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Published November 1, 2002 | public
Journal Article

Re-Os and Pd-Ag systematics in Group IIIAB irons and in pallasites


Using improved analytical techniques, which reduce the Re blanks by factors of 8 to 10, we report new Re-Os data on low Re and low PGE pallasites (PAL-anom) and IIIAB irons. The new pallasite samples nearly double the observed range in Re/Os for pallasites and allow the determination of an isochron of slope 0.0775 ± 0.0008 (T = 4.50 ± 0.04 Ga, using the adjusted λ^(187)Re = 1.66 × 10^(−11) a^(−1)) and initial (^(187)Os/^(188)Os)_0 = 0.09599 ± 0.00046. If the data on different groups of pallasites (including the "anomalous" pallasites) are considered to define a whole-rock isochron, their formation would appear to be distinctly younger than for the iron meteorites by ∼60 Ma. Five IIIAB irons (Acuna, Bella Roca, Chupaderos, Grant, and Bear Creek), with Re contents ranging from 0.9 to 2.8 ppb, show limited Re/Os fractionation and plot within errors on the IIAB iron meteorite isochron of slope 0.07848 ± 0.00018 (T = 4.56 ± 0.01 Ga) and initial (^(187)Os/^(188)Os)_0 = 0.09563 ± 0.00011. Many of the meteorites were analyzed also for Pd-Ag and show ^(107)Ag enrichments correlated with Pd/Ag, requiring early formation and fractionation of the FeNi metal, in a narrow time interval, after injection of live ^(107)Pd (t_(1/2) = 6.5 Ma) into the solar nebula. Based on Pd-Ag, the typical range in relative ages of these meteorites is ≤10 Ma. The Pd-Ag results suggest early formation and preservation of the ^(107)Pd-^(107)Ag systematics, both for IIIAB irons and for pallasites, while the younger Re-Os apparent age for pallasites suggests that the Re-Os system in pallasites was subject to re-equilibration. The low Re and low PGE pallasites show significant Re/Os fractionation (higher Re/Os) as the Re and PGE contents decrease. By contrast, the IIIAB irons show a restricted range in Re/Os, even for samples with extremely low Re and PGE contents. There is a good correlation of Re and Ir contents. The correlation of Re and Os contents for IIIAB irons shows a similar complex pattern as observed for IIAB irons (Morgan et al., 1995), and neither can be ascribed to a continuous fractional crystallization process with uniform solid-metal/liquid-metal distribution coefficients.

Additional Information

© 2002 Elsevier Science Ltd. Received February 13, 2001; accepted in revised form May 16, 2002. This paper is dedicated to Dick Holland, who has been known to one of the authors, since his early radiation damage days, and to the other authors, since he began exhaling atmospheres and bringing forth oceans. We thank Richard J. Walker and Uli Ott for substantive and detailed reviews and attention to the paper. This work was supported by NASA Grant NAG5-8251. This is GPS Division Contribution No. 8730(1061). Associate editor: U. Ott

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