RioM‐1: A New Calcite Reference Material for U‐Pb LA‐ICP‐MS Geochronology

Determining absolute ages of carbonate diagenesis, faulting, fossil formation, speleothem growth, carbonate‐hosted hydrocarbon deposits, vein mineralisation and hydrothermal alteration has become increasingly accessible through LA‐ICP‐MS U‐Pb dating of calcite, complementing traditional isotope dilution methods still applicable to certain materials (e.g., speleothems via micro‐drilling). However, well‐calibrated reference materials for LA‐ICP‐MS calcite geochronology remain scarce. Here, we characterise the Rio Maior calcite, designated ‘RioM‐1’, as a potential reference material for U‐Pb dating by LA‐ICP‐MS. Fragments (0.1 to 1 cm 3 ) from a single scalenohedral crystal were analysed by ID‐TIMS (U‐Pb), LA‐ICP‐MS (U‐Pb and 87 Sr/ 86 Sr), and SIMS (O isotopes). RioM‐1 displays high U mass fraction and low, though variable, proportions of common Pb. Combined ID‐TIMS analyses from two independent laboratories yielded a Tera‐Wasserburg lower intercept date of 63.93 ± 0.11 Ma (2 s , MSWD = 1.3, n = 16). LA‐ICP‐MS U‐Pb data from three independent laboratories are concordant with the TIMS age, producing a pooled date of 64.10 ± 0.12/1.2 Ma (2 s , MSWD = 7.7, n = 708) and an initial 207 Pb/ 206 Pb ratio of 0.85 ± 0.01. Other calcite reference materials, when normalised to RioM‐1, yielded dates within uncertainty of their published values. SIMS measurements returned a mean 18 O/ 16 O of 0.002044450 ± 181 (1 s ) and δ 18 O SMOW of 19.57 ± 0.92‰ (1 s ), while LA‐ICP‐MS strontium isotope measurement yielded a mean 87 Sr/ 86 Sr of 0.708177 ± 9 (2 s ).