| Summary: | <p>Latest advances in laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS) allow
for accurate in situ <span class="inline-formula">U−Pb</span> dating of carbonate material, with final age
uncertainties usually <span class="inline-formula">>3 <i>%</i></span> <span class="inline-formula">2<i>σ</i></span>. Cross-laboratory reference materials (RMs) used for
sample-bracketing are currently limited to WC1 calcite with an age of <span class="inline-formula">254.4±6.5</span>
(<span class="inline-formula">2<i>σ</i></span>). The minimum uncertainty on any age determination with the LA-ICPMS method is
therefore <span class="inline-formula">≥2.5</span> %, and validation by secondary RMs is usually performed on in-house
standards. This contribution presents a new reference material, ASH-15, a flowstone that is dated
here by isotope dilution (ID) thermal ionization mass spectrometry (TIMS) analysis using 37 sub-samples, 1–7 <span class="inline-formula">mg</span> each. Age
results presented here are slightly younger compared to previous ID isotope ratio mass spectrometry (IRMS) <span class="inline-formula">U−Pb</span> dates
of ASH-15 but within uncertainties and in agreement with in situ analyses using WC1 as
the primary RM. We provide new correction parameters to be used as primary or secondary
standardization. The suggested <span class="inline-formula"><sup>238</sup>U∕<sup>206</sup>Pb</span> apparent age, not corrected for
disequilibrium and without common-lead anchoring, is <span class="inline-formula">2.965±0.011</span> <span class="inline-formula">Ma</span> (uncertainties
are 95 % confidence intervals). The new results could improve the propagated uncertainties on
the final age with a minimal value of 0.4 %, which is approaching the uncertainty of typical ID
analysis on higher-<span class="inline-formula">U</span> materials such as zircon. We show that although LA-ICPMS spot analyses
of ASH-15 exhibit significant scatter in their isotopic ratios, the down-hole fractionation of
ASH-15 is similar to that of other reference materials. This high-<span class="inline-formula">U</span>
(<span class="inline-formula">≈1 ppm</span>) and low-<span class="inline-formula">Pb</span> (<span class="inline-formula"><0.01 ppm</span>) calcite is most appropriate as
a reference material for other speleothem-type carbonates but requires more-sensitive ICP-MS
instruments such as the new generation of single-collector and multi-collector ICP-MS. Reference
materials with high-<span class="inline-formula">Pb</span> and low-<span class="inline-formula">U</span> or both low-<span class="inline-formula">U</span> and low-<span class="inline-formula">Pb</span> compositions are
still needed to fully cover the compositional range of carbonate material but may introduce
analytical challenges.</p>
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