Single-zircon evaporation combined with Pb⁺ emitter bedding for ²⁰⁷Pb/²⁰⁶Pb-age investigations using thermal ion mass spectrometry, and implications to zirconology

Abstract

The internal precision of Pb isotope analyses using single-zircon evaporation in a double-filament solid source mass spectrometer (Kober 1986) can be improved combining the evaporation of Pb directly from the single grain with a suitable Pb⁺ emitter-bedding technique. This is most easily done by step-wise evaporating the investigated grain at temperatures of 1700–1800 K generating on the ‘cold’ ionization filament a deposit of radiogenic Pb together with further elements and compounds derived directly from the crystal. The heating of the deposit on the ionization filament to 1400–1500 K results in long-lived and stable Pb⁺ ion beams. The ‘activating reagents’ in the deposit are HfO₂ and SiO₂. Their release from the zircon grain together with the radiogenic Pb, which presumably is sited in the crystalline zircon domains as Pb⁴⁺, is probably due to disintegration reactions of trace-element silicates hosted in the grain.

In the bedding deposited on the ionization filament thermally stable Pb/Hf/SiO₂ compounds are formed (PbHfSiO₅(?)). They retain the Pb isotopes on the (Re) filament up to 1400 K–1500 K and are highly efficient Pb⁺ ion emitters similar to the ‘Si-gel’-method (Cameron et al. 1969).

The combined evaporation/emitter-bedding technique has been applied to natural zircons of different genesis and to isotope standards. Routinely, a Pb⁺ ion yield of 2*10⁻⁴-1*10⁻³ and a relative standard deviation of the ²⁰⁷Pb/²⁰⁶Pb ratio in the order of 1% have been obtained for sub-ng- to ng-amounts of Pb from standards and samples. The method rapidly can yield Pb isotope information on the ‘concordant’ zircon phases with a standard deviation of ±15–20 Ma of the derived ages also in the case of Paleozoic zircon populations.