About 129Xe* in meteoritic nanodiamonds

The analysis of excess Xe-129 in meteoritic nanodiamonds and the kinetics of its release during stepwise pyrolysis allow to Suggest that (1) in the solar nebula I-129 atoms were adsorbed onto nanodiamond grains and (or) chemisorbed by forming covalent bonds with carbon atoms. Most I-129 atoms existed in a surface connected state, but a minor amount of them was in nanopores of the grains. At radioactive decay of I-129 the formed Xe-129 (Xe-129*) was trapped by diamond grains due to nuclear recoil. (2) During thermal metamorphism or aqueous alteration, the surface-sited I-129 atoms were basically lost. On the basis of these assumptions and calculated concentrations of Xe-129* in meteoritic nanodiamonds it is shown that the minimum closing time of the I-Xe system for meteorites of different chemical classes and low petrologic types may be about one million years relative to the minimally thermally metamorphized CO3 meteorite ALHA 77307. With increasing metamorphic grade the closing time of the I-Xe system increases and call range LIP to several ten millions years. This tendency is in agreement with ail onion-shell model of structure and cooling history of meteorite parent bodies where the temperature increases in the direction from surface to center of the asteroids. (C) 2008 Elsevier Ltd. All rights reserved.