Improved in situ measurements of lead isotopes in silicate glasses by LA-MC-ICPMS using multiple ion counters

Abstract

A new technique that improves the spatial resolution and quantification limits of the measurement of lead isotope ratios in silicate glasses with <15 μg g⁻¹ total Pb by LA-MC-ICPMS is presented. The new method provides the capability of making quantitative, in situ lead isotope measurements on tiny objects of geologic interest such as mineral growth bands, melt inclusions, and accessory minerals, even where they are lead poor. The method allows for the concurrent, static measurement of ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb along with ²⁰²Hg in five Channeltron ion counters. Standard-sample-standard bracketing using USGS BCR2-G as the calibrant is used to correct for instrumental mass bias. Accuracy and precision of the method was evaluated by replicate analyses of various MPI-DING reference glasses with low lead concentrations (∼1–11 μg g⁻¹) and well-determined isotopic ratios. Spot sizes for in situ analyses were as small as 40–69 μm, providing better spatial resolution than previous LA-MC-ICPMS results reporting ²⁰⁴Pb. Measured lead isotope ratios for the MPI-DING reference glasses T1-G (11.6 μg g⁻¹ total Pb) and ATHO-G (5.67 μg g⁻¹ total Pb) agree within 0.10% and 0.15% respectively of the preferred values using 40 μm spots. For MPI-DING KL2-G (2.07 μg g⁻¹ total Pb) and ML3B-G (1.38 μg g⁻¹ total Pb) measured lead ratios agree within 0.75% of the accepted values with typical precisions of <1.9% (2RSD) using 69 μm spots; measured ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb are within 0.45% of preferred values with precisions of <0.50% (2RSD). These results demonstrate improvement over previous LA-MC-ICPMS data in terms of both quantification limits and spatial resolution, while retaining similar levels of accuracy and precision.