A Synthetic Haematite Reference Material for LA-ICP-MS U-Pb Geochronology and Application to Iron Oxide-Cu-Au Systems

In both nature and synthetic experiments, the common iron oxide haematite (alpha-Fe2O3) can incorporate significant amounts of U into its crystal structure and retain radiogenic Pb over geological time. Haematite is a ubiquitous component of many ore deposit types and, therefore, represents a valuable hydrothermal mineral geochronometer, allowing direct constraints to be placed on the timing of ore formation and upgrading. However, to date, no suitable natural haematite reference material has been identified. Here, a synthetic haematite U-Pb reference material (MR-HFO) is characterised using LA-ICP-MS and ID-TIMS. Centimetre-scale 'chips' of synthesised alpha-Fe2O3 were randomly microsampled via laser ablation-extraction and analysed using ID-TIMS. Reproducible U/Pb and Pb/Pb measurements were obtained across four separate chips (n = 13). Subsequently, an evaluation of the suitability MR-HFO in constraining U-Pb data via LA-ICP-MS is presented using a selection of natural samples ranging from Cenozoic to Proterozoic in age. The MR-HFO normalised U-Pb ratios are more concordant and ages more accurate versus the same LA-ICP-MS spot analyses normalised to zircon reference material, when compared with independently acquired ID-TIMS data from the same natural haematite grains. Results establish MR-HFO as a suitable reference material for LA-ICP-MS haematite U-Pb geochronology.