Albany K-Feldspar: A New Pb Isotope Reference Material

Abstract

Lead isotopes are a powerful and versatile tool to elucidate fundamental geological problems related to the formation and evolution of continental crust. K-feldspar is a popular target for Pb isotope measurement as it is prevalent in many rock types and tends to capture the initial Pb isotope composition of its parental magma. We present data for a new Pb isotope reference material: Albany K-feldspar; as well as updated data for Shap K-feldspar. Results of Pb double-spike TIMS for Albany K-feldspar are ²⁰⁶Pb/²⁰⁴Pb = 16.7872 ± 0.0062, ²⁰⁷Pb/²⁰⁴Pb = 15.5640 ± 0.0056, and ²⁰⁸Pb/²⁰⁴Pb = 36.6600 ± 0.0168 (2s). TIMS measurement results for Shap K-feldspar indicate two isotopically distinct Pb populations. LA-MC-ICP-MS, with a spatial resolution as high as 15 μm, indicates a homogeneous Pb isotopic composition in Albany K-feldspar. In accord with previous studies, our results show that scatter in the measured Pb isotope ratios, related to the low natural isotopic abundance of ²⁰⁴Pb, along with the effect of isobaric ²⁰⁴Hg-²⁰⁴Pb interference, increases at lower count rates. However, the mean Pb isotope ratios measured via LA-MC-ICP-MS using a range of spot sizes are in excellent agreement with TIMS results thus highlighting the feasibility of Pb isotope determination via LA-MC-ICP-MS to access geological information preserved in small crystals, including mineral inclusions.