In situaccurate and precise lead isotopic analysis of ultra-small analyte volumes (10–16m3) of solid inorganic samples by high mass resolution secondary ion mass spectrometry

Abstract

A sensitive high mass resolution ion microprobe (SHRIMP) SIMS instrument was used to analyse the Pb isotopic composition of ultra-small analyte volumes (10^–16 m³) of NIST SRM 610, an in situ technique applicable to any solid inorganic sample. The accuracy and precision of multiple analyses are comparable to those attained by TIMS and double focusing multiple collector ICP-MS. The international standard NIST SRM 610 was used to demonstrate the technique. No instrumental mass fractionation correction is necessary for these SIMS analyses, although a correction for the isobaric interference of ²⁰⁴Hg on ²⁰⁴Pb in NIST SRM 610 is necessary for accuracy of the ²⁰⁴Pb measurement, and is estimated to be 0.8%. With consideration of our values with respect to those from other techniques, we propose a set of ‘preferred’ values for Pb isotope ratios in NIST SRM 610, and support its usefulness as an international Pb isotope standard: ²⁰⁶Pb:²⁰⁴Pb 17.050±0.026; ²⁰⁷Pb:²⁰⁶Pb 0.908 56±0.000 68 and ²⁰⁸Pb:²⁰⁶Pb 2.1663±0.0015 (2s uncertainty). The advantage of being able to perform in situ accurate and precise Pb isotope analyses on ultra-small volumes allows the application of this SIMS technique to areas in the forensic and environmental sciences, archaeology and geochemistry where, in the last case, important isotopic information is obtainable on thin or shallow micro-structures such as melt films at grain boundaries and shallow internal structures in individual mineral grains. Such applications are not possible using dissolution techniques and are difficult or impossible with current laser ablation ICP-MS instruments. An application of the technique to the measurement of Pb isotopes in 5 µm sulfide crystals included within a large natural Ca(Mg,Fe²⁺)Si₂O₆ (diopside) crystal is presented. The results suggest that the source of the host rocks to the sulfides was an enriched mid-ocean ridge basalt. This result is consistent with a previous study of major and trace elements of the same sample.