Silver Isotope Fractionation in Chondrites

I n t r o d u c t i o n : The study of Ag isotopic compositions is of particular interest because of the extinct radionuclide Pd, which decays to Ag with a half-life of 6.5 Myrs. This chronometer has been successfully applied to unravel formation and differentiation processes of iron meteorites [1-3]. Compared to the early measurements by TIMS, the use of multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) brought more than an order of magnitude improvement in analytical precision for Ag isotopic determinations. This made Pd-Ag studies attractive for meteorites with low Pd/Ag ratios such as volatile-rich iron meteorites and chondrites [2]. However, the higher precision also raises the issue of whether small variations in Ag isotopic composition reflect Pd decay or instead are due to chemically-induced mass fractionation of Ag. Silver is a moderately volatile element with a half-mass condensation temperature of 952 K. In general, Ag in chondrites shows – like other volatile elements depletions in concentration with increasing metamorphic grade. These depletions may have been accompanied by stable isotope fractionation. The first Ag isotope measurements of chondrites were performed on Floyed (L4) and Rose City (H5) [1]. These TIMS analyses yielded a Ag isotopic composition identical to the terrestrial standard within the analytical precision (10 to 20 ε, where ε is the deviation of the sample relative to the standard expressed in parts per 10000). A limited number of MC-ICPMS measurements on other chondrites followed [2,3]. While the first measurement of the carbonaceous chondrite Allende was hampered by analytical difficulties [2], Woodland et al. [3] reported a value of –1.1 ± 1.8ε relative to NIST SRM978a. We further improved the analytical procedure for Ag isotope measurements using MCICPMS and report new high precision Ag isotope data on carbonaceous, enstatite and ordinary chondrites in order to investigate volatilization processes that occurred in the early solar system, Techniques: The Ag isotope measurements were performed on the Axiom MC-ICPMS at the Department of Terrestrial Magnetism. Since Ag has only two naturally occurring isotopes (Ag and Ag), standard and samples were doped with equal amounts of Pd and the mass fractionation corrected to a fixed Pd isotopic composition. This correction along with sample-standard bracketing was employed to control the mass fractionation induced by the MCICPMS. In the beginning of the study, a Cetac MCN 6000 desolvating nebulizer was used. While the external reproducibility obtained for the NIST SRM978a Ag standard was always better than 0.5ε (2σ) with this approach, the same standard solution passed through the ion exchange procedure showed variations of ~2ε. The same effect was observed, when the Ag standard solution was doped with small amounts of Zn standard. A similar difference was previously reported between the precisions obtained for samples and standards by [3], who used a Nu Plasma MC-ICPMS in conjunction with a MCN6000. However, after we switched to a conventional spraychamber (using the so called “wet plasma”), the discrepancy disappeared. This suggests that the MCN 6000 induces mass fractionation that is not identical for Ag and Pd. For this reason, the Ag isotope data reported here were all measured by wet plasma, although this requires ~30% more Ag due to the lower ionization/sample transmission efficiency of the wet plasma. In addition, an aliquot of ~10% of the dissolved sample was taken to determine the Pd and Ag concentrations by isotope dilution. Results: The results for bulk samples of 2 carbonaceous, 8 ordinary and the enstatite chondrite Abee are shown in Fig. 1. Several splits of Allende