The Re-Os System in Planetary Bodies

Jean Louis Birck Laboratoire de Géochimie et Cosmochimie, IPG, 4, place Jussieu, 75252 Paris Cedex 05, France

Claude J. Allègre Laboratoire de Géochimie et Cosmochimie, IPG, 4, place Jussieu, 75252 Paris Cedex 05, France

Among long-lived chronometers, the 187Re-187Os system occupies a particular position. With the exception of the chacophilic nature of Pb, all other radiometric couples have a strong lithophilic affinity and involved elements concentrate in planetary crusts. At the reverse both Re and Os show a highly siderophilic character in fractionation processes and concentrate in planetary cores. In systems where silicate fractionation is involved, Os displays a highly compatible character in all ocasions whereas Re has a more variable behaviour from compatible to moderately incompatible as will be discussed herebelow. Despite these unique properties, their widespread usage in isotope geochemistry has been hampered by their extremely low abundances in most silicate systems together with major difficulties to ionize Os in mass spectrometers. These experimental problems have been overcome in the last few years by using negative oxide thermoionisation mass spectrometry (Völkening et al., 1992; Creaser et al., 1991) and adapted redesign of chemical extraction methods of Os. 0.5 g of silicate material can be analysed with a Os blank of 0.13±0;01 pg. The isotopic measurement of Re and Os has a typical precision of a few permil on 1 pg of pure element.

Iron Meteorites

A high precision isochron is obtained on these Os rich objets (Coahuila, Canyon Diablo, Tlacotepec and North Chile): 4.623±0.018 Ga and 187Os/186OsI= 0.09565±0.00017. The age and initial Os are in good agreement with other laboratories but is somewhat biased toward high ages because of the uncertainty on the 187Re decay constant. The main purpose of the present study was to shed some light on the history of Kodaikanal. The silicate inclusions of this meteorite have been reset at 3.7 Ga for both the Rb-Sr and the U-Pb chronometers (Göpel et al., 1985; Burnett and Wasserburg, 1967). Four samples from 2 locations inside the meteorite plot close to the isochron defined by the other iron meteorites. As a consequence the parent body of Kodaikanal did not form at 3.7 Ga but about the same time as the parent bodies of the the majority of the other iron meteorites. The only difference is that Kodaikanal suffered a strong secondary event at 3.8 Ga which resetted Rb/Sr and U-Pb but did not fractionate the Re/Os ratio. This result has allready been suggested by our earlier work (Birck et al., 1991).


Samples of St Severin, Kernouvé and several batches of the metal phases of these chondrites were investigated and yield data which plot on or close to the here above iron meteorite isochron. However this metal is heterogeneous in Re/Os by a factor of at least 2 indicating an uncomplete homogeineization of Re/Os between metal grains during parent body metamorphism prior to 4.51 Ga. This reflects also the Re/Os heterogeneity of the metal grains in the region of the nebula from which the parent body of this meteorite originates. There is also evidence that within a given chondrite Re-Os in troilite is not in equilibrium with the metal even in the most metamorphised chondrites.

Basaltic materials

This includes: basaltic achondrites (Juvinas, Nuevo Laredo), diogenites (Roda, Tatahouine), 2 lunar basalts (15555,70017) and various terrestrial basalts. A clearcut difference appears in the data between terrestrial and other basalts: with the exception of Mars, extraterrestrial basalts give 187Os/186Os ratios below 1.5 whereas old terrestrial basalts reach up to more than 100. The concentrations of Os in all basalts are similar, whereas Re concentrations of terrestrial basalts are from 100 to 1000 times higher than in extraterrestrial basalts.This clearcut difference in behaviour of Re(Re/Os) between terrestrial context and extraterrestrial context may have several origins: differences in pressure, composition, water content in the different planetary mantles may be causes but they allways overlap at least partially with the terrestrial domain. Our prefered interpretation is that the behaviour of Re is a strong function of oxygen fugacity in the source region of basalts (Birck and Allegre, 1994) and therefrom that Re is present in a different oxydation state (ReVII) in terrestrial conditions whereas ReIV should be the dominant specie in the moon and the basaltic achondrite parent body(ies). This is illustrated also by the intermediate position of martian basalts (SNC meteorites).


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