Fractionation of Platinum Group Elements in Carboneceous Chondrites

Introduction: Although chondrites are among the most primitive materials accessible to us, their chemical compositions are highly variable among groups of chondrites, reflecting several fractionation processes including condensation and accretion processes in the early solar system. Considering that CI chondrites represent a solar system standard in the elemental abundance [1] and that calcium and aluminum-rich inclusions in CV chondrites including Allende show the oldest age [2], the study of carbonaceous chondrites gives us important constrains on nebular conditions and processes. Among elements, platinum group elements (PGEs; Os, Ir, Ru, Pt, Rh and Pd) play an important role in studying geochemical and cosmochemical processes because of their refractory and siderophile natures. PGEs are predicted to condense into solid material from a gas of solar composition at high temperature (>1250C at 10atm) [3]. In ordinary and enstatite chondrites, PGEs are quantitatively present in metal phases and their relative abundances are known to have been affected by several processes occurring in the nebula (during accretion to the parent body) and in the parent body [4]. Detailed abundances of PGE in carbonaceous chondrites were discussed by Jochum [5] based on precise PGE abundances determined by spark source mass spectrometry, showing that siderophile elements (except Pd) having similar nebular condensation temperature well correlate with each other. Later, Horan et al. [6] analyzed a suit of carbonaceous chondrites for PGEs by using negative thermal ionization mass spectrometry and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) and found that Pd/Ir ratios in carbonaceous chondrites decrease in the order of CI>CM~CO>CV. Such a fractionation can be primarily understood in terms of nebular processes, but an aqueous process cannot be ruled out in consideration of systematic variation of some PGEs in CI chondrites [7]. In this study, we aimed to extend the study on PGE abundances in carbonaceous chondrites following above-mentioned preceding studies. Considering the easiness in sample preparation, we have chosen five groups of carbonaceous chondrites; CI, CM, CO, CV and CK. As these chondrites contain few metal grains, homogeneous powder can be prepared even with a small amount of specimen. Both non-Antarctic and Antarctic meteorites were analyzed. Samples and analytical technique: Samples in chunk were ground to powder in a clear agate mortar. Sample mass for most meteorites exceeds 200mg. The quantity of fall CM meteorites (except Murchison) is very limited, ranging from 20 to 50mg. The Smithsonian Allende powder sample has been routinely used as a monitor. In addition, the powdered Orgueil also has been analyzed at every analytical run. All PGEs were determined by ICP-MS after NiS fire assay preconcentration [8]. Isotope dilution was applied to PGE except Rh, which was determined by calibration method with yield correction. Results and discussion: A total of 11 and 6 runs were performed for Orgueil and Allende, respectively. Our Orgueil and Allende data are shown in Fig. 1, where PGE contents are normalized to CI values [1]. There can be seen an excellent agreement between our data and literature data of two preceding studies for Allende. The reproducibility (%; 1 s.d.) of our 11 sets of data for Orgueil is 5.2 for Os, 4.6 for Ir, 2.0 for Ru, 4.0 for Pt, 4.1 for Rh and 2.6 for Pd. Note that these values cover both the reproducibility in ICP-MS and the heterogeneity of PGE in powdered Orgueil. CInormalized PGE patterns for Orgueil shown in Fig. 1 seems to be variable compared with those for Allende, with our data being the lowest among three sets of data including [5] and [6]. In spite of such a variation, it is notable that all patterns are parallel to each other. The absolute abundance of PGEs in Orgueil must be highly dependent on the presence of volatile components like indigenous water and/ or adhering moisture. Considering a consistency in PGE data between our values and literature values for Orgueil and, especially, Allende, we may be confident with our PGE data obtained in this study. From analytical data for fall carbonaceous chondrites, CI-normalized PGE abundances are found to be correlated with condensation temperature, with refractory PGEs being more abundant, regardless of the class of carbonaceous chondrites. Weathered meteorites tend to show anomalous abundances of PGEs, among which Pt seems to be the most heavily affected, followed by Os. This is clearly demonstrated by the data of Colony, which is known to be a heavily weathered CV find [9]. Most Antarctic meteorites seem to be not or scarcely affected by weathering while some are considerably affected like Colony. The degree of weathering effect may be monitored by Ni and Co contents; these elements are highly depleted in Colony and some heavily Lunar and Planetary Science XXXVII (2006) 1928.pdf