57-11 Characterization of the Duff Brown Tank Limestone to Improve the Accuracy and Precision of U-Pb Carbonate Geochronology by LA-ICPMS and ID-TIMS

Session: Advances and Applications in Geochronology for Interpreting Stratigraphic and Basin Records, Part II

Presenting Author:

Authors:

Abstract:

U-Pb carbonate geochronology is used to constrain the age of a range of geologic events, including carbonate diagenesis, hydrothermal precipitation, and sediment deposition. Isotope dilution thermal ionization mass spectrometry (ID-TIMS) can measure dates to ca. 0.1% precision for silicate accessory phases like zircon, but high sample failure rates have limited its application for carbonates. A recent renaissance of U-Pb carbonate geochronology has been fueled by faster data acquisition and higher spatial resolution measurements by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). However, LA-ICPMS analysis requires well-characterized reference materials for sample-standard bracketing and mass fractionation corrections. We evaluated the Duff Brown Tank Limestone (DBTL), a widely distributed reference material with internal complexities. We posit that CL imaging followed by high spatial resolution LA-ICPMS can guide an informed microsampling approach for small-volume carbonate ID-TIMS U-Pb geochronology.

Cathodoluminescence (CL) images of DBTL reveal two contrasting textures of a packstone and sparry calcite infilling irregularly shaped voids, where the spar distinctly shows alternating luminescence. LA-ICPMS measurements from the calcite spar have low U and Pb concentrations that average < 0.3 ppm and define an isochron intercept age of ca. 63 Ma. Higher measurement and age uncertainties for the calcite spar are attributed to low U and Pb concentrations. The micrite contains ca. 1-4 ppm of U and Pb with a spread of data projecting to an intercept age of ca. 62 Ma. The micrite texture in DBTL is therefore better suited for U-Pb analysis.

Higher precision measurements are needed to constrain the timing of these closely related events and to support accurate interpretation of DBTL data. Characterizing DBTL opens the door to implementing mechanical sampling for ID-TIMS, with the goal of improving age uncertainties to ca. 0.1%. We have formatted a working data reduction and uncertainty propagation algorithm, for a reduced Pb blank mass and sample U/Pb ratios from dramatically decreased sample volumes used in isotope dilution, while maintaining ionization efficiency and improving measurement precision. Carbonate U-Pb geochronology using a LA-ICPMS and ID-TIMS microsampling workflow paired with cathodoluminescence images may help improve the accuracy of carbonate U-Pb geochronology and its applications.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9437

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.