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.

Exploring the advantages and limitations of in situ U–Pb carbonate geochronology using speleothems

“…an analysis of single calcite fibres in vein structures (e.g. Goodfellow et al, 2017). Here we use an example from speleothem studies.…”

“…The primary concern for any samples of this type, and particularly when measuring by LA, is the obvious potential for contamination with environmental ("blank") Pb during analysis. Although LA rastering and extraction of age information from the resulting isotopic images have shown great promise for dating limestones with relatively high Pb abundances (Drost et al, 2018), the same approach cannot be easily implemented in speleothems where Pb contents are often in the low parts per billion range and thus where each spot analysis may be measuring total Pb amounts in the low femtogram range. For this reason, in this study, we have used spot analyses and perform a clean pre-ablation with a larger Gas blank 30 s on-peak zero subtracted using Iolite smoothing spline Calibration strategy WC-1 carbonate Data processing package used or correction for LIEF UcomPbine DRS vs. 2015.06 running in Iolite vs. 3.71 providing baseline subtraction, downhole correction and calibration against the reference material; the 207 Pb-based common-Pb method was employed.…”

Direct U-Pb dating of carbonates from micron scale fsLA-ICPMS images using robust regression

“…Since the seminal publication of Li et al (2014), the U-Pb dating of carbonates by LA-ICPMS has progressed significantly with applications as varied as the dating of tectonic fractures and veins (e.g., Roberts & Walker, 2016;Beaudoin et al, 2018;Parrish et al, 2018;Nuriel et al, 2019), carbonate deposition (Drost et al, 2018), speleothems (Woodhead et al, 2019) or https://doi.org/10.5194/gchron-2020-10 Preprint. Discussion started: 14 April 2020 c Author(s) 2020.…”

New analytical and data evaluation protocols to improve the reliability of U-Pb LA-ICP-MS carbonate dating

“…Within a day, 300-600 single points can be analysed and a minimum number of 20-30 points per sample is suggested (Beaudoin et al, 2018;Godeau et al, 2018;Yokoyama et al, 2018), although the appropriate number is strongly sample-specific and depends on the variability of the initial to radiogenic 35 Pb ratios. To maximize the variation of the initial to radiogenic Pb ratios and improve the isochrone, new approaches in the acquisition of data by imaging and data pooling were introduced and look promising (Drost et al, 2018).…”

“…Previous studies of dating carbonates, mostly calcite, focused either directly on the analytical method development (Li et al, 2014;Roberts et al, 2017;Drost et al, 2018;Yokoyama et al, 2018) or a range of applications from dating fault activity to directly dating early-diagenetic cements in ammonites (Coogan et al, 2016;Methner et al, 2016;Burisch et al, 2017;Drake et 40 al., 2017;Goodfellow et al, 2017;Nuriel et al, 2017;Hellwig et al, 2018;Walter et al, 2018;MacDonald et al, 2019;Scardia et al, 2019). Only recently, secondary RMs were routinely analysed (Beaudoin et al, 2018) with data reporting standards following more or less the community derived standards suggested for zircons (Horstwood et al, 2016).…”