched in Xe-132, Xe-129, and to a l esser extent in Xe-131. Similar large excesses of Xe-132 as well as of Xe-131, Xe-134, and Xe-129 have previously been found in material pro cessed in a natural nuclear reactor (Oklo phenomenon). Excess of these isotopes had also been encountered in MORB-glasses, in an ancient Gre enland anorthosite. Thus, this Xe-type, which had previously been term ed ''alien'' (JORDAN et al., 1980a) does not seem to be unique. To det ermine the origin of ''alien'' Xe, we analysed Xe (a) in neutron irrad iated pitchblende and in the irradiation capsule, (b) in non-irradiate d extremely fine-grained pitchblende (so-called Colorado-type deposit) , and (c) in sandstone taken from the epicentre of an atomic explosion . In addition, the isotopic composition of xenon released by stepwise degassing and after selective dissolving of rocks from the Oklo natura l reactor was determined. The results of these dedicated experiments d emonstrate that the formation of alien Xe is due to the migration of t he radioactive precursors of the stable isotopes Xe-134, Xe-132, Xe-13 1, and Xe-129. Due to this reason we now call it CFF-Xe-Chemically Fra ctionated Fission Xenon. Prerequisites for its formation are the simul taneous prevalence of two conditions: (1) fission (of U-238, U-235, an d/or Pu-244) and (2) a physicochemical environment (temperature, press ure, fluidity) at which the precursors of xenon (mainly Te and I) are mobile. Taking into account the occurrence of xenon in meteorites and terrestrial rocks, not all excesses of Xe-129 in mantle rocks and natu ral gases are necessarily connected with the decay of primordial I-129 .