Tianyu-Py pyrite: a new natural reference material for micro-beam determination of iron isotopic ratios

Abstract

The Fe isotopes of bulk Fe sulfides have become useful tracers for understanding geochemical conditions and processes ubiquitously involving Fe during both biological and non-biological processes under various conditions. However, Fe sulfides occur as intergrowth textures in most cases. This gives rise to isotopic results obtained from bulk samples representing a mixed signal. Micro-beam analytical techniques for Fe isotopic analysis using secondary ion mass spectrometry (SIMS) and laser ablation-(multi-collection)-inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) are capable of resolving Fe isotopic variations between natural minerals commonly present as thin lamellae in iron meteorites, micron scale phases in sedimentary, and zonation in hydrothermal deposits. Pyrite is the most common Fe sulfide mineral in igneous, sedimentary, and metamorphic rocks on Earth. In this study, natural pyrite (Tianyu-Py) from the Tianyu magmatic Ni–Cu massive sulfide ores (China) is evaluated as an Fe isotopic RM by multiple methods. Major elements are homogeneously distributed as verified by electron microprobe analyses. The results of LA-MC-ICP-MS iron isotopes prove the homogeneity of Fe isotopes. For in situ microanalysis, the standard deviation of iron isotopic compositions is 0.20‰ (2SD, n = 105) for the δ⁵⁶Fe value and 0.31‰ (2SD, n = 105) for the δ⁵⁷Fe value. Based on the solution MC-ICP-MS analyses, the best recommended δ⁵⁶Fe and δ⁵⁷Fe values for Tianyu-Py pyrite are 0.49 ± 0.03‰ (2SD, n = 4) and 0.73 ± 0.04‰ (2SD, n = 4), respectively. In summary, the Tianyu-Py pyrite RM is suitable for in situ Fe isotope microanalysis using a laser spot size of 16 μm and is expected to be used for tracking the formation of pyrite precipitation in biological and geological processes.