A magnetite reference material for LA-ICP-MS U–Pb geochronology from the Troodos Ophiolite, Cyprus

Abstract. Magnetite is an increasingly important target material for in situ geochemical analysis and has recently emerged as a promising U–Pb geochronometer. However, the wider application of the method is hindered by the lack of a widely available matrix-matched reference material. Here we propose TR41, a hydrothermal magnetite from the Troodos ophiolite (Cyprus), as a candidate reference material for LA-ICP-MS U–Pb magnetite geochronology. Petrographic, geochemical, and isotopic data demonstrate that TR41 is sufficiently homogeneous for laser ablation analyses and yields clearly defined regressions in Tera–Wasserburg diagram despite U concentrations below 1 µg g^-1. As independent ID-TIMS characterisation of magnetite is currently not feasible, the age of TR41 was constrained using complementary geochronological approaches. U–Pb dating of coexisting hydrothermal andradite yields a lower-intercept age of 92.04 ± 2.23 Ma. Assuming that the andradite and magnetite formed during the same hydrothermal event, four blind-test samples containing coexisting magnetite and garnet or apatite were analysed. In all cases, magnetite ages obtained using TR41 as the primary reference material agree, within uncertainty, with independently determined ages of the associated minerals. These results demonstrate that TR41 provides an appropriate matrix correction for LA-ICP-MS U–Pb analyses of magnetite and represents a practical reference material for expanding the magnetite U–Pb geochronology. TR41 is available for distribution to interested laboratories upon request from the corresponding author.