Chemically fractionated fission-xenon in meteorites and on the earth

Abstract

This is a report on the nature of isotopically anomalous xenon, which has been detected in two Ca-Al-rich inclusions of the Allende carbonaceous chondrite. It is extremely enriched in 132Xe, 129Xe, and to a lesser extent in 131Xe. Similar large excesses of 132Xe as well as of 131Xe, 134Xe, and 129Xe have previously been found in material processed in a natural nuclear reactor (Oklo phenomenon). Excess of these isotopes had also been encountered in MORB-glasses, in an ancient Greenland anorthosite. Thus, this Xe-type, which had previously been termed “alien” ( Jordon et al., 1980a) does not seem to be unique. To determine the origin of “alien” Xe, we analysed Xe 1. (a) in neutron irradiated pitchblende and in the irradiation capsule, 2. (b) in non-irradiated extremely fine-grained pitchblende (so-called Colorado-type deposit), and 3. (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 natural reactor was determined. The results of these dedicated experiments demonstrate that the formation of alien Xe is due to the migration of the radioactive precursors of the stable isotopes 134Xe, 132Xe, 131Xe, and 129Xe. Due to this reason we now call it CFF-Xe—Chemically Fractionated Fission Xenon. Prerequisites for its formation are the simultaneous prevalence of two conditions: 1. (1) fission (of 238U, 235U, and/ or 244Pu) and 2. (2) a physicochemical environment (temperature, pressure, 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 129Xe in mantle rocks and natural gases are necessarily connected with the decay of primordial 129I.