RE-OS AND PD-AG SYSTEMATICS IN GROUP IIIAB IRONS AND IN PALLASITES

Abstract

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 λ187Re = 1.66 x 10-11 a-1) and initial (187Os/188Os)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 (187Os/188Os)0 = 0.09563 +/- 0.00011. Many of the meteorites were analyzed also for Pd-Ag and show 107Ag enrichments correlated with Pd/Ag, requiring early formation and fractionation of the FeNi metal, in a narrow time interval, after injection of live 107Pd (t1/2 = 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 107Pd-107Ag 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 , and neither can be ascribed to a continuous fractional crystallization process with uniform solid-metal/liquid-metal distribution coefficients.