Calcite TLM as a new natural reference material for in situ U-Pb geochronology and its geological implications

Calcite (CaCO3) forms in a wide variety of geological environments as both a primary and secondary mineral phase and may contain sufficient U for U-Pb geochronology. The limited availability of well-characterized calcite reference materials, however, has impeded its broader application in microanalysis. In this study, a natural calcite sample (TLM) was characterized and assessed as a potential primary reference material for in situ U-Pb geochronology. U-Pb isotopic analyses were conducted across nine independent laboratories using laser ablation (Q, SF, MC)-ICP-MS and isotope dilution (ID)-TIMS, to examine the homogeneity and to determine the U-Pb age of TLM. Calcite TLM has an age homogeneity at the level of ∼ 1.0 %, as indicated by multiple analyses from different laboratories. ID-TIMS analysis of 14 aliquots yielded an intercept age of 222.5 ± 2.8 Ma (2 s) with an initial 207Pb/206Pb value of 0.8380 ± 0.0013. Calcite TLM has an age uncertainty of 1.28 %, which is considerably lower than that of WC-1 (∼2.5 %). Calcite TLM has U and Pb contents of ∼0.35 μg g−1 and ∼ 0.02 μg g−1. Additional C and O stable isotope, radiogenic Sr isotope and trace element compositional data indicate that calcite TLM is likely sourced from meteoric water and indicate that the recurrent karstification may have affected the studied area as late as the Triassic.