A high-temperature sintered cassiterite reference material for in situ determination of Sn isotope ratios

Abstract

Microanalysis of tin isotopes in cassiterite by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) has been an important tool to reconstruct the fluid evolution or discriminate petrological sources and processes. However, it is a major challenge to prepare the absolutely essential cassiterite reference material that has a homogeneous Sn isotope composition on a micron scale. In this study, we report a novel method to synthesize a homogeneous cassiterite reference material via sintering a micrometer powder. Under the optimized conditions (600 °C, 1 GPa), the synthesized cassiterite has a smooth surface and dense structure that is suitable for laser ablation analysis. The Sn signal for the synthesized cassiterite was consistent with that of natural cassiterite and lower than that of the pressed powder pellets by LA-ICP-MS, indicating that the sintered cassiterite approximated the theoretical maximal density. LA-MC-ICP-MS measurements suggest that the homogeneity (2SD) for δ¹²⁴Sn/¹²⁰Sn was improved from 0.64‰ (original cassiterite crystal) to 0.10‰ (synthesized cassiterite). The reproducibility of δ¹²⁴Sn/¹²⁰Sn was 0.07‰ (2SD) for the synthesized cassiterite. These data have demonstrated that the synthesized cassiterite has a homogeneous distribution of Sn isotopes. Therefore, we recommend combining ultra-fine powders with the high-temperature sintering technique to synthesize other minerals for microanalysis.