Solution and laser ablationMC-ICP-MS lead isotope analysis of gold

Abstract

A new approach is presented for the analysis of lead isotopes in gold using solution and laser ablation multi-collector inductively-coupled plasma mass-spectrometry (MC-ICP-MS). Repeat analyses of two gold reference materials (RAuGP3: 34.1 ± 0.5 ppm Pb and RAuGP5: 129 ± 4 ppm Pb, from SPEX CertiPrep Ltd) yield an external reproducibility of <150 ppm for ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb and <250 ppm for ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb for solution analyses after lead separation using a two-stage ion exchange chromatography methodology. External reproducibility of <600 ppm for ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb is achieved for laser ablation analyses. Due to the low abundance of ²⁰⁴Pb and the isobaric interference of mercury, the external reproducibility for ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb is strongly dependant on the ²⁰⁴(Pb + Hg)/²⁰²Hg ratio: <1640 ppm when ²⁰⁴(Pb + Hg)/²⁰²Hg ∼1 to 5, <900 ppm when ²⁰⁴(Pb + Hg)/²⁰²Hg ∼5 to 20, and <810 ppm when ²⁰⁴(Pb + Hg)/²⁰²Hg >20. Instrumental mass bias is corrected by standard sample bracketing, using NIST SRM981 for solution analyses and NIST SRM610 for laser ablation analyses. Both techniques agree within error, and they have been further demonstrated through the analyses of both geological and archaeological samples: multiple grains from two Irish alluvial gold deposits and samples of two Early Bronze Age Irish gold lunulae. The applicability of these techniques to studies concerning the sources and age of gold mineralisation, and the identification of gold sources exploited in antiquity, is highlighted.