Boreskov

PL‐2ANALYSIS OF ISOTOPE SYSTEM DATA (Hf‐W, Rb‐Sr, J, Pu‐Xe, U‐Pb)AS APPLIED TO THE PROBLEM OF PLANET FORMATION.CASE STUDY: EARTH‐MOON SYSTEMGalimov E.M.V.I. Vernadskii Institute of Geochemistry and Analytical Chemistry RAS, Moscow, RussiaIsotope data provide important geochemical constraints for models of accumulation ofplanets, including the Earth‐Moon system.Short living isotope 182 Hf decays into 184 W with T 1/2 ~ 9 million years, so that Hf‐Wsystem enlightens the very early stage of accumulation of planets. In particular, analysis ofthe 182 Hf/ 184 W system shows that the Moon could not have come into existence earlier than50 million years after formation of the Solar system. Since, on the other hand, there arecertain rocks on the Moon whose age has been estimated as approximately 70 million yearsafter formation of the Solar system, the Hf‐W system indicates that the likely interval offormation of the Moon would be 50‐70 million years.In contrast to 182 Hf 87 Rb is a very long living isotope, yielding 87 Sr. The lunar initial87 Sr/ 86 Sr suggests some higher Rb content in the Moon than it is observed. Rubidium mayhave been lost as a volatile element. But because of its high atomic weight, it cannot runaway from the lunar surface. Rb may escape only from the heated surfaces of small bodiesor particles. It makes likely that the pre‐lunar material existed in a dispersed form at thetime preceded to emerging of the Moon as a condensed body (i.e. during about 50‐70 Mlnyears).Indications on the preliminary dispersed stay may also have been inferred as regard tothe Earth accumulation. There is so called the xenon isotope paradox, the gist of which isthat the atmospheric xenon contains considerably less 129 Хе and 134 Хе isotopes generatingfrom 129 J and 244 Pu than it could be expected. It appears to be as though the radiogenic Хеthat had been generating during the initial 120 million years of existence of the Earth waslost. Хе is an atmophilic element that concentrates in the atmosphere of the planet. But dueto its high atomic weight, it cannot run away from the surface of a planet. It may have beenlost either owing to catastrophic loss of the atmosphere happened 120 millions years, or in8