Secondary Ion Mass Spectrometry Methodology for Isotopic Ratio Measurement of Micro-Grains in Thin Sections: True Grain Size Estimation and Deconvolution of Inter-Grain Size Gradients and Intra-Grain Radial Gradients

In a thin section, grains that were approximately spherical in situ appear circular in cross section, and the distribution of apparent diameters frequently assumed to be their size distribution. Scanning ion imaging by secondary ion mass spectrometry (SIMS) is capable of providing precise (< 1‰) stable isotope ratio measurements of such grains, but, importantly, also registers their rate of evolution in apparent size as they are ablated by the primary beam. By assessing rates of radius change with depth, the described methodology enables the ‘true’ size of grains to be estimated, as well as the distance of the sectioned surface from the original grain centre. Transects in three dimensions are made possible, and this capability enables better identification (and thus separation) of both inter-grain chemical signatures as a function of grain size, and intra-grain radial trends. In this example, we highlight the specific application to pyrite (FeS₂) minerals, which are frequently analysed by SIMS to determine their inter-grain and intra-grain geochemical variations, particularly in their sulfur stable isotopic ratios (δ³⁴S). Benefits of the new methodology over the Faraday cup ‘spot mode’ are described. Data correction algorithms and precision considerations are discussed.