An Image Mapping Approach to U-Pb LA-ICP-MS Carbonate Dating and Applications to Direct Dating of Carbonate Sedimentation

Abstract

We present a new approach to laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) U‐Pb dating of carbonates based on selection and pooling of pixels from 2‐D elemental and isotopic ratio maps. This image mapping technique is particularly useful for targeting subdomains in samples with complex geological histories. Key major and trace elements that are sensitive to detrital components, postformational fluid ingress, mineralogical changes, or diagenetic overprinting are measured along with the Pb and U isotopic data. Laser sampling is undertaken along successive linear rasters that are compiled into maps using the Monocle add‐on for Iolite, with one pixel in the map corresponding to one time slice of the time‐resolved signal. These element, element ratio, and isotope ratio maps can be overlain over photomicrographs or scanning electron microscopy images to spatially link compositional data to textural and structural features. The pixels corresponding to likely homogeneous age domains can be isolated by applying appropriate selection criteria (e.g., Th < 0.3 ppm, Mg/Ca < 0.004) and pooled into pseudo‐analyses using a proxy for the parent/daughter ratio (e.g., 207Pb/235U, 238U/208Pb) to retrieve the largest possible spread of the data points on isochron diagrams. The approach is best suited for analytical setups capable of rapidly or simultaneously scanning over a large mass range and can yield a precision of ±1% or better on quadrupole instruments depending on U concentration, 238U/204Pb, and age of the sample. The sample‐specific filtering criteria for selection and rejection of data and their rationale can be reported, resulting in more transparency with regard to data processing.