LA-ICP-MS U-Pb carbonate geochronology: strategies, progress, and application to fracture-fill calcite

Abstract. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb geochronology of carbonate minerals, calcite in particular, is rapidly gaining popularity as an absolute dating method. The technique has proven useful for dating fracture-fill calcite, which provides a powerful record of palaeohydrology, and within certain constraints, can be used to bracket the timing of brittle fracture and fault development. The high spatial resolution of LA-ICP-MS U-Pb carbonate geochronology is beneficial over traditional Isotope Dilution methods, particularly for diagenetic and hydrothermal calcite, because uranium and lead are heterogeneously distributed on the sub-mm scale. At the same time, this can provide limitations to the method, as locating zones of radiogenic lead can be time-consuming and ‘hit or miss’. Here, we present strategies for dating carbonates with in situ techniques, through imaging and petrographic techniques to data interpretation; we focus on examples of fracture-filling calcite, but most of our discussion is relevant to all carbonate applications. We demonstrate these strategies through a series of case studies. We review several limitations to the method, including open system behaviour, variable initial lead compositions, and U-daughter disequilibrium. We also discuss two approaches to data collection: traditional spot analyses guided by petrographic and elemental imaging, and image-based dating that utilises LA-ICP-MS elemental and isotopic map data.