Mass-independent Isotope Fractionation of Molybdenum and Ruthenium in Murchison

Introduction: Dauphas et al. [1] and Yin et al. [2] reported molybdenum isotopic anomalies in bulk undifferentiated and differentiated meteorites and this was the first time that isotopic anomalies were found on the scale of a meteorite hand-specimen. Even more recently, Chen and Papanastassiou [3,4] identified ruthenium isotopic anomalies in meteorites. Dauphas et al. [5] used theoretical production ratios in stars to show that Mo and Ru isotopic anomalies correlate as predicted by nucleosynthetic models and concluded that such a correlation demonstrates the nucleosynthetic origin of the isotopic anomalies. The predicted correlation, however, fails for one sample, the Murchison CM3 chondrite, and the present study suggests a new mechanism to account for this discrepancy. This mechanism appeals to non-mass dependent isotopic fractionation processes, which Bigeleisen [6] ascribes to the finite size of the nucleus (nuclear field shift effect). A number of experiments mostly involving solvent extraction and liquid chromatography confirmed the existence of the mass-independent effects predicted by the nuclear field shift effect (see references in [7]) In the present work, we present experimental evidence of chemical mass-independent isotope fractionation for both Mo and Ru. Then, the nuclear field shift theory is applied to the actual isotopic anomalies found in Murchison. Experiments and Results: Molybdenyl dichloride (MoO2Cl2) was dissolved in HCl to produce 0.1 M Mo(VI) solutions at various strength of HCl normalities. The organic phase was 0.1 M DC18C6 (Dicyclohexano-18-crown-6) in 1,2-dichloroethane. Aqueous and organic solutions were contacted (295±0.5 K) in a glass vial, and after the equilibrium, the two phases were separated by centrifugation. An aliquot of the supernatant aqueous solution was taken for concentration determination. Mo was subsequently purified from leftover traces of organic materials left by the DC18C6 solution by employing an anion-exchange chromatographic separation. A solution containing 200 ppb Mo in 0.05 M HNO3 was prepared for mass spectrometric analysis. Isotopic ratios of Mo in all samples were analyzed with the MC-ICPM-MS Nu plasma 500 HR coupled with a desolvating nebulizer Nu DSN-100. The instrumental mass bias was controlled by bracketing each sample with the unprocessed Mo solution. Isobaric interferences with Zr and Ru were below the detection limit of the mass spectrometer. The isotopic ratios are reported as deviations of the measured values. Hydrated ruthenium trichloride was dissolved in HCl to produce 0.07 M Ru(III) solutions at various HCl strengths. The details of the extraction procedure and purification from organic impurities follow those described for Mo. A solution containing 70 ppb Ru in 0.2 M HNO3 was created for isotopic analysis. The measurement protocol of isotope ratio was same with the case of Mo. Isobaric interferences with Pd and Mo were below the detection limit of the mass spectrometer. Let us define the isotope separation factor α as αm,m’ =([M]/[M])org/([M]/[M])aq (1) where m’ and m indicate masses of light isotope and heavy isotope, respectively. ([M]/[M])org and ([M]/[M])aq are the isotope ratios of M (M = Mo or Ru) relative to M found in the organic and aqueous phases, respectively. (m = 100 for Mo and 104 for Ru). The isotope enrichment factor ε is defined as εm,m’ = (αm,m’ − 1) × 10,000 (2)