In situ Sn isotope analysis of cassiterite (SnO2) by nanosecond laser ablation MC-ICP-MS

Abstract

Stable Sn isotopes are emerging tracers for understanding the cycling of tin on Earth. Cassiterite, the most important Sn-bearing mineral in nature, commonly contains complex zoning and intergrowth textures that record the Sn mineralization history. Thus, in situ analysis of Sn isotopes in cassiterite could provide a powerful tool for deciphering Sn mineralization. In this study, we developed a method for in situ Sn isotopic analysis of cassiterite based on a nanosecond laser ablation system and a Nu 1700 Sapphire multi-collector inductively coupled plasma mass spectrometer (ns-LA-MC-ICP-MS). The laser ablation isotopic analyses were made by sample-standard bracketing with elemental doping of Sb introduced by solution nebulization. A newly developed large and homogeneous natural cassiterite crystal (lab ID: #cas17) was used as the matrix-matched bracketing standard for laser ablation analyses. To report in situ Sn isotope data with reference to the international Sn isotope reference material NIST 3161a, a simple and reliable method was developed to decompose cassiterite for precise and accurate Sn isotope analysis by solution nebulization MC-ICP-MS, and the δ^122/118Sn_3161a of cassiterite “#cas17” was determined to be 0.36 ± 0.02‰. Based on a series of tests of different parameters (fluence, spot size, frequency, He gas flow rate, and ablation mode), the optimized analytical condition was found to be a 1 μm s⁻¹ line scan with a laser spot size of 13 μm and a laser fluence of 13.5 J cm⁻², firing at 5 Hz, in a He carrier gas flow at 760 ml min⁻¹. Under these conditions, the intermediate precision of the in situ Sn isotope analytical method was better than ±0.12‰ in δ^122/118Sn. Matrix effects from the trace elements of the natural cassiterite on measured Sn isotopic ratios were insignificant. The in situ Sn isotope analytical method was applied to different cassiterite samples, which yielded a range of 0.98‰ in δ^122/118Sn among and within these cassiterite samples, and the laser ablation results were confirmed by the solution nebulization (SN) MC-ICP-MS analyses after dissolution. Our new method provides an efficient and effective approach for determining the Sn isotopic composition of cassiterite and could be applied to studies of Sn ore deposits.