A promising primary reference material for U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) carbonate dating is analysed and reported here. The new reference material (RM) is a botryoidal cement (C1) from sample RA138. The sample was collected in outcrop strata of mid-Carboniferous age in northern Spain near La Robla, and multiple aliquots have been meticulously prepared for distribution. RA138 is characterized by variable U/Pb ratios (from ∼ 1 to ∼ 19) and a relatively high and homogeneous U content (∼ 4 ppm). This material exhibits a low age uncertainty (0.2 %, 2 s; unanchored; ID-TIMS), allowing for the establishment of a well-defined isochron, particularly when anchored to the initial Pb ratio using LA-ICP-MS. Isotope dilution thermal ionization mass spectrometry (ID-TIMS) analyses of micro-drilled C1 cement (17 sub-samples) produce a lower-intercept age of 321.99 ± 0.65 Ma, an initial 207Pb/206Pb ratio of 0.8495 ± 0.0065, and a mean square weighted deviation (MSWD) of 5.1. The systematic uncertainty of 1.5 % observed in repeated LA-ICP-MS analyses challenges previous estimations of 2 %–2.5 % based on repeated analyses of ASH-15D and JT using WC-1 as primary reference material, underscoring the precision and reliability of RA138 for U–Pb dating applications.