Measuring the isotopic composition of trace elements in zircon: application to radiogenic molybdenum

Abstract

Techniques have been developed to measure the isotopic composition of trace elements from matrices predominantly consisting of interfering isotopes. These techniques have been applied to measuring mass-independent fractionation of molybdenum in ancient zircon samples due to nuclear decays. Improvements to the digestion of large zircon samples and the ion exchange chemistry required to separate trace molybdenum from zirconium silicate are presented. The efficacy of TEVA ion exchange resin as a replacement for anion resin to improve separation efficiency and chemistry blank was studied. An algorithm was established to improve background, interference and mass bias corrections for the data analysis of Mo by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS). This enabled the isotopic composition measurement of $<$50 ng of Mo recovered from 500 mg ZrSiO$_4$ by reducing the Zr content by $>$9 orders of magnitude while retaining 63% of the Mo. With 5 ng of Zr still remaining, novel data analysis techniques enabled $<$0.1 permil precision on each Mo isotope. The relevance of this technique to extracting other atomic species is also discussed.