Re-Os ISOTOPIC AND HIGHLY SIDEOPHILE ELEMENT SYSTEMATICS OF PALLASITES

Introduction: Pallasites are of great interest to meteoriticists because they may represent samples of the interfaces between core and mantle materials in asteroids. Further, certain pallasites may be genetically related to the group IIIAB irons, based on similar ∆O values [1], potentially permitting the study of core-mantle evolution within a single body. To better understand the origin and evolution of pallasites we have examined Re-Os, Pt-Os and highly siderophile element (HSE; Re,Os,Ir,Ru,Pt,Pd measured here) abundances in a variety of pallasite metals, including those from the Main Group (PMG), including four with anomalous metal (PMG-am), as defined by Wasson and Choi [2]. We have also analyzed Cold Bay to complement our previous data for the Eagle Station grouplet (PES). The objectives of this work are to use the isotope systems to examine closure ages for HSE migration, and consider processes that may relate the metal phases via modeling of the high-precision HSE isotope dilution data. Ultimately, models that are viable from a HSE standpoint will be considered with respect to other diagnostic elements. Analytical Methods: Chemical separation techniques used were similar to previous studies [e.g. 3]. Blanks for Re, Pt, Os, Pd, Ru, and Ir averaged 2, 300, 2, 30, 50 and 1 pg, respectively. Osmium measurements were accomplished via negative thermal ionization mass spectrometry (Sector 54). The isotope dilution measurements of all other elements were done via static multi-collector ICP-MS (Nu Plasma) using faraday buckets. Fractionation was monitored and corrected via interspersal of samples with standards. All concentration data are +0.5% or better. Results: Concentrations of Ir and Os measured here are generally similar to those reported previously for pallasites [e.g. 1-2,4]. As has been previously noted, the ranges of concentrations within the PMG of Ir and Os are large (Fig. 1a-b), though they range considerably less than for the group IIAB and IIIAB irons. Platinum and Pd abundances vary much less over the range of Ni or Au contents represented by the PMG. The Re/Os ratios for PMG vary from 0.310 to 1.029. Discussion: A Re-Os isochron regression for the 12 PMG examined yields an age of 4548 ± 87 Ma and initial Os/Os = 0.09515±0.00091. There is substantially more variance of data about this isochron than for any of the IIAB, IIIAB, IVA, IVB iron groups analyzed previously [3,5-6]. The observation that some pallasites plot well to the right of the isochron (not shown) suggests substantial opensystem behavior of Re and/or Os well after initial crystallization. This may have important bearing on the interpretation of other HSE abundances. One previous study of Re-Os isotopes in pallasites also reported modest non-isochronous behavior [4].