Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.324b&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 324
Other
Chondrites, Ll, Dating, Isotopes, Lunar Rocks, Meteorites
Scientific paper
The unique potentiallity of the ^187Re-^1870s radiochronometer stems from the chemical properties of the 2 elements. Their siderophilic nature has made the dating of the metal phase of iron meteorites and chondrites possible [1,2]. In silicate crystallization or melting, Os behaves as a strongly compatible element [3]. For Re the results hereunder show that its behavior is highly variable, depending on context. The development of the highly sensitive negative thermal ionization mass spectrometry [4,5] made possible the extensive analysis of silicate materials. We developed a chemical separation method allowing the analysis of 0,5 g of silicate with an Os blank of 0.3 pg (1pg=10^-12 g). The isotopic measurement of Os has a typical precision of a few permil for 1 pg of Os. A variety of samples has been investigated. Iron Meteorites: The main purpose of this study was to shed some light on the history of Kodaikanal. The silicate inclusions of this meteorite were reset at 3.7 Ga for both the Rb-Sr and the U-Pb chronometers [6,7]. The data from Coahuila, Canyon Diablo, Tlacotepec, and North Chile plot within 1% on the same isochron along with 3 samples from 2 locations inside the meteorite of Kodaikanal. As a consequence the Kodaikanal parent body did not form at 3.7 Ga but at about the same time as the parent bodies of the the majority of the other iron meteorites (~4.5 Ga). The only difference is that Kodaikanal suffered a strong secondary event at 3.7 Ga, which reset Rb/Sr and U-Pb chronometers but did not fractionate the Re/Os ratio. This result has already been suggested by our earlier work [8]. The St Severin Chondrite: A bulk sample and several batches of the metal separated from this chondrite were analysed. They plot on the iron meteorite isochron. However this metal is heterogeneous in Re/Os by a factor of at least 2 and indicates an uncomplete homogeinization of Re/Os between metal grains during parent body metamorphism. Basaltic Materials: Basaltic achondrites (Juvinas, Nuevo Laredo), diogenites (Roda, Tatahouine), lunar basalts (15555, 70017) and various terrestrial basalts were analyzed. So far all extraterrestrial basalts give ^187Os/^186Os ratios below 1.3 whereas old terrestrial basalts reach up to more than 100. The concentration of Os in extraterrestrial and terrestrial basalts are similar. There is no correlation with age. Re concentrations of terrestrial basalts are about 1000 times higher than extraterrestrial basalts. This difference in behavior of Re between terrestrial context and extraterrestrial context may have several origins: combined with pressure differences in the water content of the different planetary bodies may be one cause, but our preferred interpretation is that Re is present in a different oxidation state (ReVII) under terrestrial conditions. In contrast Re(IV) should be the dominant species in the Moon and in the basaltic achondrite parent body(ies). References: [1] Herr W. et al. (1961) Z. Naturforsch., 16a, 1053. [2] Luck J. M. and Allegre C. J. (1983) Nature, 302, 130. [3] Roy-Barman M. (1993) Ph.D. thesis, Paris. [4] Greaser R. A. et al. (1991) GCA, 55, 397. [5] Volkening J. et al. (1991) Int. J. Mass Spec. Ion Proc., 105, 147. [6] Gopel C. et al. (1985) Nature, 317, 341. [7] Burnett D. S. and Wasserburg G. J. (1967) EPSL, 2, 137. [8] Birck J. L. et al. (1991) Meteoritics, 26, 318.
Allègre Claude J.
Birck Jean-Louis
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