Accurate measurement of silver isotopic compositions in geological materials including low Pd/Ag meteorites

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Abstract

Very precise silver (Ag) isotopic compositions have been determined for a number of terrestrial rocks, and high and low Pd/Ag meteorites by utilizing multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). The meteorites include primitive chondrites, the Group IAB iron meteorites Canyon Diablo and Toluca, and the Group IIIAB iron meteorite Grant. Silver isotopic measurements are primarily of interest because 107Ag was produced by decay of the short-lived radionuclide 107Pd during the formation of the solar system and hence the Pd-Ag chronometer has set constraints on the timing of early planetesimal formation. A 2σ precision of ±0.05‰ can be obtained for analyses of standard solutions when Ag isotopic ratios are normalized to Pd, to correct for instrumental mass discrimination, and to bracketing standards. Caution must be exercised when making Ag isotopic measurements because isotopic artifacts can be generated in the laboratory and during mass spectrometry. The external reproducibility for geological samples based on replicate analyses of rocks is ±0.2‰ (2σ).

All chondrites analyzed have similar Ag isotopic compositions that do not differ significantly (>0.3‰) from the ‘terrestrial’ value of the NIST SRM 978a Ag isotope standard. Hence, they show no evidence of excess 107Ag derived from 107Pd decay or, of stable Ag isotope fractionation associated with volatile element depletion within the accretion disk or from parent body metamorphism. The Group IAB iron meteorite samples analyzed show evidence of complex behavior and disturbance of Ag isotope systematics. Therefore, care must be taken when using this group of iron meteorites to obtain chronological information based on the Pd-Ag decay scheme.

Introduction

Silver is a trace element and is found at the ppb level in most terrestrial and extra-terrestrial materials. The exceptions to this are rare ore-deposits where Ag may occur as a native metal. In such deposits, the Ag is rarely pure and often forms alloys with Au, Cu, Te and Sb (Frueh and Vincent, 1967). Silver most commonly occurs in nature as a univalent cation and it is a moderately volatile element that can display both siderophile and chalcophile behavior.

Silver has just two naturally occurring isotopes, 107Ag (51.4%) and 109Ag (48.6%) and a value of 107Ag/109Ag = 1.07638 ± 0.00022 was determined for the Ag isotope reference material NIST SRM 978a in carefully calibrated mass spectrometric measurements. Analyses of native Ag metals from globally distributed mines have shown, that 107Ag/109Ag can vary by up to ∼0.6‰ in terrestrial ore samples, presumably due to mass dependent stable isotope fractionations that occurred at low temperatures (Hauri et al., 2000). This indicates that Ag isotopes may be a useful geochemical tracer, for example, in economic geology studies.

High 107Ag/109Ag ratios of up to almost 10 (Chen and Wasserburg, 1996) have been determined for certain meteorites, but these variations are thought to be due to radiogenic decay of 107Pd rather than stable isotope fractionation. The nuclide 107Pd decays to 107Ag with a half-life of only 6.5 Ma and in stellar nucleosynthesis it can be produced through both the s- and the r-process pathways. Many iron meteorites of the groups II, III and IV show good correlations between excess 107Ag and Pd/Ag ratios. These correlations are thought to be due to the presence of live 107Pd in the early solar system and in situ decay of this short-lived nuclide (Chen and Wasserburg, 1996). Palladium is both more refractory and more siderophile than Ag. Hence, volatile element depletion in the solar nebula and/or planetary core formation may have caused the requisite early Pd/Ag fractionations necessary for the system to be useful as a chronometer. The best examples of this are the iron meteorites, where the Fe-Ni metal typically has much higher Pd/Ag ratios than co-genetic sulfides, due to the highly siderophile nature of Pd. Isochrons can thus be constructed using these phases.

Early Ag isotopic measurements were carried out by TIMS (Chen and Wasserburg, 1983). The precision of TIMS analyses was limited to ∼±1‰ to 2‰, however, due to the difficulties in applying an instrumental mass bias correction because Ag has just two naturally occurring isotopes. Such data were sufficient for the study of meteorites with high Pd/Ag ratios, which developed large Ag isotopic anomalies. However, more precise measurement techniques are required to resolve the very small differences in Ag isotope compositions that are expected for meteorites that display low Pd/Ag. Carlson and Hauri (2001) addressed this problem and extended the Pd-Ag chronometer to low Pd/Ag materials using MC-ICPMS (multiple collector inductively coupled plasma mass spectrometry). By utilizing admixed Pd to correct for the instrument-induced mass discrimination of Ag, they were able to achieve a reproducibility of ∼±0.13‰ for the determination of 107Ag/109Ag.

This study builds on the work of Carlson and Hauri (2001) and it readdresses some of the difficulties associated with Ag isotopic measurements. Using a refined approach for the chemical separation of Ag from the bulk sample matrix, it has been possible to obtain accurate and precise Ag isotope data for samples with as little as 20 ng of Ag. This enables analysis of terrestrial samples with low Ag concentration and of low-Pd/Ag meteorites such as chondrites and group IAB iron meteorites.

Section snippets

Reagents and materials

All acids used in this study were purified by subboiling distillation. A second distillation (in a Teflon still) was found to greatly reduce Ag blanks. The water used was of 18-MΩ grade from a Millipore purification system (termed MQ water hereafter).

Two different standards were utilized in this study. The National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 978a is an Ag isotope standard, which is supplied in the form of an AgNO3 salt. This material was

Accuracy and precision of Ag isotope ratio measurements

An average 107Ag/109Ag of 1.08048 ± 0.00042 was measured in this study for NIST SRM 978a Ag, which is identical, within error, to the less precise TIMS value of 107Ag/109Ag = 1.0811 ± 0.0017 determined by Chen and Wasserburg (1983). Our result however, is higher than the value of 1.07916 ± 0.00052 reported by Carlson and Hauri (2001), who utilized a Plasma 54 MC-ICPMS. This small apparent discrepancy between the MC-ICPMS data probably reflects differences in the relative fractionation factors

Conclusions

Silver isotope ratios have been analyzed to high precision using admixed Pd to correct for the instrumental mass bias by external normalization. Application of external normalization together with standard-sample bracketing enables reproducibilities of ±0.5 ε and ±2 ε (2σ) to be achieved for Ag isotope ratio measurements of pure standard solutions and geological samples, respectively. The procedure of external normalization does not correct for fractionations that occur in nature or during

Acknowledgments

The authors would particularly like to thank the Smithsonian Institution, Washington DC, who kindly provided many of the meteorite samples used in this work. In addition, we would like to thank Erik Hauri and two anonymous reviewers whose comments greatly improved the original manuscript. Thanks are also due to Rick Carlson for many useful suggestions regarding silver isotope analyses.

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    Associate editor: R. J. Walker

    Present address: Department of Earth Sciences, Parks Road, Oxford OX1 3PR, UK.

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