Laser-ablation MC-ICP-MS lead isotope microanalysis down to 10 μm: application to K-feldspar inclusions within zircon

Abstract

The geochemical information preserved in mineral inclusions within zircon provides new ways to decipher the evolution of the continental crust. We present a routine procedure to analyse Pb isotopes in K-feldspar minerals and mineral inclusions in zircon by LA-MC-ICP-MS, with a spot size down to 10 μm diameter. Natural K-feldspar grains from the Shap granite (north England) were selected for their homogenous Pb isotope composition to validate the method. The Faraday cups of the Thermo Scientific Neptune Plus MC-ICP-MS were used to collect the beams for spot sizes of 150 μm and 40 μm. The average ²⁰⁶Pb/²⁰⁴Pb ratios were respectively 18.255 ± 0.27% and 18.24 ± 1.3% (2 s.d.), and they agree within their analytical uncertainties. Ion counters (MIC) were used at 10 μm to allow better determination of the low intensity signal for Pb isotopes. The ²⁰⁶Pb/²⁰⁴Pb average ratio is 18.29 ± 1.3% (2 s.d.), which is within error of the average values at spot sizes of 150 μm and 40 μm. For a 10 μm spot size internal errors on individual measurements are typically better than 1.1% (2 s.e.). K-feldspar inclusions in zircons from a monzonite intrusion in Dronning Maud Land (East Antarctica), were analysed by MIC-LA-MC-ICP-MS (10 μm spot and ∼12 μm depth). The Pb isotope ratios are similar to those previously obtained by SIMS with a Cameca IMS 1270 (∼40 × 20 μm elliptic spot and ∼2 μm depth). These SIMS data are used to validate the robustness of MIC-LA-MC-ICP-MS measurements in minute mineral inclusions.