Performance Optimization of a Large Geometry High Resolution-Secondary Ion Mass Spectrometer (HR-SIMS) for High Precision Measurements of Oxygen Isotopic Composition (δ18O) and U-Pb Geochronology in Zircon

Abstract

A large geometry high resolution secondary ion mass spectrometer (HR-SIMS) has been established as a part of the National Geochronology Facility (NGF) at Inter-University Accelerator Centre (IUAC), New Delhi. The performance of the instrument related to high spatial resolution, high mass resolving power (MRP), sensitivity of the instrument to measure low abundant isotopes, and sensitivity of the instrument for ²⁰⁶Pb signal under different conditions are optimized and presented in this paper. We report the precision of the order of ~ 0.2‰ for oxygen isotopes measurement in 91500 reference material zircon and measured δ¹⁸O value of 10.08‰ ± 0.18‰ (2SD), which is in agreement with the recommended values. For U-Pb age measurement in zircon, Plešovice and FCT reference materials are measured as unknown and their ²⁰⁶Pb/²³⁸U ages agree with the reported values within the uncertainties. A long-term evaluation of ²⁰⁶Pb/²³⁸UO and ²⁰⁶Pb/²³⁸U isotopic measurement ratio is also presented.

Keywords: HR‐SIMS; U–Pb dating; isotope ratio measurements; precision; zircon.