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<p>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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">U</mi><mo>/</mo><mi mathvariant="normal">Pb</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="30pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="162ad8a7dbace0ccfbe34a90b6fdeeec"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-6-465-2024-ie00001.svg" width="30pt" height="14pt" src="gchron-6-465-2024-ie00001.png"/></svg:svg></span></span> ratios (from <span class="inline-formula">∼</span> 1 to <span class="inline-formula">∼</span> 19) and a relatively high and homogeneous U content (<span class="inline-formula">∼</span> 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 <span class="inline-formula">±</span> 0.65 Ma, an initial <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi/><mn mathvariant="normal">207</mn></msup><mi mathvariant="normal">Pb</mi><msup><mo>/</mo><mn mathvariant="normal">206</mn></msup><mi mathvariant="normal">Pb</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="63pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="19914d6347e3d645b3e75a119433d985"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-6-465-2024-ie00002.svg" width="63pt" height="15pt" src="gchron-6-465-2024-ie00002.png"/></svg:svg></span></span> ratio of 0.8495 <span class="inline-formula">±</span> 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.</p>