Micro-scale (∼10 μm) analyses of rare earth elements in silicate glass, zircon and apatite with NanoSIMS

Abstract

Micron-scale (∼10μm) analyses of rare earth elements (REEs) in silicate glass, zircon and apatite were carried with nano-scale secondary ion mass spectrometer (NanoSIMS) operating at high mass resolution mode (m/Δm=∼10000, 10%), which eliminating isobaric interferences of heavy REEs by light REE monoxides. To achieve high lateral resolution, the primary O− probe was optimized to a current of ∼1nA and a diameter<2.5μm, scanning over the analysis area of 5×5μm2 or 10×10μm2. The analyses were conducted at a combined mode of multi-collection and peak-jumping to improve the analytical efficiency. Fifteen ion species including rare earth elements and a reference element (Si or Ca) were counted through 4 times of magnetic field switch. The standards of NIST glass, apatite and zircon were analyzed in this study, which represent flat patterns, LREE-enriched and LREE-depleted and show different matrix effects. The relative sensitivity factors (RSF) of REEs are higher in zircon than NIST glass by a factor of 20–50% referred to 30Si+, and they are higher in apatite than NIST glass by a factor of 2–3 referred to 42Ca+. Therefore, the absolute concentration of REEs could be only calibrated with the matrix-matched mineral standards in the SIMS analysis. The analytical reproducibilities (1SD) were <5% for silicate glass (NIST SRM 610), <10% for apatite (Durango) and <15% for zircon (M257). The analytical accuracies determined from the measurements of the standards NIST glass (NIST SRM 612), apatite (MAD and Otter lake) and zircon (91500) were better than 10%, 20% and 30%, respectively, except for La interfered by ZrSiO and ZrO3 for zircon.