150-2 U-Pb dating of dolostone: am I projecting?

Session: Geochronology of Critical Mineral Deposits with Special Reference to U-Th-Pb Dating of Common-Pb-Rich Minerals (Posters)

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Abstract:

U-Pb dating of dolostone via LA-ICPMS is an emerging technique, which is potentially attractive due to dolostone’s relatively strong weathering-resistance and relatively high U concentrations.  This study seeks to: 1) test the viability of U-Pb dating of dolostone on samples of known age, 2) identify a matrix-matched reference material for LA-ICPMS dating, and 3) assess differences in matrix-induced fractionation for calcite vs. dolomite.  To that end we have analyzed hundreds of dolomite and dolostone samples, including:  1) roughly thirty ‘gemmy’ hydrothermal dolomite specimens from around the world, 2) Paleozoic dolostones from the Champlain Valley of Vermont, and 3) late Proterozoic and Paleozoic dolostones from the Kingston Peak Range of Nevada.  Initial results suggest that hydrothermal dolomite tends to be enriched in common-Pb and generally unsuitable for isochron dating.  In contrast Proterozoic and Paleozoic sedimentary dolostone often has measurable ²³⁸U/²⁰⁶Pb ratios that define an isochron, but ratios are rarely above ~5.  Curiously, isochrons constructed from these lower ²³⁸U/²⁰⁶Pb ratio shots tend to consistently underestimate the true age of the dolostone, rather than scatter randomly.  In contrast, we obtain relatively accurate isochron ages from dark-colored, presumably organic-rich, dolostones that produce the highest ²³⁸U/²⁰⁶Pb ratios with the most accurate isochron ages. Efforts to identify a suitable dolostone reference material have so far failed given the typically low ²³⁸U/²⁰⁶Pb ratios of dolostone.  However, experiments using a TIMS-dated dolomite specimen as a primary reference material demonstrate up to 20% differences in ²³⁸U/²⁰⁶Pb fractionation between calcite and dolomite analyzed in the same analytical session.  These differences are magnified at low laser energies and underscore the potential pitfalls of U-Pb dating of dolostone relative to a calcite reference material.

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

doi: 10.1130/abs/2025AM-10384

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