HR: 08:15h
AN: V21F-02    [Abstracts]
TI: Use and misuse of extinct and long-lived radioactivities to reconstruct the early hystory of the Solar System.
AU: * Allegre, C J
EM: allegre@ipgp.jussieu.fr
AF: IPGP, 4 place jussieu, Paris, 75005 France
AU: Manhes, G
EM: manhes@ipgp.jussieu.fr
AF: IPGP, 4 place jussieu, Paris, 75005 France
AU: Moreira, M A
EM: moreira@ipgp.jussieu.fr
AF: IPGP, 4 place jussieu, Paris, 75005 France
AB: In the recent years, with the progress of experimental technics in Isotope Geology, the use of extinct radioactivities to reconstruct the early Solar System evolution has been generalized. In several cases, their records have been presented in contradiction with the independant results obtained by long-lived radioactivities. We claim that in most cases, these contradictions result from the differences in recording properties of the two types of radioactivities. Extinct radioactivities have an high chronometric resolution but a limited recording time scale. At the opposite, long-lived radioactivities have a relatively low chronometric resolution for earliest time but they deliver a robust time record. Those two types of properties have to be used simultaneously to reconstruct a coherent scenario rather to choose systematically the informations derived from the extinct radioactivities. We give three examples. One is the case of Earth's atmosphere formation with the records of 129Xe in one hand, of 40Ar in the other hand and with the 244Pu between. The second example is the chronology of the formation of the Earth's core derived from 182Hf-182W and U-Pb systematics. The third case is the differential chronology of an early object like meteorites parent bodies, by 146Sm-142Nd and 147Sm-143Nd. We shows that for a perturbed system, a concordance is obtained, but it corresponds to a wrong age. The second set of problems concerns the informations which are derived from the comparison of measured daughter isotope abundances in distinct single samples of different planetary objects. The significance of these records is related to two fundamental unknowns : the age of the objects and the chemical parent/daughter ratios. We show that for recently published studies concerming 26Mg and 142 Nd abundances, the results have no direct and simple chronological significance unlike what was published whitout the necessary care. The taking in consideration of the whole of these mesarements in a standard quantitative modelling of radioactive systems furnish a new look of the early Earth history.
DE: 1027 Composition of the planets
DE: 1030 Geochemical cycles (0330)
DE: 1040 Radiogenic isotope geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005