Ruthenium Isotope Fractionation During Planetesimal Core Crystallization

Introduction: The chemical compositions of most iron meteorites are consistent with crystal-liquid partitioning during fractional crystallization of metallic melt [e.g., 1]. On this basis, these 'magmatic' iron meteorites (e.g., groups IIAB and IIIAB) are thought to represent samples of the metallic cores of small differentiated planetary bodies. The process of fractional crystallization during core solidification should in principle also have induced an isotopic fractionation between earlyand late-crystallized irons from a given core. During crystallization some elements are continuously removed from the liquid metal and so even a small isotopic fractionation between solid and liquid metal would lead to a large overall fractionation as crystallization proceeded. Thus, the identification of stable isotope fractionation trends within a given group of magmatic irons would provide strong evidence for the formation of these samples through fractional crystallization of metallic magma. However, until now no stable isotope Rayleigh fractionation trends have been identified in iron meteorites. Here we use Ru stable isotopes to investigate the possible isotope fractionation during solidification of planetesimal cores. Ruthenium is a highly siderophile element and as such has been quantitatively removed into the cores of differentiated bodies. Thus, the Ru isotopic composition of the bulk core should be similar to that of the undifferentiated body. Moreover, Ru is strongly fractionated within most iron groups, where early-crystallized irons have high and late-crystallized samples have low Ru contents. This strong chemical fractionation makes Ru a promising element to search for isotope fractionation induced by core crystallization. For this study we selected six IIAB and seven IIIAB irons covering almost the entire crystallization sequence of both groups. We also analyzed four ordinary chondrites to assess the Ru isotopic composition of undifferentiated material. Additional analyses of IVA and IVB iron meteorites are currently underway. Analytical Methods: Iron meteorite samples (0.05 to 0.2 g) were spiked with a Ru-Ru double spike before digestion with 20 ml of reverse aqua regia in closed Teflon beakers at 120°C for 24 hours. Ordinary chondrites (0.5 g) were spiked prior to Carius tube digestion using reverse aqua regia at 220°C for 48 hours [2]. After digestion, Ru was separated from the sample matrix using a combination of cation exchange chromatography and a Savillex PFA distillation unit; the Ru cut from the cation exchange column is oxidized in a solution of H2SO4 and CrO3, and volatile Ru was trapped in H2O2 [3]. The Ru isotope measurements were conducted using a Neptune Plus MC-ICPMS at Münster combined with an ESI Apex-IR desolvating system. All measurements were performed using 200 ppb Ru solutions. Each measurement consisted of an on-peak baseline measurement on a solution blank (40×4.2s) followed by 50 cycles of 4.2s of sample or standard measurements. Isobaric interferences were monitored using Mo and Pd. The data reduction was made off-line using the Double Spike Toolbox [4]. The Ru isotope results are given in δRu as the permil deviation from an Alfa Aesar plasma standard solution (Lot#61300952). The external reproducibility (2 s.d.) of the Ru isotope measurement is 0.04 ‰ for δRu. Results: The Ru stable isotope compositions measured for ordinary chondrites and iron meteorites are shown in Fig. 1. The four ordinary chondrites have indistinguishable δRu averaging at ca. –0.02 ‰. Iron meteorites, in contrast, show variable δRu with values up to ~0.85 ‰. In Fig. 1, the IIAB and IIIAB samples are sorted by decreasing Ru content from bottom to top within both groups. Of note, samples with high Ru contents have Ru stable isotopic compositions similar to ordinary chondrites, while samples with lower Ru contents display increasingly heavier compositions (Fig. 1).