ON ORIGIN OF Xe-HL IN METEORITIC NANODIAMONDS

Xenon in the Presolar Diamonds: The finest grains of nanodiamonds identified in the carbonaceous and nonequilibrium ordinary chondrites are the most abundant relic grains, for which, as well as for the silicon carbide and graphite, the presolar origin is assumed [1].They could be apparently formed by condensation in cool stellar atmospheres or in the extreme PT-conditions of reprocessing the matter by shock waves at the supernova explosions [2]. The supposition of the presolar nature of the nanodiamond grains is based on the distinctive isotopic anomalies of the noble gases, which were trapped by the grains at their synthesis and growth or they were impregnated into the grains later [1]. Apparently, the xenon anomaly is the most intriguing and the widely discussed one. In the case of the presolar diamonds the bimodal behavior of the xenon release is observed: mainly as the Xe-P 3 component with the normal isotopic composition in the low temperature range, and as the anomalous Xe-HL component with an exotic isotopic composition in the high temperature range [1]. In comparison with the solar xenon, the Xe-HL component is about twice enriched with the light neutron-deficient isotopes 124 Xe, 126 Xe, as well as with the heavy neutron-rich isotopes 134 Xe, 136 Xe. In the 9-isotopic system of xenon, different isotopes were formed in different nucleosynthesis processes. The heavy neutron-rich isotopes 131,132,134,136 Xe are the products of the r-process; they can also be formed in the fission of 244 Pu and 238 U. The 128–130 Xe isotopes are the products of the s-process, but 129 Xe can also be radiogenic because of the decay of the extinct radionuclide 129 I. Finally, the neutron-deficient isotopes 124,126 Xe are products of the p-process. At the same time, all isotopes of Xe may have cosmogenic components, formed in the spallation reactions of Ba, Cs, Ce, and La with high-energy particles. The protosolar matter was mainly formed from matter of a giant gas–dust nebula, which, during its 10-Myr lifetime before collapse had been uniformly mixed with the products of nucleosynthesis of about ten supernovae by supersonic turbulence [3]. Namely from the results of such mixing, one should deduce the primordial isotopic composition of many noble gases, and, first of all, the heavy xenon. In the process of further evolution of the matter, the isotopic relations of gases might be changed, being subjected to different mechanisms of fractionation, which, for the most part, …